Abrading – Abrading process – Abradant supplying
Reexamination Certificate
2000-03-10
2001-10-23
Hail, III, Joseph J. (Department: 3723)
Abrading
Abrading process
Abradant supplying
Reexamination Certificate
active
06306020
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatus and methods for grinding and polishing optical elements, such as lenses, prisms, windows, mirrors and similar optical systems. This invention could also be used to polish ceramics and semi-conductor surfaces.
2. Description of the Related Art
A recirculating slurry system
10
of the prior art for grinding and polishing of optical parts is shown in
FIG. 1. A
slurry tank
12
holds a slurry
14
, which usually consists of deionized water and grinding or polishing particles of fairly uniform size, and a pump
16
. The slurry
14
is pumped from the tank
12
through pipe
18
to either a delivery pipe
24
or a bypass pipe
26
. If valve
20
is open, the slurry
14
is returned to the slurry tank
12
, which helps keep the grinding particles of the slurry
14
in suspension by agitation.
Valve
20
is usually kept open during the grinding and polishing phases, which include grinding away the surface roughness (stock removal) along with the subsurface damaged material (usually
7
times the depth of the surface roughness (peak-to-valley) is removed). When valve
22
is opened, the slurry
14
is pumped to the grinder/polisher
30
, which grinds or polishes (depending on the size of the particles used) the surface of a part (not shown). For example, a loose abrasive grinding could be performed by using a grinder with a hard lap (metal or glass). The polishing could be performed by using a polisher with a resilient material such as pitch or polyurethane. The part being ground and polished could be an optical lens or a semiconductor blank inserted in the grinder/polisher
30
. During the final polishing stage, the valve
20
is usually shut or partially closed to allow the slurry to begin to settle. The slurry in the grinder/polisher
30
is returned to the slurry tank
12
via return pipe
28
. It is desirable to shut valve
22
after polishing the part is complete so that slurry will circulate through bypass pipe
26
in order to avoid the settling and caking of the polishing compound.
The loose abrasive grinding used in the slurry is typically an aluminum oxide or silicon carbide with mean grit sizes between 9-30 &mgr;m. Polishing slurry usually uses cerium oxide or zirconium oxide with a mean grit sizes between 1-3.5 &mgr;m. For polishing slurries, the range of polishing powders on the market is usually between 0.4 to 3.7 &mgr;m APS (average particle size). However, a compound with a specific particle size will have a range of different sized particles based on standard distribution curves. For example, a 2.5 &mgr;m APS compound has particles ranging from 0.5 to 8.0 &mgr;m in size. Although the larger particles may be a weak agglomeration of smaller particles, the surface of the polished object may be scratched if the larger particle do not break apart. In another example, a 12.5 &mgr;m APS compound was tested to find that the size of the particles ranged from 7 to 25 &mgr;m and some particles were as large as 40 &mgr;m. Although these very large particles will most likely create scratches, these particles fortunately either settle down quickly or are too large to penetrate between the lap and the part being polished.
In order to avoid some of these problems, manufacturers perform two polishing operations: the so called “pre-polishing” process and the “final polishing” process. This is time consuming method of polishing parts because it requires each part to be removed, washed, and transferred into the final polishing machine.
FIG. 2
shows another recirculating slurry system
40
of the prior art where the pump
46
is located outside of the slurry tank
42
. The slurry
44
held in the slurry tank
42
is removed via pipe
48
and valve
50
. The slurry is pumped via pipe
56
to either delivery pipe
60
via valve
58
or bypass pipe
52
via valve
54
. The grinder/polisher
62
receives the slurry
44
from delivery pipe
60
and returns the slurry to the slurry tank via return pipe
64
.
The recirculating slurry systems shown in
FIGS. 1 and 2
will pump relatively rough grit into the attached polisher. While this is advantageous during most of the polishing or grinding process, it is objectionable during the final stages of the polishing because the surface finish of substrates depends on the grit size being used. Instead of removing the partially polished part to insert into a separate “final polishing” machine, the lap is flushed with deionized water to perform a “water polishing” step. It is commonly believed that the remaining polishing particles will embed themselves in the lap material, thus exposing just the tips of the grains, which are obviously smaller than the whole slurry grains. Thus, a finer polished surface will result. However, the pH of the deionized water (pH of 7) is usually different than the optimal value for the slurry. Therefore, the slurry compound will begin to aggregate and the continued polishing of the part will create scratches. It has been demonstrated that just 15 minutes of “water polishing” deteriorates the part's surface and scratches may appear after 30 minutes. In addition, the unnecessary water added to the slurry tank will change the density of slurry and affect the polishing of subsequent parts.
The recirculating slurry systems shown in
FIGS. 1 and 2
can produce an optical part with a smoothness of only 4-5 Å RMS.
SUMMARY OF THE INVENTION
The present invention discloses a slurry system that draws slurry from a slurry tank via one of several intake pipes. Each intake pipe has an intake opening at a different depth in the slurry tank. The process begins by continuously removing slurry from the intake pipe with an intake opening at the deepest level. The slurry is returned to the slurry tank via a bypass pipe in order to continue the agitation of the slurry. The slurry is then diverted to a delivery pipe, which supplies slurry to the polisher. When the surface roughness is smoothed out, the flow of slurry in the bypass pipe is stopped in order for the slurry in the slurry tank to begin to settle. As the polishing continues, slurry is removed from different pipes so that the final pipe is at the shallowest depth such that the finest grit is pulled from the slurry. When the polishing is complete, the flow of the slurry in the bypass pipe is resumed to start agitating the slurry and the intake pipe with the intake opening at the deepest level is used again.
The present invention also discloses a slurry system that draws slurry from a slurry tank via an adjustable intake pipe. The process begins by continuously removing slurry from the intake pipe at a predetermined level. The slurry is returned to the slurry tank via a bypass pipe in order to continue the agitation of the slurry. The slurry is then diverted to a delivery pipe, which supplies slurry to the polisher. The flow of slurry in the bypass pipe is stopped in order for the slurry in the slurry tank to begin to settle. As the polishing continues, the adjustable intake pipe is adjusted such that slurry is removed from predetermined levels where each level is shallower than the previous level. When the polishing is complete, the flow of slurry in the bypass pipe is resumed to start agitating the slurry and the intake pipe is returned to its original position.
An object of the invention is to provide progressively finer polishing grit to a polisher.
Another object of the present invention is to avoid using water as a final polishing step when polishing surfaces of optical ceramic and semiconductor elements.
Other objects and advantages of the present invention will become apparent when the apparatus of the present invention is considered in conjunction with the accompanying drawings, specification, and claims.
REFERENCES:
patent: 5490809 (1996-02-01), Jones et al.
patent: 5755614 (1998-05-01), Adams et al.
patent: 5791970 (1998-08-01), Yueh
patent: 6126531 (2000-10-01), Iida et al.
patent: 6183352 (2001-02-01), Kurisawa
patent: 6189621 (2001-02-01), Vail, III
patent: 6203412 (200
Fuchs Baruch A.
Hed P. Paul
Caress Virginia B.
Daubenspeck William C.
Drew Gary R.
Hail III Joseph J.
Ojini Anthony
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