Method for surface finishing of difficult polish surfaces

A method for finishing a surface of a workpiece, having the steps of agitating the workpiece with a first mixture including a first plurality of discrete, homogeneous compressed felt chunks having a first particulate abrasive coating thereon, and then agitating the workpiece with a second mixture including a second plurality of discrete, homogeneous compressed felt chunks having a second particulate abrasive coating thereon with an abrasive size smaller than an abrasive size of the first particulate abrasive coating.

BACKGROUND OF THE INVENTION 
The present invention relates generally to finishing of the surfaces of 
workpieces, and more particularly is directed to the use of finishing 
media loaded into a barrel with the workpieces. 
Finishing the surface of a workpiece usually involves polishing and 
abrading. Abrasion refers to the removal of larger portions of the 
surface, primarily to alter the overall contour of the surface. Abrasion 
is often performed in a wet process for a grinding, deburring, aggressive 
smoothing or other material removal operation. Polishing refers to the 
removal of small portions of the surface of a workpiece, in a scratchlike 
manner, primarily to alter the visible finish. Polishing is often 
performed in a dry process, resulting in surfaces with reflectivity 
approaching the quality obtained from manual buffing. Usually, automatic 
polishing is accompanied by at least a small amount of abrasion due to the 
manner in which it is performed. 
Methods for automatically finishing the surface of workpieces employ a tub 
into which workpieces and finishing media are loaded. The tub is moved to 
impart motion to its contents, and the resulting contacts between the 
media and workpieces remove portions of the surfaces of the workpieces. 
Automatic finishing methods include rotary barrel finishing, vibratory 
barrel finishing, centrifugal barrel finishing and centrifugal disk 
finishing. 
Rotary barrel finishing relies on gravitational forces. During rotation, 
the contents of the barrel move upwards until gravity causes the contents 
to slide downwards. The majority of the finishing occurs when the contents 
slide down. The contacts between the media and workpieces tend to be long 
scratches similar to those obtained using a buffing wheel. This technique 
is good for smoothing sharp exterior edges and corners (radiusing), but is 
not particularly effective for inside surfaces. 
Vibratory barrel finishing relies on kinetic energy. A vibratory motion is 
imparted to the contents of the barrel. The finishing occurs during the 
short strokes of contact between the media and workpieces. This technique 
is reasonably good for polishing interior surfaces but is not particularly 
effective for corner or edge finishing. Also, vibratory finishing does not 
produce a particularly refined finish. 
Centrifugal barrel finishing relies on centrifugal pressure. The barrel is 
rotated while it revolves around an axis, exposing the contents of the 
barrel to high centrifugal forces. The finishing occurs when the media 
press on the workpieces. This technique is good for producing refined 
surfaces in short times. This technique is also appropriate when the 
identity of each workpiece must be maintained, as each workpiece may be 
loaded into one of several barrels which simultaneously rotate around 
their respective axis and revolve around a central axis. 
Centrifugal disk finishing also relies on centrifugal pressure. Here, a 
containment vessel has a rotating disk as a base and a non-rotating 
cylindrical vertical wall. Media and workpieces are thrown against the 
wall and slide down. Finishing occurs both when the media press on the 
workpieces while they are pressed outward and during the downwards 
sliding. This technique is good for precision finishing in short times, 
but requires a large amount of monitoring. 
Traditional finishing media include hardwood or resin preforms used with 
abrasive paste, and plastic or ceramic shapes with embedded abrasive. 
Substantial deterioration of the media occurs during finishing due to the 
abrasive action of the media upon itself, such as between two preforms or 
two media shapes. Typically, plastic finishing media lose their mass at a 
rate of about 3% per hour of use, and ceramic finishing media lose their 
mass at a rate of about 3-5% per hour of use. Thus, such media are not 
durable. 
When the preforms or shapes impact the surface of a workpiece, a portion of 
the surface of the workpiece may be abraded or removed from the workpiece. 
Sometimes this is desirable, as when removing marks or radiusing. However, 
in some cases, a workpiece has been carefully brought to its present size 
and shape, and it is desirable only to polish the workpiece, that is, not 
to abrade or cut down its surface. With conventional media, if the 
finishing process is controlled so that media contacts do not abrade the 
workpiece surface, then the finishing intervals become very long, 
rendering the finishing process relatively expensive. 
