PROPER SHAPE FOR STANDARD SHOP TOOLS
267 As stated in paragraph 1 (Part 1) of this paper, our principal object in carrying on the investigation has been to obtain the knowledge required in fixing daily a definite task, with a time limit, for each machinist. It is evident that this involves the use of standard cutting tools throughout the shop which are in all respects exact duplicates of one another.
A FEW CAREFULLY CONSIDERED TYPES OF TOOLS BETTER THAN GREAT VARIETY
268 In our practical experience in managing shops we have found it no easy matter to maintain at all times an ample supply of cutting tools ready for immediate use by each machinist, treated and ground so as to be uniform in quality and shape; and the greater the variety in the shape and size of the tools, the greater becomes the difficulty of keeping always ready a sufficient supply of uniform tools. Our whole experience, therefore, points to the necessity of adopting as small a number of standard shapes and sizes of tools as practicable. It is far better for a machine shop to err upon the side of having too little variety in the shape of its tools rather than on that of having too many shapes. And the energy of the department for maintenance of tools should be directed rather toward securing an ample and uni- form supply of tools limited to a few shapes than to a great variety. The writer can hardly lay too much stress upon this point and it, therefore, becomes all the more important to use the greatest care and judgement in selecting the standard shape which is to be used for roughing tools.
STANDARD TOOLS ILLUSTRATED
269. In Folder 5, Figs. 20a to 25e, are illustrated the shapes of the standard tools which we have adopted, and in justification of our selection the writer would state that these tools have been in practical use in several shops both large and small through a term of years, and are giving general, all-round satisfaction. It is a matter of interest also to note that in several instances changes were introduced in the design of these tools at the request of some one foreman or superintendent, and after a trial on a large scale in the shop of the suggested improvements, the standards as illustrated above were again returned to. These shapes may be said, therefore, to have stood the test of extended practical use on a great variety of work.
270 It would seem to be of such great importance to explain fully our reasons for adopting these tools as our standards, that the writer will describe in the pages immediately following a number of the experiments, the results of which have perhaps had the greatest influence in determining the curve or line of the cutting edge. With this end in view, it has been found necessary to record these experiments out of the order and the place in this paper in which they would otherwise be properly and logically described. These remarks refer particularly to experiments described in paragraphs 292 to 306.
CONFLICT BETWEEN THE DIFFERENT OBJECTS TO BE ATTAINED IN CUTTING METALS WHICH AFFECT THE DESIGN OF STANDARD TOOLS
271 Our standard tools may be said to represent a compromise in which each one of the elements given in paragraph 272 has received most careful consideration, and has had its due influence in the design of the tool; and it can also be said that hardly a single element in the tools is such as would be adopted if no other element required consideration.
272 The following, broadly speaking, are the four objects to be had in mind in the design of a standard tool:
a The necessity of leaving the forging or casting to be cut with a true and sufficiently smooth surface; b Removing the metal in the shortest time;
c The adoption of that shape of tool which shall do the largest amount of work with the minimum combined cost of grinding, forging, and tool steel;
d Ready adaptability to a large variety of work.
273 As we go further into this subject the nature of the conflict between these four objects and of the sacrifice which each element is called upon to make by one of the others will become apparent. To illustrate the nature of these compromises:
274 Generally speaking, we have been obliged to adopt as our standard shape a tool which can be run at only about, say, five- eighths of the cutting speed which our knowledge of the art and our experiments show us could be obtained through another tool of entirely different shape if no other element than that of cutting speed required consideration.
275 We have been obliged to sacrifice cutting speed to securing smaller liability to chatter; a rather truer finish; a greater all-round convenience for the operator in using the tool; and a comparatively cheaper dressing and grinding. The most important of the above considerations, however, is the freedom from chatter.
276 On the other hand, we have been obliged to adopt a rather more elaborate and expensive method of dressing the tools than is usual in order to provide a shape of tool which allows it to be ground a great many times without redressing, and also in order to make a single Taylor-White heat treatment of the tool last longer than it otherwise would. And again, the shape of the curve of the cutting edge of the tool which we have adopted—first, to insure against chatter, and second, for all-round adaptability in the lathe,—calls for much more expense and care in the grinding than would be necessary if a more simple shape were used. This necessitates in a shop either a specially trained man to grind the tools by hand to the required templets and angles or preferably the use of an automatic tool grinder.
277 Before describing in detail the considerations which led to the adoption of our standard tools, it will be necessary to explain some of the more important experiments which have a direct bearing upon this subject. The shape of our standard tools will be again treated in paragraphs 325 to 332
ELEMENTS AFFECTING CUTTING SPEED OF TOOLS IN THE ORDER OF THEIR RELATIVE IMPORTANCE
278 The cutting speed of a tool is directly dependent upon the following elements. The order in which the elements are given indicates their relative effect in modifying the cutting speed, and in order to compare them, we have written in each case figures which represent, broadly speaking, the ratio between the lower and higher limits of speed as affected by each element. These limits will be met with daily in machine shop practice.
279 (A) The quality of the metal which is to be cut; i. 0., its hardness or other qualities which affect the cutting speed. Proportion is as 1 in the case of semi-hardened steel or chilled iron to 100 in the case of very soft low carbon steel.
280 (B) The chemical composition of the steel from which the V tool is made, and the heat treatment of the tool. Proportion is as 1 in tools made from tempered carbon steel to 7 in the best high speed tools.
281 (C) The thickness of the shaving; or, the thickness of the spiral strip or band of metal which is to be removed by the tool, measured while the metal retains its original density; not the thickness of the actual shaving, the metal of which has become partly disintegrated. Proportion is as 1 with thickness of shaving 135 of an inch to 31} with thickness of shaving 3‘; of an inch. l
282 (D) The shape or contour of the cutting edge of the tool, chiefly because of the effect which it has upon the thickness of the shaving. Proportion is as 1 in a thread tool to 6 in a broad nosed cutting tool. ,
283 (E) Whether a copious stream of water or other cooling medium is used on the tool. Proportion is as 1 for tool running dry to 1.41 for tool cooled by a copious stream of water.
284 (F) The depth of the cut; or, one-half of the amount by which the forging or casting is being reduced in diameter in turning. Proportion is as 1 with Q inch depth of cut to 1.36 with § inch depth of cut.
285 (G) The duration of the cut; i. c., the time which a tool must last under pressure of the shaving without being reground. Proportion is as 1 when tool is to be ground every 11} hours to 1.207 when tool is to be ground every 20 minutes. -mill». - —
286 (H) The lip and clearance angles of the tool. Proportion is as 1 with lip angle of 68 degrees to 1.023 with lip angle of 61 degrees.
287 (J) The elasticity of the work and of the tool on account of producing chatter. Proportion is as 1 with tool chattering to 1.15 with tool running smoothly.
288 A brief recapitulation of these elements is as follows:
(A) quality of metal to be cut: 1 to 100;
(B) chemical composition of tool steel: 1 to 7;
(C) thickness of shaving: 1 to 3.5;
(D) shape or contour of cutting edge: 1 to 6;
(E) copious stream of water on the tool: 1 to 1.41;
(F) depth of cut: 1 with 1/2 inch depth to 1.36 with 1/8 inch depth of cut;
(G) duration of cut: 1 with 1.5 hour to 1.20 with 20-minute cut;
(H) lip and clearance angles: 1 with lip angle 68 degrees to 1.023 with lip angle of 61 degrees;
(J) elasticity of the work and of the tool: 1 with tool chattering to 1.15, with tool running smoothly.
