Grinding and Polishing
Procedure Comparison
Back
to:
Polishing
Gateway
Polishing
Supplies Gateway
Abrasives
Gateway
In
1953, Shor developed and introduced the world's first modem polishing
systems, with isolated motors and bag type dust collection systems. Within 10
years, virtually every industrial polishing system in the world was a Shor
system or a copy of one. Today's polishing systems are not much changed from the
original one's we introduced 40 years ago. Though the polishing equipment hasn't
changed much, still there are important new procedures, accessories and axially
equipment now available that can make a world of difference in labor time,
finishing time, quality of finish and gold recovery. Fortunately, none of the
accessories or equipment is expensive (with the exception of electropolishing
and centrifugal tumbling, equipment that is for very high production and will
not be covered in this discussion). In many cases, you may already have the
necessary equipment and accessories and lack only the knowledge of how to get
the most out of them. In other cases, this guide may only serve to confirm that
your present procedures are correct or perhaps this guide can offer some
alternatives that you may want to try, In any case, if you have any questions or
need more detailed instructions on one or more of the procedures, please feel
free to call, write or fax us at Shor International.
There are many different procedures with which raw castings
may be taken and brought to a final finish. The procedures a shop chooses to use
and the order in which these procedures are accomplished, is the prerogative of
each individual shop. The following instructions are intended as a general guide
toward this end. We believe the following procedures and the order in which they
arc accomplished to be the best way to finish raw castings. However, this is
just a guide. The best way for your shop is the way that suits your needs best
(and if that means doing things like wearing your lucky t-shirt when finishing
or some other unorthodox procedure, then go with it).
Firescale Removal:
Many shops begin the finishing procedure before the castings
are even cut off the tree. Investment removers will go a long way toward
deoxidizing your castings. Pickling in Safe-T-Pickle, Sparex or other pickling
agent is quite common (virtually all dry pickling agents like Sparex are
simply expensive trade names for a very inexpensive chemical compound, sodium
bisulfate). another common procedure many shops employ is to deoxidize and
brighten their castings by bombing. A few shops sand blast their castings.
Investment removers and pickling agents will not take off any
metal from the castings. Bombing will dissolve some of the metal into the
bombing solution. Sandblasting with glass beads will not remove any metal, but
sandblasting with any other media will remove metal. Sandblasting with glass
beads will brighten clean castings but is not very aggressive in removing
firescale. For more detail on bombing and sandblasting procedures and gold
recovery from them, refer to separate Shor instructions on these
procedures.
We recommend pickling for fire scale removal. Pickling will do
a fine job of removing the firescale yet it will not remove any It is a very
fast and easy procedure which requires little or no equipment. In addition, the
entire tree can be pickled at the same time.
Sprue, Parting Line and Other Flaw Removal:
When a casting is cut from its tree, there still remains a bit
of the sprue left (no matter how close you cut to the ring). This remnant of the
sprue must be removed, and with it, any parting line (if it exists) and other
blemishes (like little pimples, bumps etc). Most large shops grind these flaws
off. Most small shops use a variety of tools including hand files and flexible
shaft machines and other tools.
Even for the smallest shops (even for I man shops), we
strongly recommend the use of MX grinding wheels to remove sprues, parting lines
and other blemishes on the outside of the ring. MX wheels do a much faster and
much better job than files and flex shaft machines. They have the following
advantages:
Fine grit MX wheels leave only very fine, easily removed
grinding lines (lines that are actually finer than many sanding lines.
MX
wheels do not clog nearly as quickly as other wheels.
The edge of the MX wheel should be prepared, after purchase,
by cutting it with a diamond dresser to conform to the curvature of your rings
(see Shor MX wheel instructions for this procedure). When this has been done,
the sprue and parting lines can be removed in just a few seconds and the
curves of the ring will be much more perfect than if the grinding had been done
with files or a flex shaft machine.
MX wheels are run on a polishing motor. That leaves both hands
free for better control. The high torque of the motor results in very fast
removal of the flaws. And last, but certainly not least, removal of the metal on
a polishing motor makes it much easier to control metal losses.
