Grinding the superhards.
Superhard materials are characterized by superior wear resistance, high temperature tolerance, thermal stability, and low weight. The aerospace industr was the first to benefit from their qualities, but now others including automotive, medical, oil, gas and chemical, electronics, and metals processing industries, are increasing demand for them.
The very same properties that are so functionally advantageous present major problems to the manufacturing engineer who has the responsibility of shaping "the superhards" into the form required. Grinding is the traditional process involved, but the hard and brittle nature of ceramics, for instance, makes them prone to cracking and chipping and, more important, sub-surface cracking.
The problem is the exceedingly high penetration (normal) forces generated between the wheel and workpiece compared with grinding conventional metals. Although the tangential forces are very similar, the ratio of normal to tangential forces for metal can be as low as 3:1; for ceramics it is substantially higher.
Tests have shown that in grinding a tool steel under creep-feed conditions, the tangential force is approximately 50 lb and the normal force 400 lb. Grinding ceramics under the same conditions, the tangential force stays at about the sam level of 50 lb, while the normal force rises to approximately 1000 lb.
These conditions dictate that a very stiff and stable grinding machine is required to form a quality surface. Lack of stiffness will cause spindle deflection that stimulates chatter, which subjects the part surface to continuous "hammering" leaving it shattered and pitted.
Another requirement for the successful grinding of hard materials is in the typ of abrasive used. Diamond is the standard choice for ceramics, carbides, and glass materials, although the softer carbon graphite and ferrite ceramics can b machined with conventional abrasives and CBN (cubic boron nitride). CBN is also increasingly used to grind metals in the hard state.
The means of bonding the diamond in the wheel also influences the grinding process. The choices available are metal plated, vitrified, or resin bonded. A resin bonded wheel has good vibration dampening properties due to the softer nature of the bond. The rigidity of a plated wheel can cause damage to the surface being ground, particularly on a machine with poor stiffness.
The third element in grinding is the cutting fluid. Today, technical performanc tends to be offset by environmental and safety concerns. While oils give superior results to water-based fluids in grinding hard materials, there is a move to the water-based fluids because of their reduced effect on the environment and operator health and safety.
Filtration is also a major consideration, particularly for ceramics, which can generate swarf as small as 1 or 2 microns. Synthetic fluids are much easier to filter than the viscous cutting oils, adding another reason for the increasing use of water-based cutting fluids.
Gaining superhard experience
Finding the correct combination of machine tool, grinding wheel, and cutting fluid is critical to successfully machining the superhards. While many of Jones & Shipman's existing surface and cylindrical grinding machines are suitable for the process, the company continues to develop new types of machines to shape superhard materials as well as to take advantage of the superior machining properties of superabrasives. Here are examples of our experience in machining these materials across a wide range of applications:
* EG&G Sealol, a European facility of a major American pump manufacturer, is using three Jones & Shipman Format 15 cylindrical machines equipped with Allen Bradley CNCs to completely finish grind ceramic pump components.
* Morgan Matroc, a European manufacturer of ceramic products, is successfully grinding a wear component using a Jones & Shipman cylindrical grinder. The aluminum oxide wire drawing cone is a complex part with multiple steps tapering from 6" to 2" in diameter.
One advantage of grinding ceramics with diamond wheels is that large volumes of material can be removed without redressing or conditioning the wheel. This give comparatively high material removal rates and short cycle times. In the Morgan Matroc example, it took only 30 minutes to remove 0.200" inches from each of 20 diametrical steps and 9 faces under CNC control.
* British Telecom (BT) Laboratories' Crystal Processing Unit at Martlesham Heath, England, is abrasive machining ceramic and glass materials that are bein increasingly used in the manufacture of fiber optic cables.
These materials can be ground, but only with cuts of a few microns, necessitating long slow cycles even when using diamond wheels. Much of this wor is now done on a Jones & Shipman 540X microprocessor-controlled surface grindin machine. This machine retains the flexibility required by the type of skilled craftsmen employed by BT but can be set to carry out the extended grinding cycles automatically, freeing the operator for other tasks.
Typical of BT's work is the production of preforms, measuring about 1" in diameter from which fiber-optic cables are extruded for research. The preform i ground flat on the 540X to produce a D-form cross-section that is retained afte extrusion so that the engineers can study transmission of the light beam. A hig degree of accuracy is required.
A creep feed solution
The typical surface or cylindrical grinding machine is designed for multiple-stroke, light stock removal operations. To machine the superhard materials in this way requires very light cuts, as the work at the British Telecom Laboratories demonstrates. Under these conditions material removal is through brittle fracture, which is not ideal for high integrity surfaces.
The most efficient material removal method for ceramics is in the ductile regime, which can be attained with very high stiffness machine tools and small chip loads. These kinds of conditions are generated in creep-feed grinding, in which a large depth of cut is employed with a relatively slow feed rate, simila to milling or turning.
Machines designed for creep-feed grinding are characterized by high stiffness and high power. The Jones & Shipman Hi-Tech machine, for example, has a static stiffness of 70 N/micron and a 60 hp spindle drive. High rates of material removal can be achieved with superior surface finishes. The capability to take large depths of cut allows machining from solid which alleviates the problem in ceramic manufacture of firing to a near net shape. Creep-feed grinding machines are also capable of running at the high speeds--around 9000 sfm--needed in ceramic grinding.
A striking example of the metal removal capability and stability of the Hi-Tech creep feed grinder is the application seen here. A test piece of alumina cerami was ground with a Norton AD 150 R75 B99 diamond wheel at a speed of 9000 sfm using Master Chemical VHP E200 cutting fluid at a pressure of 100 psi and flow rate of 80 gpm.
The piece was ground to a depth of 0.160" at a feed of 10 ipm in one pass, removing approximately half the width of the piece. Next, the wheel was pitched over its full width plus 0.010" and the process repeated to leave a tang 0.010" wide x 0.160" deep. This demonstrated the absence of any vibration or wheel run-out. Any spindle growth would have broken the tang. The limitation on the width of the tang was not set by the grinding machine, as it was possible to machine a tang as small as 0.007", but a tang this small would be broken by the force of the cutting fluid.
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|Title Annotation:||very hard structural materials|
|Publication:||Tooling & Production|
|Date:||Mar 1, 1994|
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