Productivity Science of Machining V - Industrial Engineering Research by Taylor Part 5

Lesson 6 of Process Industrial Engineering ONLINE Course (Module)

Lesson 49 of  Industrial Engineering ONLINE Course

IE Research by Taylor - Productivity of Machining  - Part 1 - Part 2  - Part 3  - Part 4 - Part 5 

Lesson 48 -  Productivity Science of Machining for Process Improvement Part 4.

Lesson 49A - Productivity Science of Machining - Stephenson - Agapiou 2006  

Paper Published in the Proceedings of IISE 2020 Annual Conference.

Frameworks for Productivity Science of Machine Effort and Human Effort

Rao, Kambhampati Venkata Satya Surya Narayana. IIE Annual Conference. Proceedings; Norcross (2020): 429-434.

https://www.proquest.com/openview/5786c4e6edff56abf808b4db26f083b3/1.pdf

The machining productivity science points discussed in this lesson.

(H) lip and clearance angles: 1 with lip angle 68 degrees to 1.023 with lip angle of 61 degrees;

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.

(J) elasticity of the work and of the tool: 1 with tool chattering to 1.15, with tool running smoothly.

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.

Shape for Cutting Tools for Productivity 

Productivity Impact of lip and clearance angles: 

1 with lip angle 68 degrees to 1.023 with lip angle of 61 degrees;

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. 

(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.

340 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 angle 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 off or 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 off or 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 hardness 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.

Productivity Impact of elasticity of the work and of the tool: 

1 with tool chattering to 1.15, with tool running smoothly.

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.

CHATTER OF THE TOOL 

633 The following are the general conclusions arrived at on the subject of chatter of the tool: 

CHATTER CAUSED BY THE NATURE OF THE WORK 

634 (A) Chatter is the most obscure and delicate of all problems facing the machinist, and in the case of castings and forgings of miscellaneous shapes probably no rules or formulae can be devised which will accurately guide the machinist in taking the maximum cuts and speeds possible without producing chatter. (See paragraph 648) 

635 (B) It is economical to use a steady rest in turning any piece Of cylindrical work whose length is more than twelve times its diameter. (See paragraph 669) 

CHATTER CAUSED BY THE METHOD OF DRIVING THE WORK 

636 (C) Too small lathe-dogs or clamps or an imperfect bearing at the points at which the clamps are driven by face plate produce vibration. (See paragraph 659)

CHATTER CAUSED BY CUTTING TOOLS 

637 (D) To avoid chatter, tools should have cutting edges with curved outlines and the radius of curvature of the cutting edge should be small in proportion as the work to be operated on is small. The reason for this is that the tendency of chatter is much greater when the chip is uniform in thickness throughout, and that tools with curved cutting edges produce chips which vary in thickness, while those with straight cutting edges produce chips uniform in thickness. (See para- graph 661) 

638 (E) Chatter can be avoided, even in tools with straight cutting edges by using two or more tools at the same time in the same machine. (See paragraphs 664 and 665) 

639 (F) The bottom of the tool should have a true, solid bearing on the tool support which should extend forward almost directly beneath the cutting edge. (See paragraph 663) 

640 (G) The body of the tool should be greater in depth than its width. (See paragraph 662) 

CHATTER CONNECTED WITH THE DESIGN OF THE MACHINE 

641 Chatter caused by modifications in the machine may be classified as follows: 

642 (H) It is sometimes caused by badly made or fitted gears. 

643 (J) Shafts may be too small in diameter or too great in length. 

644 (K) Loose fits in the bearings and slides may occasion chatter. 

645 (L) In order to absorb vibrations caused by high speeds, machine parts should be massive far beyond the metal required for strength. (See paragraph 656) 

THE EFFECT OF CHATTER UPON THE CUTTING SPEED OF THE TOOL 

646 (M) Chatter of the tool necessitates cutting speeds from 10 to 15 per cent slower than those taken without chatter, whether tools are run with or without water. (See paragraphs 671 to 677) 

647 (N) Higher cutting speed can be used with an intermittent cut than with a steady cut. (See paragraphs 678 to 680) 

648 Of all the difficulties met with by a machinist in cutting metals, the causes for the chatter of the tool are perhaps the most obscure and difficult to ascertain, and in many cases the remedy is only to be found after trying (almost at random) half a dozen expedients. 

