ACTION OF TOOL AND ITS WEAR IN CUTTING METALS THE ACTION OF THE NOSE OF THE TOOL
153 On Folder 8, Figs. 42, 43 and 44, is illustrated in enlarged views the action of a tool in cutting a chip or shaving from a forging at its proper normal cutting speed. It may be said in the case of all “roughing cuts ” that the chip is torn away from the forging rather than removed by the action which we term cutting. The familiar action of cutting, as exemplified by an axe or knife removing a chip from a piece of wood, for instance, consists in forcing a sharp wedge (i.e., one whose two flanks form an acute angle) into the substance to be cut. Both flanks of the wedge press constantly upon the wood, one flank bearing against the main body of the piece, while the other forces or wedges the chip or shaving away.
154 While a metal cutting tool looks like a wedge, its cutting edge being formed by the intersection of the “ lip surface” and “clearance surface” or flank of the tool, its action is far different from that of the wedge. Only one surface of a metal cutting tool, the lip surface, ever presses against the metal. The clearance surface, as its name implies, is never allowed to touch the forging. Thus “cutting” with a metal cutting tool consists in pressing, tearing or shearing the metal away with the lip surface of the “ wedge” only under pressure, while in the case of the axe and other kinds of cutting, both wedge surfaces are constantly under pressure.
155 After the cut has once been started, and the full thickness of the shaving is being removed, the action of the tool may be described as that of tearing the chip away from the body of the forging and then shearing it up into separate sections; the portion of the chip which has just been torn away, and which is still pressing upon the lip surface of the tool, acting as a lever by which the following portion of the chip is torn away from the main body of the metal.
156 It may be of interest to analyze to a certain extent the nature of the forces to which a chip and the forging from which it is being removed are subjected through the tearing action of the tool. The enlarged view of the chip, tool and forging, shown in Folder 8, Fig. 42, represents with fair accuracy the relative proportions which the shaving cut from a forging of mild steel (say, 60,000 lbs. tensile strength and 33% stretch) finally assumes with relation to the original thickness of the layer of metal which the tool is about to remove. It is, of course, impossible to accurately determine the extent to which various parts of the chip and forging close to the tool are under compression and tension, but in general the theory advanced is believed to be correct.
157 Referring to Folder 8, Fig. 42, the forging being cut and the nose of the tool which is removing the chip are shown on an enlarged scale. The thickness of the layer of metal about to be removed is indicated by L between the dotted line and the full line which represents the outside of the forging. It will be observed that the chip is in process of being torn apart and broken up into three sections: Section 1, which is adjoining the forging; section 2, which comes next to it, and in which rupture or cleavage has started and proceeded a a little way up from the bottom of the chip and on the left hand side, the shearing action having progressed as far as T,; section 3, in which shearing has progressed about two-thirds of the way to the top of the chip and is taking place at T,. Section 4 has been entirely sheared from its adjoining section, and has already left the lip surface of the tool.
158 On examination of the proportions of the chip it will be noticed that the width of the sections into which the chip breaks up is at their base about double the thickness of the original layer of metal which is to be removed, and that their upper portions are not enlarged to the same extent. These sections are about three times as high as the original thickness of the layer of the metal to be removed.
It should be clearly understood that the dimensions of the section of the chip will vary with each hardness of metal which is being cut, and also to a certain extent with the side and back slopes of the lip surface of the tool. The harder the metal of the forging, the less will each section into which the chip has been broken up be found to be enlarged. In other words, if the same shaped tool be used in each case the chip from soft metal enlarges or distorts very much more than the corresponding chip from hard steel. This will be referred to in paragraph 506, in explaining the reason why the total pressure on the tool has but little relation on the one hand to the cutting speed, and on the other hand to the hardness of the metal which is being cut.
159 The chip bears on the surface of the forging, say, from point H to point G, and throughout this distance is under constant compression from the lip surface of the tool. This compression is transmitted through each of the sections 1 and 2 of the chip, in the direction indicated by the small arrows, to the upper portions of these sections, which are still unbroken and act like a lever attached to the upper part of section 1 to tear‘ section 1 away from the body of the forging, as indicated at point T,. The tearing away of section 1 is also assisted by the pressure of the tool upon its lower surface.