289 (A) The quality of the metal which is to be cut is, generally speaking, beyond the control of those who are in charge of the machine shop, and, in fact, in most cases the choice of the hardness of metals to be used in forgings or castings will hinge upon other considerations which are of greater importance than the cost of machining them. This subject will be further treated in paragraph 1129.
290 (B) The chemical composition of the steel from which the tool is made and the heat treatment of the tool will, of course, receive the most careful consideration in the adoption of a standard tool. No shop, however, can now afford to use other than the “high speed tools," and there are so many makes of good tool steels, which, after being forged into tools and heated to the melting point according to the Taylor-White process, will run at about the same high cutting speeds, that it is of comparatively small moment which particular make of high speed steels is adopted. This subject will be further dealt with in paragraph 965, etc.
(C) EFFECT OF THICKNESS OF SHAVING ON CUTTING SPEED THE MOST IMPORTANT SUBJECT FOR EXPERIMENT
291 It is the THICKNESS OF SHAVING, then (item C in the list above) which must be first considered, as this element has more effect upon the design of our standard tools, and in fact upon the whole problem of cutting metals than any other single item which is completely under the control of those who are managing a shop.
EXPERIMENTS SHOWING EFFECT UPON CUTTING SPEED OF VARYING THE THICKNESS OF THE SHAVING, A TOOL WITH STRAIGHT- EDGE BEING USED, REMOVING A SHAVING IN ALL CASES EXACTLY ONE INCH LONG
292 The following experiments were made to determine the effect upon the cutting speed of varying the thickness of the shaving. For this purpose a number of broad nosed tools with straight cutting edges, similar to that shown in Folder 7, Fig. 35, were forged from ordinary tempered carbon tool steel. Straight cutting edges were used in order that the shaving should be of the same thickness throughout. The corner of the tool, however, which cuts at the smaller of the two diameters of the forging was rounded to a radius of exactly 1/8 of an inch. This was necessary in order to thin the shaving down sufficiently at this point to absolutely insure that part of the tool which gives the required smoothness to the forging from giving out before the tool dulls along the straight line of the cutting edge.
293 In these experiments the tool was set in the lathe as shown in Folder 16, Fig. 111, so that exactly one inch of the STRAIGHT PORTION of the cutting edge was at all times under pressure of the shaving. In all cases a feed of 8/100 of an inch was used, so that the thickness of the shaving was in each case directly proportional to the depth of the cut.
294 These experiments were made upon a forging of the following chemical composition:
Carbon . . . . . . - - . . . . . . . . . . . . . . . . . . . . . . . .0.369 per cent
Manganese . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.517 per cent
Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.238 per cent
Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.043 per cent
Sulphur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.051 per cent
Tensile test bars actually cut from the body of the forging showed the following physical properties:
Tensile strength . . . . . . . . . . . . . . . . . . . . . . . . . .82,947 lbs.
Stretch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21.50 per cent
Contraction of area . . . . . . . . . . . . . . . . . . . . . . . .30.00 per cent
295 A heavy stream of water was thrown throughout the experiments upon the shaving just at the spot at which it was being removed from the forging by the tool.
296 Depths of cut of exactly 1/8, 3/16, 1/4 and 3/8 inch were used, corresponding to thicknesses of shaving of 0.01, 0.015, 0.02 and 0.03 inch.
297 The speeds corresponding to these cuts, each of which was of 20 minutes’ duration, are given in Folder 16, Table 113. These cutting speeds are plotted on Folder 16, Figs. 114 and 115, and a curve corresponding to the following formula is drawn approximately through these various points. They are also plotted on logarithmic paper, on Folder 16, Fig. 113, on which the speed points lie approximately in a straight line.
V = 1.54/(t^(2/3))
in which
V = cutting speed in feet per minute for 20-minute cut;
t = thickness of shaving in inches.
298 On Folder 16, Table 113, will be seen also the ratio between the cutting speeds of each of these thicknesses of shaving, from which it will be noted, for example, that by dividing the thickness of the shaving by 3, the cutting speed is increased in the ratio of 1 to 1.8. For further description of these experiments, see paragraph 761.
EXPERIMENTS SHOWING EFFECT UPON CUTTING OF VARYING THE DEPTH OF THE CUT, A TOOL WITH STRAIGHT- EDGE BEING USED, REMOVING IN ALL CASES SHAVING 0.03 INCH THICK
299 Tools of the same shape as those shown in Folder 7, Fig. 35, were set in the exact position shown in Folder 17, Fig 35, and in Folder 17, Fig. 116, we show in section lines the sizes and pro- portions of the chips, so that with a feed of 0.08 inch the tool in all cases removed a shaving 0.03 inch thick. The experiments were made upon the same forging, and with otherwise exactly the same conditions as those described just above for determining the effect of thickness of shaving upon cutting speed. Such depths of cut were used as to bring consecutively § inch, Q inch and 1 inch in length of the straight portion of the cutting edge under the shaving.
300 These experiments were made upon a forging of the following chemical composition:
Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.492 per cent
Manganese _ , _ . . _ _ , _ . . . . . . . . . . , , . . . . . . . .0.477 per cent
Silicon . . . . . . _ _ _ _ . . . . . . . _ . . . . . . _ _ . . . . . .0.194 per cent
Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . .0042 per cent
Sulphur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.039 per cent
301 Tensile test bars actually cut from the body of the forging show the following physical properties: Tensile strength . . . . . . . . . . . . . . . . . . . . . . . . . . . .97,719 lbs.
Stretch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 per cent
Contraction of area . . . . . . . . . . . . . . . . . . . . . . .23.2 per cent
302 - The cutting speeds corresponding to these three depths of cut are given on Folder 17, Table 117. These speeds are plotted on Folder 17, Fig. 119, and a curve corresponding approximately to the following formula is drawn through these various points. They are also plotted on logarithmic paper, on Folder 1'/, Fig. 118, on which the speed points lie approximately in a straight line. 12.22 V = __ Liv in which V = cutting speed in feet per minute for 20-minute cut; L == length of shaving in inches.
303 On Folder 17, Table 117, will also be seen the ratio between the cutting speeds of each of these depths of cut, from which it will be noted, for example, that by dividing the length of the cutting edge by 3, the cutting speed is increased in the ratio of 1 to 1.27.
304 Attention is called to the fact, however, that with round nosed tools as the depth of cut becomes more shallow, there is a greater increase in the cutting speed than in the case of tools having straight line cutting edges, because, as explained in paragraphs 307 and 311, with a round nosed tool the thickness of the shaving becomes thinner and thinner as the extreme nose of the tool is approached. In the case of round nosed tools, therefore, when the depth of the cut is diminished, the cutting speed is increased for two entirely different reasons: First, because the chip bears upon a smaller portion of the cutting edge of the tool, and Second, because the average thickness of the chip which is being removed is thinner in the case of round nosed tools with a shallow depth of cut than it was with the deeper cuts.
