Home made abrasive stones/tiles

[CharlP6]

I'm slowly indoctrinating myself to one day grind my own mirror, I just don't have the luxury of time and much less the space to do it at the moment.

As I read up on how things are done, the gears in my head automatically go into overdrive to think of easier ways to do tedious tasks. 

I was thinking if moulding my own grinding stones with specific grits and custom shapes and sizes and I don't find any evidence of it on the forums. The idea is to mix some grinding powder with an epoxy resin(much more grit than resin) then add a few drops of the required hardner and cast into desired moulds. Almost like the body filler you get for people too lazy to panelbeat to fill dents in cars.

My thinking is to make up few cement or plaster tools, each with its own roughness of grinding stones  set in it, as opposed to having one tool with ceramic tiles and having to make sure all of the courser grits are cleaned from the grooves between grit progressions. I'm not sure how readily available grinding stones would be and what it would cost, but I think this might be cheaper.

Not having any experience in mirror grinding at all, what would you foresee are the pitfalls/benefits of doing this? Why would or would you not make and use this? Obviously this should be tried and tested and optimised if it's any good, but that's the fun part.

 

P.S.

From my observation, amateur mirror making seems quite like the civil engineering and construction industry, innovation is slow and generally unrevolutionary and people seem to stick to the 'same-old-same-old'. In the modern times(strangely), people seem complacent with what's established. 

[Chriske]

The method you're planning is a very good idea, only it will not work at all. There are a few reasons why.

1- Tools made out of resin/abrasive are not stable enough.
2- The difference in expansion coefficient glass/tool differ to much.
3- Most important reason : at every point in the process the curves of all tools should be exactly the same as your mirror. So when you go from, say grit #320 to #240 and the curves do not match perfectly you'll end up scratching your mirror very badly. Making scratches you will be forced to go back to (again) another tool that doesn't fit, so more scratches.

4- this is only guessing : I try to imagine what would happen during grinding. Normally a slurry of grit/water is transported all the time between tool and mirror and in the end most of the slurry will be transported to the perimeter of the tool. But doing this with a resin tool, what happens with the resin particles..? Will the slurry water/grit/resin be transported to the perimeter as usual or will it rather stick to the mirror or just embed itself in the tool..?  That's for you to find out...  (Whatever you do, keep us informed...)

I'd stick to the old method if I were you, be it glass, ceramic tiles...

Chris

[Peter Drew]

Good to question techniques, without innovation nothing moves forward. However, mirror making has been going on for 100's of years and by now a better method would probably have surfaced. Modern commercially made mirrors are usually either moulded or diamond wheel pre- generated which cuts out most of the effort suffered by amateurs, the final stages still have to be fine ground and polished independently. The problem with your suggestion is that the mirror curve is a developing shape and depth, you would need a large number of pre-made tools to account for this, the grinding grit, despite its hardness, soon breaks down under pressure. 

[Astrobits]

It won't work.

Firstly, as Chris says, you need the tool curved to your final mirror curve so you need to generate a suitable mould in the first place.

Secondly, the normal abrasive grit, Silicon Carbide, breaks down to smaller bits so you would need to keep renewing the surface with fresh grit or the grinding action will very quickly grind to a halt ( pun intended ).

The only grit to remain for long enough is Diamond. This would be expensive, as would be the cost of all the resin needed to make half a dozen tools for the different grits.

Commercially, the curve is generated with diamond tools. A cup shaped tool with diamond grit edges will generate any curve you want. Look for Diamond concrete grinding discs in your local builders merchant. You will still need to use finer grits with a matching tool before you can polish the mirror.

Have you contacted the mirror making group in the Johannesburg ASSA centre? They can give you help and advice.   http://astronomyjhb.co.za/telescope-making-classes-atm/

Nigel

 

[Chriske]

If you want to go fast you should consider a sine-table. A while ago I tested it and this method works really fast.
In the movie you see the mirror is completely immersed in water. To allow smooth action during grinding the mirror is supported by a rotating table.
The coring drill is about 6" in diameter.  A 10" blank is hogged out to f/4 in about 10 to 15 minutes.
No need to use a large diameter coring tool as in the movie. It can also be done with a much smaller coring drill.

[CharlP6]

All the comments make a lot of sense, thanks for the insights. Just shows why you are the pros, and me just a pleb.

I didn't take the tool shape into account, which is obviously very important.

@Astrobits, thanks for referring me to the ASSA, I'll definitely give them a shout. I was wondering if there are any astronomy/atm clubs in SA.

5 hours ago, Chriske said:

If you want to go fast you should consider a sine-table. A while ago I tested it and this method works really fast.
In the movie you see the mirror is completely immersed in water. To allow smooth action during grinding the mirror is supported by a rotating table.
The coring drill is about 6" in diameter.  A 10" blank is hogged out to f/4 in about 10 to 15 minutes.
No need to use a large diameter coring tool as in the movie. It can also be done with a much smaller coring drill.

This looks looks like quite an appealing and satisfying grinding method, I first had to look up what a sine table is...

Well, I'll keep churning out crazy ideas, and maybe somewhere there will be a real diamond between all the grit

[Astrobits]

I don't know whereabouts in Gauteng you are but there is a group on the West Rand and one in Pretoria as well as the J'oburg lot. There might be more established since I left SA.

Check out the main ASSA web site.

Nigel

Edited June 2, 2017 by Astrobits