160 After this tearing action has started, the further breaking of the chip into independent sections would seem to be that of simple shearing. It should be borne in mind that in shearing a thick piece of steel the whole piece is not shorn or cut apart at the same instant, but the line at which rupture or cleavage takes place progresses from one surface of the piece down through the metal until within a short distance from the other surface, when the whole remaining section rather suddenly gives way.
161 In shearing steel, the metal at the point of rupture is pulled apart under a tensile strain, although on each side of the shearing line the metal is under heavy compression.
162 As each of the sections of the chip successively comes in contact with the lip of the tool, its lower surface is crushed, and the metal flows and spreads out laterally until it becomes about twice its original thickness. As in all shearing, when the full capacity for flowing of the metal has been reached, it tears apart under tensile strain from the body of the adjoining metal of the forging. The compression on the chip from the tool still continues, however, and the chips continue to flow and spread out sideways at a part higher up; i. e., farther away from the surface of the tool, at the portions marked F. In the same way shearing continually takes place at the left side of the portion of the chip which is flowing or spreading out sideways.
164. Dr. Nicolson’s dynamometer experiments show that the pressure of the chip on the tool in cutting a chip of uniform section varies with wavelike regularity, and that the smallest pressure of the chip is not less than two-thirds of the greatest pressure. From this it is evident that shearing must be taking place along at least two lines of cleavage at the same time; since if each of the sections into which the chip is divided were completely broken off before the tool began to break off the following section, it is evident that there would be times when there was almost no pressure from the chip on the tool.
165 It is at first difficult to see how it is possible for the chip to be shearing at two or three places at the same time. It should be noted, however, that above the points T1, T2, T3, the metal of the chip is still a solid part of the forging, and moves down at the same speed as the forging in a single mass, or body, toward the lip surface of the tool; and with sufficient force to cause each of the three sections of the chip to flow or spread out at the same time at the parts indicated by the three letters F. According to the laws which govern shearing, rupture or cleavage in each case must take place as soon as the maximum possibility for flowing has been reached, and in each case shearing must occur at the left of the zone where the metal is flowing.
166 It is probable that after the shearing action has progressed in section 3 to about the point indicated by Ta, that the whole of this section gives way or shears with a rather sudden yielding of the metal from T, , to the upper surface of the chip. It is this rather sudden shearing point which undoubtedly causes the wavelike diminution in the pressure of the chip indicated in Dr. Nicolson’s experiments, referred to in paragraph 316.
167 By suddenly stopping one of our standard tools when cutting at full speed, we have clearly seen this shearing action taking place at two sections of the chip at the same time, and rupture had not completely taken place at the extreme upper part of the third section. In making an observation of this kind great care must be taken before stopping the lathe to be sure that in the tool to be observed there is not the slightest tendency to chatter, and the lathe driving parts should be massive in proportion to the sized cut being taken, as other-wise the elasticity left in the driving shafts causes an abnormal action of the tool at the instant the work stops, and produces a chip, of thicker section than the normal, which is completely severed from the body of the metal.
168 While the proportions of the chip and the section into which it is broken are fairly true representations of fact, yet it must be borne in mind that the lines illustrating the strains to which the chips are submitted, as well as the progress made in the shearing of the chip, are merely submitted as illustrating a theory advanced by us to explain the facts. We are, however, more or less suspicious of all theories, our own as well as those of others.
169 Throughout this paper the important facts stated by us have been verified by many and carefully made experiments; and we therefore trust that our readers, if unable to accept our theories, will at least have respect for our facts.
ACTION OF CUTTING EDGE OF TOOL IS THAT OF SCRAPING - CUTTING EDGE NOT UNDER HEAVY PRESSURE
170 It would appear that the chip is torn off from the forging at a point appreciably above the cutting edge of the tool and this tearing action leaves the forging in all cases more or less jagged or irregular at the exact spot where the chip is pulled away from the forging, as shown to the left of T1. An instant later the line of the cutting edge, or more correctly speaking, the portion of the lip surface immediately adjoining the cutting edge, comes in contact with these slight irregularities left on the forging owing to the tearing action, and shears these lumps off, so as to leave the receding flank of the forging comparatively smooth.
