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Spin Polishing - making a perfect edge

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#1 mark cowan

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Posted 22 July 2008 - 03:29 PM

This subject appears to need its own thread. In the recent past both here and elsewhere (Zambutomirrorgroup posts starting at 6989 (July '06) and then again at 9553 (Dec '07)) I've described the method I use to prepare large fast mirrors that are polished out for figuring. At least a couple of people have picked up on this and used it. :)

From an earlier description (see previous thread from a few months ago) I'll repeat the description.
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Basically, inside the sweet spot (SS) the curve goes shorter, outside the SS the curve goes longer, at the SS the curve stays the same. Assuming excellent contact with the tool or lap to the mirror, that is. Grinding or polishing, the principle is the same.

For any given sub-diameter lap that's about 55% or more of the mirror diameter there is indeed a SS position. If the lap is too small you will not be able to cover the center of the mirror while offset. The smaller the lap % the smaller the overhang % is at the SS, and vice versa.
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I typically get spheres using pure static-position polishing ("spin polishing" at the "sweet spot") that null out uniformly, right off the machine. Granted, I do some simple lap prep and follow a particular protocol for time, which I've detailed a couple times over on the Yahoo Z******MirrorGroup, mostly about how to obtain a perfect edge. Overhang % and excellent lap contact is key.
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For spin polishing (or grinding) OTOH the lap (or tool) is free to follow the curve of the glass, but other than that its pivot is just fixed in space above the rotating mirror.

A picture being some K better, here's one of a mirror that's undergoing final polishing prep before figuring today. This is a fixed quill (static position) polishing machine (Derek) doing edge preparation on a 14.7" f/4.4 fused silica mirror. The aft end of the overhead channel pivots via an automobile ball-end joint. The fore end is clamped onto a reinforced support table, and that's what takes the force load (which can be quite high). The table spins clockwise at about 42 rpm, the lap is AccuLap medium on an epoxied concrete base, and there's 25 lbs of weight on it.

to be continued

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  • 2534638-derek.jpg


#2 mark cowan

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Posted 22 July 2008 - 03:32 PM

And here's a cropped Foucaultgram of the above mirror now ready for figuring - this image is with the KE well advanced and using a 50 micron pinhole source, it shows the condition of the edge. An image like this is a definitive test for edge condition, with the KE advanced a little beyond the center of the mirror.

Note particularly how the first diffraction band (which is about 2/3 wave from the actual KE shadow) just tracks straight to the edge. The small scale ripples right at the edge are diffraction ringing off the edge. The mirror doesn't need to be spherical to start figuring, very smooth to the outside with shallow correction (any conic b slightly less than 0) will do, which is what this is. ;)

It shows around 1/4 wave, I get maybe -.07 for the conic. There's a certain amount of turbulence in this photo, it's an average of several video frames taken over about 30 seconds, in the dark with a pretty dim source. Most anything imprinted by the tool towards the inner half of the mirror vanishes during figuring (but see next post). Note that this is not true of defects near the edge! If you don't start with a smooth edge without artifacts, you won't end with one, if using normal sub-diameter parabolizing techniques (from the outside in).

For those curious, this parabolized mirror will have 3.54 waves of correction, compared to a sphere!

to be continued

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  • 2534648-edge finish.jpg


#3 mark cowan

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Posted 22 July 2008 - 03:47 PM

Detail on how to do the lap prep for spin polishing:

Spin polishing, I use a 75-80% lap most of the time but it works with smaller or larger laps. The lap is held in a fixed position so it can only rotate with around 15% overhang (diameter). This is the easiest way to grind by machine and it works just the same for polishing as well.

You need excellent contact while you're doing this and it's mostly useful to finish a mirror that has the typical rolled edge that comes from polishing out with a spinning lap over long sessions.

Wire brush the lap under hot water to get a roughened surface. You don't need much in the way of faceting for this, as it works mostly from the scratches. I channel the lap while avoiding the center and angling each channel so it doesn't put zones on the mirror. The pic is of an AccuLap lap made that way, but channeled for extended polishing. For finishing the channels would be narrower and allowed to almost close.

With a generous shot of slurry on the mirror, press it briefly while the lap is still hot - use a lot of force, and then check there's good contact. If there isn't do it again until there is.

