grinding machine design elementals

stephend · #1 22-06-2009, 03:54 PM

When I hand-ground a mirror once upon a time, it wasn't too difficult, but that was a 6" mirror. I want to make a machine to grind a 20" mirror.

If I remember rightly, I circled the circular bench every 10 mins or so (motion A), and each back and forward stroke took say two seconds (motion B).

Motion (A) just repositioned the stroke, and motion (B) did the work. To make a motorised machine to do something similar would be fairly easy.

Would a two motor machine be better? One motor rotates the mirror blank (A), a second one moves the tool back and forth (B).

In this case, could (A) be much faster, say 10 rpm, and contribute most of the energy needed. And could (B) therefore be a much shorter stroke, because the blank is turning under it? In fact, could the tool driven by (B) be much smaller than the mirror blank?

I think having two motors should make it easier to avoid patterns and keep the natural randomness of hand grinding. Any comments much appreciated.

cristian abarca · #2 22-06-2009, 04:19 PM

Hi Stephenend http://www.oblivion.net/~astro/ click on the triangle in the black bit at the top of the page to turn to the following page. Here is another one http://www.mirror-o-matic.com/machines.html and another one
http://www.mirror-o-matic.com/atm.html

Regards Cristian

AstroJunk (Jonathan) · #3 22-06-2009, 04:20 PM

This will do you. http://mirror-o-matic.com/

There are a few designs about and most look easy enough to do. There is no way I'm going to hog out and grind another mirror from scratch again, but it would be fun to use one of these.

Good luck!

stephend · #4 22-06-2009, 05:56 PM

Thanks guys

I'll send off a cheque to Dennis at mirr-o-matic and get the CD. He's a nice bloke isn't he, virtually giving away his plans. I still wonder if two separate motors might have advantages.

Also I'm intrigued that he suggests making the tool surface out of bathroom tiles; I thought it would be necessary to have tool and mirror with equal hardness.

Cheers

Satchmo · #5 22-06-2009, 06:36 PM

Stephen, I would recommend that you make a smaller mirror first , say an 8" and then a 12" by hand before you even consider a larger mirror by machine. Working with machines is a bit like 'knitting' by remote control. If you don't allready have an intimate feel of the look , sound and smell of a polishing tool that is in good contact with a mirror , you would have no hope of learning how to figure a 20" mirror with a machine. In any case , the best opticians I know use both machine and hand polishing to make mirrors to perfection.

Regards the tool materials, go for the hard porcelain white tiles , not the soft clay tiles with porcelian finish.

AstroJunk (Jonathan) · #6 22-06-2009, 06:44 PM

I think that they are similar - around 7 on Mohs scale.

Whilst you can probably find everything you need on the internet about grinding, polishing, figuring and testing, there is a wonderful series of books:

http://www.amazon.com/Amateur-Telesc.../dp/0943396484

in three volumes. The originals were published in the 1930's when they knew how to do this stuff! They are a joy to read...

(I know that Sirius Optics have a set in stock - I saw them there last weekend!)

Bolts_Tweed (Mark) · #7 25-06-2009, 08:31 PM

Gday - I built an Elgin machine a few years ago (well about 10 or 15 to be honest) and it happily sits outside now as the ugliest garden gnome you have ever seen (gotta dump that thing).

This does not detract from its performance - it was fatastic. I even polished to spherical with it and then hand figured on a drum.

I had two windscreen wiper motors driving both the blank and the mirror on top. Rotation speed of the tool was controlled with a little pulse width modulation unit I built which allowed varying the speed to avoid patterns. The rocking arm that moved the mirror was connect to a disk on the drive motor with a series of holes that allowed variation of stroke depending on mirror size.

I never did a mirror above 12 incjes but. W/W motors have a mile of torque but so I dont think weight would be a problem.

Anyway - it was cheap to build and very efficient and would be worth considering.

Mark

stephend · #8 25-06-2009, 09:39 PM

Thanks,

Satchmo raises the issue of grinding vs finishing. As I understand it, grinding produces a spherical surface. The longer you grind, the smaller the radius of the sphere. The surfaces obey universal entropy; spherical is the profile of lowest energy they can mutually assume. It's easy to understand and fairly easy to do. This I have done.

Finishing turns this cross-section into a parabola. I haven't ever done this. Can someone explain what principle is involved in turning the spherical profile into a parabolic one. Also, what sort of work. I gather it can be done quite quickly but takes some skill.

Astrojunk, those books sound great ... it's just the ... cost!

Bolts_Tweed, if that Elgin machine worked, what's it doing in the garden collecting rust? Why aren't you grinding mirrors with it?! Must say I'm surprised a car windscreen wiper motor has enough power. Ingenious!

Cheers

Geoff45 (Geoff) · #9 26-06-2009, 06:42 PM

Quote:

Originally Posted by stephend

Thanks,

Satchmo raises the issue of grinding vs finishing. As I understand it, grinding produces a spherical surface. The longer you grind, the smaller the radius of the sphere. The surfaces obey universal entropy; spherical is the profile of lowest energy they can mutually assume. It's easy to understand and fairly easy to do. This I have done.

Finishing turns this cross-section into a parabola. I haven't ever done this. Can someone explain what principle is involved in turning the spherical profile into a parabolic one. Also, what sort of work. I gather it can be done quite quickly but takes some skill.

Astrojunk, those books sound great ... it's just the ... cost!

Bolts_Tweed, if that Elgin machine worked, what's it doing in the garden collecting rust? Why aren't you grinding mirrors with it?! Must say I'm surprised a car windscreen wiper motor has enough power. Ingenious!

Cheers

Three ways
1. Deepen the centre
2. take material off the edge
3. Do a bit of both.

Method 1 is the "classical" way of doing it by lengthening the polishing strokes so that the centre gets more work done on it and so deepens.

AstroJunk (Jonathan) · #10 26-06-2009, 07:45 PM

Quote:

Originally Posted by stephend

Astrojunk, those books sound great ... it's just the ... cost!

Nobody loves these books any more - get the originals for next to nothing on e-bay.

Roger Davis · #11 14-07-2009, 02:22 PM

You could try "Advanced Telescope Making Techniques" by MacIntosh. There are several grinding machines plans given as examples.
I too would recommend the making of a couple of smaller mirrors to get the feel of how things "blend" when using a machine. I used to remove the arm from my machine and finish off the polishing by hand on the rotating table. You will have to learn a bit about sub-diameter laps as a 20" pitch lap is not the easiest to lift and push around. See Sachmo's comments and take it from those who have "been there and done that" many times over.

LeoLion (Len) · #12 14-07-2009, 06:35 PM

Hi from the UK . The mirror making machine related posting pointed me back a few years . A couple of Liverpool lads had (if memory served me right) used an angle grinder on an arm to hog out a BIG mirror . It was usable (but astigmatic) and their old TROK website had gone. They have moved on to newer BIG scopes , methods and a new website .
The bit reffering to the new grinding/polishing machines is here:- http://datscope.wikispaces.com/New+grinding+machine
Plenty of interest to me on their website . You may find the same !

Best wishes ,
Len E