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Whiskers toughen ceramics for hardest cuts.

Outstanding hardness of whisker-reinforced ceramic inserts has made possible the machining of many aerospace materials previously workable only by grinding. Greatest savings to date have been shown in the heat-treated alloy steels, die steel, weld overlays, hard facings, and hard irons.

Though first introduced to machine aerospace nickel-based alloys, whisker-reinforced ceramic inserts are also demonstrating their ability to turn hard materials other than nickel alloys in the 45-65 Rc range.

Machining speeds using the whisker-reinforced ceramic WG-300 material can be increased up to eight times those of uncoated tungsten carbide tools and four times those of coated carbide tools.

The secret of whisker-reinforced ceramics is their ability to withstand high temperatures while maintaining strength and hardness. Heat generated in the shear zone ahead of the tool has been traditionally thought of as a negative factor in machining and associated with heat-related failure of cemented carbide cutting tools, unless, of course, cutting speed was reduced to a point where carbide inserts would give acceptable life.

Ceramic inserts dramatically affect the mechanism for chip formation, which can be likened to the sideways slide of a deck of cards caused by the rake face of the tool. The chip is formed first by grain boundary distortion in front and below the shear plane, followed by grain boundary dislocation. This results in a chip which is always thicker than the layer of material being removed.

A large amount of shear stress is required to cause plastic deformation and shear to occur in the shear zone. This results in the generation of significant quantities of heat. In fact, as much as 80% of the heat generated during cutting is produced in this way. The other 20% comes from the sliding of the chip over the tool rake face and the contact of the flank of the tool with the workpiece.

Most of the heat generated during metal cutting is dissipated by the chip carrying it away. As cutting speeds increase, however, the process reaches a point at which heat generated in the shear zone cannot be conducted away during the very short time in which the metal passes through this zone, and the heat attacks the insert.

Because whisker-reinforced ceramic inserts can withstand high temperatures while maintaining strength and hardness, there is an optimum cutting speed--outside the range of carbide tools--where the heat generated lessens the cutting forces by softening the metal and aiding in grain boundary dislocation.

When applied to most forged nickel-base alloys, for example, optimum speeds can be achieved with temperatures exceeding 1000 C. Excellent thermal shock resistance of WG-300 results in a cutting material which can be used either dry, wet, or even intermittently cooled without fear of catastrophic tool failure from thermal cracking.

Why whiskers?

The basic concept involves reinforcing a hard ceramic matrix with extremely strong, stiff silicon carbide crystals, called whiskers. These whiskers, which are grown under carefully controlled conditions for purity and lack of grain boundaries, approach the theoretical maximum strength obtainable, on the order of 1 million psi tensile.

The super-strong whiskers are dispersed into a matrix of fine-grained aluminum oxide where they act much like glass filaments do in fiberglass, adding tensile strength and improving the fracture toughness of the brittle matrix.

The increase in fracture toughness of the material is such that inserts are now offered without hones as a standard, making them suitable for finish cuts on most forged nickel-base alloys without smearing. Identifiable by small hairlike particles embedded in the finished surface, smearing is caused by nickel, which is very gummy in nature. Nickel builds up on the flank of the tool and is then swept past a worn, chipped, or honed area on the insert under great pressure and pressure-welded or embedded in small fragments into the finished surface.

Rethinking processes

Physical properties, however, are only a rough indicator of cutting tool performance. Whisker-reinforced ceramics should always be evaluated in actual service where the synergisms of material properties and cutting tool engineering can be seen.

The correct application of ceramic tooling on a CNC machine necessitates reprogramming of the part. Here are some tested methods to approach machining using ceramic cutting tools:

* Use the strongest insert shape possible. In declining order of corner strength, the strongest inserts are round, 100 deg diamond, square, 80 deg diamond, triangle, 55 deg diamond, and 35 deg diamond. Always use the strongest possible shape to maximize corner strength and metal removal capability.

* Use the largest corner radius possible. The larger the corner radius, the stronger the corner. Do not attempt to do all roughing operations with a small corner radius just because the finished fillet calls for a small radius. Use a round insert or large radius insert for roughing, and change the tool for the final cuts.

* Use the correct edge preparation for the application. In ceramic tool applications, edge preparation is critical to tool life and surface integrity. Edge preparations are used to change the shear forces at the edge to compressive forces, thereby guarding against chipping and breakage. For most nickel alloy operations involving light roughing and finishing in clean material, the T1 edge preparation (0.002-0.003 x 20 deg) should be standard. No hone is used. For all roughing other than very heavy-duty machining, a T2A edge preparation should be used (0.006-0.008 x 20 deg + 0.0005" hone).