Put it onto the machine. I use a lot of weight and try to set it up with the overhang and start it going quickly. That's so there's no chance for a groove to set in from the overhang position - this will kill the mirror's edge. Run it 15-30 minutes, less time as you get closer to a good edge. Very important point - the lap must turn just a little slower than the mirror, which shows proper contact. Faster than the mirror or a lot slower, it's not working, so take it off and try again.

Once the time expires, take it off and redo it. You can check it if you want, but it will push a good sphere slowly out to the very edge with anywhere from 2-5 reps in most cases. It's the repeated establishing of the good contact that does it, and the lap only spins that way reliably until the microfaceting from the brushing gets smoothed out.

No particular mystery about it but you really have to experiment a bit to see what happens and then work with it.

Caveats and more blather:

I need to emphasize that I'm talking about a very specific technique in the final stages of spin polishing. To produce a perfect edge I'm not talking about just putting the lap on and letting it run for hours, I'm talking about a carefully prepped lap set at the overhang position where it raises the typical TDE coming off of polishing out or extended polishing. And you must check the results with the KE to see when to stop applying it, but that's not hard. Be aware that it's only the combination of these factors that gets to the results documented in the earlier thread, and I've tried to be careful to describe it in detail each time I've described it.

A summary sort of:

Perfecting the edge is a perennial issue in prepping large mirrors that everybody encounters (see this post. ). I'm not sure I can say much more about it, the method is very simple and reliable. It does what it's supposed to and it does it extremely well.

Best,
Mark

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  • 2534694-periodic spin polish example.jpg


#4 PrestonE

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Posted 22 July 2008 - 05:13 PM

Mark, Nice detailed write up and very similar to how Carl taught me over the internet back in 2003ish...

It works very well if as you say, one watches the rotation speed to know that things are working or not.

Regards,

Preston

#5 Mark Harry

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Posted 22 July 2008 - 06:10 PM

1. Mark, how come you don't respond to PMs?
2. Do you test for astigmatism, and if so, what is the ocurrence of it appearing?
3. Do you note the appearance of astigmatism when dealing with multiple factors when dealing with polisher size? (particularly, by factors of 2,3+4) and with similar low numerical "polishing swirls"?
"I need to emphasize that I'm talking about a very specific technique in the final stages of spin polishing. To produce a perfect edge I'm not talking about just putting the lap on and letting it run for hours, I'm talking about a carefully prepped lap set at the overhang position where it raises the typical TDE coming off of polishing out or extended polishing."
********
I don't fully understand this statement. The edge area should have a good deal of "pre-correction" if you can find a prefiguring stroke that -CAN- run for hours if needed with only normal polisher correction and preparation; and little other attention to setup parameters. If it's critical, with lots of hoops to jump through as to polisher prep, it's my opinion the method could stand improvement. Wouldn't that be a sensible assumption?
Preston, just my opinion, if CZ told you this particular method, it's the long way around the barn, it works, but it's somewhat critical and there's a shorter more reliable and "safer" way.
Regards, Mark

#6 Mark Harry

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Posted 22 July 2008 - 06:18 PM

Another way of thinking about this perverbial "perfect edge" a spherical conic ain't it! You should have a curve that varies as the square of the distance from the mirror's axis, right out to the very bevel. It will lessen the work tremendously when it comes to deepening the mirror fully to proper correction.
M.

#7 kfrederick

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Posted 22 July 2008 - 10:29 PM

I would like to get other opticans ideas on spin polishing the two opticans i talked to who have made lots of optics think spin polishing is a BAD way to polish! they both said there are much better ways to polish! and in my work on my 26 it polihed better with stroke polishing.

#8 rwiederrich

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Posted 22 July 2008 - 11:16 PM

I think what I see here is a technique sensative process.

I bet dollars to donuts the process works just as well as Mark claims it does...and I also bet it works just as bad as some as the others have claimed.

In my line of business it is readily known that there are plenty of ways to skin a cat, and many of them are prefered when performed by skilled hands, and the same is regected when performed by the novice.

I can't say what is and isn't good yet, and I think I will give this technique a spin (ha ha).

I can always return to my stroke technique if I can't figure it out.

Thanks for all the great info and photos..... :grin:

Between my time making the scopes OTA, and its mount, and mirror..I'll try anything..if it works.

How fast does the table HAVE to spin for this technique to work?

Rob

#9 mark cowan

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Posted 23 July 2008 - 12:11 AM

1. Mark, how come you don't respond to PMs?
2. Do you test for astigmatism, and if so, what is the ocurrence of it appearing?
3. Do you note the appearance of astigmatism when dealing with multiple factors when dealing with polisher size? (particularly, by factors of 2,3+4) and with similar low numerical "polishing swirls"?


