Mirror Grinding - School Project

[saac]

Major milestone achieved - we have now passed the 1mm mark! Yes I believe in celebrating small successes! We've done approximately 3 hours of rough grinding so far – seeing as we are using a full size tool and 180 grit I guess we can expect progress to be a bit slower than had we used a sub tool or larger grit size. Having said that, I’m finding the whole thing surprisingly adicative. I’ve been grinding at home this weekend using the steel drum setup; definitely steadier than the plastic drum we are using at school. I’ve also played around building a home made spherometer; we make passing reference to them in the Advanced Higher Physics course at school so I thought it would be a good opportunity to show how they are used in anger. Quite pleased with the result, although I can’t fully reconcile the measured depth of cut taken by the spherometer (the local sagitta) and that measured across the full face diameter using feeler gauge and straight edge. I suspect I need to be more accurate in measuring the distance between the spherometer’s ball feet.

Apart from allowing calculation of the developing radius of curvature the spherometer has been useful to confirm how the sphericity is shaping up. I hope I’ve interpreted the results correctly (see video) but I’m assuming that the consistent deflection on the spherometer across the blank face indicates a uniform curve. If anyone knows otherwise then please let me know. The curve however does seem to be taking its time to progress fully across the whole circumference. The picture shows the pattern of area that has yet to develop the same “density” of frosting as the rest of the disc. I really hope its not early signs of astigmatism! I’m hoping it’s just taking time because of the 180 grit and full size tool. I’m off now to search for a supplier of 180 grit – think I’m using too much in each wet – still finding it hard to get the right consistency.

http://youtu.be/ETbnsl-IbAo

Jim

[saac]

Forgot to mention, if anyone is looking for an optical reference flat for their project then Argos have a pack of 4 granite place mats (13 mm thick) for £7.99. Watch out they are heavy!

jim

[Glasspusher]

Excellent stuff Jim, I am following with interest, hope this thread inspires others to have a go!

John

[kenmyers]

Not sure if you still need this help but to stop your barrel wobbling, their is a little known fact that if you turn your barrel 45 degrees, it will no longer wobble.

sounds rediculous but try it. IT DEFINATELY WORKS!

[saac]

Not sure if you still need this help but to stop your barrel wobbling, their is a little known fact that if you turn your barrel 45 degrees, it will no longer wobble.

sounds rediculous but try it. IT DEFINATELY WORKS!

I managed to sort out the wobble by placing some wedges around various points of the base. Thanks anyway for the tip.

Doing some more grinding tonight, hoping to get close to another mm off and to test hoe the spehericity is developing with the spherometer.

Jim

[Glasspusher]

Jim, after hogging in the curve with a chordal stroke you will have a curve deeper in the middle than the edge. Working centre thu centre with about 1/3 D stroke length will bring the mirror to sphericity.

John

[saac]

Jim, after hogging in the curve with a chordal stroke you will have a curve deeper in the middle than the edge. Working centre thu centre with about 1/3 D stroke length will bring the mirror to sphericity.

John

Thanks John, that ties in with what I'm seeing and from testing with the spherometer. At the moment we have just under 0.5 of our target sagitta (3.7mm). The curve has spread out to approx 1/3 of the mirror diameter with a less dense frosting of the glass around the circumference. Should I widen the curve now with the 1/3 D centre through centre strokes or do I wait until I’m closer to my final depth?

This weekend has been rather frustrating - I had intended on pushing on with a couple of hours on the glass but having got everything set up I discovered I had left the grit at work! I guess it had to happen sometime! I made use of the time instead by re dressing the beveled edge on the mirror and tool - the edge on the tool was getting particularly sharp. I also made a curvature template as suggested by the notes from Galvoptics. The template is simply cut to the same design target radius of curvature for the mirror (twice the focal length). The template is then placed on the blank to monitor the developing sphericity of the curve.

[saac]

Here's a video and some pictures of the curvature template. The notes from Galvoptics suggested cutting the template using tin snips but I couldn't get an accurate enough line nor an acceptable level of finish that way. In the end I used a router attached to a make shift trammel pivoted at a 3m radius. The router cut through the aluminium sheet (2mm) easily enough using about 3 passes with small cuts each time. If you are going to do this I would recommend a new router bit to avoid any snagging on the aluminium. After a light sanding to deburr I was pleased to see that the finish was perfect. I tested the template by drawing around it using a fine pen and sure enough the profile was smooth. I'm not entirely sure how useful the template will turn out to be but I'll give it a go - can't do any harm!

Mirror Curvature Template - YouTube

[Glasspusher]

Jim, If you continue with the chordal stroke until the mirror is ground to the edge and to the required sagitta then switch to centre thru centre stroke you will reduce the radius of curvature a little. This happens because the chordal stroke produces a curve deeper at the centre than the edge. Switching to centre thru centre stroke blends these curves to produce a spherical curve with a slightly shorter radius of curvature. The solution is to go a little deeper than required with the chordal stroke. Hope this helps.

John

[saac]

John that makes perfect sense, thanks again. I can see now that it would be near impossible for the curve to hit sagitta, be spherical and flow precisely to the edge all at the same time. Going that bit deeper means you are only chasing one variable at a time. I hope to push on to get the curve to depth by the end of this week. Then I guess the real work starts!

Jim

[saac]

Here's a short video showing how to maintain the bevel on the mirror and tool edge. It had become really noticeable how sharp the tool edge had become so I couldn't put it off any longer. Hesitation was soon overcome by blind faith and ignorance and some help from the Stellafane web site!

Bevelling Mirror Edge - YouTube