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Spin Polishing - making a perfect edge

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#26 Shrader

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Posted 24 July 2008 - 05:14 PM

And to think all thin time I thought the eccentric was the nut that ran the machine... :-)

Eric

#27 rwiederrich

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Posted 24 July 2008 - 05:21 PM

And to think all thin time I thought the eccentric was the nut that ran the machine... :-)

Eric


Indeed...you have to be an eclectic eccentric. :grin:

#28 Glig

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Posted 24 July 2008 - 06:40 PM

And to think all thin time I thought the eccentric was the nut that ran the machine... :-)

Eric

It is when I'm running it... ;) :crazy: :grin:
BTW nice thread Mark C. and company. Keep this thread going!

#29 Mark Harry

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Posted 24 July 2008 - 08:26 PM

OK Rob, just replace "rotating table" with "spindle" and you'll have it.
To show the valuable benefit of getting some pre-correction, I did a personal first this afternoon. I had 2 mirrors, both ~64.5" focal length. They were 6+8" diameter, and the 8 had a healthy amount of pre-existing correction. The polisher was 5" diameter, and I used it on both, TOT with a black/red rouge mix.
The 6" came out a nice smooth hyperboloid, magnitude of 1/25th PVW away from the perfect parabola. The 8" came out nice and smooth and corrected to 1/15th PVW. Started at 4PM, and finished at 8PM, including mounting, cleaning, and wrapping them up for the coater. Both came out with nice, hairline diffraction rings at the edge.
Now you may have gotten 2- 6" done from a sphere, but not the 8, unless it was sheer luck.
There was -NO- testing intervals while working these 2 mirrors. I just ran them, and then took 'em to the rack, and they were done.
Certainly, if they were faster F/D, there would be several figuring sessions. I usually average about 12-16 sessions for an F/4.5. But generally, I can get about 50% correction between the outer 2 zones fairly easily while finishing up polishing.
Now where's MC?

#30 Ed Jones

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Posted 24 July 2008 - 08:27 PM

Can you show us an uncropped Foucaultgram. You do have a zone maybe an inch wide that looks turned down if your knife edge is inside focus.

#31 Mitchell

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Posted 26 July 2008 - 12:33 PM

A very informative discussion, thanks everyone!

#32 mark cowan

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Posted 28 July 2008 - 11:15 PM

I'll see if I still have a corresponding Foucaultgram for the f/4.4...no, sorry, only that one edge image. It was well inside ROC BTW.

Here's a series of photos including uncropped Foucualtgrams on a 14" f/4.66 .8" quartz mirror that I'm making for myself. The mirror was partially figured last year but it had a problem with a lagging edge and I started over on it. I ran it on the polisher to remove the correction (spin polishing) until it was mostly a decent sphere with a rolled edge (this again is typical). The next few steps take care of that condition, and will raise the edge if continued too long.

This is the surface after the first 20 minute session (in all of these there are reference Sharpie spots used to align the video Foucautlgrams in Registax). You can see swirl marks from the lap:

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  • 2547509-1st session 20 minutes surface.jpg


#33 mark cowan

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Posted 28 July 2008 - 11:22 PM

Here's the surface after an additional 30 minute session as detailed in the first part of this thread:

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  • 2547516-2nd session 30 minutes surface.jpg


#34 mark cowan

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Posted 28 July 2008 - 11:23 PM

Here's the edge of the mirror after the 2nd session:

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  • 2547517-2nd session 30 minutes edge.jpg


#35 mark cowan

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Posted 28 July 2008 - 11:37 PM

It's looking pretty good, without any swirl marks, and the diffraction edge shows well in the Foucaultgram, but it still has somewhere around 1/8th wave TDE in the last quarter inch. The mirror could be figured from this state with little difficulty, but I'm too picky. So I ran it again and this time did an experiment with a new half-micron cerium sample (SRS426) but it showed no difference. Went back to the usual compound with the usual brief lap prep ran a 25 minute session - same lap speed all the way through, just slightly lagging the turntable (I think I mentioned this is important, as that's what telling you it's enforcing a sphere on the mirror). Here's the result:

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  • 2547537-4th session 25 minutes surface.jpg


#36 mark cowan

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Posted 28 July 2008 - 11:41 PM

You can see marks about 1/4 way up from the bottom that were left by the rinse water as it dried, and a few mild zones about halfway out. These are of no consequence for figuring and will disappear after a session or two. The image is taken at almost an exact null. Here's the corresponding edge image. It shows a slight undercorrection corresponding to the previous Foucaultgram, the amount of that measured by the first fringe (which lies about 3/4 wave from the shadow) is about 1/10th wave:

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  • 2547542-4th session 25 minutes edge.jpg


#37 mark cowan

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Posted 29 July 2008 - 12:01 AM

You can see in the shadow edge what looks like a slight hook. That close to the edge diffraction ringing off the edge itself adds spurious detail. If you examine the first fringe instead it tracks straight, and I've spent time in the past trying to eliminate that apparent defect.