If the workpiece has an intricately contoured shape, its interior surfaces 
may not be adequately polished. For example, if the surface includes a 
U-shaped region, conventional media tends to abrade the tops of the 
U-shape, but not reach the surface of the bowl at the base of the U-shape. 
The preforms or shapes may have tips. When a tip perpendicularly contacts a 
workpiece surface, the tip digs a pit in the surface of the workpiece. The 
finished surface has a scratch pattern of peaks and valleys which diffuse 
or diffract light, resulting in a dull, foggy, matte finish, quite unlike 
a bright, shiny, highly reflective finish that is often desired. 
FIG. 1 shows a spherical workpiece 100 being finished by conventional media 
105A, 105B, 105C. Media 105B is seen to be sliding along the surface of 
workpiece 100, creating a long scratch 110, which desirably finishes the 
surface of workpiece 100. 
The workpiece 100 has a U-shaped socket 130. Media 105C is seen to be 
eroding the edges of the socket 130. It will be appreciated that even if 
one of the tips of media 105C entered into the socket, negligible 
finishing occurs, as it is not possible for the media to slide along the 
surface of socket 130. 
A tip of media 105A contacts workpiece 100 normal to the surface thereof 
and digs a small pit 120. In FIG. 1, pit 120 is enlarged for ease of 
illustration. During finishing, the surface of workpiece 100 becomes 
undesirably pitted. 
A portion 150 of the surface of workpiece 100 is shown enlarged. The 
surface contains pits 151 and long scratches 152, corresponding to the 
action of media 105A and 105B, respectively. 
It is expected that certain abrasives will break down during a finishing 
interval, so that the finishing interval begins with coarse abrading and 
concludes with finer polishing. However, this type of finishing cannot be 
precisely controlled. Furthermore, this type of finishing is not linear 
with time, that is, during a six day polish interval, the finishing during 
an hour of the first day is substantially different than during an hour of 
the sixth day. 
Workpieces may be sensitive to the size of abrasive used in a finishing 
process. Specifically, a certain range of abrasive size may cause skin 
fractures perpendicular to the surface of a workpiece, giving the 
workpiece an undesirable shattered look. During the remainder of the 
finishing interval, the workpiece surface must be abraded sufficiently to 
remove these fractures, lengthening the finishing interval and changing 
the size of the workpiece. Alternatively, the finishing process must be 
controlled so as to remove abrasives in the undesired size range. 
Multiple step finishing processes which do not substantially rely on 
abrasive breakdown have been used for attaining smooth surface finishing 
of metallic articles or parts. Typically, a first step, abrasive cutdown, 
removes excess material and provides a coarse finish, while a second step, 
burnishing, provides a smooth finish and a third step, polishing, provides 
a finely polished surface. Sometimes a fourth step, waxing, is used to 
produce a surface with maximum reflectivity. 
U.S. Pat. No. 2,185,262 (Lupo) describes a process for finishing metallic 
articles in a tumbling barrel including a first step of tumbling the 
articles with hard bony pellets, such as vegetable ivory chips, bone 
chips, synthetic resin chip or hard tree root chips, and a hard coarse 
abrasive, such as ground pumice, emery or carborundum of 180 to 200 mesh, 
to effect a cutting operation for removing tool, grinding or sand marks. 
In a second step, the articles are tumbled with hard bony pellets and a 
hard fine abrasive, such are pumice, emery or carborundum of 320 to 400 
mesh to effect polishing, and, in a third step, the articles are tumbled 
with fibrous fragments, such as wood pegs, including a fine abrasive of 
500 to 800 mesh to impart high luster to the metal articles. 
The process described in Lupo has several drawbacks. The pellet and wood 
peg media are abraded during finishing. The workpieces are abraded during 
each step, that is, more surface portions are removed than minimally 
necessary for polishing. The fibrous fragments, namely, the wood pegs, are 
rigid enough to dig pits in the surface of the workpieces which may be, 
e.g., malleable metals. Complex surfaces are not uniformly polished. A 
finishing process takes a long time, since a rotary barrel is used. 
