D.M. Procida wrote:
The Natural Philosopher wrote:

D.M. Procida wrote:
I'm restoring an old tape recorder. The capstan flywheel is very
lightly pitted. I'd like to polish this out.

What would be a suitable polish? Or if it's likely to be too hard
for a polish, could it perhaps be sanded first?

It will need to be the very finest abrasive paper, because it needs
to have an extremely smooth surface.

It doesnt have to be THAT smooth.

However the trouble is, that anything you take off it will slow the
tape down.

Speed it up, not slow it down.

However, I'd bet that it would make an unmeasurable difference. The
pitting's very, very light.

Th 'pro' approah wuld be to get the spindle off and make a new one,
or turn it down and fit a machined sleeve to it.

But for non critical applications, go to Halfords and get some wet
and dry of the finest grade - about 600 grit, cut strips and simply
wind round the capstan while running.

It's not the capstan, it's the flywheel, that needs attention.

It would help if you properly described what bit you are actually talking
about. Do you mean the peripheral surface of the part the tape wraps round,
against which it is held by a pinch wheel and thus pulled past the recording
heads?

If so then I'd leave it alone unless the pits have sharp edges which might
damage the tape. The capstan surface should be almost mirror smooth. Some of
the advice you've been given such as 180 grit it bleedin' absurd. 180 grit
is rough as a dog's arse. 1000 grit is what you'd call a fine grit but even
that, or in fact any grade of abrasive paper will destroy the original
finish. You get surfaces that smooth by fine polishing with things like high
speed felt mops containing very mild abrasives in a slurry form.

If you remove material and decrease the capstan's diameter then the tape
speed will slow down NOT speed up. This is hardly rocket surgery. Smaller
diameter = lower peripheral speed for a given diameter not the reverse. It's
the capstan that drives the tape not vice versa!

If you remove the pits then the tape will no longer run at the right speed
which is fairly pointless. You'll also never remove them without machining
even if they're only a few thou deep. You can't polish off large amounts of
metal like that by hand and have any hope of keeping the capstan truly
circular. I've spent half a lifetime polishing crankshaft journals in race
engines so I know what it takes to remove a given amount of metal and what
surface finish you end up with. Half a thou is a shed load to remove by any
sort of polishing even when you're rotating the part on a lathe. You're
normally only talking about the odd tenth of a thou or even fractions of a
tenth for the final polishing operation.

The most I'd do just to make sure the pits didn't have sharp edges is go
over the surface very lightly with a fine metal polish, or even toothpaste,
and a cloth. Try on an old capstan first to make sure that even this doesn't
just lead to scratches.
--
Dave Baker