Re: ATM First posting - making a washer covered tool

["Bill T." <twentiethwave@hotmail.com>]

> This as my first posting (although I've had a look at the list archives
from
> time to time) and I've just started grinding my first mirror, a 5" f10
made
> from plate glass. As there seems to be have been quite a bit of discussion
> about making tools I thought I'd add how I made a washer covered tool. I
cut
> two disks of fairly thick MDF, one disk I covered with a thin layer of
> plasticine, then covered that with a layer of plastic food wrap. I
squeezed
> the disks together in a vice to flatten the surface of the plasticine then
> peeled off the plastic and stuck on a sheet of sandpaper which I trimmed
> back to the edges of the disk. I used the sandpaper covered disk to sand
the
> other disk with a centre over edge action. This sanded the bottom disk to
a
> reasonably spherical shape while the sandpaper and plasticine were slowly
> stretched and squeezed to a shallow curve. The stiffness of the sandpaper
> seemed to stop the plasticine deforming too much. I then epoxied on a
packet
> of steel washers and varnished the edges and back to seal them from
> moisture. So far it seems to have worked well - all the washers are making
> full contact except for one near the edge which only seems to be making
> about 50% contact (although this is slowly increasing as I keep grinding).
I
> was inspired by Steve Lee's web page to try this method.

Nick,

Pretty ingenious!

One thing to look out for with soft and/or lightweight tool substrates is
that the tool's curve can get very slightly distorted in proportion to the
weight applied, so that the curve imprinted on the mirror may vary during
the grinding -- sometimes a sphere, sometimes a zoney horror.

You might want to end up your grinding MOT, with the tool on a piece of rug
or packing foam, with movement pressure applied only from the sides of the
mirror with your fingers.  You should probably place a one or two pound
weight on the back of the mirror, preferably something round and flat.  Do a
series of Sharpie tests until the marks disappear going quickly from the
center out.  Don't try to "extend" the grits used for the final wet.  I'm
sure you'll have no problem as long as the applied weight is reasonably
consistent by whatever means.

This may not be a big issue with a 5", but I have (painfully!) seen regrinds
arise from using less than rigid grinding substrates on large mirrors.

Bill T.