U.S. Pat. No. 3,504,124 (Kittredge et al.) relates to a finishing process 
carried out in water in a vibratory barrel, using media having a hardness 
which depends on temperature. Articles to be finished and the media, 
comprising a rigid plastic binder with abrasives having average particle 
diameters below 15 microns such as alumina, quartz or silicon carbide, are 
loaded into vibratory equipment for a first finishing operation at low 
temperatures of about 35.degree. to 50.degree. F. The temperature of the 
water is increased to about 100.degree. to 125.degree. F. in a second 
finishing operation, which produces articles having a finish in the range 
of one to 5 microinches (0.025 to 0.13 microns). A third step of final 
polishing is indicated as necessary, but no particular way of performing 
this final polishing is provided. 
The process described in Kittredge et al. has several drawbacks. 
Importantly, a final polishing step, such as manual polishing, is 
required. A water supply is necessary, including a way to control the 
water temperature in a range from very cold to warm. The finishing media 
are not durable. The workpieces are abraded during each step. Complex 
surfaces are not uniformly polished. 
At present, there is no known method of finishing surfaces which can be 
accomplished in a short finishing interval, uses durable media, polishes 
workpieces with minimal abrasion, polishes complex surfaces, provides a 
lustrous and highly reflective surface, is easy to control and is linear 
with time. 
OBJECTS AND SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a method for 
surface finishing which avoids the aforementioned disadvantages of the 
prior art. 
Another object of the present invention is to provide a method for surface 
finishing with at least one of the following advantages: use of durable 
media, polishing of workpieces with minimal abrasion, finishing of complex 
surfaces, provision of a highly reflective surface, ease of control, 
linearity with time and short finishing interval. 
In accordance with this invention, a method for finishing a surface of a 
workpiece, comprises the steps of: first agitating the workpiece with a 
first mixture including a first plurality of discrete, homogeneous 
compressed felt chunks having a first particulate abrasive coating 
thereon; and second agitating the workpiece with a second mixture 
including a second plurality of discrete, homogeneous compressed felt 
chunks having a second particulate abrasive coating thereon, the second 
particulate abrasive coating having an abrasive size smaller than an 
abrasive size of the first particulate abrasive coating. 
The above, and other objects, features and advantages of the present 
invention will be apparent in the following detailed description of the 
preferred embodiments of the present invention when read in conjunction 
with the accompanying drawings in which corresponding parts are identified 
by the same reference numeral.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is a multiple step method for surface finishing of 
workpieces which are difficult to satisfactorily finish using conventional 
media and conventional finishing techniques. 
The present invention uses, as finishing media, compressed felt chunks 
coated with abrasive, as described in U.S. Pat. No. 5,140,783, having a 
common inventor with the present invention, the disclosure of which is 
incorporated by reference herein. These compressed felt chunks will now be 
described in connection with FIG. 2, which shows a workpiece 200 being 
finished by abrasive coated compressed felt chunks 205A, 205B, 205C. 
Each felt chunk is about 1 inch in at least one dimension, and may be a 
cube, pyramid, triangular or other shape. The compressed felt has a 
density of about 20 to 45 lbs. per cubic foot in the dry condition. The 
abrasive coated compressed felt chunks are inexpensive to manufacture. 
Felt is formed by matting together fibers, rather than weaving fibers, 
under pressure. The fibers are preferably wool. As shown in FIG. 3, a wool 
fiber has a somewhat coiled spine 300 with hairs 310A, . . . , 310D 
extending therefrom. A hair 320 is shown as entangled with a hair 310D. 
The hair 310D is connected to the spine 300, whereas the hair 320 is 
unconnected, being previously attached to the spine 300 or to a spine of 
another fiber. Compressed felt chunks tend to retain all their mass when 
used in a finishing operation, since the chunks do not abrade each other. 
Thus, this is a fairly durable finishing media. 
Importantly, after a hair is detached from one spine, it tends to become 
entangled with other hairs. Therefore, compressed felt chunks have a 
self-renewing action, exhibiting substantially improved durability 
relative to conventional finishing media. Specifically, after contacting 
the surface of a workpiece, hairs from a felt chunk may detach from their 
respective spine, but then such hairs become enmeshed with hairs from the 
same or another felt chunk and are available for another contact with a 
workpiece surface. Similarly, the spines of fibers may tangle, so that 
fibers lost from one chunk can become part of another chunk. In contrast, 
when portions of conventional finishing media separate from the main body 
of the media, the separated portions are waste material which are no 
longer usable and must be removed during the course of a finishing 
operation. 
