Q: I’m thinking of buying a bench grinder for my studio, so I can grind the edges of silver shapes and do some tool shaping. I wonder if I can also use it for polishing. Is this the right tool, and if so, which ones do you recommend? How much horsepower do I need, how fast should it go, and is variable speed important? Do I need a hood and fans for dust collection?
A: First. lets be sure we’re talking about the correct machines and terms. “Bench grinders” usually refers to bench mounted motors, usually without any dust collection or filtering means, with hard grinding wheels, usually a fine one on one side and a coarse one mounted to the other, rotating within a fitted protective shield. These most commonly are used for grinding and sharpening, often with wheels optimized for things like steel.
Polishing machines, however, while there is a similar motor, have tapered spindles on one or both sides, to allow the use of buffs and various other wheels, like rotary brushes, with soft centers that can screw onto the tapered thread. These often have hoods behind the spindles fitted with dust collection apparatus, normally a fan pulling air through a filter from behind the wheel.a The reason I mention this is that machines normally sold as grinders are not meant for, or suited for, polishing in most cases, because it’s more difficult for the wheels to be interchanged, which needs to be done frequently in polishing. You definitely shouldn’t try to use any machine that you’ve used for polishing to grind materials like steel, since the hot sparks that are generated can cause a fire in accumulated polishing dust.
These two types of machine, while similar in the sense both have a motor, differ in their intended use by quite a bit. There are also small multipurpose units, some with a grinding wheel or a multipurpose tapered spindle on one side and a flex shaft on the other. Generally they’re less capable in any role than the larger dedicated machines, but at their lower capabilities, they can fill more roles. Bench grinders are easy to find in hardware and tools stores. They’re good for rough-sharpening chisels and drill bits. They will also grind silver, but it’s not their intended use, and the results may not be quite what you hope. On soft metals like silver, these machines tend to leave very rough edges, and the wheels tend to clog up quickly. They’re really intended for steel.
For a grinding wheel, most often there is an optimum speed. Going slower can help to avoid overheating the metal, especially steels, where it messes up the heat treatment and hardening, if any. Most grinders run at 3450 rpm. For polishing, again, most full size/professional machines also run at 3450 rpm. Some are dual-speed, also allowing the slower 1725 rpm, which can be nice for some uses, and some types of wheels, notably some texturing wheels and brushes, which work better slower. The smallest of the polishing motors, the Foredom bench type and a few others, may offer variable speed. Sometimes this might be useful, but most of the time, for straight buffing purposes, it’s not needed.
The polishing machines are usually only found at specialized jewelry suppliers, and are specifically intended for polishing metal. That means getting buffs of various types and compounds to go with them. Polishing machines need dust control of some sort. Polishing compounds often are very fine powders, sometimes containing silica that is not good to breathe—neither is the dust thrown off the felt or muslin wheels. This stuff gets into the air, all over everything, and you don’t want it in your lungs either. In both cases, if you are on a budget, you can take care of the safety issues with a good dust mask. As far as the mess from a buffing machine is concerned, well, suit yourself. But most people prefer some sort of collection means, and with the cost of precious metals what it is, collecting and refining it eventually pays for itself, even with silver.
For grinding, dust collection is less essential. Grinding dust tends to be coarse, and doesn’t fly into the air. Dust collection helps you to prevent what does get into the air from getting into your lungs, and helps to recover precious metals if that’s what you’re grinding. Again, if you’re grinding metals, especially ferrous ones, don’t use the same dust collector that you use for polishing, to avoid the risk of fire.
The horsepower rating relates to how big a wheel you can run with a full load. More is better, but it’s not always needed. Most full-size polishing machines usually need at least ¼ hp or so, maybe slightly less for light-duty versions. A bigger motor won’t slow down when you bear into a larger buff. But if you’re talking about grinding, that may present less of a load to the wheel, so more horsepower may not be needed. At the same rpm, a six inch diameter wheel cuts more than twice as fast as a 3 inch one. Larger wheels mean a higher surface speed, so faster grinding or polishing. And they last longer. But both do the job. It’s just a question of how fast.
Again, the main thing is to understand that you seem to be lumping together two distinctly different types of equipment. Grinders use wheels that are not intended to be changed back and forth often, so they’re removed only when the wheel is worn and needs changing. Polishing motors, on the other hand allow you to work with many different types of buffs and wheels, and the tapered threaded spindles make that possible. The motors can also be fitted with fixed arbors that mount a wheel the same as a grinding wheel, but this is not their most common use. Tapered spindles require that the center hole of the wheel is soft, and generally small enough to “jam” it on the spindle. Many are sold with only a small hole to start, which ends up widening up as it’s used. Wheels intended for fixed mounting usually have a uniform center arbor hole, often ½ inch or some similar size, that get mounted between flanges and secured with a nut on the threaded shaft of the arbor.
With all the above said and done, I’d like to digress with another suggestion. If what you wish to do is to literally grind silver edges, with no intent to polish the metal, then I’d suggest that you’re actually looking at the wrong type of machine. Bench grinders are rather harsh for what you wish, and the wheels tend to clog up when used with soft metals. They’d do the job, but I think they’re not the best choice.
Rather, I’d suggest you look at small bench-mounted belt-sanding machines. Dremel makes one, and there are others. These use an easily replaced belt of sanding cloth, running over a couple vertically mounted pulleys. Usually there is a table with a fixed support behind the belt to let you rest the sheet metal or whatever on the table to feed into the belt, and if you need to work on curves, the upper part of the belt is unsupported, and can conform to a curve. I’ve seen machines like this for not much more than the cheap grinders, and you can get a wide range of grits and abrasive belts for them. The types of abrasives used will be more resistant to clogging, and may cut faster for this than a grinding wheel. You’ll have to replace the belts as they get worn, but they’re cheap. Some of the machines are made with a disk sander on one side too, which can also be useful especially if you wish to get a straight edge on something.
My suggestion, for the specific task you describe, of removing metal from the edges of sheet metal shapes, would be one of these machines. They work well, faster and are easier to control than a normal bench grinder for this use. And, because the motor is below the level of where you’re working, it doesn’t get in the way with larger pieces of sheet or stock. You can do almost any of the normal grinding tasks you might normally do on a bench grinder as well. But again, each tool has those tasks for which it’s best. Sharpening and shaping steel tools, gravers, drill bits, etc, is easier on the rigid fixed wheels of a bench grinder. Free-form trimming of sheet metal edges is often easier on the belt sander. And for actual polishing or refining a surface or an edge, you need a polishing wheel or buff, which generally means using that type of machine…
by Peter W. Rowe M.F.A., G.G.