WO1991008863A1

WO1991008863A1 - Methods of preparation of surfaces and applications thereof

Info

Publication number: WO1991008863A1
Application number: PCT/GB1990/001952
Authority: WO
Inventors: Kevin Lindsey
Original assignee: British Technology Group Plc
Priority date: 1989-12-14
Filing date: 1990-12-14
Publication date: 1991-06-27
Also published as: KR920703270A; JPH05504730A; GB8928299D0; EP0505425A1; GB2241203B; GB2241203A; GB9027130D0

Abstract

A method of machining a workpiece (14) using an active fluid and a tool (12) positioned substantially out of contact with a surface of said workpiece (14) which is to be machined utilises the combination of the shape of said tool (12) and its position relative to said surface cooperate to cause local enhancement of the activity of said active fluid, thereby locally enhancing the rate of removal of material from said surface.

Description

Methods of preparation of surfaces and applications thereof This invention relates to the preparation of surfaces and applications thereof. It finds particular application in relation to materials cutting tools such as grinding wheels and to preparation of bearing surfaces such as plain bearings, rolling journal bearings and gas bearings, and transition or transfer interfaces such as heat exchanger surfaces and chemical catalysts.

In Western Electric Technical Digest No. 50 pp 25-26 April 1978, there is described a lapping plate for producing smooth surfaces. The lapping plate is provided with grooves to facilitate uniform distribution of slurry over the workpiece.

However, very few applications require a functional surface to be topographically featureless (i.e. smooth) or to have a random surface texture. The texture of manufactured surfaces is often far from that which would produce optimum function. For many very diverse functions, there is an individual optimal surface texture or pattern which can produce a significant improvement. It is an object of the present invention to produce surfaces with such optimal textures.

According to the present invention there is provided a method of machining a workpiece using an active fluid and a tool positioned substantially out of contact with a surface of said workpiece which is to be machined wherein the combination of the shape of said tool and its position relative to said surface cooperate to cause local enhancement of the activity of said active fluid, thereby locally enhancing the rate of removal of material from said surface.

In a specific embodiment of the invention, a tool superficially resembling a grinding wheel or lapping tool, the tool waveform (superficially equivalent to the protruding cutting/abrading points/grains in a grinding wheel) does not physically contact the workpiece but come sufficiently close to raise the temperature and/or pressure in a slightly chemically active "cutting fluid" so that the chemical removal rate of the workpiece in the chemically slightly active cutting fluid is significantly, or greatly, enhanced. Preferably, the tool waveform/pattern/texture and the composition of the chemically active fluid, and the working conditions are tailored so that the temperature and/or pressure resulting at the workpiece surface are as high as possible to attain high stock removal rate, but below the temperature and/or pressure at which the workpiece material would be degraded at all, or beyond that acceptable for the workpiece function.

The invention will now be particularly described with reference to the accompanying drawings in which:-

Figure 1 shows in schematic form an embodiment of the invention comprising a "hard-loop" machine, Figure la is a section through a tool used in the embodiment of Figure 1,

Figure 2 shows in schematic form an embodiment of the

Invention comprising a "hard-loop" machine, Figure 2a is a section through a tool used in the embodiment of Figure 2,

Figure 3 is an alternative tool section

Figures 4a and 4b are waveforms of temperature and pressure distributions across a tool,

Figures 5a and 5b is a slicing or sawing tool (plan and side elevation) incorporating lateral support,

Figures 6a and 6b is a slicing or sawing tool (plan and side elevation) without lateral support and Figures 7a and 7b is a slicing or sawing tool (plan and side elevation) with partial lateral support. The optimal surface texture for the bearing/cutting tool cases is very different from that of others, so the description of specific embodiments will be divided into two categories - a) bearings/cutting tools and b) transition and transfer interfaces, e.g. catalysts and heat exchangers. When materials are mechanically machined, structural modification and/or deformation and/or cracking almost invariably result. Such workpiece material damage is of great significance with components such as semiconductors (e.g. chips), optics (e.g. mirrors and lenses) and high-technology engineering ceramics (e.g. gas turbine blades).

