CA1329988C - Ultrasonic polishing - Google Patents

Abstract

ABSTRACT
Ultrasonic polishing without substantial loss of fine resolution and detail is provided by employing a tool of a more ultrasonically abradable material than the workpiece, as the oscillating driver of a liquid abrasive slurry. The tool is preferentially eroded and conforms to the pattern of the workpiece continuous self-dressing during polishing.

Description

~AT~NT

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The present invention relates to the polishing of hard materials such as metals and the like by means of ultrasonic oscillatory vibrations. More particularly, this invention relates to the ultrasonic polishing of the surface of a workpiece by means of a comparatively more ultrasonically abradable tool which, during polishing, develops a form which is a complement of the form of the surface of the workpiece. The oscillatory vibrations of the tool are imparted to a liquid abrasive slurry disposed between the tool and workpiece.
The present invention is particularly adapted to polishing compound surfaces and complex shapes having fine or intricate detail where a reduction in surface roughness is needed without loss of resolution or detail.
BACKGROUND ART
Ultrasonic machining is a well known machining process whereby the surface of a workpiece is abraded by a grit contained in a slurry circulated between the workpiece surface and a vibrating tool adjacent thereto, vibrating at frequencies above the audible range, i.e. typically within the range 19,500 to 20,500 cycles per second. The amplitude of vibration is less than 0.01 mm, and typically within the range 0.001 to 0.005 mm. The tool is made of a material having high strength and good ductility, in order to impart a high degree of impact resistance to minimize .

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abrasion of the tool itself. The abrading tool face is provided with a three-dimensional form, while a negative compliment thereof is machined onto the workpiece surface.
Since the tool does not contact the workpiece, the actual cutting is done by the abrasive particles suspended in the slurry. These particles are driven with a percussive impact against the workpiece surface by the tool, ultrasonically vibrating perpendicular to the workpiece surface. This process finds particular utility in its ability to work difficult materials, such as glass, ceramics, calcined or vitrified refractory materials and hard and/or brittle metals, which are not susceptible to machining by any other traditional technique, or even such nontraditional techniques such as electrical discharge machining, electro-chemical machining or the like.
Ultrasonic machining has proved particularly ` advantageous for reproducing complex shapes which could not be obtained by traditional machining, or by non-traditional techniques such as electrical discharge machining, electro-chemical machining, or the like because of the nature of the materials to be worked.
Ultrasonic machining imparts some abrasive erosion of the tool as well as the workpiece, so that there is an ongoing and increasing loss of fine detail and resolution as the tool is used and worn. For this reason it has been considered rather important that the tool material be one that is comparatively tough and ductile, i.e. not readily abradable by ultrasonic machining, so as that the tool will ~' . --.

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be abraded to a much lesser degree than the workpiece. For example, tools are commonly made of materials such as cold rolled steel, austenitic stainless steel, copper, aluminum and the like.
In addition to the above, ultrasonic machining in its normal practice, only abrades areas of the workpiece which are most adjacent to the tool face surfaces. Therefore, if ultrasonic machining is to be used on a workpiece that is already formed, or formed in part, as in a polishing operation, it is very important that the tool and workpiece be aligned and registered as accurately as possible, least the workpiece be abraded or polished nonuniformly and possibly even destroying the workpiece as desired.
Setting-up the tool and workpiece with the necessartly accurate indexing and registration is a time consuming and laborious procedure .
The foregoing limitations have generally resulted in the choice of other machining techniques when the nature of the materials to be worked permit, and has generally required the use of other techniques for polishing operations. Any of the polishing techniques in common use are historically labor intensive, time consuming and expensive operations, and in addition typically require skilled workers and often produce rather inconsistent results. Polishing by any method requires the removal of a very small amount of workpiece material, and ideally a very uniform removal thereof. Manual polishing, vibratory finishing, buffing, brushing and even extrusion honing :

