Tool Wear and Failure
CONTROL OF THE CAUSES OF TOOL WEAR AND FAILURE
The design of cutting tools is not a pure science, involving only computations to be carried out in functional isolation. Of itself, a cutting tool is only a piece of metal of special shape and construction, although frequently a very expensive piece of metal.
Good tool design is accomplished with consideration of many inseparably related factors: the comparison, hardness, condition, and shape of the workpiece material; the rate and volume of the specified production; the type, motions, power, and speed of the machine tool to be used; the toolholders and workholders available or be designed; the specified accuracy and surface of the finished workpiece, and many other factors, common or specific to the particular operation.
The following discussion is, therefore, very much the business of the good tool designer. He cannot escape responsibility by saying that some of the factors leading to undue tool wear, or to tool failure, are the responsibility of the process or methods functions. He should know, or anticipate, the possible application difficulties that lie ahead for the tool he is about to design, and then accommodate the difficulties by tool design, or consult with the other manufacturing functions as to changing certain conditions, or both.
Rigidity Set-up rigidity is vital to the maintaining dimensional accuracy of the cut surface, since the tool shifts into or out of the cut with the accumulation of static deflections and take-up of loose fits. Rigidity also maintains surface-finish quality, avoiding the marks made by elastic vibration and free play of loose fits and backlash. In the control of vibration, rigidity of the part and cutting tool can make the difference between success or failure of the machining operation.
Increasing mass reduces vibration amplitude and resonant frequency, while dampening reduces amplitude by dissipating vibratory energy as frictional heat. Since each part of the cutting system (i.e. the machine, the fixture, the tool and the workpiece) can affect the mode and amount of vibration most should be made oversize and broadly supported. This provides design latitude for those members having more severe costs and space limitations. Designers should be generous with rigidity, anticipating fast, efficient cuts.
Strength The strength of each member can be considered separately and related to the magnitude and application of the forces it will transmit. It should clearly be sufficient to prevent breakage or deformation beyond the elastic limit when the operation is performed correctly. The designer must also consider overloads and damage that may be encountered, providing abundant strength wherever economically possible. In particular, generous size and material specification should give good working life to areas subject to abrasive wear and work hardening under impact loads. But chatter, packed chips, or binding due to set-up misalignment can multiply normal operating forces many times. Tool failure, mechanical malfunctions, and operating errors threaten destructive casualties even with costly overdesign.
Weak Links A common practice is to protect the structural chain with weak links in anticipation of casualties, and to confine or limit the possible damage. Suitable low strength with high rigidity is illustrated by the common soft shear pin. But these weak links must be strong enough to withstand normal operation and overload if possible. The permanent members are made unquestionable stronger by comparison and are protected by location. Identical design criteria make weak links an ideal combination with wearing details. They should be comparatively cheap, with duplicates widely stocked or readily produced, and of a form of easy, accurate replacement mounting. Mass-produced, delicate workpieces are of themselves natural weak links, though cutting-tool inserts are the typical wear and breakaway members. Indexable insert cutting tools, using replaceable backing seats, are ideal examples of wear and damage protection.
Force Limitations Operating forces may be obviously limited by a weak-link member, as in the case of a delicate workpiece. A machine tool such as hydraulicoke planer or broaching machine may be related in terms of force, with the ratings understated but subject to measurement and control. Some saws, mills, and grinders regulate the feeding force instead of the cutting force in the direction of cutting velocity. This causes the machine to stop feeding if an accident occurs or if forces begin to exceed a certain safe level.
Speed, Feed, and Size A machine tool’s speed and feed ranges, its cutting tool adaptor capacity, and its working clearance establish restrictions on tool design and production rate. The effect of forces is indirect but inevitable. A milling cutter with few teeth contacting a delicate workpiece exerts only a few times the cutting forces of one tooth, which high speed permits rapid completion of the cut. Limited cutter diameter and a low speed range would require more teeth and more force or a longer cutting time. Variable infeed rates can be used to speed rough stock removal and then minimize distortion while finishing, as in the case of the spark-out of a grinding wheel. The important thing is to design around limitations and take full advantage of flexibility.
