I recently made a little sharpening table for my shop. Its purpose is to grind drill bits and HSS lathe tool bits. Here is the article. I wrote this article and posted it a week or two after I posted the above to the Web. I was very happy with the way my sharpening table went together but I had a problem. When I put on the new grinding wheels I'd purchased for this project, the grinder sounded awful and shook so much it was unusable for precision work. Without the wheels the grinder ran smoothly. With the wheels installed it vibrated excessively. So I knew I'd have to face up to the problem of how to balance grinding wheels.
I know I am not the first guy with this problem. I hear about it all the time. People blame their bench grinder when their grinding wheels are the cause of the problem. If your bench grinder vibrates, try running the grinder without the wheels. If it still vibrates then of course it is the grinder that's the problem. Otherwise, you need to balance your grinding wheels. I have wondered about this problem for many years. In this article I found out two ways to balance grinding wheels. Both are practical and straightforward.
The first approach I came up with myself. I've heard about beads that they sell to put into truck tires, and about the rings they put around modern washing machines partly filled with water. These are designed to automatically balance the system they are installed in. Here is a quick overview of how a self-balancing system of this type works. Any rotating mass rotates around its center of gravity. If that center of gravity coincides with the center of the axis of rotation, the system is balanced. If not, there is a heavy end and a light end, and the system will be unbalanced. The center of rotation will be towards the heavy end. So the heavy end's arm of rotation is shorter than the light end's. Since the moving weights are free to move, they will move towards the longer rotation arm; i.e. towards the light end. If the mass of the moving weight is sufficient, the system will balance automatically.
I found this concept very appealing. I wouldn't have to buy or make a balancing stand, and I wouldn't have to patiently balance the wheel somehow. So I came up with a sketch and ground up a lathe bit and here's what happened:
Here is the form tool I ground (freehand):
Here is a piece of aluminum held in soft jaws, with a circular groove:
Here are a pair of self-balancing washers. The matching circular grooves form a torus (donut) shaped void inside the washer pair.
Here are some bearing balls in the groove, with some light grease:
I cut open an old bearing and got a small handful of bearing balls 0.219" in diameter. They roll freely around the circular groove when it's clean. I used very light grease in varying amounts to act as a damper. Eagerly, I mounted the autobalancing setup on the grinder next to one of my imbalanced wheels. I turned on the grinder and was disappointed to find that the vibration seemed about the same with or without the autobalancing washers and balls. I started with just a few tiny dabs of grease and tried several times, each time adding a bit more grease to increase the damping. It just didn't seem to work at all. I had been hoping to avoid having to explicitly balance the wheels, but now there wasn't any choice, so I moved on to plan B.
Years ago I had a motor from an import drill press that vibrated excessively. I bought a piece of ground flat tool steel and used my disc sander to grind one edge to form a pair of "balancing knives". They worked well to balance that motor, and I decided to pull them out and use them again. The best way, of course, to check a wheel for balance, is to use a commercial balancing stand like this one. Since I don't have one, and don't really have room to store one, I put one together as needed. I carefully level a small surface plate. Then I dig out four matched 1-2-3 blocks with holes threaded 3/8-16. I use bolts and washers to clamp the knives loosely. Then I carefully level the knives themselves. Here is what that looks like:
Notice the cigarette paper shims. I made up an arbor from 5/8" drill rod. I decided to make the arbor spring-loaded, so I used a compression spring which butts against a spring roll pin. This keeps the arbor assembly together by friction. At the other end, I use a ground pin that slips through a hole. So I put the arbor together, compress the spring, and insert the ground pin. Then the spring tension keeps everything in place.
My idea was to use teardrop-style balancing washers. an idea I got from this web posting.
Anyway, I designed a pair of teardrop-style balancing washers and made them from 16 gauge sheet steel. Here are a few pictures:
A pair of teardrop balancing washers drawn in layout ink:
then with 5/8" holes punched and hand reamed to a close fit on the arbor:
with the excess sheared away:
then sanded to line with a 2x72" belt grinder:
then stamped with identifying numbers (I'm a lousy stamper)
With the arbor and pair of balancing washers fabricated, I mounted and balanced a grinding wheel using the washers. The balancing knives/1-2-3 blocks setup worked very well. In the beginning, I put the wheel with no balancing washers on the arbor. The imbalance makes the wheel roll all by itself, and it oscillates back and forth until the heavy spot settles in the lowest position. Then I mark the light side, and then install the balancing washers and start moving the tips symmetrically towards the light side mark. The spring introduces enough friction to keep the washers in place but lightly enough so I can still move them. It takes just a few minutes. When you get to where you give the wheel your lightest touch and it moves part of a revolution and then just stops and doesn't start back in the other direction, the wheel is balanced. Then I mark the location of the balance washers with a sharpie:
and install the balanced wheel on the grinder. Here is my worst wheel with its washers:
and here is the other one:
As you can see, the worse the wheel is, the closer together the points of the balancing washer are to balance it. By the way, I have seen guys on the Web telling young guys if their grinder vibrates they can solve the problem by buying a Norton grinding wheel. Don't you believe it! The white wheel shown above is a Norton, and as you can see it needed a fair amount to balance it.
