surface finish problems

I'm running my 1953 Reid 6 X 18 grinder with a 5hp Phase-A-Matic converter I bought used. It was made around 1991. I'm having trouble getting a good surface finish and want to eliminate everything I can before I consider spending $2K for a rebuilt spindle. One possibility is a problem with the phase converter. Is there a simple (with a voltmeter) way to check the output to the motor to see that the phases are balanced? Chuck is flat & spindle runout is under 2 tenths. I'm also about to install a VFD just in front of the motor to get the sfm down to a reasonable speed. I'd appreciate and and all advice.
Bill McConnell

Please don't be insulted if this is too basic, your question indicates you might not have tried it before. With it off/disconnected, take the face off your starter. Do a dry run at L1, L2, L3 (I'm assuming these will be the supply from your convertor). What you are going to do is probe across L1 - L2 L1 - L3, & finally L2 - L3. I'm suggesting a dry run so you can be familiar with the open box, what you are going to check, and hopefully where not to put your hands when it is hot.

Turn the phase convertor on, and turn the grinder spindle on. (the balance will change depending on the load) Now check as above across each terminal, and verify the voltages across each pair are within 10% of each other. Hopefully better.

FWIW, I doubt that a minor "unbalanced" voltage condition will affect the finish significantly. Unlike the pulsing or resonant effect that can occur from the line frequency with single phase, "unbalanced" 3 phase is still a continuous effect on the rotor. Finish problems are more likely to be from an unbalanced wheel, or a poorly dressed one. If you dress the wheel, then turn the grinder off and set up the work, then turn it back on again, the inertia can shift the wheel enough to negate the effects of dressing.

smt

I run a DoAll 6X12 from a static (read capacitor starting) phase converter without surface finish difficulty.

I'm not sure if this relates to your issue, but speaks to electrical supply issues.

CalG

Try letting the spindle run for at least half an hour, then true/dress the wheel, then grind without stopping the spindle and see if it is any better.

Sometimes the spindle can center up differently when the bearings warm up a little. If you dress the wheel with cold bearings, things can change as the spindle warms up.

Hello Caliper I had the same problem as you for years I got a poor finish on my surface grinder tried every thing new spindle bearings ,balancing still poor finish. I was using a phasematic then tried a rotery phase converter it helped a little. Then I got rid of the phase converters and put a vfd on single phase in and 3 phase out and my finish was like a mirror.

Tim

Not enough data for a meaningful answer, but...

caliper said:

I'm having trouble getting a good surface finish and want to eliminate everything I can before I consider spending $2K for a rebuilt spindle.

Click to expand...

What problem does the finish exhibit? Most grinder handbooks and machining college texts, have a series of photos describing finish problems and their likely causes: what kind of wheel are you using; what material are you grinding; do you allow the spindle to come up to operating temp. first; are you using flood coolant; if flood coolant, do you shut the coolant off first and allow the wheel to wring itself dry, before shutting down the spindle; are you dressing the wheel before you grind; have you removed the mag. chuck and cleaned underneath; is the chuck bolted down tight; have you reground the chuck; etc.

While its not the most advanced of fixes, dont forget to check drive belt tension and that its not too stiff from age. I currently have that a bit as my grinder sits out in the cold, you can feel it turning the stone over by handthe belts taken a set ands a bit stiff. A new belt when i can get around to it should be a easy fix.

Surface Grinder finish problem

Thanks all for the useful information. I checked the voltage balance and found it on the ragged edge of acceptable. I've bought a vfd (Action Master X44-33364; 3-phase in, 3 out) which I'll place just ahead of the motor. This unit will allow speed variation (via frequency variation which is linear to rpm variation). It will also allow slow start and remove the voltage variations (I'm told by the tech support guy). I'm aware of all the other potential problems and I'm working them out one by one.
Thanks,
Bill McConnell