Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Ive had a mad idea for some time now about cutting relief channels into a pitch lap, but have never tried it. Instead of cutting channels, why not drill a series of holes spaced apart, in a grid formation, using a 1/2 inch drill driven by a the slow speed on a screw driving drill. This would be a lot easier to do than channeling. Comments please, and has anybody tried the same idea and actually used it.
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perfessor
sage
Reged: 12/30/07
Posts: 499
Loc: Northern Illinois
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Are you talking about walking the drill bit along, perhaps joining previously drilled individual holes? I would imagine that a lot of flaking would occur, and you wouldn’t get a clean cut. Then also, the bit would heat up, melt the pitch and completely foul the flutes of the bit. Maybe at that point, your heated bit would melt its way through the pitch to create a channel. Might as well use a soldering iron, in that case. So it doesn’t sound promising to me. But feel free to try it out and give us a report.
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Tom
"Don't always know what I'm talkin about"
8" f/7
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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John, that sounds like an interesting idea. Maybe a Forstner or custom bit would work, and soft pitch might need chilling?
I'm wondering if those holes might act like suction cups? If so maybe some light scratched channels between them would take care of that?
Maybe I'll experiment tonight.
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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I have actually tried a specimin pitch tool, cast flat on a glass plate, the pitch was about a 1/4 inch thick,I then drilled holes in rows to form a matrix, I did not drag the bit or connect the holes with channels. I spaced the holes about 3/4 inch apart, and the depth of the holes I kept to about 1/4 inch. The drill was 3/8 inch diameter. By using a slow speed of about 50 RPM I succeeded in drilling the holes without flaking or fracturing. The drilling heated up the pitch locally and prevented this from happening. The drill did clog but it made no difference. Afterwards I lightly scraped a steel rule over the holes to remove the burrs. Making a lap this way was easier than channeling, and it was quick. I have not yet tried it on an optic. The clog in the drill flukes was easily removed by a screwdriver. Subsequent dressing of the lap is easier too, as all you have to do is lightly go over the holes with the drill; the speed of the drill must be low.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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"I have not yet tried it on an optic."
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I think you'll be disappointed when you do. Probably have very high/inconsistent drag, but I might be wrong.
M.
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So many projects, so little time!
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Yeah! you are probably right.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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John,
I tried a 5/8" Forstner bit - with very gentle pressure - at about 600 RPM in the drill press and it seemed to work fine with the build up around the edge of the hole coming off easily.
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Gary, are you going to try it on a mirror polish, it would be interesting to see what happens when you test it, probably get a load of garsely ring zones. I am not making mirrors at the moment, so I cannot do this for myself, anyone else want to try? John.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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John,
I just did one cut as a test but it so happens I made a new lap last night and it's not channeled and I'm curious so may try it. If disk-shaped facets can work it would seem reasonable that round holes might also. This pitch is softer than what I tested though so may not cut nicely.
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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I await your verdict with bated breath. John.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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John,
Bearing in mind that I'm no expert, here's my report:
I made about 35 5/8" holes in a 6 1/2" lap using a Forstner bit in the drill press. It does melt the pitch pretty quickly but most of the pitch rides up on the bit and there's just a slight rim around the hole.
At first I cleaned the bit after one or two holes but then found I could drill a bunch and just pick off pieces from the bit without bothering to clean it.
So that only took a few minutes. Removing that rim is easy, either fingers or a piece of steel as you described. I put a board on the press table and a plastic bag on that to contain the pitch. Altogether not too messy. The pitch is on the soft side so I was surprised the holes came out well.
I pressed with mesh and scratched a few criss cross lines with a single edge blade then ran it fixed post style with my 9" mirror on the bottom at about 36 RPM with about 10lbs weight.
Because of the softness I could only do about 10 min before it needed the mesh and a press. I did 3 sessions and then redrilled some of the holes to open the up. They'd reduced to about half their original size. Fewer holes would have been better as I couldn't open all of them.
Anyway, one more session after that. As far as I can tell, the lap behaved pretty well. The mirror isn't quite polished out and had been very zoney but is considerably improved.
My concern about vacuum proved to be unwarranted.
I did a few strokes by hand and it felt very good. Perhaps mostly due to the soft pitch but still.
I'm going to continue with this at least a bit more.
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Gary that is encouraging, if you can complete the polish, and figure the mirror with the tool then we have something. The holes will not close up so rapidly with a harder pitch of course, but you have found that it is easier to rout out the holes than to re channel a standard lap. You can zonally tune a perforated lap by arranging the holes to reduce the surface area on parts of the lap, say more holes closer together at the center one third than the rest to reduce a torus, and vice versa to reduce a hump. I feel that one can fill up the holes with pitch and redrill then press far more easily than completely rechanneling. John.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Quote:
if you can complete the polish, and figure the mirror with the tool
Still a ways to go...
Here's what the lap looked like with 10 min of work after redrilling some holes.
A close-up showing the finish the bit leaves and also how the other holes look closed up somewhat.
And the bit after some of the accumulated pitch has been broken off.
I think I forgot to mention it earlier but my first session was with just the mesh microfaceting and no additional scratching. That seemed to work fine but since the pitch was a little too soft I wanted to extend the time a bit.
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Gary two of your pix fail to show. Here is a diagram of the perforated lap showing a regular polisher, and two ' tuned ' laps. The black and white holes are just for emphasis.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Gary your pix are now showing,it's about the result I am getting, spacing wise. John.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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John,
It's odd about pictures, sometimes they show up after waiting awhile, and like now, needed a screen refresh.
I'm thinking maybe 5/8" was a bit big for this lap so might try 3/8" which would make the variable spacing you describe easier to manage.
Also want to try hand turning the bit. That would be more work but less heat and easier to control for rechanneling. An end mill might work too and might be easier to remove the clog of pitch from.
Gary
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Kalle
newbie
Reged: 06/29/09
Posts: 2
Loc: Germany
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How messy was your action? What about pieces flying around?
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Kalle
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Quote:
How messy was your action?
Not bad. I had the plastic bag on a 12"x12" piece of scrap plywood, and that on the press table and just let the bag edges up an inch or so to contain the pieces. Mostly the pitch melted in contact with the bit. Certainly much less mess than with a blade, but not as neat as melting channels.
Quote:
What about pieces flying around?
Even with the hard pitch I tested first the pieces didn't go far.
Possibly a larger lap with 1" or bigger holes might be more of a problem.
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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No I had no problem with bits flying around as the bit tended to melt the pitch locally thus holding particles in place, the flukes are deep and broad and will allow a lot of pitch to stick to the drill, which can be peeled off. A 3/8 twist drill, as used in machine shops is about the right size, I use a Bosch drill/ screwdriver set on screwdriving, you can vary the speed by pressing the trigger in and out, The speed should be about 50RPM to make the holes. The drill is a stub about an inch long, a short drill allows better control
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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John,
You've probably been through all these but just in case, hand turning an end mill didn't work well - chipped a lot even with very light pressure. I'm not going to bother trying it powered.
I'd previously tried a 5/8" Forstner bit and last night switched to a 3/8". That seemed to work fine too, though I suspect the twist drill you suggest would be easier and maybe safer to clean.
I may repour tonight and start fresh with the smaller holes. One problem with the larger holes was chunks breaking off where the holes were too close to the edge.
Gary
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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: 665
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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: 665
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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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