If the castings are rings, you may want to grind the inside of
the shank. We recommend the use of a rotary file for this purpose. However,
there are other options: emery paper mandrels, wood emery paper mandrels,
shoulder and standard mandrels, emery ring shells and cartridge rolls. While
these other options are fine, rotary files are better because: they give a
consistent finish, cut quickly (even the fine) and last for a very long time. We
recommend the use of a fine grade rotary files because they leave only fine
grinding lines which are easier to remove.
Sanding should not be necessary if using a fine grade of MX
wheel and rotary file. If there is a need to sand, then the best way to sand
large areas quickly and consistently is with a cone-lok drum sander, This is a
rubberized steel wheel that accepts strips of emery cloth and fits on your
polishing motor.
We recommend that MX wheels and all grinding, rotary filing
and sanding be done on a motor, hood and dust collector devoted only to this
purpose. If it is not practical to devote a dust collector for this purpose,
then don't use a dust collector for grinding. The grindings are heavy enough to
obviate the need for a dust collector. If it is not practical to devote a motor
to grinding, then devote just a hood to grinding. Before grinding, sanding or
filing, remove your polishing hood and replace it with a grinding box (a closed
hood with no exhaust vent). We recommend the use of the Shor locked
grinding box. This is a box with a screen at its base and a locked drawer
underneath the screen. While there is no way to insure that gold grindings walk
out of your shop with your workers, the locked grinding box goes a long way
toward this assurance. If you are doing the grinding yourself or do not want to
invest in a prefabricated grinding box, any plastic box of the right size and
with a light in it will do.
Ultrasonic and Ionic Cleaning:
| It is a good general policy to clean the pieces between
each finishing procedure. This could be done by hand, but a much faster way
is to use an ultrasonic. Ultrasonics have come way down in price since
Shor first introduced them to the jewelry industry back in the early
fifties. Now virtually every shop can afford one. An inexpensive and very
effective alternative to the ultrasonic is the ionic cleaner. Ionic
cleaners, in general, are just as effective as ultrasonics and they appear
to have a better longevity track record. |
 |
Brightening Crevices and Hard to Reach Areas
Both how and when to brightening crevices will vary
from shop to shop. Brightening of crevices can be done at any time up and
including the time of the final finish. There are 3 basic ways of brightening
the crevices:
- Bombing or stripping
- Tumbling with steel pins
- Polishing with brushes
Each technique has its advantages and disadvantages, which
will be outlined as follows. Please refer to the appropriate Shor
instructions for "how to" details.
Bombing and Stripping-
These techniques both use baths that are primarily cyanide and water to brighten
jewelry (non-cyanide stripping solutions are available but they do not work as
well and the primary ingredient, thiourea, is a suspected carcinogen). Each has
its advantages and disadvantages.
Both bombing and stripping have the advantage that they allow
a shop to do many pieces at one time very quickly and with a minimum of labor.
Both have the disadvantage that they work with cyanide solutions (cyanide
solutions can be safely used and disposed of only under tightly controlled
conditions-see the appropriate Shor instructions for these conditions).
Stripping has the advantage that its solution can be reused many times. Bombing
solutions, only once.
The most important difference between the two techniques, however, is that
bombing removes metal from and
brightens the entire piece evenly but stripping tends to remove more metal from
the high points (like prongs). This means that stripping, in general, will
remove more metal than bombing from the castings (greater losses) and may
actually damage the prongs on very light weight pieces. For this reason, we
recommend bombing over stripping. See the appropriate Shor instructions
for how and when to bomb and strip how to recover gold from bombing and
stripping solutions and how to destroy the cyanide.
Rotary Tumbling with Steel Pins-
The advantage to tumbling is that it brightens large numbers of pieces will
little labor (mass finishing). Tumbling requires much more
time than bombing or stripping but requires slightly less labor. Tumbling with
steel shot
removes no metal. In general, tumbling does not get into the crevices as well as
bombing.