305 To make it more apparent that the element affecting the cutting speed the most is the thickness of the shaving, the writer would call attention to the fact that dividing the thickness Of the shaving by 3 increases the cutting speed in the ratio of 1 to 1.8, while dividing the depth of the cut by 3 only increases the cutting speed in the ratio of 1 to 1.27.
306 From these ratios it will be seen that the thickness of the chip has about three times as great an efiect in modifying the cutting speed as has the depth of the cut. For further description of these experiments, see paragraph 761.
WHY CUTTING EDGE OF TOOL SHOULD BE CURVED PRINCIPAL OBJECT IN HAVING THE CUTTING EDGE OF TOOLS CURVED IS TO INSURE AGAINST DAMAGE TO THE FINISHED SURFACE OF THE WORK
307 The above experiments described in paragraphs 293 to 303 upon the effect of thickness of shaving on cutting speed enable us to explain from the theoretical standpoint the well known fact that each properly designed roughing tool should have the line or contour of its cutting edge curved as it approaches the extreme nose of the tool or that portion of the tool which insures a good and true finish of the work. A tool whose cutting edge forms a curved line of necessity removes a shaving which varies in its thickness at all parts. The only type of tool which can remove a shaving of uniform thickness is one with a straight line cutting edge. The principal object in having the line of the cutting edge of a roughing tool curved as that part of the cutting edge which does the finishing is approached, is to thin down the shaving at this point to such an extent as will insure the finishing part of the tool remaining sharp and uninjured even although the main portion of the cutting edge may have been ruined through overheating or from some other cause.
308 The effect of a curved cutting edge upon the thickness of the shaving will be seen by inspecting Folder 16, Fig. 112, in connection with Figs. 111, 113, 114, and 115.
309 On Folder 16, Fig. 112, is shown a view of the curved cutting edge of a tool enlarged to 16 times its full size; thus at points 0.005, 0.01, 0.02, and 0.04 inch on this curve, the shaving is enlarged in each case so as to be respectively just 16 times as thick as the shavings shown in Fig. 111, shavings 0.005, 0.01, 0.02, and 0.04 inch. ‘
310 It will be observed that the quality and accuracy of the finish left upon the work will depend upon maintaining sharp and uninjured throughout the cut that portion of the cutting edge of the tool which extends, say, from about point 0.005 inch to point 0.02 inch.
311 By examining the standard cutting speeds noted opposite each of the straight-edge shavings, Fig. 111, it will be seen that the
standard or ruining speed at point 0.04 inch, for instance, on the curve of our enlarged tool, is 13 feet. From this it is evident, then, that if the tool enlarged in Fig. 112 were run at a cutting speed of 13 feet per minute so as to just ruin it at point 0.04 inch, the tool at point 0.01 inch would then be running at less than one-half of the cutting speed which would be required to ruin it. It would be, therefore, well within its safe limit of cutting speed, and would remain sharp and uninjured until the edge at 0.04 inch had entirely broken down. It is obvious then that a curved line cutting edge insures the finishing part of the tool from damage and for this reason, as above stated, the cutting edges of all tools should be curved, at least as that portion of the edge of the tool is approached which leaves the work of the proper size and with the proper finish.
TOOLS WITH BROAD NOSES HAVING FOR THEIR CUTTING EDGES CURVES OF LARGE RADIUS BEST TO USE EXCEPT FOR RISK OF CHATTER
312 Upon appreciating the increase in the cutting speed obtained through thinning down the shaving, as shown in our experiments with straight cutting edge tools, described in paragraphs 292 and 303, the tools shown on Folder 7, Figs. 32, 33, and 34, were made, and used on roughing work for years in the axle lathes of the Midvale Steel Company. The gain in cutting speed of these standard broad nosed tools over our standard round nosed tool, as shown in Folder 5, Figs. 24 and 23, is in the ratio of 1.30 :1.
313 This general shape of tool continues to be extensively used, but it is subject to the disadvantage that it is likely to cause the work to chatter, and so leave a more or less irregular finish.
314 Were it not for this difficulty, added to the fact that our standard round nosed tool has a greater all-round adaptability and convenience, the tools illustrated on Folder 7, Figs. 32 to 34-, would undoubtedly be the proper shapes for shop standard. This matter will be further discussed in paragraph 665. A method is there described of using two or more broad nosed tools so as to be free from danger of chatter even upon work which is especially liable to chatter.
REASONS WHY CUTTING EDGE WITH COMPARATIVELY SMALL RADIUS OF CURVATURE TENDS TO AVOID CHATTER
315 The avoidance of chatter in the tool plays such an important part in the design of the curve of the cutting edge for standard shop tools that we quote in full from that portion of Dr. Nicholson’s admirable experiments which appears to the writer to offer an explanation for one of the important causes for chatter.
316 Dr. Nicolson’s experiments, which were made with special apparatus (of his own design) for weighing the pressure of the chip upon the tool, were described in his paper (published in Transactions, Vol. 25, pp. 672, 673, 674), as follows:
317 The experiments (numbered 725 to 732 inclusive), the results of which are given in Table 9 are of special interest in regard to: First, the variation of the cut- ting force as the cut progresses at a very low speed; second, the variation of the cutting stresses with large ranges of speed variation.
318 These experiments were made with a tool having a 55 degree cutting and a 67} degree plan angle; a cut § inch deep by §inch wide being taken.
319 For numbers 725 and 726 the lathe was turned round at a cutting speed of about 1 foot in 5 hours, by means of a wire rope made fast round the large cone pulley, and hauled upon by a man operating a winch.
320 A pointer about 5 feet long was clamped upon the forging, and the four dynamometer gages were read at every half an inch of motion of the end of this pointer, 12. e., at about six on&hundredths (0.0625) of an inch of the cut. The vertical force varies from 9080 to 8920 every § of an inch of motion of the tool, the same wave length characterizing the variation of the surfacing and traversing forces. The observations have been plotted in Fig. 340 [see Folder 12, Fig. 86, present paper] on a base of actual relative tool motion.
321 A similar experiment, No. 636, carried out with the first dynamometer, is shown in Fig. 341. [See Folder 12, Fig. 82. present paper.] Here the cut was heavier, Q inch by 1» inch, and the tool had a 45 degree plan, and 60 degree cutting angle. The wave length of the force-curve is about 0.6 inch for this experiment, and it varies between 13.000 and 8000 pounds. It will be observed that the force attains a. maximum soon after the cutting commences to crack or shear across, and that it drops to a minimum when the small piece of cutting falls off the forging. At such a slow speed as this the cutting has time to shear off right across in separate fragments, whereas it forms a continuous curl of -considerable rigidity when the cutting speed is higher than a few feet per minute. These fragments measured, in this experiment, about } inch across the widest part of their surface -next the top of the tool in the direction of motion.
322 In these experiments Dr. Nicolson cuts the metal at a speed of one foot in five hours. By referring to Folder 12, Fig. 82, (Fig. 341, of his paper) it will be seen that the pressure on the tool increases and diminishes in the ratio of about 8 to 13 at comparatively regular wave-like intervals, and by comparing this diagram with Folder 12, Fig. 86 (Fig. 340 of his paper) in which a thickness of feed only one-half as great is used, it will be seen that the wave lengths are in this case very much shortened. From this it appears that each thickness of shaving has its own corresponding wave length for the periods of maximum and minimum pressure on the tool.