1) I will eventually, Mark. No offense! Call me if it's more urgent and I'd be happy to discuss anything (without the risk of misunderstood e-mails). (503) 884-3832.

2) Astigmatism doesn't occur, unless the flotation pad under the mirror manages to come unstuck (it's attached with Duro spray adhesive). Yes, I check for astig, I think I documented that pretty thoroughly in another thread... ;) I've got a personal 14" f/4.7 under prep right now, I can shoot some pics of how it goes as well.

3) Hmmm. Good question(s), if I understand what you mean properly. The last first - polishing "swirls". I've seen that, but it takes excessive action on the lap to produce them, as in an almost dry working condition. By applying a wire-brushed texturing (I got this from Carl Z BTW :) ) and minimally hot pressing the surface comes out extremely smooth and the contact will stay perfect throughout the short session.

If by the first you mean are lap size ratios correlated to astig? I see what you might be thinking there but in practice I don't see it happening.

"I need to emphasize that I'm talking about a very specific technique in the final stages of spin polishing. To produce a perfect edge I'm not talking about just putting the lap on and letting it run for hours, I'm talking about a carefully prepped lap set at the overhang position where it raises the typical TDE coming off of polishing out or extended polishing."
********
I don't fully understand this statement. The edge area should have a good deal of "pre-correction" if you can find a prefiguring stroke that -CAN- run for hours if needed with only normal polisher correction and preparation; and little other attention to setup parameters. If it's critical, with lots of hoops to jump through as to polisher prep, it's my opinion the method could stand improvement. Wouldn't that be a sensible assumption?


I don't want a lot of pre-correction on the edge, but I'll accept some. ;) The method I use for hand figuring (to be applied robotically soon) corrects the entire mirror at the same time evenly, save some portion of the center (varies with f/ratio). Because of that, if I have anything near the edge that isn't the same smooth fractional conic as the rest of it it will very faithfully end up as an error on the finished optic - and for fast mirrors that's the same time it becomes hard to detect. A sphere or very close to it (on the weak negative side) is the best starting point for me. I also want the maximum amount of working time for figuring to get the results that I'm after.

Maybe I haven't yet explained this procedure properly - the short sessions are used to take a mirror with some form of correction (either a narrow turned edge, a wide turned edge, or a strong negative correction) and transform it to a sphere. It does it from the inside out. The lap overhang must be set properly for this to occur - and it happens to be exactly at the position where it would neither lengthen nor shorten the ROC of the mirror - ie, the "sweet spot", which is around 15% give or take for an 80-85% sub lap. It's easy to see what happens during multiple short sessions on the bench, as the turned edge shrinks in size while the spherical center grows outwards. Reliable as sin, at some point along the way the turned edge disappears completely. That's all there is to it. You want to stop when this happens - I think that's obvious, right?

So looking at it a different way - the polishing out is done with the same lap running for hours on end between dressings. It may (and sometimes does) result in a good sphere or fractional negative conic ready for figuring, but typically it results in a turned edge with zones and what have you. So there are only two branches on the decision tree - use it if it's good, or finish it by applying the short steps if it's not. The short steps always work and require little thought process (except perhaps as to why! ;) ) and mere repetition of the sequence always results in a mirror ready to figure. That's why I'm calling it a simple process. I don't see anyplace it needs improving, it works and it works well.

Preston, just my opinion, if CZ told you this particular method, it's the long way around the barn, it works, but it's somewhat critical and there's a shorter more reliable and "safer" way.


I doubt it was this method. When I posted it over there (the Zambutomirrorgroup) the first time, Carl had been complaining about never quite being able to get pure spin polishing to work, and spending a lot of time fixing edges as they came off the machine method they were using. That was one of the reasons I explained it, as apparently it was not a method that he (or anybody I've heard about) was using. Perfect edges aren't easy - isn't making large fast mirrors hard enough already?

Best,
Mark

#10 mark cowan

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Posted 23 July 2008 - 12:15 AM

I think what I see here is a technique sensative process.


As are they all! :grin:

How fast does the table HAVE to spin for this technique to work?


I don't know. I've designed all tables to run at a fixed speed, 42 rpm. It's either heresy :mrevil: or the Answer to The Question. :lol: I haven't tried it at other speeds.