This method worked fine. It does involve a balancing fixture and the making of a balancing arbor as well as a pair of teardrop washers for every wheel you want to balance, but as long as there is enough eccentric mass to counter the imbalance in the wheel, it will always work. I found that I do have to use a little more force when tightening the nuts holding the wheel onto the grinder. Otherwise the balancing washers can move. Once you've balanced a wheel and marked the position and numbers of the teardrop washers, though, you can remove the wheel and washers and quickly replace them and it will still be balanced.
After I got the grinder balanced, I did a test grind on a drill bit and found that the 60 grit Norton wheel was too coarse. A pro machinist told me recently that Radiac makes good wheels, so I bought one of those in 100 grit. It was about as out of balance as the Norton. I set up the balancing jig, put the wheel and the teardrop washers on the balancing arbor, and had it balanced in about 12 minutes. So my conclusion is that this approach works but that it's a bit slow and fiddly.
Anyway, even though the teardrop method was more work, the grinder ran just fine. I'm no videographer, but I shot a little video so you can see for yourself.
With the grinder finally balanced to my satisfaction, I reflected for awhile on why the self-balancing (aka autobalancing) washer design didn't work. Having once been an engineer, I did a fair amount of shop math, calculating the net moment of inertia of nine balls spread around a circular groove with a 7/8" radius. I came up with a moment of 4.58. To put this a different way, an ideal mass of 6.53 grams at a radius of 0.70" from the center of rotation would behave the same way as the nine balls. I estimated the moment of the pair of balance washers for the worst case (where the tips are both pointing exactly at the light spot; i.e. the washers coincide) and came up with a moment of 4.49, equivalent to a mass of 5.99 grams at a distance of 0.75" from the spindle axis. Comparing, my nine balancing balls should have been able to balance a grinding wheel.
Of course, when I had tried it before, the self-balancing setup did not work. However, this was with light grease applied to the circular groove. I guessed that the grease was preventing the balls from moving. With nothing to lose, I cleaned off all the grease and put the self-balancing washers on the spindle with the white wheel in place of the teardrop washers. Lo and behold, it actually worked! Hooray! Emboldened by the success of my first video (above) I shot another one.
I think the self-balancing washers work slightly better than the teardrop ones. However, after shooting the above video where they worked, I turned the grinder off and knocked off for the day. The next morning I turned it back on and it vibrated again.
I figured that my design of the self-balancing setup was in some way marginal. In other words, that the design wasn't correct enough to work every time. So I returned to my other balancing method. However, after I got the teardrop-shaped washers to work for me, after some point they stopped working too and I realized there was something else going wrong, something not related to unbalanced grinding wheels. Eventually I figured out that my grinder had a "soft foot". It was mounted on a table that, coming from a random piece of 3/8" plate wasn't quite flat. When the grinder was bolted down it torqued the case of the grinder just enough to induce vibration. I'm not sure why, but it didn't really matter. I shimmed between the grinder's MDF block and the table until I could tighten the grinder's mounting bolts and have it still run smoothly. Then I figured I was finally done. But then I realized that maybe now my self-balancing washers would work after all and it turned out they do work and work just great, time after time.
I believe that a self-balancing system will work if it meets two requirements: the moving weights must move freely enough so that the forces on them in an unbalanced condition cause them to actually move, and also the moving weights must have sufficient mass to counter the wheel's imbalance. If those two conditions are met, I believe that such a system will indeed balance itself.
It is wonderfully quick and convenient when installing a new grinding wheel to just mount a pair of self-balancing washers along with it. Then you don't have to worry at all about balancing that new wheel. You can just turn on your banch grinder and it will run nice and smoothly all by itself.
Here are some notes about the fabrication of the self-balancing washers. I made them from 3/8" aluminum plate that was quite flat. I just sheared away excess metal until I had an irregular piece a bit larger than my desired size. Then I used a drill press to drill the approximate center with a center drill.
I put the faceplate on my lathe. I put a live center in the tailstock and used it to push the part against the faceplate. I used a small piece of wood behind the part to protect the faceplate. The part was just held by friction and the live center point in the center hole. Then it was easy to turn the outside diameter to size. After repeating this for both washers, I put my 3-jaw back on the lathe and installed soft jaws. With the soft jaws bored to size, I could hold the part by its edges and still have it be on center. Then I drilled and bored the 5/8" arbor hole in the center and used the form tool I ground to turn the circular groove.
I ground the form tool by hand. It has a 1/8" radius on its tip. It cut into the aluminum easily and cleanly. Although theoretically I should have been able to use 1/4" balls, in practice they didn't roll freely due to minor imperfections in my form tool grinding. So as I mentioned above I used 0.219" balls from a used ball bearing. They are a little rattly but they move very freely and in fact work fine to balance a grinding wheel.
In retrospect, it's roughly the same amount of work to make the self-balancing washers as it is to make the teardrop washers. Also, you only have to go through the balancing process for the teardrop washers one time for a particular grinding wheel. After that it's just as quick to install either system. So I like both ways and which way you decide to use is up to you.
Thank you for reading this! This was a fun project and I learned quite a lot.