Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Try and avoid the edge, leaving a land around the periphery, just stagger the holes to avoid zoning. A twist drill is the best option, as the pitch peels out cleanly from the flutes, also the flutes are large and deep in a twist drill. John.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Here's the latest:
This time with a 3/8" twist bit. That did work better. The pitch rides up and accumulates. I pulled some off the bit twice. Almost no dust or chips. Not enough time tonight to try it out.
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Good luck when you do, you might have to experiment with hole distribution to avoid zoning. John
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Update:
For the time being I've gone back to a previously made harder full-size lap with square facets and hand work at 3 RPM. And had to lay off polishing for the past week or so.
The round hole or "perforated" lap was giving a smooth surface but maybe was just too soft for the particular setup I was trying to use it with? Or too much weight? Or too high a platter speed? Lots of variables not under enough control.
My plan for now is to get the mirror back to spherical and fully polished out before trying the sub diameter perforated lap again. It could be that the lap I'm using is a bit too hard and I may end up redoing that. If so I will try that one perforated.
In any case it seems like a method worth experimenting with - just maybe will show better results in the hands of someone more experienced and also with more time to devote...please stay tuned...
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Looks interesting! I might try it on my spindle lens grinding machine, I am doing some small work at the moment.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Update:
My full size square faceted lap seemed too hard so I melted that and the soft pitch from the sub diameter lap together and poured a new one. I used a 3/8" bit with about 1/2" spacing. Took about ten minutes to make the holes taking my time. Very little chipping or dust. A light scraping took off slight rims where they formed. For some reason they form on some holes and not others. Microfaceted with onion bag mesh.
I've been doing MOT, CoC and after a couple of ten minute sessions the figure is improving, I'd say predictably, finally. The lap feels good. Not much different than a square faceted one.
Anyone else experimenting with this?
Gary
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Update:
Not a lot to report; I've had even less time to work the past few weeks but do continue to make steady progress.
I've opened up the holes with the same setup as for drilling them. A couple times I chipped the surface around the hole -- probably advanced the bit too quickly -- but most were clean and didn't create any ridge. Aside from those chips, there really wasn't any appreciable dust as the pitch is heated and rides up on the bit. It does flake a bit when you break it off but that just falls straight down.
The lap -- microfaceted with onion bag mesh -- continues to give a smooth surface and I'm getting very close to spherical with the TDE gone or almost so.
Gary
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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One reason to channel the pitchis. The edge can flow out and if you channel the pitch in the center can also flow out .your hole way does not let this happen .i think .
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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So far the holes seem to close up evenly, the generous bevel is diminishing, and the wear on the microfaceting -- which really is a form of channeling, isn't it? -- appears to be even so I think the pitch is flowing. Probably not as much as with traditional channels but at least for the approximately 10 min sessions I've been doing contact seems to remain very good.
Gary
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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just the pich at the edge has nothing to keep it from flowing out where the pich in the center cannot move .If it works great //just a thought
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Looks good so far.
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