323 Since the thickness of the shaving is uniform with straight edge tools, it is evident that the period of high pressure will arrive at all points along the cutting edge of this tool at the same instant and will be followed an instant later by a corresponding period of low pressure; and that when these periods of maximum and minimum pressure approximately correspond to or synchronize with the natural periods of vibration either in the forging, the tool, the tool support, or in any part of the driving mechanism of the machine, there will be a resultant chatter in the work. On the other hand, in the case of tools with curved cutting edges, the thickness of the shaving varies at all points along the cutting edge, as we have pointed out in paragraphs 292 and 303. From this fact, coupled with Dr. Nicolson’s experiments, it is obvious that when the highest pressure corresponding to one thickness of shaving at a given point along a curved cutting edge is reached, the lowest pressure which corresponds to another thickness of shaving at another part of the cutting edge is likely to occur at about the same time, and that therefore variations up and down in pressure at different ‘parts of the curve will balance or compensate one for the other. It is evident, moreover, that at no one period of time can the wave of high pressure or low pressure extend along the whole length of the curved cutting edge. For this reason a curved cutting edge tends to prevent chatter.
324 Dr. Nicolson’s experiments afford us a much needed theoretical explanation of what has for years been a well recognized fact, namely, that tools with straight cutting edges are much more likely to chatter than those with curved edges, and if Dr. Nicolson’s apparatus and experiments had given no further information than this, they would be well worth all of the trouble and time expended upon them.
REASONS FOR ADOPTING THE PARTICULAR CURVES CHOSEN FOR THE CUTTING EDGES OF OUR STANDARD TOOLS
325 Having explained the necessity for curved cutting edges in standard roughing tools, it is desirable to give our reasons for the adoption of the particular curves of our standard tools, illustrated in Folder 5, Figs. 20a to 25e, and Folder 17, Fig. 120. It will be noted that as the body of the tool becomes smaller, the radii of curvature of the cutting edge also become correspondingly smaller. This change in the curve of the cutting edge is rendered necessary by the fact that the smaller tools are used in the small lathes, which, generally speaking, work upon small forgings, from which cuts are removed which are both shallow in depth and have comparatively fine feeds.
Forgings which are small in diameter are quite as liable to chatter as the larger forgings which are machined in larger lathEs, and in order to avoid this chatter, it is necessary that a curve for the cutting edge should be chosen which will give a variation relatively in the thickness of shaving even in small depths of cut. Thus, for the avoidance of chatter, the curve of the cutting edge should be small in proportion as the depth of cut and feed which it it normally takes are small.
326 As will be seen later, the smaller radius of curvature of the cutting edge involves a diminution in cutting speed. Therefore, with larger sized tools it becomes important, on the other hand, to take as large a radius of curvature for the cutting edge as is compatible with freedom from chatter. The coarser feed which usually accompanies the larger tool also calls for a larger radius of curvature at the nose of the tool, in order that the ridges left by the spiral path of the tool along the forging shall be as low as practicable.
327 The all-round adaptability of the standard tool to a variety of uses also calls for a smaller radius of curvature the smaller the tool, since standard roughing tools are continually required to run up against a shoulder or into a corner on the work, and the fillet in this corner is normally small in proportion as the forging or casting is small.
328 Generally speaking, also round nosed roughing tools of the type shown do not require the special care in adjusting in the tool post that would be demanded of tools designed with a straight line cutting edge for the purpose of producing a smooth finish, etc.
329 By comparing curves of the tools on Folder 5, Fig. 21a, with Folder 5, Fig. 21b, it will be observed that tools which are to be used for cutting cast iron and hard steel have slightly larger radii of curvature than those which are to be used for the softer steel and wrought iron. The reason for this change is that much slower cutting speeds must be used in cutting hard steels than for soft, and this is also to a less degree true for cast iron as compared with soft steel. 330 It is a matter of common experience that the slower the cut- ting speed, the less the liability of the tool to chatter. It therefore becomes safe from the standpoint of chatter to use in cutting hard steel and also cast iron a larger radius of curvature than would be permissible in cutting soft steel. The fact has already been pointed out that the larger the radius of curvature, the thinner the shaving, and therefore the higher the cutting speed, and in the interest of economy, it is of course particularly desirable in cutting hard steels to increase the necessarily slow cutting speed as much as practicable.
331 For the following entirely different reason, also, the radius of curvature for tools to be used in cutting cast iron is made larger than in tools to be used in cutting soft steel. Cast, iron is cut, as will be seen in paragraphs 491 and 512, with less cutting pressure or resist- ance to the tool than is required for soft steel. Therefore, in a given lathe a greater depth of cut and coarser feed can be taken on cast iron than on soft steel; and, as explained above, the coarser the feed, the greater should be the radius of curvature of the extreme nose of the tool in order to leave an equally smooth finish.
332 In many machine shops a very considerable portion of the work consists of cuts to be taken upon pieces of cast iron; the depth of the cut being comparatively shallow and the strength and rigidity of the casting begin so great that in order to use even approximately the full pulling power of the lathe or planer, etc., broad feeds must be taken. In our standard shop tools, as illustrated in Folder 5, Fig. 21a, the extreme end of the noses are rounded with too small radii to take a very broad feed, and yet at the same time leave a reasonably smooth finish. It is therefore desirable in all such shops to have standard tools available which are especially designed for work of this character.
333 In Folder 5, Fig. 22, is illustrated a single size of the type of tool which we recommend for this purpose. It will be observed that in form it corresponds exactly to our other standard tools for cutting cast iron and hard steel except that the extreme nose of the tool is widened out so as to have a curve of very large radius, approximating to a straight line.
(H) LIP AND CLEARANCE ANGLES OF TOOLS
334 Contrary to the opinion of almost all novices in the art of cutting metals, the clearance angle and the back slope and side slope angles of a tool are by no means among the most important elements in the design of cutting tools, their effect for good or evil upon the CUTTING SPEED and even upon the pressure required to remove the chip being much less than is ordinarily attributed to them.
CLEARANCE ANGLE OF THE TOOL
335 The following are our conclusions regarding the clearance angle of the tool. a For standard shop tools to be ground by a trained grinder or on an automatic grinding machine, a clearance angle of 6 degrees should be used for all classes of roughing work. (See paragraphs 336 to 340) b In shops in which each machinist grinds his own tools a clearance angle of from 9 degrees to 12 degrees should be used. (See paragraph 339)
336 In seeking for the proper clearance angles for tools, we have as yet been unable to devise any type of experiment which would demonstrate in a clear cut manner (as, for example, the experiments cited for lip angles in paragraphs 361 and 367) which clearance angle is the best. The following, however, are the considerations which affect the choice of clearance angles.