I'll run another session on it just to see what happens :mrevil:

Keep in mind (please!) I'm not suggesting people adopt this method, nor am I making any claims about it being superior or smarter or more efficient or pretty much anything else. I'm documenting something that works and works reliably for me in a production environment, and that has been adopted by a few other people I know of who had trouble getting a better edge.

Also note that the way I'm working overall I don't care about or want correction put on the glass when it comes off of polishing prep. It's easier to verify a sphere than any other surface, and using a figuring technique that works over the whole mirror has no surprises when starting from spherical. For my work and robotic methods, figuring is an entirely separate process from polishing prep, with different laps, different methods, different machines, different strokes and different controls. The polisher only does one thing, polish. If I had problems with the polishing prep I would look for alternative methods to produce the original surface...

Glad some people find it interesting. :grin:

Best,
Mark

#38 mark cowan

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Posted 29 July 2008 - 03:39 AM

Mark H.,

One other thing I noticed, the polishing machine doesn't really strike me as being an Elgin, and I also see a -LOT- of weight being used on the mirror, which I assume to be cleated to the table's spindle. I have several of those self-same weights, and the only thing I use 'em for is cold-pressing. I remember doing blocks of flats, about 50 at a whack, and the use of all that weight anywhere near the finishing up of those quartz flats meant edge trouble, regardless of the type of stroke used. It also doesn't do the microfinish any good, either. That, in combination with the cleats makes it very hard for me to swallow that you don't have 'stig' issues. Without blocking up, or other ways to support the piece other than tossing it on a platter with just cleats, I've seen too many times, where the part(s) are botched. Installed loosely with cleats, it's only gonna hit in 3 places on the bottom. Sure, you can rotate the mirror, but you'll only get something that approaches having minimal 'stig'.
Regards, M.


Just catching up on this thread, now, while running that next session. The polisher is made only for fixed quill, it's what I've built and used consistently since day one - it's just a machine with a lot of torque and rigidity to use it. The mirror is not "cleated" to the table, I've gone over all of that many times in this forum. It sits on a rug-backing flotation pad that's attached to the table with spray adhesive. There are three cleats for centering but there is adequate clearance at all times. I've used "lots of weight" quite successfully for quite some time as well. I've gone over that many times in this forum as well.

Whether you wish to "swallow" that I "don't have 'stig issues" or not is immaterial - you either are saying I'm unable to detect it, which is patently untrue, or that I see it but don't care, which is ridiculous. In either case it's an offensive comment.

Best,
Mark

#39 mark cowan

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Posted 29 July 2008 - 04:12 AM

Mark H,

Mark C.,
I'm looking at the polisher picture, and I note the grooves are not very deep at all- and the groove spacing is larger than usual- hence, the polisher isn't going to change much at all as far as the ROC is concerned. Owing to the fact that AL isn't a very compliant pitch medium, basically, you're working with a 'solid face' tool, and relying on the brush scratches as mini-grooves. It's typical for those to close up after 15-30 minutes- use. (As revealed in your polishing period duration.) Since the polisher is conformed well previously, and starts to likely run erratically after the mini-grooves close up, I think your stressing the conformity of the polisher is a bit off. It doesn't need repressing, just re-scratching with the wire brush treatment to renew the neutral acting characteristic you're looking for.
Just me thinking out loud, if you cut the grooves deeper, and spaced them a little closer; you'd induce more correction with perhaps less overhang with the better compliance of the improved tool; and have a leg-up on accomplishing a bit more correction near the edge; but you'd still need the mini-grooves for smooth polisher operation. Due to the increased compliance of the polisher, you'd have to back off on the weights used somewhat.
I also think you'd have to modify either approach you use if the F/D is increased to any mirror over F/5, and especially with F/6 and longer.
Do you think that's a good assessment of what's going on?
M.


