This article has many more pictures than were used when published in Rock and Gem.

I am sure you have heard the old joke that when a customer asks an engineer (any kind) to design some new product “faster, better, and cheaper,” the client gets to pick only two of the three criteria. I want to show you a design for a lapidary grinder that meets all three criteria and uses inexpensive saw blades. We have been happily using our “SuperGrinder” for two years at the Houston Gem & Mineral Society shop for rough grinding. The machine greatly reduced the load on the cabbing wheels and increased their life. The sphere makers particularly enjoy the improved grinding speeds for removing the corners of their initial cubes, and there is always a line for the unit in the shop.

The key items are eight robust 10-inch diameter 40-grit diamond saw blades intended for concrete work. We get these from our local Harbor Freight store (Item # 42043-1VGA, $27 list but there are sales) or from their http://www.harborfreight.com Web site. An alternative mail order-only site is http://www.mcgillswarehouse.com/ that offers cheaper blades (#DB10, $8 for the 10-inch size blade) which we have not tested.

The eight blades are stacked on the arbor and ganged as a unit. We run these on a donated Foredom grinder that has had its aluminum splashguard raised to accommodate a larger wheel size than the machine’s original 8-inch silicon carbide wheels. We added a separate single blade to the machine at the request of people who do stone carving. We are using a standard 1½ horsepower. There is so much mass in the nine saw blades that the machine takes seconds to get to full speed, and it draws more than 20 amperes while doing so. A future improvement will be to replace the existing redwood 2x12 base (this machine is so old that it uses old growth redwood!) with a steel plate. The wood base is probably too flexible.

SuperGrinder Schematic

SuperGrinder with the splashguard raised

The motor is positioned behind the SuperGrinder unit.

The motor powering the SuperGrinder is mounted on a hinged board with a threaded rod to keep the drive belt tight. The splash guard has been raised with two metal plates.

The thing you must do is bore out the blades from the delivered arbor size to the standard 1-inch shaft size of the grinding arbor. In our shop we asked one of our members to do this on a lathe, but you could also do it on a drill press where the blade is first positioned using a tapered pin and then clamped down before it is drilled. Don’t just try to enlarge the hole with a hand-held drill, and be sure to try the drill on a piece of scrap to determine whether it drills a hole that fits on your arbor! Don’t be fooled into believing that the size stamped on the drill is the actual size. You need to make a hole that matches your arbor, and that may require a reamer for final adjustment. You can buy very satisfactory large reamers from Harbor Freight for $50.

An unexpected benefit of having the wheel made of stacked blades is that the wheel can be effectively “trued up” by just loosening the compression on the blades and rotating them separately. This needs to be done about once every four months because the “bumping” of big rocks against the blades causes them to wear out of round, but truing the wheel takes only minutes to do.

Users of the SuperGrinder get wet and muddy using this machine even with the additional splashguards made of vinyl flooring. The high rotational speed causes the water to mist and drift everywhere. Since the blades run very close to the bottom of the drip pan, we glue (using E-6000 cement) in a flattened piece of PVC pipe just in front of the blade to keep dropped rocks from getting wedged under the blades. This protection has worked perfectly in the year we have been using the machine.

The PVC pipe shown at the bottom of the image to the left is an important safety feature that keeps rocks from getting trapped between the blades and the bottom of the water trough.

We have experimented with faster rotational speeds, but the machine vibration seemed excessive (possibly due to the flexible wooden base), and faster speeds are unnecessary because the current machine cuts 10 times faster than anything else in the shop. There may be issues with higher speeds reducing bearing life, but the cost of replacing the bearings is not significant. We use modern ball bearings with 4-bolt cast iron mountings and have never had to replace any in our grinders. You may wish to build a SuperGrinder with smaller blades running at a higher speed. For example, if you used the inexpensive 7-inch blades, you would be able to trade off the smaller amount of diamond on the blade for the ease of mounting on a grinder that is constructed to use 8-inch carbide wheels. You would have to run the wheels at 3700 RPM to have the same surface speed that we get with our 10-inch blades. Roughly, this would mean running at twice motor speed and would require a pulley on the motor that is twice the size of the pulley on the shaft. You would also decrease the bearing life.

We are also experimenting using sixteen 7-inch diamond saw blades (item # 41743-3VGA, $10) running at 1725 RPM just to replace a plated wheel. The blades are stacked tightly together with 0.04-inch thick washers to eliminate the grooving produced by the SuperGrinder. The 40 grit in the blades produces a surface finish that is too rough to go to a 140 grit sanding wheel, so users have to spend a moment on an 80-grit plated wheel to prep the surface for the next step, a 140-grit diamond sanding wheel. In spite of this extra step, this wheel is still very popular because it grinds at 9 grams/minute, much faster than plated wheels. This 7-inch diamond saw also is already bored for mounting on a 1-inch arbor, the standard in our shop.

SuperGrinder Jr. (shown on the left) uses sixteen 7” diamond saw blades to replace a metal bond grinding wheel.

With use, grinding wheels typically dish because users tend to avoid the edges of the wheel and concentrate the wear in the center. Now the wheel no longer can be used to grind a flat surface. The wheel made from sixteen blades stays much flatter than the one made from only eight blades. You might think that we could change the order of the blades to flatten the curvature, but we should have done that when the curvature was much less. It will be interesting to see how well the grinder works when the center blades have worn to bare metal while the edges still have diamond.

So did we meet our goal of Faster-Better-Cheaper? These two grinders certainly grind much faster that a plated wheel and last much longer. They are not any cheaper on a purchased price basis but are certainly cheaper on grams of ground up rock per dollar basis. The surface finish is definitely not better, but our club members think that faster is better, our treasurer thinks that the increased life of plated wheels is better, and our shop foreman thinks the increased use of the shop is better. This unit is perfect for a club shop where there are lots of machines, and one can be dedicated to be a SuperGrinder. See our club Web site www.hgms.org for a discussion of the grinding rate of various wheels, grinder plans, and suppliers.

Comments as of 10/14/05

Washers: