Hogging out mirror using an angle grinder disc

[dark star]

I have started hogging out my 14 inch mirror. I am using 80 grit and a metal 6 inch telescope counterweight which should be comparatively fast.

However I read an article on line which recommends using a diamond encrusted angle grinder disc attached to a counterweight. The problem is that I know nothing about angle grinders and cannot decide which disc to buy. Can anyone advise on a particular disc or type of disc and where to buy it that would work for this?. The article says it is quite fast using this method so I would like to try it. I am not brave enough to use an actual angle grinder! 

The article is here: www.webtreatz.com/resources/Grinding_a_14inch_mirror.pdf‎

[crashtestdummy]

I would totally avoid this if you don't know what you are doing and I would doubt how regular you could get the curve using a grinder hand held..also you are better using a disc that isn't flat for grinding the front.

Hopefully someone with more knowledge than me will come along with a better oppinion

[Peter Drew]

I'm not sure if I've fully understood the OP. Could it be that the suggestion was to bond a diamond faced disc to a weight to act as the conventional tool, the premise being that the diamonds would not wear as the conventional tool would thereby speeding up the hogging out process. 

[Glasspusher]

By the time you have set it all up and worked out the best approach you would have had the curve ground in with 80 grit and a sub-diameter tool. There are pit falls with the method, and as said it is best avoided.

John

[dark star]

I probably didn't make myself clear. Peter was correct, the diamond disc would be bonded to the telescope weight and used as a sub-diameter tool, hand held. It would be used for hogging out to the required sagitta only. I would then make a tool conforming to the shape of the mirror for the rest of the grinding. I think that hogging out a 14 inch mirror to the correct sagitta will take a long time, even with a metal tool.

But I could be wrong, as this the first time I have ground such a large mirror. 

David

[billhinge]

When I was 16 in the late 70's we didn't have the internet and access to fancy tools so my first f6 8" pyrex mirror was hogged out by hand with a grinding stone (not a flat circular one but literally gouging out the centre of the glass by circular gouging ). Of course the gouged hole was not circular in depth ...

i didn't have a glass blank either so I used a circular slab of marble with #80 abrasive -  It was a bit bumpy at first, but after a while both glass and marble took on a perfectly circular shape and worked fine through to polishing and ended up a pretty decent mirror.

 I reckon your idea would work, but its your mirror though, don't know how much time it would save

[fwm891]

I probably didn't make myself clear. Peter was correct, the diamond disc would be bonded to the telescope weight and used as a sub-diameter tool, hand held. It would be used for hogging out to the required sagitta only. I would then make a tool conforming to the shape of the mirror for the rest of the grinding. I think that hogging out a 14 inch mirror to the correct sagitta will take a long time, even with a metal tool.

But I could be wrong, as this the first time I have ground such a large mirror. 

David

Glue some ceramic tiles to your weight and use that with the #80 grit, once down to sag you can wash the tile tool off and continue with the next grit size...