Re: ATM Flat edges during hogging out

[Cary Chleborad <bennu@ns.net>]

Nik Cain wrote:

> ...I carried on, and got the ground part
> to almost half an inch from the edge, but now the sagitta is way past
> what I wanted. I then did COC with the tool on top, thinking that
> this would still grind the mirror to the edges, while bringing the
> sagitta back to what I wanted. I did about three wets, and then
> wondered if this was wrong, and decided to write here before carrying
> on. I'm only testing the curvature with feeler guages, since I
> couldn't find a 12 foot long stick to make a template. Should I be
> worried about not making a sphere, or more to the point, worried
> about not knowing whether I've achieved a sphere?
>

Ok...at this point go with TOT 1/3 w strokes, this will extend the
ground area to the edges and reduce the ROC.  This will also form a
sphere.  Don't forget to keep the mirror turning too.

> On another point, it took me a while to realise I was using grit that
> was too wet, and washing a lot out. This meant I've virtually run out
> of 80 grade SiC now. Since most of the grit at the bottom of my
> washtub is probably still good, can I reclaim it by floating out the
> rubbish?

You could just scrape it up and use it as is, washing it is better
though.  Also you can probaply find some grit locally, check to see if
you have a lapidary (rock polishing)shop in your town, they'll have sic
grit in bulk.  Another source may be a welding supply or air-tool shop,
they'll know it as sand blasting grit.

-- 


-Cary Chleborad                 Sacramento, CA
Sacramento Valley Astronomical Society - http://www.calweb.com/~svas/