Last first, no.

I don't recall ever saying the lap runs erratically after the first 15-30 minutes, that would be your speculation. It will run for hours on end at the proper offset because - that's right! - the ROC isn't changing. Often I'll pull it off after an extended session which closes the channeling almost completely.

I emphasize the conformity of the polisher because experience has shown me that if the lap doesn't spin just slightly slower than the table it will not be enforcing a sphere. Scratching the lap to get micro grooves is about the easiest way to produce that conformance without extended pressing, which I abandoned years ago. The duration of the hot press is about 10 seconds - this probably is not in disagreement with what you're thinking.

I've experimented with all manner of weight levels and channel patterns. I note what works best for me and adopt it. The channels aren't even necessary for spherical polishing and they do eventually close on their own. Making them wider or more numerous can induce circular traces on the mirror if they happen to match up over one area and you are not using strokes during the polishing, I've stroked mirrors by hand in the past to avoid that. I found it was much easier to use a non-periodic channel pattern as shown to avoid channel traces, as defects go they can be quite sharp and persist through figuring.

If I want to get correction into the mirror it's easy, all I have to do is move the pivot point inwards. I've had the polisher produce a commercial grade 14" f/4.7 all by itself (admittedly one of those accidental cases, but isn't a 1/6th wave mirror that size a sort of happy accident?), but I wasn't satisfied with its "decision" as to how to finish the edge so I redid it to my own specs.

Finally, I've used the same technique with different lap aspect ratios all the way out to f/18. F/5, f/6 is easy and works quite well.

Best,
Mark

#40 mark cowan

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Posted 29 July 2008 - 04:35 AM

mark why do you need a bearing on the end of your arm if it doesnot move maybe for figureing? thanks


That's just a pivot for the overhead arm. The arm clamps to a anchor platform on the right (not shown) that takes the load of the polishing force. I originally considered converting it to a controlled offset (eccentric) but never bothered as the results from pure spin polishing were more than acceptable.

So let's see what this thread has produced so far - various unnamed opticians think spin polishing is a BAD way to polish; it's not legitimate to polish out glass this way (I gather), even though it works; it must generate astigmatism and zones near the edge of the mirror, even though it doesn't; using weight to polish like this must generate a poor microfinish, even though it doesn't; it won't work with slower mirrors, even though it does; single examples of it not working for somebody else mean it shouldn't be used; single examples of traditional methods working better for one person mean it shouldn't be used.

Well hey, that's about par for the course, then. :tonofbricks:

Sure am glad I didn't start out knowing all that or I'd never have bothered to develop such a useful and practical system and apply it on a daily basis. :lol:

Best,
Mark

#41 Owen

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Posted 29 July 2008 - 06:45 AM

Mark,

Its a great piece of lateral thinking, one outside of the knowns... placing it very definitely in the unknown category.

The geometry hangs together, so why should it not work...


Owen

#42 kfrederick

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Posted 29 July 2008 - 07:06 AM

hi mark nice thread i didnot have good results spinpolishing /i am a farmer/ not a optician but i have talked to two opticans who make a lot of mirrors and they both said / in there opinion it was a bad way to polish / diferent strokes for diferent optcians .it workes for you great / thanks for your opinion .if you wont you might try one mirror there way it might be easer.

#43 Mark Harry

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Posted 29 July 2008 - 07:08 AM

"So let's see what this thread has produced so far - various unnamed opticians think spin polishing is a BAD way to polish; it's not legitimate to polish out glass this way (I gather), even though it works; it must generate astigmatism and zones near the edge of the mirror, even though it doesn't; using weight to polish like this must generate a poor microfinish, even though it doesn't; it won't work with slower mirrors, even though it does; single examples of it not working for somebody else mean it shouldn't be used; single examples of traditional methods working better for one person mean it shouldn't be used."

**********
No, I'd say that no two opticians will do any particular thing the same, and if you get them in the same room, there will usually be a conflict resulting from the 2 opposing methodologies they happen to use on any particular job.(the big rub, one will prove better timewise, and/or with less inherent defects- the reason for my raising the issues.) The only truth here that I see, is that it works for -YOU- with these hard, fast mirrors. M.

