2. Polishing with diamonds: Radial arm polishing
The broad majority of stone fabrication shops in the USA have time and higher pressures exerted by the operator. As for
a radial arm polishing machine, used primarily to resurface granites, which are much harder, polishing is uneconomical,
stone slab surfaces damaged through handling. Without this often impossible. Silicon carbide is simply incapable of cutting
machine, a shop has to send the damaged slab to somebody else the granite surface without extremely heavy pressure. Some
with the machinery to repolish large surfaces. Radial arm polish- radial arm polishers are incapable of these higher pressures and
ers are not only great cost savers, they are quite rugged durable. those that are can be impossible to control.
Most units in operation today were purchased anywhere from Let’s look at the mechanics of what happens. In manual polish-
several years to several decades ago. The typical radial arm ing with right-angle grinders, the 4" diamond disks spin at 4000
polisher consists of articulated arms supported from a station- RPM. The velocity of the outside edge of that disk is about 4000
ary post. An electric motor drives a plate supporting the abra- feet per minute.
sives by means of pulleys and belts. A handle permits the
operator to guide the whole assembly across the face of the Attach the same 4" disk to a typical radial arm polisher rotating
stone to perform the polishing operation. at 300 RPM, and the outside edge will be traveling at only 300 feet
per minute. Even the standard 10" disk will have a peripheral
An acceptable polish on marble has traditionally been achieved speed of only 800 feet per minute. That’s better, but not nearly
with three or four grit sizes of silicon carbide bricks, followed by fast enough for optimum performance. Diamond abrasives re-
a buffing operation. The resulting finish usually matches the quire much higher speeds. The hand-held, right-angle grinder
factory finish although the process is quite time consuming. yields a peripheral velocity of 4000 feet per minute, and that
It’s a different story for granite. Most shops are unable to match should be the goal for a radial arm machine.
the factory finish with a radial arm polisher and silicon carbide, The best option is to increase the rotational speed of older
and experienced shops will no long even make the attempt. machines with a different combination of pulleys. The motors on
Yet, it stands to reason that if granite be finished with a hand- radial arm machines have enough power to operate under as
held grinder and flexible diamond disks, the average shop much as 200 pounds of pressure on the abrasive block. With
should be able to do the same thing with a radial arm polisher diamond abrasives, that power can be used to obtain higher
and diamond abrasives. Indeed, it can be done, but some changes speeds because far less pressure is needed. New machines
are required. already have the option for high speeds
Marble is easily polished when it is relatively soft. The silicon Today
carbide abrasive, the slow 300 RPM speed, and the 100-200
The use of diamond abrasives on the plate of a radial arm
pounds of pressure—typical working parameters of a radial arm
polisher offers new possibilities for the fabricating shop. Dia-
machine—are adequate.
monds are much more aggressive than traditional silicon car-
Harder stones, however, begin to cause problems. Green marbles bide abrasives. They will perform as well on granite, and far
or serpentine marbles require more work, usually in the form of faster on marble, And polish both with far less pressure.
ABRASIVE TECHNOLOGY, INC. 8400 Green Meadows Dr. Westerville, OH 43081 USA (614) 548-4100 Fax: (614) 548-7617 1-800-964-TECH (8324)
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3. Disk use 1. With hook and loop fasteners (e.g., Velcro) glued directly onto
For rough work, most shops will use a flat steel plate to which a clean metal support plate.
have been welded segments of metal-bonded diamonds similar 2. With a special adaptor and rigid plates designed for this
to the segments attached to circular saw blades. Such plates are specific purpose. One is required for each grit size.
well suited to removing saw marks or taking off a flawed polish.
Resin-bonded diamond disks don’t work as well because they The latter method (the single adaptor and the rigid support
aren’t sufficiently aggressive. plate) has distinct advantages:
For polishing operations, the best choice is a resin-bonded, 10" • Optimum cushioning of the diamond for the longest
diameter disk made specifically for radial arm machines. These disk service life.
disks range in grit sizes from a coarse #30 to a very fine #8500. • The proper rigidity for an even and perfectly flat polish.
Most shops use #50, #120, #220, #400, #800, #1800, and #3500 for That rigidity also allows the polishing plate to extend
processing granite. Marble polishing is less demanding, and the well beyond the edges and the corners of the work to
combination of #50, #220, and #800 grits usually yields the ensure a uniform finish over the entire surface.
desired finish.
• The simplest means of attachment for fast and easy grit
The technique of polishing with diamonds is not that much changes.
different then the technique used with silicon carbide. The
standard practice of overlapping passes and crosshatching is • Easy and hassle-free switch-over from diamond to
the same. The only real difference is that far less pressure is silicon carbide if it is necessary to use traditional
needed. In fact, excessive pressure will shorten the life of a abrasives.
diamond disk. For machines fitted with Frankfurt type abrasive bricks, all-
For polishing marbles, a good flow of water is needed to flush diamond resin-bonded bricks may be used. They are made in the
away the abrasive cuttings. Adjust the water flow when using a same grit sizes as the 10" disks. Unfortunately, they cannot reach
coarse diamond disk such as #30 grit. The water coming off the to the corners of the slab with the same degree of success as the
disk should be milky, as opposed to clear or pasty. The water 10" disc configuration.
flow rate that is right for that first, coarse grit will also right for Costs
each succeeding grit all the way up to the finest.
On the basis of materials only, diamond polishing tools are
For granite, the water flow should be a little less than it takes to significantly more expensive than silicon carbide. The overall
make the diamond disk hydroplane. That’s usually slightly less advantages, however, clearly favor diamonds—less grinding
than the ideal flow for marble, but consistency of the slurry is not time, improved production rates, better finished surfaces, longer
a concern for granite. disk life—and add up to improved profitability.
The results with diamond disks are very predictable. Processing For polishing with radial arm machines, the advantages of
obstinate, tricky, and soft Negro Marquina marble, for instance, diamonds include:
becomes a routine task, without even the usual variations in
• The ability to resurface and polish harder stones,
gloss over the surface of the slab. And, for the first time, granite
including granite, to acceptable finishes.
processing to a high gloss is not only achievable, but repeatable.
• The ability to polish stones with variable surface
With marble or granite, polishing will be enhanced by buffing to
characteristics and achieve a uniform polish.
bring out the full gloss and depth of color of the stone.
• Doing more work in-house and sending less work to
Attachment
better-equipped shops.
Attaching the 10" disks onto the faceplate on the radial arm
polisher can be done in either of two ways:
ABRASIVE TECHNOLOGY, INC. 8400 Green Meadows Dr. Westerville, OH 43081 USA (614) 548-4100 Fax: (614) 548-7617 1-800-964-TECH (8324)
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