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, cannot remove the workpiece material to the extent of uniformity often desired, particularly in the case of complex workpiece surfaces. Burnishing, on the other hand, tends to redistribute -the workpiece surface material causing dimensional changes.
DISCLOSURE OF THE INVENTION
According to the present invention, there is provided a method for ultrasonic polishing of a workpiece by means of a tool which is significantly more ultrasonically abradable than the workpiece and therefore, need not be preformed to provide a complement of the surface of the workpiece. Instead, a blank tool face can be used. When the tool is vibrated, imparting its vibrations to an abrasive slurry disposed in the gap between the tool and workpiece, the tool is quickly eroded in such a fashion that it quickly develops a complementary form of the workpiece with a high degree of resolution and detail.
Thereafter, the tool will continue to be abraded at a comparatively high rate while continuing to maintain its high degree of resolution and detail, while at the same time the surface of the workpiece is abraded to a much lesser degree so that it is merely polished while the tool is being progressively abraded down, but at all times maintaining its high resolution and detail complimentary work surface.
The present invention may be employed to polish any material more resistant to ultrasonic erosion than the material of which the tool is made. In this fashion, the :
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tool will be re-dressed continuously and inherently to the complementary form of the workpiece, by virtue of the fact that the tool will be eroded to a greater extent than the workpiece. The preferential working of the tool results in a constant or even increasing conformity to the fine detail and resolution of the workpiece, so that as polishing of the workpiece occurs, there is no loss of resolution.
By the present technique, ultrasonic polishing is made applicable even to relatively soft and easy to work materials, such as bronze, brass, or gold, to polymeric materials, and a wide diversity of other materials which were not heretofore thought to be appropriate for ultrasonic techniques, in addition to very much harder materials, including those where ultrasonic machining techniques have been employed previously, as discussed above.
With ultrasonic polishing in accordance with the present invention, surface finishes can be attained, depending on the extent of polishing, of substantially any desired degree, regardless of the material and in any degree of intricacy and fineness of detail without substantial change in detail or resolution. Surface roughness can be reduced to as low as about 0.1 microns Ra, although such high degree of polish may not always be required and a lesser extent of polishing may often suffice for a given application.
Because the process of this invention does remove a very uniform layer of material from a workpiece surface, '~ . .

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the process is also ideally suited to the removal of thin layers of unwanted material from a workpiece surface, such as an EDM recast layer of material which is normally 0.003 to 0.006 mm thick. In addition, the process of this invention can be used to remove burrs from a workpiece surface or to radius the edges thereof.

An aspect of the invention is as follows:

A method of ultrasonically working a workpiece surface having a desired configuration performed therein to remove a very thin and uniform layer of material from the workpiece utilizing a tool which need not be preshaped comprising:

A. forming a tool from a material that is more ultrasonically abradable than the workpiece;

B. mountiny said tool in a ultrasonically vibra~able relationship to said workpiece;

C. applying a liquid abrasive slurry between said tool and said workpiece;

D. causing a relative vibratory motion between said tool and said workpiece at an ultrasonic frequency and amplitude sufficient to abrade and shape said tool into relative conformity with the negative image of the surface configuration of said workpiece while at the same time imparting a polishing action on the surface of said workpiece; and E. stopping said vibratory motion when the surface of the workpiece has been polished to the degree desired.

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BRIEF DESCRIPTION OF THE DRAWINGS
An ultrasonic machine tool as employed in the present invention is shown schematically in Fig. 1.
Figs. 2 and 3 are ~Ibefore~ and "aftex" photomicrograph~
of a carbide compacting die illustrating the effectiveness of this inventionO

MODES FOR CARRYING OUT ~H~ INVENTION
Ultrasonic machine tools are known to the art and the present invention is generally applicable for use with any such machines, utilizing typical parameters for vibrational frequency, amplitude and abrading particles. Typically, such equipment comprises a frame adapted to hold a workpiece and a tool holder including an ultrasonic driver which vibrates the tool at a ~requency of about 20 KHz up to, in some applications, 10 to 40 KHz, most often about 19 to 22 KHz. The tool holder is adapted to advance the tool from a retracted position into working position and, during working, into the workpiece. The equipment will ordinarily be furnished with abrasive slurry handling means so that ~ 20 the slurry can be disposPd between the tool and the s workpiece. The slurry will often be pumped through the gap ' between the tool and workpiece to continuously provide ., . . .