Compressed felt chunk 205B of FIG. 2 is seen to be sliding along the 
surface of workpiece 200, creating a long scratch 210, which desirably 
finishes the surface of workpiece 200. 
The workpiece 200 has a U-shaped socket 230. A portion 235 of compressed 
felt chunk 205C is seen to be deforming its curvature to finish the 
interior surface of the socket 230. That is, the flexibility of the felt 
chunk allows it to conform to the surface of the workpiece, so that the 
abrasive carried by the felt chunk has an opportunity to finish the 
surface of the workpiece. Thus, complex surfaces may be satisfactorily 
finished using compressed felt chunks as finishing media. 
During contact between abrasive coated compressed felt chunks and the 
surface of a workpiece, workpiece surface portions removed mainly consist 
of the substance formerly in the scratches removed by the abrasive 
embedded in the felt chunk. That is, although the workpiece surface is 
polished, since the felt is itself deformed, there is a cushioning effect 
with minimal abrasion of the workpiece surface. Consequently, compressed 
felt chunks are inherently non-abrasive and are good for finishing a 
surface without changing the contour of the surface. 
When a tip 206 of felt chunk 205A contacts workpiece 200 normal to the 
surface thereof, the tip 206 bends during contact with the workpiece, 
imparting a short scratch 240 to the workpiece. A short scratch pattern 
results in negligible light diffraction, that is, a highly reflective, 
bright, apparently flawless surface finish. In contrast, conventional 
media dig pits in workpiece surfaces that diffract light and result in a 
dull finish. 
The felt chunks are resilient, compressing under pressure and uncompressing 
when the pressure is removed. The compression and uncompression of the 
abrasive coated felt chunks creates very short scratches in the surfaces 
of workpieces, which further finishes these surfaces. 
A portion 250 of the surface of workpiece 200 is shown enlarged. The 
surface contains short scratches 251 and long scratches 252, corresponding 
to the action of compressed felt chunks 205A and 205B, respectively. The 
surface also contains very short scratches 253 formed during compression 
and uncompression of the abrasive coated felt chunks. 
The amount of abrasive breakdown is fairly small when using abrasive coated 
compressed felt chunks as finishing media, since the abrasive is 
substantially separated from other abrasive. There is no need to remove 
broken down abrasive during a finishing operation using compressed felt 
chunks. In contrast, the abrasive paste used with conventional finishing 
media promotes abrasive breakdown due to the contact of the abrasive with 
itself. The finishing action of abrasive coated compressed felt chunks is 
substantially linear with time. Also, since the abrasive size of abrasive 
coated compressed felt chunks remains relatively constant during 
finishing, it is easy to avoid use of an abrasive size range which causes 
surface fracturing of a workpiece. Thus, there is a reduced need for 
external monitoring of the finishing process, and it is easier to obtain 
consistent results. 
The compressed felt chunks may be used in any type of automatic finishing 
barrel and in a wet or dry process. A surface finished with abrasive 
coated compressed felt chunks in a rotary barrel finisher, centrifugal 
barrel finisher or centrifugal disk finisher has a scratch pattern as 
shown in the enlarged portion 250 of FIG. 2. 
If a workpiece surface is finished with abrasive coated compressed felt 
chunks in a vibratory barrel finisher, then it has a scratch pattern 
mainly comprising very short scratches, such as very short scratches 253 
of FIG. 2, and is substantially devoid of longer scratches. Specifically, 
during a finishing operation the workpiece stays in a central portion of 
the mass of finishing media, rather than on or towards the outside of the 
mass of finishing media. Thus, although some of the finishing media at the 
outside of the mass exhibit rolling motion, the workpiece does not, and so 
it is devoid of the long scratches that occur in a conventional process. 
This very short scratch pattern produces a surface with substantially 
better reflectivity than the surface reflectivity obtainable through any 
conventional process, including manual buffing. The exceptionally high 
reflectivity of a surface finished in this manner is perceived as an 
extraordinarily shiny and flawless finish. 