Best,
Mark

#11 mark cowan

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Posted 23 July 2008 - 12:22 AM

I would like to get other opticans ideas on spin polishing the two opticans i talked to who have made lots of optics think spin polishing is a BAD way to polish! they both said there are much better ways to polish! and in my work on my 26 it polihed better with stroke polishing.


Must be heresy then. :mrevil: It'll be used on a 20.2" f/4 fused silica mirror next, BTW. ;)

It works for me, I've got no reason to use another method to do this job. I'm not suggesting that other people should use it, BTW. I'm just saying, here it is, this works for me, have a look at it. I kept the method in-house only for quite a while, until I noticed some people having problems that indicated they'd never quite come to terms with how to get the last critical step accomplished.

Also, it shouldn't need to be said again, but experiment with it. Don't think the bare description is going to be enough to take it and make it work, or that because somebody doesn't like something that they gave the same name to that that means it's the same thing. Optical work isn't like that, it's hard to get the nuances right. Use what works, but try little variations to see if you can improve it - that's how I came up with this in the first place!

Best,
Mark

#12 Shrader

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Posted 23 July 2008 - 12:45 AM

Hello Mark (H),

Could you explain what the shorter, more reliable and "safer" way around this barn is? I have used spin polishing on my machine and usually need to do some corrective long stroking to get things right to begin figuring in earnest. Always looking for a more reliable method.

Cheers, Eric

#13 Mark Harry

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Posted 23 July 2008 - 03:01 AM

I'll say this much about processes to get a sphere, which I had to serve an apprenticeship to learn about. Basically, you learned the foolproof way to obtain a sphere. It's not really dependent on the stroke/RPM relationship, and only minimally on a few other basics, such as offset, and spindle speed. A wide, sweeping stroke is pretty valuable. There are many combinations that will rub that glass, eliminate astigmatism and edge defects, and produce a sphere as far as stroke types are concerned. But you can't get too wild about the RPMs. Just to show the differences between Mark C's approach and mine, I hardly ever use a spindle speed over 6RPM, and it's generally down around 1/2-1RPM. (yup, you read that right!) Elgin polishers (the predominant type found in legitimate shops) are made to "stroke" the glass, and to just make the eccentric stationary, you might as well save some dough and make a tub grinder. Those -are- designed for spinning.
My regimen may slow down the "polishing out the pits" phase, but it's getting a leg up on the correction, and eliminating astig. and any possible roughness induced by using urethane. When I'm doing multiples, I can generally get the correction to 75-95% correction from the .707 zone outward with a 7/8ths size polisher.
So in doing 3 things at once, I do manage to shortcircuit some of the time off of getting a mirror ready to finish up. Since we have a Vendor here, I am somewhat reluctant to "give away" information particulars that took years to develop. But with some basic determination, and the goal of accomplishing more than one good thing at once is kept in mind, you can explore this aspect and easily develop a stroke that can do it all, or just get a smooth 'stig-free sphere.
And Mark, I don't buy the thing about PMs, this thread here is virtually the same thing. But thanks for the input about CZ's having found it can be a bit touchy with spinning, for I believe he would never recommend that method. I think that he'd know better, but that's my guess.
*****
As to what I'd accept on the mirror, I learned a long time ago to accept whatever's given to me. But the hard issues is when, say a mirror comes out of a prefigure session, it's corrected to 1/16th wavefront error, just what exactly it was that ocurred to make that happen, and if it's repeatable! It happened with a 5", and an 8" mirror, while using a somewhat smaller polisher(~5/8) with the 8, and a full size (???) with the 5!!! But I have been able to find out what corrects it very well on the outer half of area consistently and reliably.
Later, Mark

#14 Mark Harry

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Posted 23 July 2008 - 03:17 AM

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.

#15 Ed Jones

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Posted 23 July 2008 - 07:23 AM

but other than that its pivot is just fixed in space above the rotating mirror



This may fix the edge but it also invariably creates a zone along the inside edge of the lap. Granted the defect on the inside is easier to deal with in parabolizing.

#16 rwiederrich

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Posted 23 July 2008 - 09:09 AM

I think what I see here is a technique sensative process.


As are they all! :grin:

How fast does the table HAVE to spin for this technique to work?


I don't know. I've designed all tables to run at a fixed speed, 42 rpm. It's either heresy :mrevil: or the Answer to The Question. :lol: I haven't tried it at other speeds.

