METHOD AND APPAH ATUS FOR ULTRASONIC WORKING

This invention relates generally to a method and apparatus suitable for the working of area of an article such as a surface of a metal article, and to articles having a surface worked thereby.

In one particular respect the invention relates to such methods and equipment useful for polishing the internal surface of the cavity of a metal die, as used for molding of synthetic polymeric material.

The method and the equipment of this invention operate using ultrasonic energy and usmg a flowable mixture of polymeric material including particulate abrasive material.

In the following, it will be helpful to consider on the one hand ultrasonic machining and on the other ultrasonic polishing and related procedures.

Ultrasonic machining is a well-known machining process, involving the removal of significant amounts of workpiece material. In this process the surface of a workpiece is abraded by a particulate abrasive material contained in a slurry. This slurry is located between the workpiece surface and a tool vibrating at ultrasonic frequencies in a direction perpendicular to the surface. The frequencies used are typically about 20 kHz. A typical amplitude of vibration is less than 0.1 mm, and usually an amplitude in the range of 0.002 to 0.05 mm is adopted. The tool itself is made of a material having high strength and good ductility in order to impart a high degree of impact resistance with the particles and thus minimize abrasion of the tool. The tool face is configured with a desired three-dimensional shape. The effect is to abrade a negative of that shape in the workpiece surface. The tool does not contact the workpiece, and the actual cutting of the workpiece is done by virtue of the abrasive parUcles suspended in the slurry, driven with a percussive impact against the workpiece surface. The tool does not contact the workpiece, and the actual cutting of the workpiece is done by virtue of the abrasive particles suspended in the slurry driven with a percussive impact against the workpiece surface. This ultrasonic machining process is useful in the working of difficult materials such as glass, ceramics, and calcined or vitπfied refractory material, together with any other hard and/or brittle materials which are not susceptible to traditional machining techniques or to such techniques as electrical discharge machining or electrochemical machining. Ultrasonic machining is particularly advantageous for reproducing complex shapes which could not be obtained by such traditional or non-traditional techniques. Ultrasonic polishing is a polishing process and as such leads to the removal of a very small amount of workpiece material, in an essentially uniform manner over a flat or configured surface.

Typical known polishing processes are labor intensive, and are tune-consuming and expensive as a consequence. They moreover require skilled workers and even then can produce rather inconsistent results. Thus, manual polishing, vibratory finishing, buffing, brushing and extrusion honing cannot always remove the workpiece material to the desired degree of uniformity, especially when the surface configuration is complex.

Conventional ultrasonic working has a number of disadvantages, most notably the need to place the ultrasonic tool very close to the surface to be worked (at least 0.1 mm). This in turn means

that conventional ultrasonic procedures are intolerant of changes in the shape of the tool due to abrasion and require very accurate fit and registration of the tool and the surface to be worked.

There are two characteristic features of conventional ultrasonic procedures which limits their usefulness in polishing procedures. Firstly, ultrasonic procedures of the machining type, if applied to a polishing procedure impart some abrasive erosion of the tool as well as of the workpiece, so that there is an ongoing and increasing loss of detail and resolution over the period during which the tool is in use. For this reason the tool material is conventionally a material which is comparatively tough and ductile, that is to say not readily abradable by ultrasonic machining, and tool wear is much less than the workpiece configurational change. Tools are commonly made of material such as titanium, cold rolled steel, stainless steel, copper, aluminum and the like. While this limits tool wear it still does not readily provide a degree of accuracy of metal removal needed for polished surfaces.

Secondly, ultrasonic procedures of the machining type only abrade areas of the workpiece which are adjacent to the tool face surfaces. The workmg of surfaces is constrained by the limited area of ultrasonic tools and the need, in workmg complex surfaces, to employ multiple tools and multiple ultrasonic machining operations.

Ultrasonic polishing, while a known technique, is therefore of specialized application as shown for example in European patent 0403537. In this prior disclosure is shown a method of polishmg a workpiece utilizing a tool which is formed in a matenal more ultrasomcally abradable than the workpiece. The relative vibratory motion is effected at an ultrasonic frequency and at an amplitude sufficient to cause the workmg surface of the tool to be abraded into a negative form of the workpiece. This relative vibrating motion is continued m order to abrade the tool during the peπod in which the tool reforms and maintains this negative complementary form. At the same time it imparts a polishmg action on the workpiece itself. While the technique is very effective, it is limited to polishmg operations and is not useful for more general workmg requirements. Another limitation of that technique is that the tool is consumed rather rapidly. The value of the polishmg operation must exceed the cost of the tool tip replacement.

