Mirror Grinding advice needed

[DavidH]

Hello All,

Having never ground a mirror I decided to give it a go as much for the educational value as anything. Obtained a 6" mirror kit fron telescopes and astronomy and am nearly half way with my rough grinding. However, at this stage, the mirror appears as if it is grinding faster in one direction than the other. If you look at the image below, you can see that the grinding pits extend right out to the edge at the bottom right and top left of the mirror and finish about 5mm in from the edge at the bottim left and top right.

My question is whether this will result in astigmatism in the final product, and what if anything I can do to correct it in future grinding. Sagitta measured in both directions appears to be about the same at around 0.030". I am aiming for around f5 which puts my final sagitta at 0.075" I think.

Regards,
David.

[cristian abarca]

Hi David It's good to see someone else trying mirror making. firstly what strokes are you doing to reach the depth required. If you are using 1/3 strokes then it might be an error in your technique. If you are using another method (Mirror on top and overhanging 50%) then try varying the steps you take.(if you are walking around a drum). If you are turning the mirrors instead again vary this a little. You want to make it as random as possible. By this I mean don't end up in the same place all the time. This is not a major problem because as you go through the fine grinding process the mirror will even itself out. If you are using 1/3 strokes then it will take you much longer to reach the depth. I would not be using this stroke to reach the depth. When you reach your depth, using the same grit change your stroke to 1/3 strokes for a number of wets this will even out any irregularities you may only have to do this for 10-20 minutes. Hope this helps. If you have any further questions regarding1/3 strokes or any thing else let me know.

Regards Cristian

[DavidH]

Thanks for the reply Cristian, it is reassuring to have advice from someone who has been there and done that.

At this stage I am using chordwise centre over edge strokes with 80 grit. Am following Texereau's book. Looking at the mirror closely, it may not be as bad as I thought. The areas which are ground to the edge are only just to the edge and I expect that the other areas will be there shortly. You and the book both indicate that minor figuring errors at this stage will be corrected with fine grinding and 1/3 W type strokes when the final depth has been reached.

Which brings me to another question. At what point do I start switching to finer grit? I was thinking maybe 5-10 thou short of the final depth.

Thanks for your help,
David.

[cristian abarca]

David you can stop at about 10 thou or go to the depth you require. As long as you are close it shouldn't be an issue. If you go over use the tool on top and if you are under use the mirror on top. You can keep alternating keeping control of the depth as you go along. Remember that the finer the grit the longer it will take to reach the correct depth.

Regards Cristian

[DavidH]

Thanks Cristian,

Your help is appreciated.

Regards,
David.

[bojan]

When I was doing this long time ago, at some stage the mirror and tool get stuck (I guess the vacuum developed in the space between, I think I forced it too much.
I panicked quite a bit, then I started to apply warm and cold water.... and they separated suddenly, I almost dropped the whole thing in the vanity....
Bojan

[DavidH]

Hi Bojan,

Yes, I am experiencing some of this too. Appears to be caused by the volume of the gap between the tool and mirror changing (increasing) as the mirror moves over the tool. If there is some air in the gap, you probably won't get it stuck completely, but if there is only water, you can get a vacuum seal that is hard to break. I have read elsewhere that this is best tackled by soaking the tool and mirror in a bucket of water for a short period and then attempting to slide the pieces apart.

Regards,
David.