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I’m around 10 hours into polishing, and it looks like I’m very close to polished out! I can see barely any tiny pits under a microscope. I think I’m done.
Next up, figuring. I can test a mirror’s shape easily. I have been doing that already despite not quite being done with polishing.
The problem with a thin mirror is that glass (and anything else) will happily bend a few hundred nanometers under its own weight.
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Polishing continues. I’ve noticed that my short center over center strokes seem to be creating a hill in the center of this mirror, so I’ve switched to longer 1/2-diameter strokes and 1/3 side to side motion in order to wear down the center of the mirror. It seems to be working, according to these ronchi test images before and after. I’m jumping the gun a little by figuring while polishing but there’s a long way to go so I should be fine.
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My pitch lap polishing tool wasn’t making good contact with the center of my mirror. I can tell because I’m just beginning polishing and so I can see that the mirror’s outside is more reflective than the inside. Also, the tool looks unevenly worn.
I tried heating up the tool using a tub of hot water so the pitch would flow and then pressing the tool against the mirror some more (pictured) but it seems to have only helped slightly.
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I finally started polishing this 12" mirror! It has taken so long and many failures to get here. I took the pitch lap tool that I made and carved some bonus channels into it, squirted on cerium oxide mixed with water, and got polishing. An hour and a half later, the outside of my mirror is polished and now is smooth enough to reflect light! It looks like my pitch lap wasn’t pressed for long rnough to make good contact with the mirror, though, because I can see that only a ring around the outside is being worn down, not the center.
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I ran out of leftover 12 micron, so I’m going to 5 micron. The longer scratch seems to be entirely gone, and the short scratch is now two tiny slices less than a millimeter long. Hopefully it’ll grind out.
Current cumulative grinding time: 46 hours.
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Went back to 12 micron grinding. 1 hour and 45 minutes later, the scratches are now little dots spaced along a line. There aren’t any more lines trailing in from the edge, at least, so I hope that counts as a success for my beveling.
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Tragedy!
I thought beveling the edge would help, but… I see scratches. Again. And only after 20 minutes of grinding. (I drew pencil marks next to them for visibility). Long, too.
Maybe my sanding made particles of 400 grit fly into my tool? Aaaaargh
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I bought some 400 grit alox sandpaper and glued it to a failed 3D print to act as a bootleg grinding stone. Hopefully this will stop the bevel from introducing particles.
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Another hour of 5 micron grinding and, unlike the first time, no scratches!
But then I looked closer. By using my phone flashlight, I saw one very thin and not very deep scratch (a “sleek”). That’s fine. I also saw some thicker scratches around 1-2mm long intersecting the edge - see the photo.
Maybe those marks are from bits of glass that were torn from the beveled edge? This adds credence to my theory that my previous scratch problems were bevel problems, not a speck of coarse grit contaminating.
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Mirror grinding with the newly refurbished tool is going extremely well! After two hours I ran out of 30 micron aluminum oxide grit and moved on to 12 micron. (You can move on when you’ve eliminated pits from the previous bigger grit size, but since previously I was at 5 micron, I didn’t have any pits and could move on whenever I wanted, but I also wanted to grind the tool as smooth as possible).
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