The present invention sets out to overcome these limitations and problems and to provide ter aha a method and associated equipment for ultrasonic workmg which can be used with great efficiency and productivity over large workpiece surfaces without the requirement for changing tools, without the indexing and registration required by many procedures m the prior art, and which can be adapted to machining and/or polishmg of surfaces at will. .

The present invention provides m a first aspect a method of workmg a surface of an article, compπsmg directing a flowable abrasive medium comprising a mixture of abrasive particles and polymeric material transversely across the surface and, at the same time, caus g vibratory movement of particles of the abrasive medium by means of an ultrasomcally vibratmg tool, the abrasive medium being flowed through a gap between an operative end surface of the tool and the surface, whereby material is removed from the surface both by vibratory movement of the particles of the abrasive medium and by the transverse movement of the abrasive medium across the surface.

In another aspect, the present mvention provides a method of workmg a surface of an article comprising directing a flowable abrasive medium comprising a mixture of particulate abrasive material and polymeric material transversely across the surface and, at the same time, causmg vibratory movement of particles of the abrasive medium by means of an ultrasomcally vibrating tool, the abrasive medium bemg flowed through a gap between an operative end face of the tool and the surface, wherem the polymeric material comprises a viscoelastic, preferably rheopectic polymer, such as a polyborosiloxane or the like.

The surface area is typically a metal surface area, presented in the form of a sintered, cast, forged, machined or other worked metal. It can also be a glass, ceramic, or polymer surface. We have observed during the practice of the method of both aspects of the mvention that the said gap can be greater than the gap conventionally used for the process of ultrasonic machining, or for the known ultrasonic polishing process discussed above. We believe that the effectiveness at substantial gaps is m part attributable to the greater efficiency with which the vibratory motion is imparted to the abrasive grains in the medium of the present mvention, and because of the additional metal - removmg effect of the flow of abrasive medium, and possibly a contribution by cavitation, although we do not intend that the scope of our patent should be limited by any statement as to the believed or possible nature of operation. S ce, however, a larger gap is possible, a steerable and moldable tool, not shaped or dimensioned as an exact complement of the surface area, can be used. Further, abrasion of the tool may have less effect on the workmg process because of the larger gap.

Whatever the cause of the enhanced workmg efficiency and because of the added flexibility in the development of tool shapes and sizes and the use of the steerable tool, the process is effective in a broader range of machining operations than usual in ultrasonic machining processes. For example, the method can be employed to machine and grind a workpiece, to debur and to radius edges, to grind recast surface layers, and to polish the surfaces to reduce surface roughness and wav ess. All these operations can be performed in an integrated operation, desirably with computer controlled operation of the apparatus and method. The operation can be continued until any desired degree of finish are obtained.

It is possible to employ media with relatively coarse abrasive while machining and grinding operations are bemg performed to obtain acceptable cuttmg rates, and then change to a finer abrasive medium during polishmg and surface finishing operationsto obtain the finest surface finish. The cuttmg rate is directly proportional to the gap and to the particle size of the abrasive grains m the medium. The time required to attain a given amount of surface workmg of the workpiece is inversely proportional to the gap and to the particle size of the abrasive grains. The optimum size of the gap may vary with the nature and roughness of the article to be worked, the tool composition, the vibratory frequency and amplitude, the abrasive type, particle size and loading, the polymer type, and finish required, but is typically within the range of 0.2 to 3mm; at these spacmgs, highly accurate mdexmg between the tool operabve end face and the surface area is

not generally necessary, and the shape of the tool surface is generally mdependent of the conformation of the workpiece surface or the desired shape to be attamcd.

Similarly, the size of operative end face and of the worked surface of the article, and the relative sizes thereof, can vary. Without limiting the broad scope of the mvention, a typical s ze of the end face may be from 0.5 to 25 sq cm, but is more usually from 5 to 10 sq. cm. In the operation of the process this can cover from 60% down to as little as 5% of the surface area to be worked.