171 Thus in this tearing action, particularly in the case of cutting a thick shaving, while the cutting edge of the tool is continually in action, scraping or shearing off or rubbing away these small irregularities left on the forging, yet that portion of the lip surface close to the cutting edge constantly receives much less pressure from the chip than the same surface receives at a slight distance away from the cutting edge. This allows the tool to run at higher cutting speeds than would be possible if the cutting edge received the same pressure as does the lip surface close to it.
172 There are many phenomena which indicate this tearing action of the tool. For example, it is an everyday occurrence to see cutting tools which have been running close to their maximum speeds and which have been under cut for a considerable length of time, guttered out at a little distance back of the cutting edge, as shown in Folder 3, Fig. 17e. The wear in this spot indicates that the pressure of the chip has been most severe at a little distance back from the edge.
173 Still another manner in which in many cases the tearing action of the tool is indicated is illustrated in Folder 3, Fig. 17a, in which a small mass of metal D is shown to be stuck fast to the lip surface of the tool after it has completed its work and been removed from the lathe. When broken off, however, and carefully examined, this mass will be found to consist of a great number of small particles which have been cut or scraped off of the forging, as above described, by the cutting edge of the tool. They are then pressed down into a dense little pile of compacted particles of steel or dust stuck together and to the lip surface of the tool almost as if they had been welded. In the case of the modern high speed tools, when this little mass of dust or particles is removed from the upper surface of the tool, the cutting edge will in most cases be found to be about as sharp as ever, and the lip surface adjacent to it when closely examined will show in many cases the scratches left by the emery wheel from the original grinding of the tool.
174 With roughing tools made from old fashioned tempered steel, however, and which have been speeded close to their “standard speeds," in most cases after removing this “dust pile” from the lip surface, the cutting edge of the tool will be found to be distinctly rounded over. And in cases where the tool has been cutting a very thick shaving, the edge will be very greatly rounded over, as shown in the enlarged view of the nose of a tool in Folder 7, Fig. 41.
NATURE OF WEAR ON TOOLS DEPENDS UPON WHETHER IT HAS BEEN CHIEFLY CAUSED BY HEAT
175 The appearance of tools which are worn down so as to require regrinding differs widely according to whether or not the heat produced by the pressure of the chip has been the chief cause of wear; and according to the part which heat has played in producing the wear, worn out tools may properly be divided into three classes.
THE FIRST CLASS
176 Tools in which the heat, produced by the pressure of the chip, has been so slight as to have had no softening effect upon the surface of the tool.
177 Tools in which the heat has been so great as to soften the lip surface of the tool beneath the chip almost at once after starting the cut, and in which, therefore, heat has played the principal part in the wear of the tool.
WEAR OF THE SECOND CLASS
178 Tools in which the heat only slightly softens the surface of the tool during the greater part of the time that it is cutting, while during the latter part of the time heat is the chief cause of wear because, as described in the third class, it greatly softens the lip surface under pressure of the chip.
179 In the FIRST CLASS, in which heat plays no part in the wear of tools, all tools (whether made from carbon tempered steel, or from the old style self-hardening steel, or from the modern treated tools) wear in about the same manner. Namely, the lip surface just back of the cutting edge is slowly rubbed or worn or ground down through the friction of the chip, as shown in Folder 3, Fig. 17d.
180 As the surface of the tool through the long rubbing of the chip becomes slightly roughened, the tool wears away somewhat more rapidly, but the increase in the rapidity of wear is in this case by no means marked.