The tool waveform and working conditions can be further tailored so that a limited acceptable exceeding or workpiece-modifying conditions can be used as an early-stage or penultimate-stage operation, taking advantage of the further-enhanced stock removal rate; the final machining stage is then so arranged to remove the predictable, limited workpiece damage/modification resulting from this earlier/penultimate stage. The requirements, then, for the construction and materials of the tool are chemical inertness (to produce, in principle, zero or near-zero tool wear rate and generally high stiffness/elastic modulus.

If the conditions in the working volume - from tool waveform crests through the chemically active fluid through to the workpiece surface - are arranged to be elastohydrodynamic, with the fluid composition and structure tailored to exhibit viscosity increase with the raised pressure (resulting from the elastohydrodynamic condition), then the pressure/temperature enhancement is improved; the pressure from the elastohydrodynamic condition; the temperature from enhanced work done in fluid viscous shear. The condition enhancement would be reduced if the stiffness of the tool were low, allowing a reduction of elastohydrodynamic (EHL) realised pressure, and thus temperature.

At surface speeds often used in conventional grinding, hydrodynamic forces are generated between the tool and the workpiece tending to separate the two. Such a separating force added to the EHL force would be undesirable, so, for machining at significant speeds, the tool waveform for producing optimal texture is further modified, e.g. by "cutting away" the waveform valleys to reduce this hydrodynamic (henceforth called HL) separating force.

This EHL-optimal-texture tool can be used in two modes - a) "hard loop" and b) "soft loop". In hard loop machines such as lathes and grinding machines the machine has a rigid frame (i.e. loop) connecting the (back of the) workpiece to the (back of the) tool, and where the geometry of the workpiece is typically generated by machine motions (slides, etc.) within that hard loop. In soft loop machines such as lapping/polishing systems there is no, or only a soft, loop, and the geometry of the workpiece is produced by mirroring the prefixed geometry of the rigid tool (the only motion in the system being usually the notation of the tool, relative to the workpiece).

Hybrids of a) and b)are also possible.

Resisting the HL and EHL separating forces can be accomplished via the "hardness" of the loop in mode a), and via an added closing forcer (spring, weight, etc.) for the soft loop mode. Often in the lapping, soft-loop mode an acceptable workpiece stock removal rate is produced by large. area coverage of the workpiece by the tool, and not necessarily by using high surface speeds. In this case the cutaway-valley waveform would not be needed for the tool.

The two machining geometries and examples of tool waveforms are shown in Figs. 1 and la and 2 and 2a, which show a "hard-loop" and a "soft-loop" machine respectively. In the hard loop machine, a frame 11 supports an optimal texture tool 12 by means of a rotary bearing and in-feed 13. A workpiece 14 is mounted on a translatable support 15. The surface of the tool has peak regions 16 to act as a cutting enhancer and valleys 17 where cutting does not take place. The transition between the two has a sharp leading 18 and trailing 19 edges. The soft loop machine has a closing force applicator 21 a workpiece 22 and an optimal texture tool 23 mounted on a rotary bearing 24. The surface of the cutting tool has peaks 25 and valleys 26 with leading and trailing edges 27,28. To remove pressure and/or temperature spikes at the leading edge transition and trailing edge transition of the crest of the waveform (see Fig 3), slight radiusing of these parts of the waveform may be required with a high stiffness tool waveform. This "radiusing" can also be achieved by tailoring the tool material so that the waveform naturally elastically recedes to this form under EHL conditions.

The optimising pressure/temperature enhancement in the working volume equates to flat temperature and pressure distributions as shown in Fig. 4(a). Undertaking penultimate-stage machining in (control lably) slightly damaging condition with the same tool /waveform could produce temperature- and pressure distribution shown in Fig. 4(b).

Such optimal texture tools can be produced by mi crofabri cation methods, I.e. masking plus deposition and/or etching technology, or scanning modulated etching and/or etching technology. It can be approximated to by modification of conventional grinding or lapping tools.