Flow rates are most conveniently expressed as mass flow m gram/mm, and we have found that a wide range of mass flow rate, from 0.01 to 100 gram/mm can be used. The flow can be intermittent but is more preferably contmuous. Under some circumstances, it is found that microscopic mspection of a surface worked by the method of either aspect of the mvention shows stπations or lines on the surface of a length which appears to be determined by the frequency of vibration of the ultrasonic source and by the speed of flow of the abrasive medium. Without limiting the scope of the present patent to any believed or possible nature of operation, it is considered that particles are moved both normal and parallel to the surface and thereby cause abrasion of the surface.

As well as the workmg function exerted by the supply of new particles and the transport through the gap, the flow of abrasive medium also moves away particles of metal, and dulled abrasive particles, from the gap. The rate and continuity of flow to achieve this is also most preferably arranged to be such as to exert a cooling effect on the operative end face of the tool and on the surface area, as well as bemg such that the medium itself does not overheat.

The flow of abrasive medium is preferably achieved by a circulatory pumped supply of the abrasive medium. It will generally be desirable to periodically add fresh, undulled abrasive to the re-circulating medium to assure that the cuttmg rate is maintained.

Preferably, the rate of flow of the abrasive medium is such that some workmg of the surface would occur if there were no ultrasomc vibration of the abrasive particles. However, under typical conditions, the rate of flow of abrasive medium would cause negligible workmg of the surface of most materials without the application of ultrasomc vibration of the abrasive particles.

Uniformity of flow over the gap is improved if abrasive medium is fed through a duct at or near the center of the operative end face of the tool, although of course other arrangements are possible. Preferably, the medium is delivered through the duct to the gap.

The tool end face can be flat m cross section. Alternatively it can show a convexly arcuate cross section, e.g. as part of an ellipse, or it can be part spherical e.g. hemispherical m shape. The exact choice of tool shape and size will depend upon the exact nature of the surface, which can be flat, recessed (e.g. as a die cavity) or otherwise configured. The frequency of ultrasomc vibration can also be chosen dependence upon the other parameters of the workmg process as discussed above. Typically, it can be from 10 to 40kHz. A suitable procedure operates at 19 to 22 kHz, i.e. a nominal 20 kHz. The amplitude of vibration is usually less than 0.1 mm, e.g. 0.002 to 0.05 mm.

The method of the mvention is particularly effective and preferred for application over a workpiece surface to remove a uniform thin layer of metal, glass, ceramic, or even a material such as gallium arsenide or the like. For example, an EDM re-cast layer e.g. from 0.003 to 0.006 mm thick can readily be removed. However, the process also has application in removmg, as part of a polishmg process, burrs arising during fabrication of the original workpiece and also of radiussing the edges of a workpiece to be polished.

The nature of as suitable abrasive medium is generally that described from abrasive flow machining and polishing procedures of the prior art.

Thus abrasive particle size is typically 0.015mm to 0.175mm (i.e. 1,000 to 100 mesh) and more preferably around 0.032 mm (320 mesh). Preferably moreover the abrasive medium contains from 25 to 75% by wt of such particles.

Abrasives which can be used clude tungsten carbide, silicon carbide, aluminum oxide, boron carbide, boron mtπde or diamond powder as the harder abrasives; and for the less resistant surfaces, alumina, corundum or garnet and the like as softer abrasives. The polymeric material component of the abrasive medium is preferably a semi-solid material, of a putty-like consistency, flowable with some difficulty and thus capable of providmg a substantially solid matrix to carry the abrasive particles. A number of such materials are known in the art of abrasive flow machining, including both natural and synthetic polymers. The preferred medium is the viscoelastic and rheopectic polyborosiloxane based formulations. Such media in a variety of viscosities and with a variety of abrasives and abrasive particle sizes are commercially avadable from Extrude Hone Corporation of Irwin, Pennsylvania, and from Extrude Hone Limited of Shannon, Ireland.

The workpiece and tool are mounted on a machine frame to provide relative movement of the tool over the surface area to be worked. Typically, the workpiece will be mounted on a mount providmg two axis movement, ordmanly horizontal movement m x,y axes, and desirably rotation about a vertical, z axis. The tool is earned in an articulating mount that provides vertical movement in the z axis to alter spacing or to accommodate changes in the configuration of the surface area of the workpiece, and desirably also provides additional articulation, including rotation m x, y, and z axes and x and y axis translation, to effectively engage the tool with the surface of the workpiece, such as within cavities and particularly those havmg undercut regions. In some operations, it may be desirable to provide a regular or random orbital or planetary motion of the tool relative to the workpiece.