THIRD CLASS
181 On the other hand, tools which wear according to the THIRD CLASS begin to distinctly deteriorate within from one to three minutes after the chip has started to cut, depending upon the length of time required for the friction of the chip to raise the tool from its normal cold state to the high temperature which corresponds to the combination of pressure and speed which produces the heat. And the moment the nose of the tool has reached a degree of heat at which the metal under the chip becomes distinctly soft, the wear then proceeds with great rapidity. Sometimes after arriving at a certain degree of softness, the heat remains approximately constant, and the wear upon the tool continues at a uniformly rapid rate until a comparatively deep groove or gutter has been worn into the lip surface. At other times after the lip surface of the tool begins to soften, it appears to become rougher and cause a still greater amount of friction and heat, in which case the wear of the tool proceeds at an increasingly rapid rate, and the tool is soon destroyed. There are rare instances in which after the rapid wear has started, the friction between the chip and the tool, for some unaccountable reason, appears to become less and the tool slightly cools down. Cases have come under the observation of the writer in which tools which had been running with their noses at a visible dark red heat, cooled off to such an extent that the chip which had been very dark blue in color changed to a color but slightly darker than a brown. This indicated a very marked diminution in friction, although the cutting speed was maintained at a uniform rate throughout. This case, however, is of rare occurrence.
182 While a deep groove worn by the chip is a characteristic of wear of the third class, by no means all of the tools in this class wear into a deep groove. Most of them give out before the groove has had time to wear deep. After wear of the third class has started, tools will generally be completely ruined in a time varying from 20 seconds to 15 minutes, and the time which elapses between the softening of the lip surface and the final mining of the tool is exceedingly irregular. One of two tools—which have been proved through standardization to be uniform within, say, 1 or 2 per cent, may give out within one minute after this action starts, while the other may last 15 minutes. On the other hand, occasional lots of tools are found which. after having been proved uniform through standardization, will last under this softening speed for approximately the same length of time.
REASON FOR ADOPTING A STANDARD TEST PERIOD OF 20 MINUTES
183 It is this irregularity in the ruining time of tools belonging to the third class which has led us to adopt a trial period of 20 minutes as being the SHORTEST RUINING TIME from which it is safe to draw any correct scientific conclusions from tests in the art of cutting metals. See paragraphs 703-704.
184 A cutting speed which causes the tool to be ruined in a shorter period than 20 minutes is accompanied by such a high degree of heat as to produce irregularity in the ruining time; on the other hand, a speed which ruins at the end of 20 minutes is accompanied by that degree of heat at which tools, generally speaking, can be depended upon to wear uniformly. In other words, it represents the degree of heat at which a lot of uniform tools will all give out at about the same time.
185 Cutting speeds which are sufficiently slow to cause the tool to wear as described in the first class are entirely too slow for economy. On the other hand, tools when run at the high cutting speeds which produce wear of the third class last so short a time that these high speeds are entirely out of the question for daily shop use.
186 It is then with cutting speeds causing wear of the SECOND CLASS that we are chiefly concerned; as it is within this range of cutting speeds that almost all roughing tools in every day use should be run for maximum all-round economy. CUTTING SPEEDS OF THIS CLASS ARE REFERRED TO AS “ ECONOMICAL SPEEDS” on “ MOST ECONOMICAL srasos. ” Our experiments, therefore, have been practically confined to a study of cutting speeds of the second class.
187 By referring to paragraph 701, and Folder 15, Table 105, it will be noted that a cutting speed which will cause a given tool to be ruined at the end of 80 minutes is about 20 per cent slower than the cutting speed of the same tool if it were to last 20 minutes. By referring also to paragraph 717, it will be noted that, on the whole, we have concluded it is not economical to run roughing tools at a cutting speed so slow as to‘ cause them to LAST FOR MORE THAN ONE AND ONE-HALF HOURS without being reground. Tools which are ruined in one and one-half hours, however, are still within the second class as far as the causes for wear are concerned; and the wear of tools of the second class may be again referred to as being during the greater part of the life of the tool similar to the wear of the first-class tools, in which the heat is not sufficiently great to materially soften the lip surface. In the second class, however, the cutting speeds are so high as to cause the lip surface of the tool to gradually become rougher, and rougher, and as this surface roughens, the heat increases so that in their final wearing out, say, during the last five to fifteen minutes of their life, tools of the second class wear in a manner similar to those of the third class. Thus it is seen that wear in tools of the second class is a combination of the type of wear of the first class with the type of wear of the third class, the first class type of wear extending through the greater part of the time that the tool is cutting.