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fresh, unworn abrasive to the working surface and to flush away eroded material and debris. The slurry may be processed to remove debris and recirculated. The transducer will most typically be an electronically driven stack of piezoelectric element or a magnetostrictive transducer.
The abrasive slurry will ordinarily be formed of hard abrasive particles disposed in a liquid carrier. The abrasives are typically silicon carbide, aluminum oxide, boron carbide, boron nitride, diamond and the like, although it should be noted that when polishing softer materials in the present invention, softer abrasives may be used, such as alumina, corundum, garnet, and the like. The liquid carrier must be one capable of transmitting ultrasonic vibrations and should be chosen to be compatible with the workpiece and the electrode materials. Water is the best such transmitters, although other liquids such as cutting oil or fluid and the like may be used. When water is used, it may be necessary to add rust inhibitors. In polishing operations according to this invention, as opposed to machining according to the prior art, a relatively modest movement of the abrasive particles is preferred. Therefore, liquids other than water, such as cutting oil, can be used to effect a low amplitude particle movement, or in the alternative a lower power can be used with the water as the transmitter.
Generally, the particle size of the abrasive is not critical as long as the particle s-ize is such that it can .. . .

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be held in suspension. It is generally preferred, therefore, to use small particle sizes, less than 0.075 mm (200 mesh), and preferably, 0.025 to 0.015 mm (600 to 1000 mesh), with a particle concentration of from 10 to 20 volume percent of the fluid to attain the highest levels of polish.
The workpiece to be polished can be substantially any material which, contrary to prior art practices, is ultrasonically harder than the tool material, typically, a metallic workpiece. The extent of polishing required will be determined by the initial surface roughness of the workpiece and the finish required after polishing. Both an advantage and a limitation of the procedure of the present invention resides in the fact that the configuration of the workpiece will not be altered during the polishing operation. It is thus important to recognize that the present invention will not improve resolution of fine detail, and the quality of the final product will, except for surface finish, be determined by the initial workpiece.
Thè tool, as previously noted, must be formed of a material that is considerably more ultrasonically abradable than the workpiece material. An ultrasonically more abradable material does not mean one that is softer, but usually one that is more brittle. To understand ultrasonic abradability, it should be realized that in the ultrasonic machining of a surface, the tiny abrasive particles suspended in the fluid are impinged against the workpiece surface at an ultrasonic velocity, so that the tiny .~ .

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particles microscopically chip-away at the workpiece surface. To be chipped away in this fashion, the workpiece must have some degree of brittleness, whether or not the material is hard. It should be apparent that soft or resilient materials such as tough and ductile steels could not be readily machined in this fashion because the tiny abrasive particles would merely bounce therefrom.
Accordingly, for the polishing of most metal workpieces, which would include everything from mild steel to hardened alloys and refractory metals such as ~itanium and tungsten, an ideal tool material would be a material having a significantly greater degree of ultrasonic abradability, such as graphite, glass, quartz and other such materials which have normally been considered ideal workpiece materials but not normally tool materials.
When employed with suitable equipment, the tool may be provided with passages communicating with the gap through which the abrasive slurry may be pumped to provide flushing of debris from the gap. In the present invention, the debris will predominantly be tool material particles eroded from the tool combined with minor amounts of material polished from the surfaces of the wor~piece. In addition, the pumping will serve to provide fresh abrasive slurry to the gap so that cutting edges are not excessively worn during use.
Contrary to prior art practices, it is not necessary to start the polishing process of this invention with a pre-shaped tool, and accordingly, indexing and registration d . ~ :