337 On the one hand, it is evident that the larger the clearance angle, the greater will be the ease with which the tool can be fed (wedged or driven) into its work, the first action of the tool when brought into contact with the forging being that of forcing the line of the cutting edge into the material to be cut. On the other hand, every increase in the clearance angle takes off an equal amount from the lip angle, and therefore subjects the tool to a greater tendency to crumble or spall away at the cutting edge, as indicated on Folder 6, Figs. 31a and 31b. It must be remembered also that the tool travels in a spiral path around the work which it is cutting in the lathe, and that the angle of this path with a perpendicular line in the case of coarse feeds taken upon small diameters of work becomes of distinctly appreciable size. In all cases, therefore, the clearance angle adopted for standard shop tools must be sufficiently large to avoid all possibility from this source of rubbing the flank of the tool against the spiral flank of the forging. The clearance angles for roughing tools in common use vary between 4 degrees and 12 degrees. We have had experience on a large scale in different shops with tools carefully ground with clearance angles of 5 degrees, 6 degrees and 8 degrees. In the case of one large machine shop which had used clearance angles ground to 8 degrees through a term of years, they finally adopted the 6 degrees clearance angle with satisfaction. For many years past our experiments have all been made with the 6 degree clearance angle, and this has been demonstrated to be amply large for our various experiments. On the other hand, a 5 degree clearance angle in practical use in a large shop has appeared to us through long continued observation to grind away the flank of the tool just below the cutting edge rather more rapidly than the 6 degrees angle. We have, therefore, adopted the 6 degrees clearance angle as our standard.
A CLEARANCE ANGLE OF FROM 9 TO 12 DEGREES SHOULD BE USED IN SHOPS IN WHICH EACH MACHINIST GRINDS HIS OWN TOOLS
339 It should be noted, however, that in shops systematized by us the cutting tools are invariably ground either on an automatic tool grinder, or by special men who are carefully taught the art of grinding and provided with suitable templets and gages, and that in this case the clearance angle for every tool is accurately made to 6 degrees- 3-10 In shops, however, in which each lathe or planer hand grinds his own tools, a larger clearance angle than 6 degrees should be used, say, an angle of from 9 degrees to 12 degrees, because in such shops in nine cases out of ten the workmen grind the clearance and lip angles of their tools without any gages, merely by looking at the tool and guessing at the proper angles; and much less harm will be done by grinding clearance angles considerably larger than 6 degrees than by getting them considerably smaller. It is for this reason that in most of the old style shops in which the details of shop practice are left to the judgment of the men or to the foreman, that clearance angles considerably larger than 6 degrees are generally adopted.
LIP ANGLE OF THE TOOL
341 The following are the conclusions arrived at regarding the angl§ at which tools should be ground:
342 (A) For standard tools to be used in a machine shop for cutting metals of average quality: Tools for cutting cast iron and the harder steels, beginning with a low limit of hardness, of about carbon 0. 45 per cent, say, with 100,000 pounds tensile strength and 18 per cent stretch, should be ground with a clearance angle of 6 degrees, back slope 8 degrees,and side slope 14 degrees, giving a lip angle of 68 degrees. These angles are used in the tools illustrated on Folder 5, Figs. 21a and 25e. (See paragraphs 358 to 359)
343 (B) For cutting steels softer than, say, carbon 0.45 per cent having about 100,000 pounds tensile strength and 18 per cent stretch, tools should be ground with a clearance angle of 6 degrees, back slope of 8 degrees, side slope of 22 degrees, giving a lip angle of 61 degrees. These angles are used in tools illustrated in Folder 5, Fig. 25b. (See paragraph 361)
344 (C) For shops in which chilled iron is cut a lip angle of from $6 degrees to 90 degrees should be used. (See paragraph 365)
345 (D) In shops where work is mainly upon steel as hard or harder than tire steel, tools should be ground with a clearance angle of 6 degrees, back slope 5 degrees, side slope 9 degrees, giving a lip angle of 74 degrees. (See paragraph 360)
346 (E) In shops working mainly upon extremely soft steels, say, carbon 0. 10 per cent to 0. 15 per cent, it is probably economical to use tools with lip angles keener than 61 degrees. (See paragraphs 368 to 370)
347 (F) The most important consideration in choosing the lip angle is to make it sufficiently blunt to avoid the danger of crumbling or spalling at the cutting edge. (See paragraphs 352 to 356)
348 (G) Tools ground with a lip angle of about 54 degrees cut softer qualities of steel, and also cast iron, with the least pressure of the chip upon the tool. The pressure upon the tool, however, is not the most important consideration in selecting the lip angle. (See paragraphs 374 and 367) ‘
349 (H) In choosing between side slope and back slope in order to grind a sufficiently acute lip angle, the following considerations, given in the order of their importance, call for a steep side slope and are opposed to a steep back slope: a With side slope the tool can be ground many more times without weakening it; (See paragraphs 379) b The chip runs of sideways and does not strike the tool posts or clamps. (See paragraph 380) c The pressure of the chip tends to deflect the tool to one i side, and a steep side slope tends to correct this by bringing the resultant line of pressure within the base of the tool, as explained in paragraph 382. d Easier to feed. (See paragraphs 383 and 384)
350 (I) The following consideration calls for at least a certain amount of back slope. An absence of back slope tends to push the tool and the work apart, and therefore to cause a slightly irregular finish and a slight variation in the size of the work. (See para- graph 386)
351 (J) For conclusions as to clearance angle, see paragraph 385.
352 Before it is possible to discuss the proper lip angles for tools, two ways in which the cutting edge gives out should be described.
353 On Folder 6, Fig. 31a, is shown on an enlarged scale the manner in which the sharp end of the wedge of the tool spalls offior crumbles away, when the lip surface of the tool right at the cutting edge is subjected to great pressure. In pars. 516 to 519, later in the paper, it will be pointed out that in the case of cutting very hard metals and also in cutting all qualities of cast iron, the pressure of the chip is concentrated very close to the line of the cutting edge,
and the harder the metal to be cut and the smaller its percentage of extension, the greater will be the concentration of the pressure close to this line, and the greater will be the tendency of the cutting edge to spall of or crumble away.
354 On Folder 6, Fig. 31b, is shown another way in which the metal of the lip surface of the tool spalls offior crumbles away when the line of the cutting edge of the tool is subjected to great pressure in feeding or forcing the tool into the forging. In this case the hard- ms of the metal into which the tool is being fed is the chief element causing this type of injury to the cutting edge.
MOST IMPORTANT CONSIDERATION IN CHOOSING LIP ANGLE IS TO MAKE IT SUFFICIENTLY BLUNT TO AVOID DANGER OF CRUMBLING OR SPALLING OFF AT THE CUTTING EDGE
355 In deciding upon the acuteness of the lip angle of a tool the absolute necessity of guarding against the spalling or crumbling of the cutting edge from both of the foregoing causes becomes by far the most important of all considerations. In this connection’ the essential fact to be borne in mind is that the harder the metal to be cut, the blunter must be the lip angle of the tool. In the case of chilled iron and semi-hardened steel, for instance, the lip angle must be made from 86 degrees to 90 degrees. A smaller angle than this will cause the metal at the extreme cutting edge to spall off or crumble away (quite is much on account of the feeding pressure as from_ the pressure of the chip) and thus ruin the tool. As the metal to be cut grows softer, however, the lip angle can be made keener without danger of spalling, until with standard tools intended to cut the softer steels, say with a high limit for hardness of about 100,000 pounds tensile strength and 14 percent to 18 per cent stretch, the smallest lip angle which, in our iudgment, it is on the whole wise to use would seem to be about 61 degrees.