649 This paper is chiefly concerned with chatter as it is produced or modified by the cutting tool itself. Some of the other causes for chatter, however, may be briefly referred to. These may be divided into five groups:

(A) The design of the machine; 

(B) the nature and proportions of the work being operated upon; 

(C) the care and adjustment of the parts of the machine; 

(D) the method of setting the work in the machine or of driving it; . 

(E) the shape of the cutting tools, manner in which they are set in the machine and the speeds at which they are run. 

Causes (A) and (B) are outside the control of the machinist. Elements (C), (D) and (E) are or should be to a large extent under the control of the management of the shop. 

650 (A) Referring, now, to cause (A), “The design of the machine” the chief elements causing chatter in the design of a machine are: 

651 (Aa) Gears which are set out of proper adjustment or the teeth of which are untrue. It should be noted that involute teeth will run smoothly whether their pitch diameters exactly coincide or not, whereas the epicycloidal teeth are almost sure to rattle unless their pitch lines are maintained in their exact proper relations one to the other. 

652 (Ab) Chatter is frequently caused through mounting the driving gears upon shafts which are either too small in diameter or too long. A large excess in the diameter of shafts beyond that required for strength is called for in order to avoid torsional deflection which produces chatter. 

653 (Ac) Lathe shafts and spindles must of course be very accurately and closely fitted in their bearings, and the caps adjusted so as to avoid all play. 

654 (Ad) For heavy work the lathe tail stocks should be fastened to the bed plates with bolts of very large diameter, and should be lightened down with long handled wrenches. 

655 (Ae) The lathe bed itself should be exceedingly massive, and should contain far more metal than is required for strength or even to resist ordinary deflections; and the moving tool supports should also be heavy far beyond what is required for strength. 

MASSIVE MACHINES NEEDED FOR HIGH SPEEDS 

656 Undoubtedly high cutting speeds tend far more than slow speeds toward producing minute and rapid vibrations in all parts of the machine, and these vibrations are best opposed and absorbed by having large masses of metal supporting the cutting tool and the head and tail stocks. It is largely for the purpose of avoiding vibration and  chatter in machines that the high cutting speeds accompanying the modern high speed tools call for a redesigning of our machine tools. While it is true that in many cases a very great gain can be made by merely speeding up a machine originally designed for slow speed tools, this increase in speed almost invariably produces a corresponding increase in the vibration or chatter, and for absorbing this, the lathes and machines of older design are, in many cases, too light throughout. 

657 (C)     Cause (C) namely, “The care and proper adjustment of the various parts of the machine” is almost entirely under the control of the shop management. It is of course evident that so far as the effect of chatter is concerned, one of the most important causes can be eliminated from the shop by systematically looking after the careful adjustment of all of the working parts of the machine to see that the caps of the bearings are always so adjusted as to have no lost motion and yet not bind, and so that all gibs and wedges for taking up wear upon the various slides are kept adjusted to a snug fit. It is our experience, however, that the adjustment of the various parts of the machine should in no case be left to the machinist who runs his lathe, but that the adjustment and care of machines should be attended to systematically and at regular intervals by the management. In large shops a repair boss with one or two men can be profitably kept steadily occupied with this work. A tickler, however, should be used for reminding the repair boss each day of the adjustment of machines and the overhauling which should be attended to on that day. 

658 (D)   Cause (D), namely, “The method of setting the work in the machine or of driving it,” is in many cases capable of being directly under the control of the machinist. 

659 (Da) One of the most frequent causes for chatter lies either in having too light or too springy clamps or lathe dogs fastened to the work for the purpose of driving it, or in having vibration at the point of contact between the lathe dog, and the face plate of the lathe, or the driving bracket which is clamped to it. In heavy work the clamps should be driven at two points on opposite sides of the face plate, and great care should be taken to insure a uniform bearing of the clamps at both of these driving points. Chatter through vibration at this point can frequently be stopped by inserting a piece of leather or thick lead between the clamps and the driving brackets on the face plate; which has the effect both of deadening the vibration and equalizing the pressure between the two outside diameters at which the clamp is driven by the face plate. 

660 (Db) A dead center badly adjusted so as to be either too tight or too loose on the center of the work, or any lost motion in the tail stock of the lathe is such an evident source of chatter that it need not be dwelt upon. 