Related Force Components Total cutting force is usually resolved into three mutually perpendicular components. Force in the direction of feeding motion of a turning, boring, facing, plunge forming, or parting tool corresponds to radial force on a peripheral-cutting milling cutter tooth or abrasive wheel or belt contact area. It is commonly taken as from one-fourth up to three- fourths of the tangential force Ft for sharp tools, the larger fraction being appropriate for extremely heavy feeds. The straight-line cutting tools, namely teeth of saw blades or broaches and planer or shaper tools, develop this force component in the direction of feed into the work.
The radial force component for turning or boring tools is normal to the finished work surface in facing, planing, and shaping, and in an axial direction for peripheral milling. It may be negative, pulling into the work as a result of large positive back rake, but it is usually a pushing force of relatively low value.
The third and most significant force component is the tangential force Ft which acts on the top of the tool tangent to the direction of rotation of the part or tool. Carbide turning tools typically have 1000 pounds (4450 N) of tangential force in general purpose machining applications.
Chip Disposal One sure way to overload a cutting tooth is to block the path of the chip flowing across its face so that the chip is re-cut. Single-point tools cutting ductile work frequently employ a pressed-in chip breaker to curl an otherwise stringy chip so that it will break in the form of a figure nine and fall away. If groove design is too weak for the size of cut being taken, it can cause edge chipping, or breakage. Small diameter coarse pitch milling cutters commonly have ample chip spaces, provided that the chips are thrown or washed out between successive passes through the cut. On the other hand, large diameter cutters taking full width cuts must carry the chip a half rotation before the chip can exit. These cuts require large chip slots. It is difficult to remove work materials like soft steel or copper alloys and titanium, whose chips tend to weld onto the tool face. Chip disposal in milling slots may demand high positive rake angles and climb milling instead of conventional cutter rotation to eject the chips. Complex selection and application methods have been developed for tapping and deep hole drilling where chip clogging, misalignment, and runout can readily break tools. Tool design should provide space for chip flow and means of disposal, which may well be the solution to many problems of tool chipping.
Uneven Motions Another sure way to overload a cutting tooth is to increase the feed rate drastically beyond its structural or chip-disposal capacity. Machine structural deflection accomplishes this is the example of a drill breaking as it breaks through the work. As the heavy thrust of the chisel edge is relieved, structural members spring back toward their unstressed shape, and the drill lips plunge into the work for an oversize bite. Feed mechanisms may employ air or hydraulic fluid whose compression is elastic; or gearing and a leadscrew nut fit may introduce backlash. Machine way motion becomes jumpy at slow speeds (“slip-stick” motion), even when heavy lubrication. A milling cutter at slow feed may actually rub until pressure builds up. It then may dig into the work and surge ahead. Adding to the difficulty, the sudden change in cutting torque adds to the pounding caused be teeth entering the cut.
Torsional vibration and backlash tend to develop in a rotary drive train. Should cutter rotation become so erratic that it momentarily stops, carbide teeth will generally break at once by being bumped into the work. With some teeth gone, the entire cutter may fail progressively as each successive tooth is unable to carry the extra load left by the preceding damaged teeth.
Chatter The rapid, elastic vibration that sometimes appears between tool and work is easily detected by marks on the work surface and by the sound that gives it the name “chatter.” Chatter is the momentary separation and the tool and workpiece and the immediate banging back into contact at an audible frequency. It is a danger signal of impending possible chipping or fracture. The remedy is to eliminate uneven motion and loose fits. Chatter is less likely with few teeth moving at high velocity taking thick chip loads, and having high rake and ample relief angles. A negative take angle may prevent pulling into the cut. In grinding, harder action or broader contact helps withstand bumping. As an extreme simplification, chatter can be combated with lower cutting forces while looseness and backlash cannot. Like all other problems in machining, chatter can be greatly reduced by proper tool design.