* Use the thickest inserts available for roughing. Increased insert thickness results in far better impact resistance, better heat dispersion, and longer tool life.

* Use a toolholder or boring bar with the largest possible cross section. Stability of the tool and freedom from deflection are paramount to consistent performance. If the tool post will accept 1 1/4" (32 mm) shanks, do not be satisfied to take a 1" (25 mm) shank and shim it to suit. This is false economy.

* Consider heavy metal or carbide bars for boring applications. Boring bars in particular usually operate with much greater length-to-diameter ratios than turning tools. In this case, "heavy" metal or solid carbide bars are often easily justified. Solid carbide boring bars have three times the modulus of elasticity of a steel bar. This means that a carbide bar will only deflect one-third as much as a comparable steel bar under identical circumstances.

* Prechamfer on entry and exit whenever possible.

* Keep toolholder overhang to a minimum. Any deflection will lead to vibrations which are particularly damaging to ceramic tools. Unnecessary tool overhang is the principal cause of vibration. It should be noted that the force required to produce a particular deflection decreases by the cube of the overhang, ie doubling the overhang will increase deflection eight times if all other conditions are constant.

* Use a toolholder system designed for ceramic inserts. In the case of negative rake tooling, the normal carbide tool geometry of -5 deg top and side rake may be changed very advantageously to -5 deg top rake -10 deg side rake for materials under 45 Rc hardness. Positive rake toolholders for use with ceramic inserts should have a neutral side rake and 5 deg positive top rake for longer effective tool life.

Finally, and perhaps most importantly, rethink the process. Here's a sampling of how ceramic inserts are being used to restructure machining operations:

Cut-off operations may be accomplished by using a WG-300 grooving tool and then completing the cut-off with a drill or boring tool in a secondary operation. This eliminates tool breakage which would occur if attempting to cut off with a ceramic tool totally. This technique works best with smaller components where the cut-off piece can be captured on the drill or boring tool.

Four-axis machining where two separate tools are engaged in the workpiece at the same time yields best results when both tools are operating at the same approximate surface speed and on the same surface. The feedrate can be doubled as long as both tools are operating on the same plane and are synchronized. The depth of cut can be doubled with one tool cutting on a higher plane than the other as long as tools are separated by a distance equal to at least half the insert diameter. The depth of cut may be divided in such a way that tools are effectively working in a "ramping" situation. Tool A cuts on a ramp; Tool B follows behind and is programmed to cut straight but, in effect, also cuts on a ramp since it straightens out the ramp left by tool A.

Interrupted cuts in turning must be met by increasing speed to get back into temperature zones which are lowered by virtue of the intermittent contact between tool and workpiece. Calculate the circumference of the part and then subtract the sum total of the interruptions. Increase rpm so that the smaller diameter value returns to the originally recommended surface speed. For example, if 50% of the material is taken away by voids or interruptions at the surface, 50% of the surface remains in contact with the tool compared with an uninterrupted part. In this case, double the surface speed to compensate.

Milling with ceramics

Milling can be compared to interrupted machining in turning. Since each insert is in and out of the cut during each revolution, the desirable temperature ahead of the tool is not easily achieved and calls for increased surface speed, reduced feed per tooth, or a combination of both. It can be surprising how much extra speed is needed in some operations to get the heat back, compared to machining the same material continuously as in turning. The increase can be many times the turning speed. This will obviously give some limitation in the use of small-diameter milling cutters, since the machine tool is often unable to reach the rpm needed for the surface speed.

For example, if the recommended speed is 1500 sfm (457 m/min) for a 2" (50.8 mm) dia milling cutter and 3000 sfm (915 m/min) to get required heat, then speeds in excess of 5000 rpm are needed, which are not available on most milling machines. As the cutter diameter increases, milling with WG-300 becomes more practical from a speed point-of-view.

With milling, unlike turning, the chip can be generated from thin to thick as in conventional milling or "up" milling, or from thick to thin as in "climb" or "down" milling. It is highly recommended to use the climb milling technique to avoid high heat in a thin section of the chip, which encourages chip welding and re-cutting of chips, causing reduced tool life.

Feed and speed recommendations are found in a nomogram that Greenleaf has developed from its experience with WG-300. The nomogram can be used to determine speeds and feeds for milling as well as machining materials such as Hastelloy, Incoloy, Monel, Nitralloy, Rene, stainless, Udimet, Waspaloy, and toolsteels where little or no work has been done to date.
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Author:Smith, Keith H.; Kraemer, Rolf H.
Publication:Tooling & Production
Date:Jan 1, 1993
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