Tumbling brightens pieces by hammering the surface with steel
"shot". Because this technique results in a fine "hammered" effect on broad flat
surfaces (an effect that must later be removed from these surfaces to get a
quality finish), we recommend that all tumbling be done prior to tripoli work.
The tripoli work will remove the hammered effect from the broad surfaces, but
will not reach the crevices where the brightening will remain. Most steel shot
has relatively few pins in it. Yet the pins are the component of the shot that
brightens the crevices. For this reason we recommend adding pins to your shot
mixture. A good ratio of pins to standard ball shot is 50/50, however, trial and
error will tell you what works best for you. For detailed "how to" instructions
on tumbling, see the appropriate Shor instructions.
Magnetic Tumbling with Steel Pins-
This is a relatively new technology and is excellent for burnishing the hard to
reach areas as well as any surface which is not very flat. Flat surfaces 
tend
to get a bit of a "haze" on them by this means of finishing. If using on
flat pieces, a quick buff with a wheel easily removes the haze. The high
G's produced in magnetic tumblers results in a burnishing action that is just
not possible with standard tumblers or vibratory machines. With media as small
as .010 in diameter and .250 in length, these stainless steel pins are able to
work in areas such as undercuts, recesses, slots etc. where no previous method
has been found suitable. Finishes the most intricately detailed parts.
Very quiet when compared with most other forms of finishing. Typical
finishing time on gold and similar metals is only about 30 minutes. This
is an excellent method of automatic finishing, producing rapid final finish
results with an absolute minimum of labor.
Polishing with brushes-
This technique, without question, gives the greatest luster. It is, however,
very labor intensive. When properly done, most retail customers cannot tell the
difference between a bombed piece and a brushed piece. For this reason, we do
not recommend the use of brushes for most pieces. To find out how to choose the
right brush for the right job, see the appropriate Shor instructions.
On balance, we recommend bombing as the best
overall technique for brightening the crevices.
Removing Grinding or Sanding Marks (Tripoli
Work)
This may be accomplished by sandblasting or tumbling or
polishing with a high (fast) cut polishing compound like tripoli. However,
we recommend the use of a vibratory finisher here. In
the early 1970's, Shor introduced the precious metal jewelry industry to
vibratory finishers. It wasn't until the '80's though, that they began to become
popular. Vibratory finishers are very inexpensive mass finisher systems that
have a great many important advantages over hand finishing, sandblasting and
tumbling.
Sandblasting-
Sandblasting does a great job. If sandblasting, use first a coarse media (like
carborundum grain or pumice or sand) to do the tripoli work. Next, change the
media to a glass bead to do the polishing. When finishing with a sandblast
machine and with these media, it is generally not necessary to bomb or tumble,
and sandblasting can replace virtually all of the hand tripoli work and almost
all of the hand rouge work. Unlike mass finishing machines, sandblasting has the
advantage of allowing the operator to use his intelligence when required to
blast a little more here or a little less there. Mass finishing machines do not
provide for this option. Another advantage of sandblasting is that the media
gets into every little nook and cranny. Mass finishing media generally doesn't
do that. On the other hand, sandblasting is more labor intensive than tumbling
or vibratory finishing. And, while sandblasting will remove much less metal than
hand finishing, it is very difficult to recover the gold from the sandblast
media.
Tumbling-
Generally speaking, tumbler cannot work well with any media except steel shot.
Steel shot, because it is not abrasive, does not lend itself well to removing
grinding or sanding scratches.
Hand polishing-
Tripoli work done by a skilled polisher will yield excellent results especially
in those circumstances where a little intelligence is necessary. And when
lapping is necessary, there is no substitute for hand lapping. In addition,
there is no faster way to do tripoli work. On the other hand, hand polishing is
very labor intensive and generally results in much greater gold losses than any
other technique. This makes hand polishing a very expensive technique that
should be reduced to a bare minimum.
