Powering a BIG Grinding Wheel

A fellow Centralian was discussing building his own grinder with a newly acquired large surplus wheel. Having some knowledge and experience in that department I thought I'd offer some advice for others contemplating a similar project.

Working backwards from the max safe operating speed on the wheel's label is a good place to start, but there are some essential preliminaries before you put the wheel in commission.

Step one: Determine the suitability of the wheel to grind steel. The most common abrasive for steel is aluminum oxide. Look on the wheel's label or a stencil on the wheel that has a series of numbers and letters. This is an industry-wide code defining the characteristics of the grinding wheel.

The first letter should be "A" for aluminum oxide or "C" for silicon carbide. If your wheel is aluminum oxide, rejoice! Aluminum oxide is much preferred for grinding steel. If it's silicon carbide, mourn. Silicon carbide (not silicone) is an excellent abrasive but it's not suited for tool sharpening.

The next number should be between 36 and 80. This is the grit size. Optimum grit size for tool grinding is 42 to 60.

The next letter denotes hardness. For heavy off-hand grinding this should be in the "M" to "O" range. If the wheel is intended for machine grinding it will be in the "H" to "L" range. Subsequent letters and numbers convey other info but not much that you really need to know except out of curiosity. Look up grinding wheel codes at the Norton website.

Step two: Is the wheel sound? Cracked? It's possible. Most grinding wheels are vitreous bonded (as with glass) and consequently brittle. You must first conduct a "ring test" to see if it is safe to use.

Blow off any debris and suspend the wheel from a rope or a stick through the hole. Look at the side of the wheel. Imagine a protractor printed on it with zero° on top. Chalk two marks at 45° from either side of zero. With a block of wood about 2x4x8" tap the wheel on the side at each of the marks using the end grain. The wheel should emit a clear musical note at each tap. Rotate the wheel 45° and repeat. If the wheel passes the ring test it's provisionally okay for full speed operation. If the tap produces a "bonk" the wheel is a dud good only for very low speed operation.

If the wheel is cracked it's unsafe for full speed operation. It can burst and, if you or anything valuable is in the plane of rotation, it can be severely damaged by debris flying at 60 miles per hour.

Step three: Determine the operating safe RPM. Vitreous bonded aluminum oxide wheels typically run at 5500 to 6500 peripheral feet per minute (SFPM). Work the formula where RPM = (SFPM * 12) /(dia * π). Working the math (5500 x 12) / (20 * 3.14) = 1050 rpm. The drive from an induction motor should be reducing and the ratio should be motor RPM / wheel RPM (for example: 1750 / 1050 or 1.67). If the motor is a 1 HP 1750 RPM (the very minimum) you could run a 3" motor pulley (no smaller) to a 5" grinding wheel pulley using an "A" V-belt. If the available motor is 3450 RPM use 2½" motor pulley (no smaller) to run with an 8" or 8½" grinding wheel pulley (8.33" is the actual number but 8 or 8½ is close enough).

Step four: Make a suitable arbor. I suggest a piece of 1¼" diameter cold rolled steel shafting about 16" long and a couple of ball bearing pillow blocks. Find a machinist to make you a hub for the wheel and some large thick washers. Everything should be fully machined and snugly firred to run in good concentricity and balance. The hub should be a very snug fit on the arbor, preferably shrunk on with 0.0015" interference. Heat the roughed-out hub to 700° (blue) and quickly slip it into position on the shaft. The hub will shrink on the shaft so tightly it will be as good as welded. Finish machine the hub after shrink fitting.

Step five: Make a stand. The stand may be of wood construction but it should be heavy and strongly constructed so vibration won't move it around. Before you start your grinding wheel, build a strong box of 2x dimension lumber and 3/4" plywood sides as a wheel guard and put it in position. Position your new grinder so nothing important is in the plane of rotation. Stand clear to the side and start the wheel. Chances are it will vibrate like crazy because the wheel is out of true and thus out of balance. Let the wheel run for five minutes. If everything is okay and holds together, true the wheel with a star dresser.

All that's left is to build a suitable permanent guard and a tool rest.

All the above is the minimum procedure for commissioning your 20" grinding wheel. It's what I'd do if I had your grinding wheel and I wanted to put it into service. Don't cut any corners. If the wheel is cracked it's cracked. Use it at 100 RPM as a low speed wheel but not at full rated RPM.

I speak from expertise here; I know about grinding wheels. I've served 40 years on the machine shop floor at several levels of responsibility, served five years on the union safety committee, and I taught the trade to many years of apprentices.

Unless you're an experienced scrounger of industrial surplus you can expect to spend about $400 on all new materials and machine shop services to make your new grinder.