661 (E)     Cause (E) namely, “The shape of the cutting tools, the manner in which they are set in the machine and the speeds at which they are run.” In paragraphs 312 and 315 we have attempted to explain the effect of a uniform thickness of chip in causing chatter, and have indicated that the proper remedy for this is to use a round nosed tool, which is always accompanied by a chip of uneven thickness. 

662 In paragraphs 415 and 425 we have also referred to the desirability of having the body of tools deeper than their width in order to insure strength as well as to diminish the downward deflection of the tool, which frequently results in chatter, particularly when the tools are set with a considerable overhang beyond their bearing in the tool post. 

663 In paragraphs 450 and 459 we have also called attention to the great desirability of designing tools with their bottom surfaces extending out almost directly beneath the cutting edge, and of truing up the bottom surface of the tools, so as to have a good bearing directly beneath the nose of the tool on the tool support. If sufficient care is taken in the smith shop and the smith is supplied with a proper surface plate, the tools can be dressed so as to be sufficiently true on their bottom surfaces for all ordinary lathe work. 

664 As indicated in paragraphs 315 to 325, it has been the necessity for avoidance of chatter which has influenced us greatly in the adoption of round nosed tools as our standard. As shown in paragraph 312, tools with straight cutting edges, which remove chips uniform throughout in thickness can be run at very much higher cutting speeds than our standard round nosed tools; but owing to the danger of chatter, from these tools, their use is greatly limited, in fact, almost restricted to those special cases in which chatter is least likely to occur. Attention should be called, however, to a method by which straight edge tools have been used successfully for many years upon work with which there was a very marked tendency to chatter. 

665 While at the works of the Midvale Steel Company we superintended the design of a large lathe for rough turning gun tubes and long steel shafts; in which tools with long straight cutting edges were used without chatter, and yet at the high speeds corresponding to the thin chips which accompany this type of tool. This lathe was designed with saddle and tool posts of special construction, so that two independently adjustable tool supports were mounted on the front side of the lathe and one on the back side. In each of these slides a heavy straight edge tool was clamped. The three tools were then adjusted so that they all three removed layers of metal of about equal thickness from the forging, and, although the tendency toward chatter,- owing to the uniform thickness of the chip,—as indicated in paragraph 315, was doubtless as great with these straight edge tools as with any others, the period of maximum or of minimum pressure for all three tools never corresponded or synchronized so that when one tool was under maximum pressure, one of the others was likely to be under minimum pressure. For this reason the total pressure of the chips on all three tools remained approximately uniform and chatter from this cause was avoided. 

666 (B)    Cause (B), namely, “The nature and proportions of the work being operated upon.” 

667 In assigning daily tasks to each machinist with the help of our slide rules, the element which still continues to give the greatest trouble to the men who write out these instructions is deciding just how heavy a cut can be taken on the lighter and less rigid classes of work without causing chatter. This branch of the art of cutting metals has received less careful and scientific study than perhaps any other. While the element is one which must always remain more or less under the domain of “rule of thumb,” since the causes which produce chatter, particularly in castings of irregular shapes, are so many and complicated as to render improbable their successful reduction to general laws or formulae, undoubtedly much can be done toward attaining a more exact knowledge of this subject, and experiments in this line present a most important field of investigation. 

668 The following rule (belonging to the order of “ rule of thumb ”) which has been adopted by us after much careful and systematic observation, extends over work both large and small, and covers a wide range: 

IT IS ECONOMICAL TO USE A STEADY REST IN TURNING ANY PIECE OF METAL WHOSE LENGTH IS MORE THAN TWELVE TIMES ITS DIAMETER 

669 When the length of a piece becomes greater than twelve times its diameter, it is necessary to reduce the size of the cut to such an extent that more time will be lost through being obliged to use a light cut than is required to properly adjust a steady rest for supporting the piece. 

670 There is one cause for chatter which would seem to be impossible to foresee and guard against in advance; i.e., chatter which is produced by a combination of two or more of the several elements likely to cause chatter. If, for instance, the natural periods for vibration in the tool and in the work or in any of the parts of the lathe and the work happen to coincide or synchronize, then chatter is almost sure to follow; and the only remedy for this form of chatter seems to lie in a complete change of cutting conditions; a change, for instance, to a coarser feed with an accompanying slower cutting speed, or vice versa. Unfortunately, for economy, higher speeds rather than slow speeds tend to produce this type of chatter, and the remedy therefore generally involves a slower cutting speed. 

THE EFFECT OF CHATTER UPON THE CUTTING SPEED 

671 A tool which chatters to any great extent must be run at a rather slower cutting speed than a tool which runs free from chatter, as will be seen by the following carefully tried experiment: 

672 A forging, 14 feet long, 4 and 5/8 inches diameter, made out of exceedingly hard steel which was especially hammer hardened and uniform, was placed in the lathe, and standard cuts 3/16 inch depth, and 1/16 inch feed (with our standard round nosed tool 7/8 inch) were taken upon it in such a way that they first ran smoothly without chattering; other cuts were then taken in such a position on the forging that the tool chattered badly throughout its cut. This was accomplished by using a steady rest in one case so as to prevent chatter, and in the other case running without the steady rest. All of the tools had been carefully standardized before starting the experiments, and proved uniform and capable of running at maximum cutting speeds. The forging had also been proved uniform, and its standard cutting speed had been shown to be between 15.5 and 16 feet per minute. 

673 In the two tables below in paragraph 674 are given the details of the cutting speeds obtained with and without chatter. In one of these experiments the tool was run without water and in the other the tool was cooled through the use of a heavy stream of water. 

674 An examination of the results of this experiment indicates in general that chatter causes a reduction in cutting speed of from 10 per cent to 15 per cent whether tools are run without water or with a heavy stream of water to cool them. 

675 The following EXPERIMENTS SHOW THAT CHATTER  CAUSES A REDUCTION IN CUTTING SPEED OF 10 PER CENT TO 15 PERCENT WHETHER THE TOOLS ARE RUN WITH OR WITHOUT TO COOL THEM. (See paragraphs 671 to 676)

676 Experiment No. 125b was made for the purpose of again showing conclusively that both the tool and the forging had been properly standardized. It will be noted that this tool, free from chatter, broke down in 14.5 minutes at a cutting speed of 17 feet 6 inches, whereas the tool just above it ran all right at 15 feet for 20 minutes, showing that both the forging and tools had been properly standardized. 

677 Accurate experiments on the chatter of the tool are difficult to make because the comparatively small diameter of work which is needed to insure chatter calls for an extremely hard piece of metal  (i. e., slow cutting speeds) in order to make the runs, which must last for 20 minutes, extend through a sufficiently short distance over the length of the forging so that the tools shall not be in danger of chattering. It was for this reason that we were obliged to make the above forging out of extremely hard metal. 

HIGHER CUTTING SPEED CAN BE USED WITH AN INTERMITTENT CUT THAN WITH A STEADY CUT 

678 An intermittent cut, however, has a. very different effect upon cutting speed from that produced by chatter. We have observed in a large number of cases that when a tool is used in cutting steel with a heavy stream of water on it (and this is the proper method of cutting steel of all qualities), a rather higher cutting speed can be used with an intermittent cut than with a steady one. The reason for this is that during that portion of the time when the tool is not cutting, the water runs directly on those portions of the lip surface and cutting edge of the tool which do the work and for this reason the tool is more effectively cooled with intermittent work than with steady work. As an example of intermittent work, the writer would cite: 

a. cutting the outside diameter of a steel gear wheel casting, in which case the tool is only one-half its time under cut;
b. or turning small pieces of metal which are greatly eccentric;
c. or, for example, all planer and shaper work which is not too long. 

679 It would seem from a theoretical standpoint that a tool would be greatly damaged (and therefore a slow cutting speed would be called for) by the constant series of blows which its cutting edge receives through intermittent work. It will be remembered, however, that in planer work (and this class of intermittent work comes to the direct attention of every machinist), the tool is more frequently injured while dragging backward on the reverse stroke of the planer than it is while cutting, and it is very seldom that a tool is damaged as it starts to cut on its forward stroke. In all cases, however, where the tool deflects very greatly, when it starts its cut on intermittent work slower speeds are called for than would be required for steady work. 

680 The above remarks on intermittent work do not, of course, apply to cast iron with a hard scale or the surface of which is gritty. It is evident that in all such cases owing to the abrasive action of the sand or scale on the tool, intermittent work is much more severe upon the tool than a steady cut.

Updated 16 July 2021

Pub 4 July 2020