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of the tool and workpiece is not required. In the process of this invention, the surface contour of the workpiece first serves to shape the tool surface into very exact registration therewith. During the subsequent polishing operation, the tool is continually eroded and will perpetually generate and maintain very exact registration in situ. The preferential erosion of the tool is the feature of the present invention which permits a high polish on the workpiece surface by a very thin, highly uniform surface removal. In some applications, particularly where the surface to be polished has deeply recessed portions, it may be desirable to utilize a pre-shaped or partially pre-shaped tool to speed up the operation by minimizing the amount of tlme it takes to shape the tool into registration with the workpiece, and to and avoid an excessive polishing action on the raised portions of the workpiece surface before the tool is worn sufficiently to start polishing the deeper recessed portions.
The relatively low mass of graphite or glass for examples, in relation to the materials most often employed for ultrasonic machining permits the employment of graphite tools of greater dimension than can be successfully driven by ultrasonic transducers.
As noted above, the process of this invention is also ideally suited to the removal of any undesired layer of material from a workpiece surface. For example, an EDM
recast layer, typically from 0.003 to 0.006 mm (0.0001 to , . ~ ., , . ~ .

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0.0002 inch) thick can readily be removed by the practice of this process with the result that the recast layer¦ is removed without any loss of resolution of detail in the workpiece surface. In a like manner, workpieces coated with material such as ceramic, can be processed as described herein to remove or selectively remove the ceramic coating therefrom without any loss of resolution of detail in the workpiece surface. In addition to these variations, the process of this invention can be used to remove burrs which protrude from the workpiece surface, or to radius sharp corners on the edges of the workpiece.
Either of these objects can be readily effected by using such an ultrasonically abradable tool without losing any of the workplece detail.

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Figure 2 and 3 are photomicrographs of a tungsten carbide compacting die taken at 3000~ magnification.
Figure 2 illustrates the original die surface as originally produced by a CNC engraving operation. The machining marks produced by the CNC machining operation are clearly visible. Figure 3 illustrates the same carbide compacting die after it had been polished in accordance with the process of this invention, utilizing a graphite tool, a grit of 15 micron boron carbide and polishing for 15 minutes to remove only 0.005 mm (0.0002 inch) of material.
In addition to the markedly improved surface finish, as is readily apparent from the photograph, the edges of the die were also radiused somewhat rounding the right angle corner as resulted from the CNC engraving operation.

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Claims (14)

1. A method of ultrasonically working a workpiece surface having a desired configuration performed therein to remove a very thin and uniform layer of material from the workpiece utilizing a tool which need not be preshaped comprising:
A. forming a tool from a material that is more ultrasonically abradable than the workpiece;
B. mounting said tool in a ultrasonically vibratable relationship to said workpiece;
C. applying a liquid abrasive slurry between said tool and said workpiece;
D. causing a relative vibratory motion between said tool and said workpiece at an ultrasonic frequency and amplitude sufficient to abrade and shape said tool into relative conformity with the negative image of the surface configuration of said workpiece while at the same time imparting a polishing action on the surface of said workpiece; and E. stopping said vibratory motion when the surface of the workpiece has been polished to the degree desired.

2. The method of Claim 1 wherein the process is utilized to polish the workpiece surface.

3. The method of claim 1 wherein the process is utilized to remove an unwanted layer of material from the workpiece surface.

4. The method of claim 1 wherein the process is utilized to remove any unwanted burrs from the workpiece surface.

5. The method of claim 1 wherein the process is utilized to radius the edges of the workpiece.

6. The method of claim 1 wherein said tool is preshaped to a form having a general conformance to the preshaped surface of the workpiece.

7. The method of claim 1 wherein the abrasive in said slurry has a particle size less than about 0.075 mm (200 mesh).

8. The method of claim 7 wherein said abrasive has a particle size of from 0.025 to 0.015 mm (600 to 1000 mesh).

9. The method of claim 7 wherein said abrasive is present is said slurry at a concentration of from 10 to 20 volume percent.

10. The method of claim 1 wherein said tool material is selected from the group consisting of graphite, glass and quartz.

11. The method of claim 1 wherein said tool material is an unformed block of graphite.

12. The method of claim 1 wherein said liquid abrasive slurry flows continuously through the gap between said tool and said workpiece.

13. The method of claim 12 wherein said liquid abrasive slurry flushes tool particles and particles abraded from said workpiece from said gap.

14. The method of claim 1 wherein said abrasive in said slurry is a member selected from the group consisting of tungsten carbide, aluminum oxide, silicon carbide, boron carbide, boron nitride, alumina, corundum, diamond, and mixtures thereof.