A standard vibratory finisher provides a vibration amplitude of 
approximately 1/16 inch. It has been found that a vibratory finisher which 
provides a vibration amplitude of approximately 3/16 inch provides 
substantially better results for a finishing operation in which abrasive 
coated compressed felt chunks are used. It is believed that the relatively 
light weight of the felt chunks is more effectively moved by the larger 
vibration amplitude. 
The present invention resides in a process for finishing the surfaces of 
objects or workpieces made of plastic, ceramic and/or metallic material. 
Surface roughness is measured normal to the nominal surface of a 
workpiece. A surface roughness of less than 1 microinch is produced using 
the present invention. 
As described below, a multiple step operation using a centrifugal barrel 
finisher, a vibratory finisher, conventional finishing media and abrasive 
coated compressed felt chunks is possible. An important advantage of the 
present invention is production of a finely polished surface with minimal 
workpiece material removal. The multi-step nature of the present invention 
greatly reduces the overall workpiece finishing time. Since the use of 
multiple steps avoids the need to rely on abrasive breakdown, consistent 
and simple control of a finishing process is possible. 
Many variations of this procedure are envisioned, depending upon the nature 
of the workpiece and its uses, and on the nature of the desired finish. 
For example, if a workpiece has holes or cavities, it is advisable to 
select the size of the finishing media so as to avoid clogging the holes 
or cavities. If the workpiece has threads, it is helpful to add a light 
coating of oil to the compressed felt chunks to encourage retention of 
abrasives by the felt chunks, reducing the amount of abrasive transferred 
to the threads of the workpiece during finishing. 
Plastic parts benefit from a finishing process according to the present 
invention, since they are made of a soft material and often are initially 
produced with deep scratches that propagate fractures resulting in an 
undesirably crazed surface. These plastic parts are effectively finished 
using gentle pressure and a multitude of contacts with the abrasive coated 
compressed felt chunk finishing media. 
The purpose of a first step is to abrade excess material from the 
workpieces, so rigid finishing material are used. The first step may be 
omitted if the workpiece is already smooth, that is, has a surface 
roughness of less than 20 microinches, or if the workpiece is fragile. For 
example, if the workpiece has voids which could become cleave points, 
producing surface fractures, it is preferred to go directly to a second 
stage using flexible finishing media, with an abrasive coating size 
selected in view of the void size. 
In the first step, a centrifugal barrel finisher filled to 50-80% of 
capacity is used. Depending upon the composition and shape of the 
workpieces, different finishing media and a wet or dry process may be 
used. For a dry process, the finishing media may be grain, such as walnut 
or corn. For a wet process, the liquid may be water, refined mineral oil 
or polyalkylene glycol, and the finishing media may be plastic or ceramic, 
such as polystyrene or urea formaldehyde combined with zirconia, silica or 
aluminum oxide. 
The first step improves sphericity, that is, abrades the surface of the 
workpieces, and reduces surface roughness of a workpiece having a surface 
which is machined or rough belt 120 grit or finer (65-70 microinches) to 
between 10-20 microinches. 
In a second step, a centrifugal barrel finisher is used. In either a wet or 
a dry process, grain and/or abrasive coated compressed felt chunks may be 
used as finishing media. For a wet process, high purity mineral oil is 
preferred due to its inertness. A combination of grain and abrasive coated 
compressed felt chunks is particularly useful when finishing parts with 
complex or discontinuous shapes, since such a combination produces a 
random media motion which eliminates cavitation and preferential 
orientation of workpieces, and promotes uniform finishing of complex 
shapes. 
The abrasive coating for the felt chunks may be comprised of silicon 
carbide, aluminum oxide, cerium oxide, or diamond of up to twice the size 
of the desired finish, e.g., 9 micron diamond for a 4.5 micron finish. 
Selection of the abrasive is performed to ensure compatibility with the 
nature of the workpiece, such as silicon carbide for plastic workpieces 
and diamond for ceramic workpieces, and its adjunctive materials, that is, 
materials with which it will be subsequently used, for example, reactivity 
to the human body. 
The second step reduces surface roughness of the workpieces to less than 4 
microinches. 