The optimal texture principles and application can be used for a wide range of machining requirements. Furthermore, since optimal texture machining can be zero or near-zero workpiece damaging, processes hitherto regarded as (fast but damaging) early-stage processes which can be replaced by optimal texture equivalents, can make later machining stage unnecessary, thereby saving on machining time, cost, and on workpiece material. An example would be sawing or "slicing" of semiconductor materials (silicon, gallium arsenide, indium phosphide, etc.) and precious stones (diamond, ruby, sapphire, etc.). Precision optimal texture slicing/sawing can give final geometrical accuracy, not just zero/near zero damage, as well as minimum material wastage. HL and EHL are not the only routes to chemical removal rate enhancement at the optimal texture waveform crests (i.e. in the working volume). The range can be stated as an enhancement of chemical or electrochemical workpiece removal rate, using chemical or electrochemical systems which only, or very preferentially, activate at the enhanced temperature and/or pressure and/or working fluid shear rate in the working volume, workpiece in contact with working fluid away from the working volume, i.e. near Optimal texture tool waveform valleys, or away from the whole tool, preferably should not be being chemical ly/electrochemically removed. A working fluid which away from the working volume is chemically inactive, or deactivates the workpiece surface (by, for example, forming a passivating layer on the workpiece) is preferable; conditions in the working volume either removing the passivating layer and/or activating the working fluid.

For sawing or slicing a tool may be in the form of a wire, tape, disc or annulus. The essentials of the arrangement and principles are shown in Fig. 5. Since minimum loss/wastage of the workpiece 51 is important for semiconductors and gems, the tool 52 will be thin, so the intrinsic lateral stiffness will be low. The tool can be stiffened and posltionally/dimensionally stabilised in the lateral direction by the application of a waveform 54 to the side surfaces of the tool to produce a hydrodynamic (self-acting) or externally pressurised/fed bearing action between the tool's sides and the workpiece being machined, which will stiffen, stabilise and self-centre the tool, maintaining symmetrical accuracy in the workpiece, i.e. in the workpiece slices formed. Since in this sawing/slicing arrangement, much of the workpiece is not in contact with working fluid away from the working zone, the side-of-tool-acting bearings working conditions must be moderate (low pressure, shear rate, etc.) so as not to cause significant removal of workpiece away from the working zone (since this represents material loss-wastage) or the working fluid can be fed only to, or close to, the working volume (for instance, entrancing at the optimal texture waveform valleys) with the rest of the workpiece and tool being in diluent, deactivator or neutral iser for the working fluid. The stabilising side-of-tool-bearings incorporation are an option. If they are not required for the attributes stated, the tool sides/back can be cut away, as shown in Fig 6.

An intermediate, with partial side-support/stiffening, is shown in Fig 7, the tool being in the form of a circular cross-section wire; the cutting tool waveform extending round the full circumference of the tool. If extra tool stiffening is needed in the in-feed direction, the back of the tool can be extended so that the cutting tool waveform Is 180° wide (on the front, i.e. working volume of the tool) rather than 360° wide, the tool then being extended behind this part for stiffening, as in Figs 5 & 6.

Bearings are a special case of the EHL cutting tool. system. One requirement common to almost all bearings is zero, near-zero, or very low wear rate. The use of optimised surface texture/waveform can eliminate wear at "high-spots" (the optimal texture waveform arranged to have no "high-spots"), can reduce wear-inducing pressure/temperature spikes, even in EHL bearing mode, and can be used as a safety addition to HL bearings or externally pressurised fluid bearings where wear-inducing solid-solid contact can result from excess loading or speed reduction.

An example of the latter would be an internal combustion reciprocating piston/cylinder assembly in which relative motion at top dead centre and bottom dead centre is zero making HL ineffective, but which can be afforded some protection by the slow squeeze-out rate of an EHL system. So in this case, an EHL-inducing waveform superimposed on the HL geometry will reduce wear rate. A crude, existent approximation to this requirement is via plateau-honing. Optimising criteria and waveforms will be similar to those for the cutting tools, and, obviously, non-chemically-active fluids are used. Optimal texture protecting EHL waveforms/textures are also highly applicable to rolling journal bearings. Structural or motion vibration damping is often achieved via viscous shear. Viscous shear work (therefore damping) generally increases with reducing fluid film thickness. In many applications, the thickness is only of the order, of the amplitude of the texture of the solid surfaces forming the boundaries of the fluid film so that with "high-spot", non-optimised waveforms, solid-solid contact can occur i.e. "stick-slip". During "sticking" there is no shear, therefore no damping. Optimised waveforms (which will be similar to those for bearings and cutting tools here identified) can be used to maximise viscous shear, especially if tailored to operate in EHL mode, to eliminate stick-slip, and to eliminate/minimise solid-solid contact.

