Also, is it necessary to have a motor specially balanced for a surface
grinder? If so, is this usually specified when the motor is ordered or


I have a Rockwell "toolmakers" grinder which is a cross between a
surface grinder and a tool/cutter grinder. The manual states that
the motor is a special balance and is also dynamically balanced with
the pulley in place. They stress that changing the motor requires
the motor and pulley be rebalanced together.

Now, does that still apply to motors made today? I thought I read
here that today's motors are much better balanced than the motors
of yesteryears; even to the point that production motors are as
good as specially balanced motors of the past. I have no idea if
this is true or NOT.

I will say that the old Rockwell motors on my mill definately have
annoying resonances when ran at certain speeds with a VFD. The worst
one is the 1/2HP 1140RPM motor on the rockwell vertical head. This
motor has numerous resonances with just the pulley. However it runs
very smooth at high RPM.

My new VFD rated 2HP Leeson motor is perfectly smooth at all frequencys
with or without the belt running the spindle of the mill/drill. However
I have also have a 2HP leeson motor on my rockwell lathe and it
is smooth except below 15Hz where the drive system has some kind
of resonance. I believe this is a resonance in the big flat drive belts
used for the mechanical variable speed and it not directly related to
the motor. The drive belt is difficult to remove so I have never
tested this theory.

I have also noticed that older motors seem sensitive to carrier frequency.
Some of mine are quite noisy with low carrier frequencies. I have not
experimented to determine if the resonances are different with
different carrier frequencies. I try to use a low carrier freq with
older motors because I THINK high carrier freqs can cause motor heating
and I think the high freqs stress the winding harder. The leeson motors
run very smooth and quite with a high carrier freq (7.5-10K) There is
more noise from the motor with lower freqs but I don't remember
any resonances when I was experimenting in the past.

Mike, I have a static phase converter if you want to try running
your grinder with it.
chuck