Late night musings...

[Dons255]

Well, more like early morning - so please keep the laughter quiet

Not that I've gotten the bug or anything you understand, but having spent some time on here reading about different aspects of making one's own scope(s) a couple of thoughts occur and I'd like your opinions (or an explanation of why it can't work where appropriate).

Mirror grinding; Why not create a plunge grinder for rough grinding / polishing?

e.g. (and apologies if incoherent, rather tired)... take an appropriate sized piece of strong ply, drill a small hole in the middle. Insert tube through hole and overlay a piece of rubber, gluing the edges to the ply.

get/manufacture a 'ring mould' the same size as your desired lens, centre over the hole in ply, clamp down firmly (water tight).

Having calculated the desired saggitus, inject through the pipe water or oil creating a nice parabola. Grease top surface and fill ring mould with plaster of paris / concrete, allow to set creating a negative of the required grinding tool.

Once set, deflate and remove negative, build dam around, line & grease, more concrete to create a grinding tool with the desired profile. Epoxy on tile fragments or mild steel washers to complete grinding surface.

Mount blank on potter's wheel, or similar, apply grit & water and lower tool to near blank (I'm envisioning a 'bench drill' type arrangement, to keep everything level), spin it up (slowly) and gently lower tool onto glass.

OK for roughing out? would such a system produce an acceptable finish for polishing (obviously by working down through grit size) or is some sort of eccentric motion necessary?

Am I mad, bored or a combination of both?

[NGC 1502]

Hi, well it's great to have ideas, & think of newer & better ways of

getting things done. If we had never done that, we would still be

living in caves.

But mirror making has a long history, and all the various methods

of achieving that 20th wave mirror surface (dream on !) have

been tried. So I do think that more conventional methods would

be best. But why not try your ideas out ? You never know, you

could teach some old dogs new tricks !

Cheers, Ed.

[FraserClarke]

I'm not quite sure I understand the details of your concept; seems you want to just want to centre the tool on the mirror, and grind away by spinning the tool? Is so, then no, that won't work -- you'll end up with a horribly 'zoned' mirror because the same bit of tool always works on the same bit of mirror. One important thing in mirror making is to have random variation in the motion of the tool over the mirror. If you have regular patterns of motion such that the same bit of tool always passes over the same bit of mirror, you'll end up grinding structures into the mirror surface -- not what you want.

However, your idea is not dissimilar to 'spin polishing', where a sub-diameter tool is held off-centre on the mirror, and the mirror spun. The tool is also free to rotate, and the relative motion between the mirror and the tool causes the right polishing/grinding action. Look up 'spin polishing' on the google and you'll find some videos explaining better than I can in words...

[Dons255]

Words were my failing too

Yes, I had wondered about just grinding out / polishing and thought it'd probably be a no-goer but couldn't think why, thanks for your explanation - off to google spin polishing