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Galloping Grinders and Chattering Cutters

Joe Gwinn

Nov 22, 2009
Boston, MA area
Prolog: Some ten years ago, I was asked why a brand new 8” Baldor model 333B dual buffer bounced and gyrated wildly, sufficient to prevent careful cleaning of rusted surfaces, even though the unit was bolted directly to a stout shop ~MDF benchtop. The motor itself weighs 55 pounds. It likely needed more mass (representing something like a cast iron stand) and more damping.

The vibration isolators arrived from McMaster (resembling their current part 93945K117) first, and so were installed. This completely solved the galloping problem, so the added weight was never installed. (It’s a 40# bit of 1” thick A36 plate, now residing in my steel stockpile.)

Later, a few years ago, I had been having great problems with a Dayton clone of the 7” Baldor bench grinder (roughly Baldor model 7306), bolted to a sheet-metal tool stand of reasonable weight. The wheels were white aluminum oxide made by Norton. That grinder just shook the planet, precluding precise work. Much time was spent on getting things exactly aligned and perpendicular, with low runout; this all helped, but not enough. The bouncing was too great to allow for proper diamond dressing of the wheels – dressing made the wheels become less and less round.

Recently, the synapses at long last clicked – these two stories have a common cause. So, I bought some vibration isolators (McMaster model 93945K124) and installed them between grinder base and the shop-made poplar top of the tool stand. Success! It still vibrates a bit, but it does not gyrate or gallop, and precise dressing and work is now easy.

The remaining vibration may be due to static imbalance; this can be fixed by adding a pair of eccentric weight washers between wheel and one flange, and adjusting for static balance. This is standard on larger grinders.

The various isolators are made from a soft rubber (Shore A 40 durometer), and are sized so that each isolator carries about a third to a fifth of its stated per-mount load capacity from its share of the weight of the grinder or buffer. The rubber is viscoelastic, and so the per-isolator load should be large enough that there will be significant alternating strain in the rubber body, to ensure sufficient dissipation of vibration energy.

So, what does all this have to do with tool chatter? Turns out that the mechanics are the same.

What we are seeing is a self-sustained oscillation where the uneven cutting force feeds back through the machine structure to make the cutting force even more uneven, until something else limits motion, and/or breaks.

The difference is frequency. With the buffer, the benchtop is the spring and the buffer motor (in the center of the bench) is the mass, and this assembly resonates at a few Hertz. With the grinder, the tool base is a resonant thick beam with grinder motor mass clamped to one end, that again resonates at a few Hertz.

With a lathe cutting tool, things are far stiffer, and so the lathe and/or tool holder is resonating somewhere in the audio range, causing audible chatter or squeal. Making the machine frame of cast iron (a lossy material) helps a lot. As does a damper of some kind built into a critical place.

I also have some galloping grinders with ordinary hard rubber feet. We don’t generally think of the feet as doing much beyond preventing sliding around, but there may be more to it than that. These feet are typically Shore A 60-70 durometer, far too hard for vibration isolation. I bet that replacement of the hard feet with larger softer feet (perhaps backed by a fender washer) will greatly help.

You learn something new every day.
I don't have anything is say about this,, but here they are.
I do remember visiting a customer and complementing a nice surface grinder they bought at auction, "Too bad it is scrap with a bad spindle"
I pulled the wheel and it ran as smooth as glass,
They were ready to call the scrapper and it was just an oil-logged wheel.
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