#44 rwiederrich

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Posted 29 July 2008 - 09:26 AM

"So let's see what this thread has produced so far - various unnamed opticians think spin polishing is a BAD way to polish; it's not legitimate to polish out glass this way (I gather), even though it works; it must generate astigmatism and zones near the edge of the mirror, even though it doesn't; using weight to polish like this must generate a poor microfinish, even though it doesn't; it won't work with slower mirrors, even though it does; single examples of it not working for somebody else mean it shouldn't be used; single examples of traditional methods working better for one person mean it shouldn't be used."

**********
No, I'd say that no two opticians will do any particular thing the same, and if you get them in the same room, there will usually be a conflict resulting from the 2 opposing methodologies they happen to use on any particular job.(the big rub, one will prove better timewise, and/or with less inherent defects- the reason for my raising the issues.) The only truth here that I see, is that it works for -YOU- with these hard, fast mirrors. M.


Very good points...all of them. I tend to agree with the notion that if it works for you...then it is good for you.

Being a technique sensative method...not everyone is gonna *get it*. Howard Hughs made the larges plane(at that time) out of plywood..when it was said (by experts), that it wasn't going to work. Any process, no matter how misunderstood by the novice or professional alike, can be perfected by some who see it happening, either by sheer will or gifted aptitude. One cannot deny the evidence presented by those who *HAve* done it.

Being a *Novice* myself..I tend to believe I can do anything I put my mind to...even if the experts say it isn't possible.

I have enjoyed this thread, and it has opened my eyes to greater applications of the materials, and techniques of the process of polishing astronomical mirrors.


Rob :bow:

#45 Joe Cipriano

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Posted 29 July 2008 - 11:18 AM

Friendly reminder: Play nice, people. Rigorous discussions are encouraged - but turn the heat down just a little, please.

Thanks.

#46 Mark Harry

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Posted 29 July 2008 - 02:57 PM

I composed this offline so I hope it all fits!
***
The last 3 mirrors I have been working on have essentially the same radius of curvature, to give between 62.375-64.5" focal lengths. All are pyrex mirrors, of 6,8, and 14" diameters to give F/10.75, F/8.063, and F/4.455 respectively.All figuring polishers are made with a blend of Gugolz, and with liquid #85 polish from Universal Photonics. The only thing that is changing, is either diameter of mirror/ focal ratios, depending on which way you want to look at this scenario.
I finished the 6+8 , and am working on the 14.
All were polished out on urethane to get rid of the pits. I kept note of the turned edge with each so far with this urethane. To polish out with this aid, left turned edges as follows:
6" ~1/4"+ 8"~ 1/8"+ 14"~1/64th inch.
These could be seen before putting the glass on the test rack, simply by looking at the reflection of a light bulb, just off of the mirror's surface. They were confirmed on of the test rack. Interesting to note about the fast 14, the extremely narrow turned edge produced a double curved bright hairline opposite the knife, depending on where my eye was positioned primarily due to diffraction, IMO.
To correct the rather severe edges with the 2 smaller mirrors, took fairly lengthy sessions with pitch and thinned compound, particularly the 6". One thing to remember about F/10s is that they are extremely sensitive to polisher overhang. 1/4" too much or too little, and the edge issue will -NOT- disappear, with any kind of sweeping stroke used with Elgin polishing machines. I don't care how you run them, on an Elgin, it's very critical.
Polishers used with the smaller mirrors was my particular pitch blend, of 66% and 63% diameter with 6+8 respectively.
The 14" lacks just a little haze about 3/4" in from the edge of the mirror, so it isn't -quite- fully polished, but I was curious to see what it looked like pertaining to the subject thread. So I haven't hit it with pitch just yet. But even with the extremely narrow TDE, it's showing a faint hairline the same side as the knife. The settings with the 10" urethane polisher is ~13 strokes/rev; an overhang of 1" which is as far as my overarm allows;(far side) and a wild amplitude overhang of just shy of 1/2 polisher-diameter.(since the edge lags the most)
Now the really nice thing about this mirror, the outer 2" of mirror has to have .193" of correction with fixed source knife edge tester. At this stage, it's showing .075" exactly. For all intents, that's 40% of my correction needed. When fully polished in about another 1.5 hours, it won't take much effort to increase the correction slightly, and change it essentially from linear to exponential.Switching from urethane to pitch on this edge area, I should be able to easily triple that amount of correction with a good offset/amplitude setting.
My conclusion:
Focal ratio plays a huge part in the condition of the edge in polishing out, and also in the figuring methods, and difficulty in getting rid of the edge defect. And I don't really favor the knife well inside of focus to tell the edge condition. It's much more sensitive at the actual ROC. The brightening of the outer zone on the 14", along with the hairline narrow TDE made a slight crescent about 4" long, with that wire-thin edge extemding to the full half diameter of the mirror opposite the knife.A whole lot easier to see/quantify.
I tried to keep everything consistently the same as I usually do and change only the size/focal ratio of the mirror to outline the differences found. Your results may vary, but i think basically these observations will hold true for the majority of mirror projects done similarly.
Regards, Mark