Major contributions to heat transfer between a heat-exchanging surface and a surrounding fluid will be conduction and convection in the surrounding fluid at, and close to, the heat-exchanging surface. Heat transfer will increase with increasing contact surface area, so long as fluid flow is not thus restricted, so reducing convection. Convection can be enhanced from a coarse scale to less than boundary layer scale by a micro-turbulating waveform, i.e. a waveform across the direction of fluid motion, superimposed on a larger-scale area-increasing waveform aligned with the direction of fluid motion. This double-waveform texture is optimised for maximum heat transfer; this optimised crossed waveform will not be of maximum surface area, nor minimum resistance to surface flow.

For chemical catalytic surfaces the requirements are similar, a micro-turbulating waveform again being used superimposed on a coarse -waveform aligned with the direction of flow to enhance convection which will enhance catalytic rate; the system being similarly optimised not for maximum surface area nor lowest flow resistance, but for maximum catalytic rate.

Claims

1. A method of machining a workpiece using an active fluid and a tool positioned substantially out of contact with a surface of said workpiece which is to be machined characterised in that the combination of the shape of said tool and its position relative to said surface cooperate to cause local enhancement of the activity of said active fluid, thereby locally enhancing the rate of removal of material from said surface.

2. A method of machining a workpiece as claimed in claim 1 characterised in that said fluid Is adapted chemically to etch the surface of said workpiece.

3. A method of machining a workpiece as claimed in claim 1 characterised in that said fluid Is adapted electrochemically to etch the surface of said workpiece.

4. A method of machining a workpiece as claimed in claim 1 characterised in that said fluid is adapted physically to abrade the surface of said workpiece.

5. A method of machining a workpiece as claimed in claim 1 characterised in that conditions In the working volume are arranged to be elastohydrodynamic, with the fluid composition and structure tailored to exhibit viscosity increase with the raised pressure.

6. A method of machining a workpiece as claimed in claim 1 characterised in that the working fluid is selected to deactivate the workpiece surface away from the working volume.

7. Apparatus for machining a workpiece using an active fluid comprising a tool adapted to be positioned substantially out of contact with a surface of said workpiece which is to be machined characterised in that said tool (12) is adapted to cooperate with said surface (22) to cause local enhancement of the activity of said active fluid, thereby locally enhancing the rate of removal of material from said surface.

8. Apparatus for machining a workpiece as claimed in claim 7 characterised in that the shape of said tool is adapted to prevent the local elevation of the temperature and pressure above a value at which degradation of the remaining workpiece material takes place.

8. Apparatus for machining a workpiece as claimed in claim 6 characterised in that the shape of said tool is adapted to reduce the hydrodynamic force separating it from the workpiece.

9. Apparatus for machining a workpiece as claimed in claim 6 having a substantially rigid frame and adapted to produce a desired geometry in a workpiece by way of machine motion.

10. Apparatus for machining a workpiece as claimed in claim 6 having a substantially rigid frame and adapted to produce a desired geometry in a workpiece by way of tool geometry.

11. Apparatus for machining a workpiece as claimed in claim 10 incorporating means for urging the tool towards the workpiece.

12. Apparatus for machining a workpiece as claimed in claim 6 including additional radiusing (27,28 Fig 3) to reduce the i nci dence of temperature and pressure spi kes .

13. Apparatus for machining a workpiece as claimed in claim 6 characterised in that the material of the tool is selected to modify the shape of said tool elastically under elastohydrodynamic load conditions.

14. Apparatus for machining a workpiece as claimed in claim 6 characterised in that stiffening is provided by the application of a waveform (54) to lateral surfaces thereof to produce a bearing action between said tool and the workpiece.

15. A workpiece characterised in that it has a surface machined by the method claimed in any one of claims 1 to 6.