Taylor AMC - Part 10 - Chatter of the Tool - Productivity Implications

Research by F.W. Taylor

Industrial engineers have to do research to discover variables that have an impact on machine work or equipment work. Taylor's work is the foundation for this task. Unfortunately, in industrial engineering discipline there is no comprehensive text, that discussed this task. So researches done by industrial engineers are not available in a consolidated form to guide use of there research results in productivity engineering of machine work and equipment work. Industrial engineering academicians have to notice this gap in knowledge base of IE and make correction for it.

Narayana Rao specifically included Research as an important function in Industrial Engineering

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.