[ATM] Wear and TDE theories

[Peter De Baan]

David, 
I think you need a smaller fan so it keeps you from getting TDE 
and from getting tue's and use ice in the cerium to keep it cool,
2 cents please > From: stainless_steel at suddenlink.net> To: melockwo at uiuc.edu; atm at atmlist.net> Date: Mon, 9 Jun 2008 19:15:53 -0500> Subject: Re: [ATM] Wear and TDE theories> > David Harbour wrote:> > Only once did I get a turned down edge; after that, no matter what> > I did, I always got turned up edge; the late Bob Goff told me it> > was because the edge of the mirror, being exposed more often (for> > longer durations) during polishing, got less polishing action, and> > also, it was mostly because of the wetness of the edge, with the> > evaporative effect, pulling the edge down; he may have been right,> > If the edge is experiencing evaporative effects, then the edge would> shrink during polishing because it is being cooled. If the mirror is> polished spherical in this thermal condition, later on when the rest> of the mirror shrinks, the edge would be HIGH, not low. Maybe that's> why you always got TUE.....> > Yes, that is exactly what Bob said; that is the exact mechanism. I always > got turned up edge, because the edge of the mirror was cooling more rapidly, > because it was exposed, as a "corner" in cross section, with a near point, > and a point, and lost heat very rapidly, with the evaporative effect, and > shrunk away from the glass, and then, when the fan was not running, and I > was not polishing, the whole mirror assumed a uniform temperature again, and > the edge turned up, because it had not received as much work. That is > exactly what Bob Goff suggested to me. But, I always tested immediately > after a work session; the TUE was there, and it was there later, also.> > So, Mike, your analysis, in the end, is essentially the same as Bob Goff's. > I will never forget it, because he was a legend; this is what he told me. It > is, in your words, a restating in slightly different language. I do not > imagine the rest of the borosilicate mirror "shrinking"; only the edge, like > one sees in the explanations of edge defects due to evaporative effects, and > the cross sections of the edge in these books. Right behind me, on my shelf. > You all have them; they are all classics. But edge defects, and why people > get them, still remain mysterious to me to this day; perhaps not for more > experienced, or more knowledgeable people; but then, where does all this > controversy come from?> > My visiting optician friend, the one who always got TDE and cursed about it, > was warned, when he came to work in my optical shop for a week, that he > might catch the "TUE virus". He just laughed. It had never happened to him. > He was flabbergasted when he not only did not get his TDE, but left my shop > with a mildly TUE. He remarked that I used a far more liberal amount of > slurry than was necessary; also, I always worked where I needed that fan. > That fan was there. In retrospect, it was not because I was so stupid, > thought that did figure into the equation a little bit, but the culprit was > that fan.> > R-101> > > ----- Original Message ----- > From: "Mike Lockwood" <melockwo at uiuc.edu>> To: <atm at atmlist.net>> Sent: Monday, June 09, 2008 11:45 AM> Subject: [ATM] Wear and TDE theories> > > Hi,> > Ted Cohen wrote:> > At any given grit, the center region gets whipped into shape first> > and the outer regions, near the perimeter, are completed last.> > This has been my experience so far on my mirror. I spend 80% of the> > time completing the outer edge whereas the inner parts of the glass> > are completed relatively quickly. Isn't this fairly typical?> > No, it depends on technique and which piece is on top, the mirror or> the tool.> > In machine work one can adjust so that the mirror polishes (or grinds)> from the edge in or the center out.> > > Conclusion – Tde is to be expected at most stages during grinding> > and polishing. It is only removed, at any given grit, when> > grinding/polishing is extended for a sufficient period of time;> > My significant and direct experience does not support that conclusion,> especially for machine work.> > David Harbour wrote:> > Only once did I get a turned down edge; after that, no matter what> > I did, I always got turned up edge; the late Bob Goff told me it> > was because the edge of the mirror, being exposed more often (for> > longer durations) during polishing, got less polishing action, and> > also, it was mostly because of the wetness of the edge, with the> > evaporative effect, pulling the edge down; he may have been right,> > If the edge is experiencing evaporative effects, then the edge would> shrink during polishing because it is being cooled. If the mirror is> polished spherical in this thermal condition, later on when the rest> of the mirror shrinks, the edge would be HIGH, not low. Maybe that's> why you always got TUE.....> > Mel Bartels wrote:> > Also, some definitions and checking of language might be in order.> > A turned edge is a narrow edge zone, typically 1/4 inch wide, that> > drops off. The actual amount of drop is often minimal, but looks> > bad because the drop occurs in only 1/4 inch. A rolled edge is a> > wider edge zone that drops off, extent might be 1-2 inches.> > For me the definition of the edge is very narrow, maybe 1/32" to 1/16"> wide. After all, it's the edge!> > A rolled edge might be 1/8" wide. Any wider than that, and you have> turned edge and a longer radius outer zone.> > > Lockwood postulates the 'plowing lap' explanation. If this were> > true, then the extreme edge would show polish first. I've seen> > this in unusual situations with polishers that have a handle or> > amateurs that use a hand position that exerts the to and fro> > movement at a moment arm above the pitch surface. But not in the> > standard situation.> > As you said in your other message, grinding typically produces a very> nice sphere. Since it is a sphere, the pitch lap will fit the> spherical mirror, and the edge will not polish first.> > What I am saying is polishing with improper technique (too short of a> stroke, possibly pressure on the leading edge) leads to an oblate> sphere (which you seem to agree with).> > However, it is this resulting oblate SHAPE of the mirror's surface> (after some working time) that causes the problem - the pitch lap> cannot flow fast enough to conform to the surface, so it plows and> knocks off the edge.> > This explanation is not a "catch-all" for TDE. It is simply a common> case that I saw and decided to write up and post. There are other causes.> > For me, the most common explanation for a turned edge is a PITCH LAP> CONTACT PROBLEM, due to technique issues, pitch lap problems, thermal> problems, etc.> > Mike Lockwood> > _______________________________________________> ATM mailing list http://www.atmlist.net/> > > _______________________________________________> ATM mailing list http://www.atmlist.net/
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