Vibratory Finishing-
Vibratory finishers do the tripoli work by using a light weight plastic media
impregnated with a
cutting compound (like carborundum, silicates or aluminum oxide). Vibratory
finishing takes longer than other techniques but will accept hundred of pieces
at the same time and requires almost no labor. The result is a very dull but
smooth piece, ready for the rouge work. Very little metal is removed in this
finishing process (approximately 1/10th of 1% per hour of running time with a
fine grade media- average time 8 hours) and the metal is reasonably easy to
recover. Vibratory finishers cut very evenly on all surfaces. That means that
the vibratory finisher will preserve details. It also means that they will
preserve flaws, so it's important to do a good job removing those flaws before
the pieces reach the tripoli stage. For more details on the process of vibratory
finishing, please consult the Shor instructions on vibratory finishing.
Polishing (Intermediates Work)
This is the step between the tripoli work and the final
polish. This step is often skipped by many shops, and it is not absolutely
necessary. Each shop must determine what works best for them.
There are several ways in which the intermediate polish can be
accomplished. A common technique is to use a tumbler with steel shot as the
media. It is our opinion that, at this stage, tumbling is unwise and should be
avoided. The shot will impinge on the broad flat surfaces of the work and leave
a beaten hammered effect (an effect that must later be removed in the final
polish in order to achieve a quality finish). In addition, the shot also has the
effect of bending or breaking prongs.
Hand polishing with intermediate compounds like graystar and
white diamond is another possibility, but hand polishing at this stage has the
same drawbacks as it did at the tripoli stage.
We recommend that, for most pieces, you should either
vibratory finish with a rouge impregnated walnut shell or corn cob- or you
should skipped this intermediates step altogether. Vibratory finishing will take
the dull smooth jewelry and transform it with a luster just short of a final
finish (for stampings and silver, this is often a final finish). Vibratory
finishing at this stage will not remove any metal at all. It does, however,
require a minimum of 24 hours processing time.
Final Polishing (Rouging)
This must be done by hand. Please refer to the appropriate
Shor instructions to choose the appropriate polishing compounds, buffs, ring
buffs, brushes, felt wheels, laps and cones to suit your particular needs (or
call us at Shor International 914-667-1100).
| Summary
Chart |
| Process |
Strengths |
Weaknesses |
| Rotary Tumbling |
Inexpensive to finishes curved surfaces and brighten
settings. Very short learning curve to gain the basics on how to use
equipment.
With a lot of experience, almost all of the weaknesses can be overcome.
Burnishing removes no metal. |
Tends to give flat surfaces a "beaten" appearance. Tends
to round off sharp edges, obscure fine details and bend thin prongs.
Uses steel shot, which tends to get stuck in settings.
Even though the basics of tumbling are learned in an afternoon. To obtain
the best results requires a lot of experience.
Not very good for cut-down process (the tripoli work) |
| Magnetic
Tumbling |
Very fast. Short learning curve.
Excellent for getting into small crevices
Burnishing removes no metal.
|
Relatively expensive. Relatively small capacity.
Gives slightly frosted look to large flat surfaces (light buffing
removes this haze).
|
| Vibratory Finishing |
Excellent for cut-down (tripoli work) Good for rouging
work.
Very inexpensive.
High capacity
Short learning curve when using proper instructions.
Preserves details. Very little rounding of edges.
Rouging removes no metal. |
Works with very light media and which is relatively slow.
Not good for crevices. Not good for inside settings. |
| Electropolishing |
Lightning fast. Finishes in seconds. Finishes crevices
and settings better than any other equipment.
Preserves details better than any other equipment.
Excellent for flat surfaces as well.
Inexpensive to operate.
Gold is recovered as flakes of gold. Does not need to be extracted from
an abrasive or polishing media. 100% recovery.
Parameters can be adjusted to polish more aggressively at high points or
in crevices. |
Uses cyanide based salts. Expensive to purchase
Relatively long learning curve to determine the best parameter settings
for your particular rings. |
Stripping |
Inexpensive to purchase and to run. Finishes in seconds
or a few minutes.
Gold is recovered as flakes of gold. Does not need to be extracted from
an abrasive or polishing media. 100% recovery. |
Uses cyanide based salts. Does not work well with white
gold.
Does not work well with low and high karat gold.
Is much more aggressive at the tips of prongs than inside the settings. |