The mvention further extends to a work piece, e.g. a molding die, havmg a surface machined, deburred, polished or otherwise worked by the above method. Another aspect of the mvention is constituted by apparatus for workmg the surface of an article, comprising

A. an ultrasomcally vibratable workmg tool havmg an operative end face,

B. means for mountmg the article with the surface to be worked opposed to and spaced from the operative end face of the tool,

C. means for supplying to the space between the surface and the operative end face of the tool an abrasive medium comprising a mixture of particulate abrasive matenal and polymenc matenal, and

D. means for imparting ultrasomc vibrations to the workmg tool, so that, m use, matenal is removed from the surface at the same time by vibratory movement of the abrasive medium and by the movement of the abrasive medium across the surface.

Other optional features of the apparatus will be apparent from the above descnption. Thus, the operative end face may be from 0.5 to 25 sq. cm e.g. 5 to 10 sq. cm. m area. Moreover, there may be a feed duct for the mixture near the center of the operative end face. This end face can be flat or convex e.g. part sphencal, or concave.

The ultrasomc vibration can be imparted by an electronically dπven stack of piezoelectnc elements, or a magnetostnctive transducer, and as mdicated above can be capable of operatmg at 10 to 40 kHz e.g. at 20 kHz.

The workpiece holder is typically arranged to possess X, Y-movement capability and a rotational facility about an axis orthogonal to the surface of the article. The tool mount and/or the workpiece holder however preferably possesses at least a Z component facility to alter the spacmg or to adjust for different surface levels of the workpiece , and preferably a tdt capacity about two axes preferably located at nght angles to a plane orthogonal to the Z axis. As noted, it may also be desirable to include a capability for random or regular orbital or planetary motions of the workpiece holder, the tool holder, or both.

The tool itself is generally formed in a matenal that is comparatively tough and ductile and accordingly not readily abradable by the ultrasomc workmg so that the tool will be abraded to a much lesser degree than the workpiece. The tool can be made e.g. of matenal such as titanium, cold rolled or forged steel, auste utic stainless steel, copper, aluminum, and the like. From the above general descnption it will be apparent therefore that the present mvention utilizes a low wear rate tool but overcomes the time consuming and labonous procedures of indexing and registration between tool and workpiece by maintaining a gap between the tool and workpiece within which an abrasive medium is caused or allowed to flow intermittently or continuously. The present mvention moreover does not require the tool profile to be an exact negative of the workpiece configuration, because of said gap filled with abrasive medium.

The mvention will be further described with reference to the accompanying single figure of drawing, which is a diagram of the equipment m side elevation.

The equipment comprises a frame (1) adapted to hold a workpiece (2) and a tool mount (3) for holding the tool (4) and vibrating this tool with an ultrasonic dnver (not shown). The equipment in the examples is to be considered as allowmg relative onentation changes between the tool holder and the dnver, and the workpiece (2). This relative onentation change is obtained by an X, Y and a rotary motion of the workpiece m a horizontal plane, and by a Z and axial tdt movement of the tool holder and dnver m the vertical plane.

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The vibratory movement of the tool, mduced by the tool mount, will most commonly be engendered by an electronically dnven stack of piezoelectnc elements. If necessary a magnetostπctive transducer can be used.

The equipment is furnished with handling means (5) for flowable abrasive medium such that the abrasive medium can be disposed between the tool (4) and the workpiece. To do this the abrasive medium is m the embodiment shown pumped through flow passages within the tool (4) and mto the gap between the tool and the workpiece so as to assist the polishing process. In so do g it continuously provides fresh unworn abrasive to the workmg surface, flushes away eroded matenal and debns, removes heat from the machining area, and provides a pressunzed medium through with to transmit the ultrasomc energy.

In one example of use, silicon carbide particles of 320 mesh (0.032 mm) are loaded mto a polyborosdoxane based earner to an extent of 50% by mass of the total mixture. This slurry is pumped through the ducts m the tool (4) to flow through the space between a tool end face of 1.7 sq. cm at a mass flow rate of 10 grams per minute. The workpiece had a surface roughness of 1.8 μm Ra and over a peπod of 20 minutes at a gap of about 0.8 mm (between 0.2 and 3 mm from the surface to different portions of the tool) reduced the surface roughness to a value of 0.6 μm Ra.