HOW CARBON STEEL TEMPERED TOOLS AND TOOLS MADE FROM OLD FASHIONED SELF-HARDENING STEEL WEAR
189 With carbon steel tempered tools at standard speeds the cutting edge begins to be injured almost as soon as the tool starts to work, and is entirely rounded over and worn away before the tool finally gives out, and the tool works well in spite of its cutting edge being damaged. While with high speed tools at standard speeds, the cutting edge remains in almost perfect condition until just before the tool gives out, when even a very slight damage at one spot on the cutting edge will usually cause the tool to be ruined in a comparatively few revolutions.
190 Carbon tempered tools and also, to a considerable extent, the old fashioned self-hardening tools (such as Mushet), when run at their “ economical ” or “ standard ” speeds, pass through the following characteristic phases as they progress toward the point at which they are finally ruined. It will of course be understood that the “rounding of the cutting edge,” the “mounting of the steel upon the lip ” and the “ rubbing away beneath the cutting edge” all progress simultaneously, although each of these phenomena is separately described.
191 The line of the cutting edge of the tool, which is at first keen, becomes very slightly dull so that it shines when held in the light, and it is then gradually rounded over until it finally loses all resemblance to a cutting edge, and after severe use frequently looks as if it had been purposely rounded over. This action is referred to in recording our experiments as “CUTTING EDGE ROUNDED.”
192 The lip, or top surface of the tool, soon becomes slightly roughened and whitened as the shaving presses against it, then the surface of the tool near the cutting edge becomes discolored as if the temper of the tool were being drawn, and this discoloration increases both in the extent of its area and in its approach toward almost a black color, until the tool is entirely ruined. This action is referred to as “ DISCOLORED BACK FROM CUTTING EDGE.”
193 The flank of the tool begins to discolor in a manner similar to the lip and this discoloration continues to increase until the tool is finally ruined. This action is referred to as "DISCOLORED BELOW CUTTING EDGE."
194 Long before the tool is ruined, also, the fine particles of steel or dust scraped of by the cutting edge begin to weld or stick to the lip of the tool and mount upon it sometimes from 1/16 inch to 1/4 inch in height, as shown in Folder 3, Fig. 17d; referred to as “STEEL MOUNTED ON LIP.”
195 The flank of the tool just below the cutting edge begins to wear or rub away slightly, and this wear gradually extends further and further below the cutting edge until the tool is ruined; referred to as “RUBBED BENEATH CUTTING EDGE.”
196 Small portions of the metal being cut wedge themselves in between the flank of the tool and the work, and are partly welded or stuck to the tool, and these lumps which are welded on to the tool, and the rubbing away of the flank as the tool grows dull, prevent the feed from being uniform in thickness, and cause the tool sometimes to entirely skip its feed for one or two revolutions; referred to as “ TOOL JUMPED.” These lumps also force the work and the tool away from one another, and so leave the work rough and lumpy in these places; referred to as “LEFT LUMPY FINISH.” Also the particles of metal which are welded on to the flank of the tool frequently leave scratches on the finished part of the work; referred to as “LEFT SCRATCHY FINISH."
197 The tool is finally completely ruined by having so much of its flank rubbed away beneath the cutting edge, that no amount of pressure can force it into the work, and when it reaches this condition it must be immediately removed from the tool post, or there is danger of breaking either the tool, the machine, or the work.
198 As a result of our experiments it became evident that in case of tools made from the “low speed” self-hardening steels, for instance, such as the old fashioned Mushet steel, the tools would in many cases run successfully long after the line of the cutting edge had become slightly injured; also that in the case of “roughing” tools made from the carbon or tempered steels, it was economical and right in daily shop practice to run these tools at such a cutting speed that their actual cutting edges rounded off quite soon after starting to do their work. These tools continued to cut with their edges rounded, somewhat as shown in Folder 7, Fig. 42, successfully through the whole cut.
199 As stated in paragraph 67, all of our experiments made with these two types of tools (carbon and Mushet types) to determine the laws for cutting metals were rendered much more difficult, owing to the judgment required in deciding the exact amount of damage which was appropriate both to the tool and the particular thickness of shaving which was being taken.
