Re: [ATM] DC Motor for grinding machine question

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Don Clement wrote:
> A periodic component of a polishing machine will manifest itself in
> the surface finish even if there are "multiple passes". Length of
> time only reinforces a  periodic components effect on surface 
> finish.

This is true if the stroke arm and turntable exhibit periodicity, that
is, the strokes repeat on the same parts of the mirror.  This
periodicity is avoided by proper machine design.

> The bearings for a polishing machine turntable should not add a 
> periodic component.

I agree with that.

> The regular arraignment of the balls in a ball bearing race can add
> a periodic component to the surface finish.  Even the artifacts 
> from the periodic arraignment of a ball bearing race can show up in
> surface finish.

You're contradicting your above statement.  I don't believe these
artificts make their way onto the mirror's finish.  There is enough
slop built into the typical machine arm, the pin and lap backer, and
the compliant layer under the mirror to absorb any slight vibration
coming from the bearings.

> The "non-issue" caveat of "motion is sufficiently random" is the
> key and is why we don't typically see amateur polishing machines
> replacing hand polishing.

Amateur machines are completely capable of making a good mirror, 
provided they are operated correctly.  I think the main reason most 
amateurs stick to hand work is that they don't need the time and labor 
savings that a machine can provide, and they enjoy the act of pushing 
glass by hand.  They also may not want to re-learn mirror making a 
second time, which is required to learn to work with a machine.

My small grinding/polishing machine uses a 1/3 HP DC motor powered by 
a variac and a full-bridge rectifier.  It operates quietly and 
smoothly, and it was inexpensive.  The larger machine I use has a 1 HP 
motor and a PWM speed control.

	Mike Lockwood

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