In a third step, a vibratory barrel finisher is used. In either a wet or 
dry process, only abrasive coated compressed felt chunks are used as 
finishing media. The finishing action in this step is characterized by low 
pressure, high repetition contacts. Workpiece surfaces are finished by the 
compression and uncompression of the media. 
The third step reduces surface roughness to less than 1 microinch. The 
surface finish has an unusually short scratch pattern. 
A fourth step may also be employed to achieve an even finer finish. This 
fourth step advantageously uses a vibratory barrel finisher with a 
finishing media of abrasive coated compressed felt chunks. The abrasive 
may be 0.3 or 0.05 (1/20 micron) alumina. 
Between finishing steps, it is preferred that any retained abrasive be 
cleaned from the workpieces, such as by manual, ultrasonic or detergent 
cleaning. 
Several examples of a finishing operation according to the present 
invention will now be described. 
In one example, ceramics such as aluminum oxide or zirconia, for example, 
zirconia balls for use in medical applications, received in an as machined 
state including lathe machine marks, may be finished in a three-step dry 
operation using a centrifugal barrel finisher operated at 320 or 325 rpm 
in each step. Due to the impingement of finishing media on ceramic, the 
length of the scratches in the scratch pattern is approximately 1/4 of the 
size of the abrasive. 
In a first step, compressed felt chunks coated with 30 micron diamond 
abrasive are used as the finishing media. The first step lasts about 1.5 
hours. The function of the first step is to smooth the workpieces to a 
surface roughness of under 15 microinches. 
In a second step, compressed felt chunks coated with 9 micron diamond 
cutting abrasive are used as the finishing media. The second step lasts 
about 1 hour. The function of the second step is to polish the workpieces 
to a surface roughness of less than 6 microinches. 
In a third step, compressed felt chunks coated with 1 micron diamond 
cutting abrasive as used as the finishing media. The third step lasts 
about 1 to 1.5 hours. The function of the third step is to polish the 
workpieces to a surface roughness of less than 2 microinches. 
Conventional media are not effective for polishing these ceramics, since 
the media are softer than the workpieces and are not an effective carrier 
for an abrasive. The compressed felt chunks act as a carrier for the 
diamond abrasive, which is harder than the workpieces and actually 
performs the finishing. 
An advantage of using a multi-step operation is reduced processing time. 
For example, a one step operation using only compressed felt chunks coated 
with 1 micron diamond abrasive is estimated to require approximately 50 
hours to achieve a surface roughness of less than 2 microinches, versus a 
total of about 33.5 hours for the three step operation described above. 
In another example, cobalt chrome workpieces for medical applications 
received in a 220 belt state or coarser, that is, a surface roughness of 
approximately 30 microinches, may be finished in a four-step operation to 
achieve a surface roughness of approximately 2 microinches. 
The first step is a wet process performed in a centrifugal barrel finisher 
operated at 120 rpm. The finishing media for this step are, for example, 
zirconia tetraform 3/8 inch cones. This first step takes about an hour. 
The second step is a wet process performed in a centrifugal barrel finisher 
operated at 120 rpm. The finishing media for this step are, for example, 
R700 media from Rosemont Industries, Cincinnati, Ohio, a polyester resin 
with a quartz abrasive. This second step takes about two hours. 
The third step is a dry process performed in a centrifugal barrel finisher 
operated at 140-160 rpm. The finishing media for this step is a mixture of 
grain media, compressed felt chunks coated with an abrasive of 1200 grit 
(9 micron) such as silicon carbide, and #169 oil. Advantageously, this 
mixture is about 80% compressed felt chunks and 20% grain, by volume. This 
third step takes about four hours. 
Grain media used by itself tends to create a flow pattern on the workpiece 
due to uneven collisions of the media across the surface of the workpiece. 
That is, grain media finishes only certain portions of the surface of the 
workpiece and tends to produce a scratch pattern with aligned scratches. 
In the above-described mixture, the felt chunks serve two purposes. First, 
the felt chunks result in a more random scratch pattern due to dispersal 
of the grain media and/or jarring the workpieces during finishing. Second, 
the felt chunks are an efficient carrier for the abrasive which performs 
finishing. 
The fourth step is a dry process performed in a vibratory barrel finisher 
providing a vibration amplitude of approximately 3/16 inch, at 1750 rpm. 