#47 Mark Harry

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Posted 29 July 2008 - 03:07 PM

To add, I think owing to the extremely narrow and abrupt TDE on the 14" mirror, it would be wise to leave as-is, and tweak the bevel width with a diamond stick or disc after completion of figuring. Probably take less than 5 minutes. It would take a very long time to remove enough glass across such a large surface, and to throw away all that "hard won" correction..... Whaddaya think?
The biggest problem encountered, is the edge on the 6, and the capacity of the machine with the 14". But IMO, the fast large mirrors should -NOT- have big issues with edge defects, in light of my previous post. If the substrate is harder, that should make the magnitude of edge problems less, but harder to fix once they do ocurr.
(off the soapbox)
M.

#48 kfrederick

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Posted 29 July 2008 - 03:25 PM

that was nice info mark h /i wish i had your skill i have worked glass by hand until i started this 26 in . i have had to learn machine work i have tried all ways of doing it / still having problems and a lot more to learn . mark cowen i should have been more careful in my post it sounded like i was not thankful of the info you were shareing didnot wont to imply that/ i like reading your posts and appreciate all you share / hope you keep writing and i will keep reading . kevin frederick

#49 mark cowan

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Posted 30 July 2008 - 01:12 AM

Time is short today and all I can say is I posted this topic originally on account of I thought it was interesting, not just to get slapped upside the head by a bunch of folks to say "hey you can't do that." I understand that when somebody says something that doesn't sound legit your first impression is to try to pick it apart from a distance.
No problem with that. :tonofbricks:

I posted the same info in condensed form over on the Zambutomirrorgroup on Yahoo a couple years ago in direct response to Carl discussing trouble with getting the edge close enough to go into figuring and how he'd experimented with spin polishing and gotten some good results, but that it wasn't reliable enough (this from memory, you can read it all there). Narrow edge defects that are smaller than the correction to be applied in figuring almost always clean up by themselves. But that can be iffy, and beveling isn't an ideal solution, although everybody ends up doing it sometimes.

It's zones near the edge that cause the most grief; I had interferometry done by James M. on a spec 16" f/4.5 some years ago just to see how it tested - it was great, except for a narrow flat zone that threw it off. Who needs that kind of grief! I started experimenting then with this technique and soon added hi-res imaging as well as the null tester to examine those narrow zones more closely and to figure out how to eliminate them. The edge is the toughest thing to get right and carries the most potential for lessened performance.

I can build any kind of machine I want, frankly. I know how the standard machines work, pretty much. The only advantage I can see to changing the offset under machine control is to control zoning, and the mild zoning that I sometimes get with the pure spin method I'm using disappears in figuring. Who knows? Maybe I'll fit that to the new automated grinder/polisher just to see how useful it is.

I'll have some more pics on the results with the 14" - yeah I did put it back on for a spin, as nothing much was happening. :grin:

Best,
Mark

#50 mark cowan

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Posted 30 July 2008 - 02:32 AM

Mark H,

And I don't really favor the knife well inside of focus to tell the edge condition. It's much more sensitive at the actual ROC.


Informative post. I don't use the knife inside of focus for determining the edge condition either, I can see everything I need from the diffraction edge at ROC for that. But the diffraction edge doesn't photograph well, while the KE inside of focus does, and it illustrates the edge condition accurately. Imaging is both more and less sensitive than the eye, in certain ways.

BTW I have a 14.7" f/6.8 (100" FL) that's one of a bino pair in process and ready for figuring last I checked - this too was prepped via spin polishing with one of the 80% 12" polishing laps - though now that I think about actually I used a 9" polisher on them both - would that be a better or worse test of your theory? :question: . I'll see if I can capture the edge on that, it's a tight fit in the testing area as is without fitting the camera in there.

Best,
Mark

edited


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