356 Dr. Nicolson with his dynamometer experiments (see Figs. 328 lad 329 of his paper) has shown that with a“cutting angle”of 60 degrees, corresponding to a -lip angle of 54 degrees, clearance angle 6 degrees, tools remove metal with the minimum of pressure. This is also corroborated in a general way by our observations in cutting dead soft steel, referred to in paragraphs 368 to 370. Therefore from the standpoint °i pressure, with a view to taking the largest cut with a. given pulling power and with the least strain upon the working parts of the lathe, ibis angle should be approached. And although, on the whole, the question of pressure on the tool has less weight than either the crumbling at the cutting edge, the cutting speed, or the proper angles for obtaining the longest life and the largest number of grindings for a given tool, still it must be considered; and it is this which has led us to choose for our standard in each case
THE KEENEST CUTTING ANGLE WHICH IS FREE FROM DANGER OR SPALLING.
357 As pointed out in paragraph 372, we believe that experiments would demonstrate the advisability of using still more acute lip angles for cutting dead soft steels. -
358 Metals which even approach in hardness chilled iron and semi- hardened steel are but seldom met with in ordinary shop practice and, therefore, in selecting the lip angles for standard shop tools, we have divided the metals to be cut in a shop into two classes: a cast iron and the harder classes of steel, say, beginning as a low limit for hardness with a steel of about 0.45 to 0.50 per cent carbon, 100,000 pounds tensile strength and 18 per cent stretch; and b the softer classes of steel.
359 Our guiding principle in selecting the lip angles for the tools to be used in cutting cast iron and the harder classes of steel has been to select what we believe to be the smallest or most acute lip angle which can be safely depended upon to run without danger of spalling off at the cutting edge while cutting the harder steels ordinarily met with in machine shop practice (such as the hardest steels used in this country for car wheel tires, say of 135,000 to 140,000 pounds tensile strength, and 9 to 10 per cent of stretch, and, for instance, unannealed tool steels, or the harder of the oil hardened and annealed forgings which are used under government specifications for making large steel cannon,etc.) ; and after large experience in cutting metals of this quality we have concluded that it would be unsafe to use a more acute lip angle than that shown on Folder 5, Fig. 20a, namely, a lip angle of 68 degrees,with clearance angle of 6 degrees, side slope of 14 degrees and back slope of 8 degrees. We have demonstrated by repeated trials that tools with the above lip angle are safe from danger of spalling or of crumbling at the cutting edge, even when cutting tire steel, gun steel or tool steel.
360 For shops which are engaged mainly in cutting steels as hard as tire steel, We should recommend as a standard tool one having 6 degrees clearance, 5 degrees back slope and 9 degrees side slope, giving a lip angle of 74 degrees. Since for this special work the tools can be run at a high cutting speed, they can be ground in less time and they can be ground more times for each dressing in the smith shop than tools with more acute lip angles.
361 The following experiment was made in 1906 with a high speed tool of the latest and best composition. The chemical composition of the tool was that of tool No. 1 in Folder 20, Table 128.
362 Repeated trials with the same tool ground first with a clearance angle of 6 degrees, back slope of 5 degrees, and side slope of 9 degrees, giving a lip angle of 74 degrees; and afterwards with a clearance angle of 6 degrees, back slope of 8 degrees, and side slope of 14 degrees, giving a lip angle of 68 degrees. No difference was indicated in the cutting speed of these two tools when used upon the very hard forging referred to in Folder 20, Table 128.
363 It is interesting, however, to note that machinists who grind their own tools and who are accustomed to machining hard tires and metals of the classes above referred to, invariably use a blunter lip angle than our standard of 68 degrees. After making a few mistakes by grinding tools with lip angles which are too acute, they are sure to lean too far toward the safe side, and adopt lip angles which are not quite sharp enough. They are influenced in this very largely, how- ever, by the fact pointed out in paragraph 124 that the less acute the lip angle, the easier it is and the less time it requires to grind a tool. A tool with a lip angle of 80 degrees for example, can be more easily ground than one with a lip angle of 70 degrees.
364 In those shops which work upon metals of average hardness and in which the tools are furnished to the machinists ground to the required shapes, and in which either automatic tool grinders are used or special grindstone men are employed to grind the tools, more work can be gotten out by grinding the tools to angles at least closely approximating ours than from the use of tools with blunter lip angles.
365 The reason for preferring the more acute lip angle of 68 degrees, for cutting medium hard metals to the angle of 75 degrees to 85 degrees adopted by the average machinist, is that the more acute angle removes the metal with a lower pressure on the tool (see paragraph 374); while repeated experiments made by us in cutting medium hard steels indicate that there is little if any difference in cutting speed between the 68 degrees lip angle and coarser angles. Our standard tools, therefore are capable of taking heavier cuts than the blunter tools, and in a given machine working to the limit of its pulling power, can remove rather more metal in a given time.
WHY TOOL FOR CUTTING SOFT CAST IRON SHOULD HAVE BLUNTER LIP ANGLE THAN TOOL FOR CUTTING SOFT STEEL
366 It may be a matter of surprise to some that we have adopted a lip angle of 68 degrees for cutting the softer grades of cast iron, while we recommend a lip angle of 61 degrees for the softer steels. It is one of the strange anomalies met with in so many of the elements of this art, however, that if we experiment with a very soft cast iron, on the one hand, and a very soft steel, on the other—the standard cutting speeds of which are each, say, 150 feet per minute with a 13¢ inch depth of cut and fir inch feed—in the case of the soft steel the highest speed can be obtained only with a cutting edge at least as keen as 61 degrees, and we believe even keener, while the lip angle corresponding to the highest cutting speed with soft cast iron is 68 degrees or even blunter. The following experiments were carefully made and have since been verified by repeated trials.
