Polishing Stone with a Radial Arm Polisher: A Practical Guide to Workshop Equipment and Operation

A large cast-iron arm mounted on a concrete base or anchored in a wall was, until recently, a mandatory piece of equipment for every professional stonemason's workshop. While this still holds true today, stonemasons use it significantly less than in the past. This is primarily due to the complete disappearance of new terrazzo elements in cemeteries. 
In the vast majority of cases, radial arm polishers are currently used to regrind old inscription slabs – monuments. This is done when the text on the monument is outdated, poorly placed, or contains an error in the text or date. Well, such things happen.

I don't think so. What I like about stonemasons is that they can help each other. Even though they are competitors, they are men who stand by one another. This isn't seen so often in other industries. Sometimes it's enough if one master in the area has a radial arm polisher. Although from a logistical point of view, it is certainly better to have such a machine ready for action in your own workshop. "Murphy’s Law" applies in these cases too. On the positive side, it can be purchased for relatively negligible money compared to other stonemasonry machines.

As with any stone surface treatment, precise and consistent work is the most important factor here. It is necessary to concentrate and not skip any area, not to stand on one spot, and to perform systematic movements. Simultaneously, you must monitor the state of the so-called "slurry." It is a sludge composed of incoming water, ground stone, and also the worn-off segment from the disc. 
A constant water flow of 10-20 liters per minute is recommended. For the tool to function correctly, the water must be fed through the axis directly to the center of the polishing disc and filtered from impurities.

1. Grinding – grits #30 to #60 | Higher pressure (2-5 bar) | Lower speed 400-600 rpm
2. Polishing – grits #100 to #3000 | Gradually lower pressure with each grit | Higher speed 800-1200 rpm
3. Buff - Felt – minimal pressure | Lower speed approx. 700 rpm | minimum water

Each machine has a specific mounting system. In the past, mounts with cardan shafts and silent blocks or spring-loaded locking systems were used. These mounts had some play to account for inequalities. 

Currently, precise diamond tools with fixed mounts and minimal play are available. New radial arm polishers can format and regrind a stone slab to an absolute plane. This was a bigger problem in the past, and with a little inattention, a "hole" could be ground into a larger stone slab. This caused puddles and early degradation of the material.

Regardless of what mounting your machine has, there are always several ways to attach a backer pad. If you didn't buy them with the machine, a skilled turner can easily handle it. In the case of new machines, it's just a matter of finances.  

In addition to the repeatedly mentioned discs with Velcro attachment or hard discs with a screw-on system, we should mention the popular discs with a snail lock attachment (SL / BOSCH / SF).

Tools with a diameter of 150 or 130 mm are very popular for polishing multiple pieces of grave frames joined into one unit, for example, with clamps. This is an extremely efficient polishing method where models like 3-step or 8-step. are used.  

There are several types of polishing sets for radial arm polishers on offer. Before I introduce them, I should mention that if you are deciding on the purchase of a new set of polishing discs, do not hesitate to contact us. We will go through it together. Often, you can save on certain segments or use older discs. At the very least, we will try, and if successful, we will save some money and set up the optimal set.

• Probably the most modern set is the 3-step. It can be said that in recent years it has been the TOP set, which I can highly recommend. In this model line, we also have extremely popular flexible and hard polishing pads for hand tools, which are undoubtedly among our bestsellers.
• The classic set with grits from #30 to #3000 is represented by the eight to nine-step RESIN set. It is a traditional and still preferred set of tools whose grits can be combined with discs from other manufacturers. This way, you can smoothly transition from a competitor's set to a new one as you need the discs. Another difference is that it is not necessary to use the entire set, and thus all grits from the coarsest #30 to the finest #3000.
• Hard discs have a great advantage, resulting in a perfect plane. Polishing discs RESIN-METAL with grits #30, 50, 100, 200, and #400 are very popular. It is not necessary to buy the whole set at once. Individual grits can be skipped or combined with soft discs. The most preferred is the combination of grits #30 and #100, which level the surface, grind out the lettering, and prepare the surface for polishing. Subsequently, you can continue from grit #1 from the 3-step set.
• The BUFF grit is a chapter in itself. Buff is very similar in composition to a felt disc, into which polishing powder is pressed. You can choose from hard or soft - flexible buff.

 

 

There are several options. In the past, discs with a diameter of up to 310 mm were used more frequently. Currently, 98% of polishing discs sold are 250 mm in diameter. Therefore, the market supply is adjusted to this trend. 

In the case of smaller diameters, hard polishing discs with SL attachment and a diameter of 150 mm are most commonly used. With this diameter, you can process long planes as well as smaller monuments – stone inscription slabs.  

We are coming to perhaps the most essential chapter of this issue. To get to the point, we should briefly mention the method of polishing on a radial arm machine.

 

Besides the fact that the surface must be absolutely thoroughly processed with every single grit included in the tool set, it is important to place greater emphasis on processing the material along its perimeter and edges. Some masters consciously go over the perimeter of the polished area more times than the center. This is to achieve a minimal, invisible radius on the surface. 

• water stays less on a rounded surface and thus drains better
• light breaks better on a rounded surface, which increases the gloss

• Soft discs with Velcro attachment have a greater tendency to bend at the edges of the polished material and thus grind the very edge more. 
• Hard polishing discs maintain a better plane even at the edges.

 

In our range, you will find hard discs where the grinding or polishing segment is firmly attached to a hard, rigid backer disc. Soft discs are attached to Velcro fabric and are slightly flexible. Even the Velcro attachment itself allows for a compression of about 2-3 millimeters. 

1. If the entire set is soft – i.e., with Velcro attachment, it is of course fine. Polishing discs with Velcro attachment will follow and polish even slight inequalities.  
2. First discs hard and the rest soft – This combination is, for me, the most optimal and very often preferred. The first grits thoroughly level the surface and prepare it for the following steps – polishing. During the polishing process, smaller and smaller layers are gradually removed, so there is no risk of deforming the surface like with the first (coarse - sharp) grits. 
3. Soft discs at the beginning and the rest hard is a significant problem. Especially for an inexperienced master who cannot control the leveling. It really takes practice and concentration to avoid stressful situations or embarrassing complaints like: "your discs can't polish the edge of the stone!" Yes, I've dealt with that too. And more than once. 

What about safety when working with a radial arm polisher?

Besides the usual safety instructions, I would highlight only one case from practice. It occurred because the master did not secure the stone (monument - inscription slab) on the work table under the radial arm polisher. 

I must admit, I probably don't know a master who would secure the stone against movement during this work. This is because when polishing granite (from #30 up to #3000), the disc almost slides over the stone without much resistance. However, when polishing with a buff or felt, high working temperatures are generated between the disc and the stone. This causes friction to increase to such an extent that the processed material may move. 

A critical situation arises the moment the material moves during buffing enough to be knocked off the table and hit the wall. The material moved an estimated 150 to 200 centimeters. This is a surprising situation even for experienced masters, but not unrealistic. In this case, it ended only with a financial loss for new material and shaping the monument. One shouldn't forget the wall repair and new cladding. It's probably clear to everyone what would have happened if the heavy stone had moved in the opposite direction.   

Conclusion