Best,
Mark


Mark another question. I heard you mention polishing nearly dry. Now what I want to know is...how much slurry does one need or how much do you use to achieve the polish you want? Is your mirror slightly wet...just wet...or running wet?

Rob

#17 rwiederrich

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Posted 23 July 2008 - 09:29 AM

Mark.....Very interesting post. Thanks.

Several things..My machine spins at a very slow RPM probably 5 rpm a minute. give or take a rev.

Any way..in my business we polish polymers, and it is always assumed that fast revolutions on the polisher are best....when in fact slow rotations of the polisher are just as fast or even better, and the friction of the fast polisher can cause heating and even zones on the product.

It is the impatient, and Hurried who jump to the fast polish...and fail to recognize the significance, and practical importance of how well slow polishing can really be superior in most cases.

Now, with that said....can that same principal be applied to polishing glass. It has been mentioned that a slow stroke can and does produce very satisfactory result.

Now to my project. With good contact, and a slow regular stroke...I have been able to polish the center(thus far) of my mirror to a very nice shine. I will now extend the stroke of the over arm and *push* that toward the edges.

Do you foresee this technique as typical or should I try something different as suggested...maybe.....

Any input? I just want to successfully polish this mirror out...and if there is a better way...then..

Rob

#18 Shrader

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Posted 23 July 2008 - 09:30 AM

Thanks Mark (H),

I didn't mean for you to give away info that you didn't want to divulge. The technique you describe though is much closer to what I have converged on, though. I spin polish fairly fast at first, and taper off the speed to very slow at the end with a healthy dose of stroke. Works for me. I'm not getting anything close to 75% correction from polish, though.

Cheers, Eric

#19 Mark Harry

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Posted 23 July 2008 - 03:09 PM

I'm not sure what you mean by slow stroke. I never meant that if you took it that way. Actually, I -think- the standard "accepted" speed of a polisher is supposed to be around 4"/sec as linear travel. I don't think I EVER go that slow, probably 3-4x that speed with deft and light pressure done by hand, or no weights on the overarm. (even with a 6" mirror!) At that speed, heavy weights/pressure will most certainly ruin the job.
If I had a spindle speed of ~5RPM , I'd shoot for a polisher that went out with an overhang of ~1/4 the diameter of the polisher, if ~1/2 mirror diameter, maybe more OH, and a stroke of duration of 1.33-2 seconds. Should get you up to 6-8 strokes/mirror rev. (I like to see more, but it might not be possible without adding a jackshaft to your spindle drive train) And do yourself a favor- put a dot on the mirrors edge, -AND- polisher to check repeatability. You don't want them to line up only occasionally as everything turns/strokes.
The wider you can make the amplitude/offset, the quicker you'll get the edge to catch up with the center. But watch the offset, for it could give a narrow tde if it gets away from you.
This is some of the basic parameters. If you check it occasionally, you should be able to fine-tune your setup -as is- fairly well to be useful.
Mark

#20 Mark Harry

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Posted 23 July 2008 - 03:20 PM

Mark C, if you don't mind, what's on the right of your polishing machine picture?
M.

#21 rwiederrich

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Posted 23 July 2008 - 03:42 PM

Sorry Mark...the 5 RPM is the table speed not the spindle speed. I don't think they mean the same thing.....? do they?

Rob

#22 Mark Harry

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Posted 23 July 2008 - 06:53 PM

What are you assuming is what you put your mirror on? (spindle!) The overarm is generally called either that, or simply eccentric, which is how the overarm is adjusted. M.

#23 kfrederick

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Posted 23 July 2008 - 10:26 PM

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

#24 Mark Harry

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Posted 24 July 2008 - 08:35 AM

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.

#25 rwiederrich

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Posted 24 July 2008 - 09:24 AM

What are you assuming is what you put your mirror on? (spindle!) The overarm is generally called either that, or simply eccentric, which is how the overarm is adjusted. M.



I might need a class on polishing machine nomenclature.

Here is a simple diagram of my machines workings.

The blue thing is the mirror(It sits on the rotating table)
The OVER ARM comes from the large eccentric wheel to the right. The smaller eccentric wheel to the bottom controls *cross* stroke. Both connect to the Spindle that sits inside of the lap(that thing on the blue mirror thing... :grin:

If I have it all wrong..please correct me. :grin:

My machine is based on the Porter machine.

Rob

Attached Thumbnails

  • 2538320-Polishing machine diagram of over arms.jpg



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