367 In 1894 before the discovery of high speed cutting tools, the standard speed for cutting soft cast iron was determined for each of two sets of tools, one set having a lip angle of 61 degrees and the other 68 degrees. These tools were made from tempered carbon steel of {Q inches by 1% inch section, having the curve of the cutting edge as shown in standard, Folder 5, Fig. 24. One set was ground with 6 degrees clearance angle, 8 degrees back slope, and 14 degrees side slope, thus giving a lip angle of 68 degrees. The other set was ground with a clearance angle of 6 degrees back slope of 8 degrees, and side slope of 22 degrees, thus giving a lip angle of 61 degrees. These two sets of tools were successively run on a carefully standardized test piece of soft cast iron of about 24 inches diameter. The standard ruining speed of the 68 degrees angle was 67 feet, while the standard ruining speed of the 61 degrees lip angle was 651} feet; thus in cutting soft cast iron, changing the lip angle from 68 to 61 degrees reduced the cut- ting speed from 67 to 65% feet, a loss of 2.3 per cent in speed. 368 On the other hand, the following result was obtained repeatedly in experiments made in 1900 upon a carefully standardized test forging made of soft steel, whose chemical composition and physical properties were about:
Carbon . . . . . . . . . . . . . . . . . . . . . . . . _ , , _ . . _ .0.105 per cent
Manganese . . . . . . . . . . . . . . _ _ . . . . _ . . . . . ..0.25 per cent
Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..0.008 per cent - Sulphur . . . . . . . . _ _ _ , . . . . . . . . . . . . . . _ _ . . 0.04 per cent
Phosphorus . . . . . . . . . . . . . . . . . . . . . . _ _ _ _ 0.008 per cent
Chromium . . . . . . . . . . . . . . . . . . . . . . . . _ . ..0.047 per cent
Tensile Strength . . . . . . . . . . . . . . . . . . . . . . . . 48,000 pounds
Elastic Limit . . . . . . . . . . . . . . . . . . . . . . . . . . .2-1,500 pounds
Extension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 per cent
Contraction of Area . . . . . . . . . . . . . . . . . . . . . 62 per cent
Taylor-White treated tools of the chemical composition of tool steel
No.27 on Folder 21 , Table 139, were used. Body of the tool, {V by 1% inch, curve of the cutting edge, shown in Folder 5, Fig. 24, with a clearance angle of 6 degrees, back slope 12 degrees, side slope 18 degrees, giving a lip angle of about 61 degrees. The standard ruining speed with a 13¢ inch depth of cut and a 1'3 inch feed was 150 feet per minute; tools in other respects exactly like these, except that the back slope was 8 degrees and the side slope 14 degrees (giving a lip angle of 68 degrees), showed in repeated trials a ruining speed of from 125 to 130 feet.
369 The effect of changing the lip angle from 68 to 61 was to increase the cutting speed from 125 to 150 feet, a gain of 20 per cent. Thus a more acute cutting angle used on a tool for cutting soft steel produces just the opposite effect from that produced in cutting equally soft cast iron.
370 The lip angle of 68 degrees used in cutting soft steel, when tried at the high speed of 150 feet, caused the chip to be much more distorted or upset, and thickened, and after running a short time at this speed, the chip began to stick to the lip surface of the tool almost as though it were welded. With the more acute angle of 61 degrees this bunching up of the chip and welding did not occur. It was also evident that when the chip began to weld to the lip of the 68 degree tool, the power required to cut the metal was so greatly increased as in a number of cases to entirely stall or slow down the lathe, whereas with the 61 degrees lip angle, the lathe was never slowed down in the least. At slower cutting speeds it was not noticeable that the 68 degrees tool consumed any more power than the 61 degrees tool.
371 It would be interesting to repeat this experiment at the very high speed with a dynamometer, carefully measuring the pressure of the chip on the tool.
372 The writer believes that it would be profitable to experiment with more acute lip angles than 61 degrees in cutting dead soft steel such as above referred to, containing about 0.10 per cent carbon, and °f 48,000 pounds tensile strength, which approximates to wrought iron. It may be that with this extremely soft steel still higher cutting speeds could be obtained with more acute angles, in which case it would be advisable of course to make special tools for cutting this quality of metal in shops where large amounts of it are used. However, a trial of this sort would not modify our selection of 61 degrees for the Standard angle for cutting the ordinary softer steels met with in the average machine shop, because as explained above, our standard shop lwls for cutting the softer steels are intended for use in cutting metals With an upper limit of hardness of about carbon 0.45 to 0.50, say, pounds tensile strength, 18 per cent of stretch or thereabouts, and if a more acute lip angle than, say, 61 degrees were used in cutting steel of this hardness, there would be danger of the cutting edge crumbling away or spalling off.
THEORY AS TO WHY AN ACUTE LIP ANGLE PRODUCES A HIGHER SPEED FOR CUTTING SOFT STEEL AND A SLOWER SPEED FOR SOFT CAST IRON
373 In all matters pertaining to this art a theoretical explanation of the various phenomena is of less importance than a clear knowledge of the facts. However, it may still be of interest, at least, to present our theory as to the opposite effects of an acute lip angle in the case of soft cast iron and soft steel.
374 Dr. Nicolson in his dynamometer experiments has demonstrated the fact that tools ground with a “cutting angle of 60 degrees” which corresponds to a “lip angle of 54 degrees” work with a smaller total pressure upon the tool than tools whose cutting angles are either larger or smaller than 54 degrees, the metals upon which he experimented being as follows: Medium cast iron, which with a 135 inch depth of cut and a 11;; inch feed having a standard speed of 49 feet per minute; and steel 60,000 pounds tensile strength and 26 per cent extension, having a standard speed of 111 feet per minute with a 135 inch depth of cut and 11}; inch feed. His experiments,however, show that although tools of these angles cut with less pressure, yet tools with larger angles can be run at higher cutting speeds. This confirms our experiments on cast iron as cited above in para- graph 367. The reason for this phenomenon appears to be as follows:
375 First, the amount of heat generated by the friction of the chip is doubtless closely proportional to the pressure of the chip upon the tool. Therefore, with the 54 degrees cutting angle and its lower pressure there will be less heat generated than with the larger cutting angles On the other hand, the heat is carried away from the cut- ting edge mostly through the metal of the tool itself (very little heat being radiated into the air); and the more acute the angle of the tool, the smaller will be the cross-section of the wedge shaped metal of the tool close to the cutting edge, so that the blunter angled tools will have also a larger section of metal for carrying away the heat. In addition to this, and of greater importance in our judgment, is the fact that in cutting cast iron the pressure of the chip comes very close to the cutting edge of the tool, as explained in paragraph 523, and the more acute its angle, the more will a trifling amount of wear or damage affect the cutting edge. These two causes working together operate to enable the blunter cutting edge to run at higher speeds in cutting cast iron. On the other hand, as pointed out in paragraphs 170 and 516, the pressure of the chip in cutting dead soft steel comes at a considerable distance from the cutting edge, so that in this case the more delicate edge of the acute angled tool is further removed from the source of heat and also subject to much less abrasive wear than in cutting cast iron; and the cross-section of the tool beneath the center of pressure of the chip is much larger. Therefore, in the case of very soft steel we have exactly the reverse effect, as described in paragraph 368, namely, the more acute their lip angles down to 61 degrees (the low limit experimented with by us), the higher the cutting speeds at which tools can be run. war
WHY TOOLS SHOULD BE GROUND WITH GREATER SIDE SLOPE THAN BACK SLOPE
376 We have endeavored above to make it clear that the para- mount consideration affecting the choice of the lip angle for standard tools has been the avoidance of the danger of spalling or crumbling at the cutting edge. Having chosen a lip angle which is sufficiently blunt to avoid danger from this cause, it must still be decided whether this angle shall be produced, say, altogether by side slope or altogether by back slope, or by a combination of side slope and back slope; and in settling this question there are several important, and, as usual, conflicting considerations. These may be divided into the following groups, which are given in the order of their importance:
a Ease and cheapness of grinding and the effect of repeated grindings upon the strength and life of the tool;
b Guiding the chip in the proper direction for convenience in operating;
c The effect of pressures produced by side slope and back slope upon the tendency of the tool to gouge or plunge either forward or sideways;
d The power required to feed.