The finishing media for this step are compressed felt chunks coated with a 
1 micron abrasive such as alumina oxide, and #169 oil. This fourth step 
takes about 2 to 2.5 hours. 
In yet another example, cobalt chrome workpieces received with a less 
coarse surface, such as 300 belt, that is, a surface roughness of 
approximately 15 microinches, may be finished in a two-step operation to 
achieve a surface roughness of approximately 2 microinches. 
The first step is a dry process performed in a traction drive centrifugal 
barrel finisher operated at about 320 rpm. This apparatus is faster than a 
centrifugal barrel finisher, and imparts greater energy to the finishing 
media contained therein. The finishing media for this step is a mixture of 
grain media, compressed felt chunks coated with an abrasive of 1200 grit, 
and #169 oil. This first step takes about 20-30 minutes. 
The second step is a dry process performed in a vibratory barrel finisher 
providing a vibration amplitude of approximately 3/16 inch, at 1750 rpm. 
The finishing media for this step are compressed felt chunks coated with a 
1 micron abrasive such as alumina oxide, and #169 oil. This second step 
takes about 2 to 2.5 hours. 
In still another example, titanium workpieces for medical applications 
received in approximately a 220 belt state may be finished in a four-step 
operation to achieve a surface roughness of approximately 2 microinches. 
The first step is a wet process performed in a centrifugal barrel finisher 
operated at 150 rpm. The finishing media for this step are, for example, 
zirconia tetraform 3/8 inch cones. This first step takes about an hour. 
The second step is a wet process performed in a centrifugal barrel finisher 
operated at 150 rpm. The finishing media for this step are, for example, 
R700 media. This second step takes about two hours. 
The third step is a dry process performed in a centrifugal barrel finisher 
operated at 140-160 rpm. The finishing media for this step is a mixture of 
grain media, compressed felt chunks coated with an abrasive of 1200 grit, 
and #169 oil. This third step takes about 3.5 to 4 hours. 
The fourth step is a dry process performed in a vibratory barrel finisher 
providing a vibration amplitude of approximately 3/16 inch, at 1750 rpm. 
The finishing media for this step are compressed felt chunks coated with a 
1 micron abrasive such as alumina oxide, and #169 oil. This fourth step 
takes about 3 to 6 hours. 
In a further example, workpieces made of hard plastic, such as acrylic, 
polycarbonate or DELRIN.TM., may be finished in a three-step operation to 
achieve a surface roughness of approximately 2 microinches, or a four-step 
operation to achieve a surface roughness of less than 1 microinch. 
The first step is a wet process performed in a vibratory barrel finisher 
providing a vibration amplitude of about 1/16 inch at 1650 rpm. The 
finishing media for this step are, for example, R700 media. This first 
step takes about two to six hours, and results in workpieces with a 
surface roughness of about 9 microinches. 
The second step is a dry process performed in a vibratory barrel finisher 
providing a vibration amplitude of approximately 3/16 inch, at 1750 rpm. 
The finishing media for this step are compressed felt chunks coated with 
an abrasive of 1200 grit, and #169 oil. This second step takes about eight 
hours, and results in workpieces with a surface roughness of about 4-6 
microinches. 
The third step is a dry process performed in a vibratory barrel finisher 
providing a vibration amplitude of approximately 3/16 inch, at 1750 rpm. 
The finishing media for this step are compressed felt chunks coated with a 
1 micron abrasive such as alumina oxide, and #169 oil. This third step 
takes about four to six hours, and results in workpieces with a surface 
roughness of about two microinches. 
The fourth step is a dry process performed in a vibratory barrel finisher 
providing a vibration amplitude of approximately 3/16 inch, at 1750 rpm. 
The finishing media for this step are compressed felt chunks coated with a 
0.05 micron abrasive such as alumina, and #169 oil. This fourth step takes 
about eight hours, and results in workpieces with a surface roughness of 
under one microinch. 
Although illustrative embodiments of the present invention, and various 
modifications thereof, have been described in detail herein with reference 
to the accompanying drawings, it is to be understood that the invention is 
not limited to these precise embodiments and the described modifications, 
and that various changes and further modifications may be effected therein 
by one skilled in the art without departing from the scope or spirit of 
the invention as defined in the appended claims.