200 In paragraph 506 will be found a further discussion of the very great part which the heat caused by the friction of the chip against the lip surface of the tool plays in ruining tools, and in para- graph 965 we refer to the peculiar property of retaining their hardness even at a high heat, which is called “red hardness,” as being the essential novelty in the discovery of new “high speed” tools. HOW MODERN HIGH SPEED TOOLS WEAR
201 As stated above, in the case of modern high speed tools, the damage caused to the tool through the action of cutting is confined almost entirely to the lip surface of the tool. Doubtless also the metal right at the cutting edge of the tool remains harder than it is directly under the center of pressure of the chip, because the cutting edge is next to and constantly rubs against the cold body of the forging, and is materially cooled by this contact.
202 Whether the lip surface be ground away at high speeds or at slower speeds, the nose of the tool is generally “ruined” in a very short time after the cutting edge has been so damaged that it fails to scrape off smoothly even at one small spot the rough projections which have been left on the body of the forging by tearing away the chip. The moment the body of the forging begins to rub against the clearance flank of one of these high speed tools at or just below the cutting edge, even at one small place, the friction at this point generates so high a heat as to soften the tool very rapidly. After a comparatively few revolutions (particularly when cutting medium or soft steel or cast iron), the cutting edge and flank of the tool beneath it will be completely rubbed and melted away, as shown in Folder 3, Fig. 17b.
203 The above characteristic of holding their cutting edges in practically perfect condition while running at economical speeds up to the ruining point is a valuable property of the high speed tools, since it insures a good finish, and the maintenance throughout the cut of the proper size of the work, without the constant watchfulness required on the part of the operator in the case of old slow speed tools with their rounded and otherwise injured cutting edges, which when run at economical speeds were likely at any minute to damage the finish of the work. -
204 When one of these high speed tools is nearing its ruining point, a very trifling nick or break in the line of the cutting edge will beat once noticed by its making a very small but continuous scratch, projecting ridge, or bright streak, on the flank of the forging (namely, upon that part of the forging from which the spiral line of the chip has just been removed). When the skilled operator notices this line, he at once removes the tool from its cut, and notes upon the record “tool began to ruin”; the abbreviation for which in our case consists of the letters B R. ‘
205 The letter R is used to indicate a tool which has been entirely ruined; the word “Good” (G) indicates a tool which is removed from the cut showing but very small damage or wear even to the lip surface; the word “Fair” (F) indicates a tool which shows considerable wear upon the lip surface, such, for instance as is shown in tool, Folder 3, Fig. 17c, the cutting edge of which has, however, not as yet absolutely started to ruin.
206 It is a curious fact that high speed tools which differ in their chemical compositions, and perhaps also slightly in their high heat and subsequent treatment, give very different indications while cutting that they have begun to ruin. For instance, the kind of scratch, shiny streak, or ridge, which on high speed tools made from steel of one chemical composition would indicate the approach of the ruining point, may possibly be no indication of the approach of the ruining point if made by another type of high speed tool.
207 It is therefore necessary for the operator with each new batch of tools which are to be used in an experiment to absolutely ruin a number of these tools so as to be thoroughly familiar with that exact appearance of the flank of the forging which unquestionably indicates the approach of the ruining point. After the operator has assured himself beyond doubt of this indication, it will not then be necessary for him to ruin each tool completely. The writer has in mind many instances, however, in which from the appearance of the scratches or shiny streaks, etc., on the flank of the forging, he was convinced that the tool was about to ruin, and in which the same tool continued to run for many minutes afterward, and even, in some cases, at an increase in cutting speed. In this element in making experiments, as in all others, nothing must be taken for granted, everything must be proved.
208 In paragraphs 353 and 354 two other reasons for the cutting edges of tools giving out are referred to: a. The spalling off or crumbling away of the tool due to the ' pressure of the chip at its extreme cutting edge, as shown in Folder 6, Fig. 31a; and b. The spalling off or crumbling away of the extreme cutting edge due to the feeding pressure at this point, as shown in Folder 6, Fig. 31b.
209 As both of these types of yielding must be chiefly considered in their relation to the acuteness of the lip angle of the tool, we refer to them later in the paper.
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