377 In the following brackets are grouped these several considerations, in the order of their relative importance, as they affect favorably or unfavorably the adoption of a steep side slope:
IN FAVOR OF STEEP SIDE SLOPE
a With side slope tool can be ground many more times with- out weakening it;
b Chip runs off sideways and does not strike tool post or clamps;
c Less tendency to force and deflect the tool to one side as it tends to bring resultant line of pressure within base of the tool, as explained in paragraph 382;
d Easier to feed.
AGAINST STEEP SIDE SLOPE
a. Danger of gouging or plunging into the work greater.
378 And in the following brackets are also grouped the same considerations as they affect favorably or unfavorably a steep back slope:
IN FAVOR OF STEEP BACK SLOPE
a. Does not push tool and work away from one another.
AGAINST STEEP BACK SLOPE
a. Grinds down into body of tool and weakens tool and allows fewer grindings for given height of tool;
b In case of gouging, the work is more apt to be spoiled through tool plunging forward as it does with steep back slope than if it plunges sideways as it does with steep side slope;
c Runs chip directly back against tool, tool post or clamp;
d Harder to feed.
SIDE SLOPE AND BACK SLOPE AS AFFECTED BY THE GRINDING
379 On Folder 7, Figs. 39a and 39b, we show the side view of two tools, in both of which views the lip angle of the tool is 61 degrees. In the case of Folder 7, Fig. 39b, thelip angle is attained entirely through backslope whilein Folder 7,Fig.39b,andFolder5,Fig.20b(standardtool for cutting soft steels), there is 8degrees of backslopeand 22 degrees of side slope. The cutting edges of both of these tools are of the same height. An inspection of the drawings will show, however, that the tool with all back slope can be ground but comparatively few times before the corner of the grindstone will begin to cut away the body of the tool, thus weakening it, and allowing a comparatively small number of grindings before the tool is redressed, while at the same time making the grinding much more expensive, as explained in para- graphs 435 to 439.
SIDE SLOPE AND BACK SLOPE AS THEY AFFECT THE DIRECTION OF THE CHIP
380 With the modern high speeds used in cutting steel the dis- position of the chip becomes a matter of no small moment, and in many cases it is absolutely necessary in designing the tool to provide against the jamming of the chip either between a portion of the tool itself and the lip surface of the tool, or between the nose of the tool and the clamps or tool post which hold it.
381 It is evident that a steep back slope tends to throw the chip either directly against the tool or against the tool post or clamps, while a steep side slope guides the chip off to one side, and this there- fore becomes one of the most important reasons for adopting a. steep side slope.
THE TENDENCY OF THE PRESSURE OF THE CHIP TO BEND THE TOOL TO ONE SIDE
382 In pars. 417 to 425, relating to the dimensions of the steel to be used in the body of the tool, will be seen the desirability of keeping the resultant line of pressure of the chip upon the tool within or as near as possible to the base of the tool. Dr. Nicolson’s experiments (Fig. 336 of his paper) show that the side pressure of the chip upon the tool diminishes as the cutting angle becomes more acute and reaches a minimum with an angle of 60 degrees. Therefore a steep side slope will tend to keep the resultant line of pressure within the base of the tool.
THE EFFECT OF SIDE SLOPE AND BACK SLOPE UPON THE POWER REQUIRED TO FEED THE TOOL
383 The diagram in Fig. 3.36 in Dr. Nicolson’s paper also indicates the desirability of a steep side slope even to the extent of 30 degrees in diminishing the power required to feed. 384 A tool ground with a slope of 30 degrees offers a resistance to feeding of but 1 per cent to 10 per cent while a tool ground with :1-5 degrees slope meets with a feeding resistance equal to from 1? per cent to 20 per cent of the total pressure on the tool.
385 For further discussion of feeding resistance, see paragraph 581.
BACK SLOPE NEEDED TO SECURE BETTER FINISH AND GREATER ACCURACY IN SIZE
386 A study of all of the above elements would lead to the conclusion that tools should be designed with all side slope and no back slope. There is, however, one element which makes it desirable to have a certain amount of back slope; namely, the fact that a steep back slope diminishes the tendency of the chip to push the tool and the work away from one another, and it is evident that the greater the pressure tending to force the tool and the work apart, the greater will be the irregularity in the finish left by the nose of the tool upon the work. This irregularity both in size and finish is particularly noticeable in those cases in which the tool and its supports are not especially rigid, and in which the depth of the cut varies from one part of the forging to another; and also when the surface of the forging is more or less eccentric or uneven owing to the irregularities left by the hammer in forging.
387 In paragraph 217 special attention has been called to the necessity for great rigidity in all parts of the lathe to be used in experimenting. There are a few important elements, however, which can only be studied through the use of a lathe in which the supports for the tool are more or less yielding, and even somewhat loose rather than rigid. These elements are: a the tendency of the tool to gouge or plunge into the work; and b the forcing of the tool and the work apart.
388 It is evident that the effect of the acuteness of the angle of slope of the tool is directly opposite in these two cases. The more acute the angle of slope, the greater the tendency to gouge, and the less the tendency to push the work and the tool apart. It may be said that in well managed machine shops the tool supports will be properly adjusted so as to avoid any lost motion or looseness, and that there- fore the tendency to gouge from this cause should not be considered. The fact is, however, that we are dealing with shops as they are, and even in many of the best shops, machines will be found whose tool supports are entirely too springy and more or less less worn or out of proper adjustment. We have made repeated careful experiments with lathes having springy tool supports and with more or less lost motion, and in such machines, providing the tool is fastened tight in the tool post, we have found that the tools ground to our standard angles, shown on Folder 5, Figs. 20a and 20b, very rarely gouge or plunge forward or sideways seriously. The danger of plunging forward, however, has been one of the reasons influencing the adoption of a back slope as small as 8 degrees.
389 The tendency of the tool and work to push apart, on the other hand, is very marked with tools designed with all side slope and no back slope. A series of experiments was tried with a set of tools,
in the one case having 6 degrees clearance, 8 degrees back slope, 14 degrees side slope; and in the other case, a set having 6 degrees clearance, a back slope of minus 5 degrees, or more properly a forward slope of 5 degrees; and a side slope of 25 degrees. The lip angle of the first of these sets being 68 degrees, while the lip angle of the second was about the same. One of the principal reasons for comparing these types of tools was that the tool when ground with 5 degrees front slope makes what is known as a shearing cut and that a shearing out has the special advantage of leaving a smoother finish.
390 The standard speeds of these two tools were found through accurate experiments to be practically the same, there being less than l per cent difference between the two in favor of the 8 degrees back slope. With these tools, however, even when used in a lathe with a comparatively rigid and a tight and well adjusted tool support, there was a. most noticeable difference in the tendency to push the tool and the work apart. With heavy cuts a much smoother and better finish was left by the tool with the 8 degrees back slope in spite of the shearing effect of the other tool; and it was evident to all of those who watched the experiment that the tool with back slope was greatly to be preferred to the other. It may add weight to understand that this particular experiment was made at the request of the superintendent and foreman of a large machine shop in which tools ground with the i degrees front slope had formerly been “standard. These men, however, were completely convinced through watching the two types of tools working under exactly uniform conditions.
