I have been trying to understand the various techniques that are used in generating and regenerating or renewing a surface plate. I think I have a good appreciation of the three plate method. And I can understand how a master flat can be used to scrape a new, flat surface by comparing the two. I have also watched more than one internet video where the technique used by the plate lapping companies will check and then lap a worn plate back into spec. And that is where I do not understand exactly what they are doing. Perhaps this is by design as they may want to keep their technique to themselves. But I do want to understand what is going on.
Two instruments seem to be used: a repeat-o-meter and an autocollimator with some kind of target. The repeat-o-meter is a way of checking to see if the curvature of a surface is uniform. BY ITSELF, it does not insure that the surface is flat. Only if it has the same curvature across it's entire surface. So these pros seem to say they do not rely on the repeat-o-meter for the entire measurement. What they seem to do is to use an autocollimator with some kind of target, which is probably a form of a mirror, perhaps with an additional trick or two.
The thing is, an autocollimatoris not basically a way for measuring surface flatness or the deviation from flat, at least not in that orientation. It is an instrument that is used to measure angles, usually small ones. One explanation that I read detailed the measurement of surface flatness with an autocollimator as the taking of a series of angular measurements of a "base" mirror by moving that base mirror by the effective length of it's base at a time and using the measured ANGLE of that base mirror at each position, along with the length of it's base to calculate the amount of rise or fall of the mirror in that distance. In other words, the autocollimator was actually measuring the angle and then trig was used to get the flatness or lack thereof. It was doing exactly the same thing that the repeat-o-meter does and nothing more. Perhaps with more accuracy, but still the same thing.
The problem that I see here is that the surface plate pros were NOT moving the "base" mirror by the length of it's base. They were moving it by much larger distances so any calculation based on the length of it's base would be wildly inaccurate. Or conversely, if they were using the actual length of these movements to calculate the deviation, then they were missing more than half the positions along the lines they were checking. And there is absolutely nothing that says that the curvature was constant along that distance. In fact, the very fact that they were getting different readings, says that the curvature was changing.
Among others, the YouTube video by OxTool seems to illustrate this.
https://www.youtube.com/watch?v=EWqThb9Z1jk
So, what is going on here? Are these "pros" just being deceptive? Perhaps trying to hide their trade secrets? Does their "base" mirror have some kind of optical trick in it? Is there some other kind of trick? Or what? Perhaps they calibrated the repeat-o-meter on a laboratory grade plate they keep on their truck, out of sight, and then go more by the readings of that secretly calibrated repeat-o-meter while the autocollimator was just for show. You know, dazzle the dumb hicks in those shops with the magic, high tech, expensive, optical instrument, but actually work with the plain-Jane repeat-o-meter.
Two instruments seem to be used: a repeat-o-meter and an autocollimator with some kind of target. The repeat-o-meter is a way of checking to see if the curvature of a surface is uniform. BY ITSELF, it does not insure that the surface is flat. Only if it has the same curvature across it's entire surface. So these pros seem to say they do not rely on the repeat-o-meter for the entire measurement. What they seem to do is to use an autocollimator with some kind of target, which is probably a form of a mirror, perhaps with an additional trick or two.
The thing is, an autocollimatoris not basically a way for measuring surface flatness or the deviation from flat, at least not in that orientation. It is an instrument that is used to measure angles, usually small ones. One explanation that I read detailed the measurement of surface flatness with an autocollimator as the taking of a series of angular measurements of a "base" mirror by moving that base mirror by the effective length of it's base at a time and using the measured ANGLE of that base mirror at each position, along with the length of it's base to calculate the amount of rise or fall of the mirror in that distance. In other words, the autocollimator was actually measuring the angle and then trig was used to get the flatness or lack thereof. It was doing exactly the same thing that the repeat-o-meter does and nothing more. Perhaps with more accuracy, but still the same thing.
The problem that I see here is that the surface plate pros were NOT moving the "base" mirror by the length of it's base. They were moving it by much larger distances so any calculation based on the length of it's base would be wildly inaccurate. Or conversely, if they were using the actual length of these movements to calculate the deviation, then they were missing more than half the positions along the lines they were checking. And there is absolutely nothing that says that the curvature was constant along that distance. In fact, the very fact that they were getting different readings, says that the curvature was changing.
Among others, the YouTube video by OxTool seems to illustrate this.
https://www.youtube.com/watch?v=EWqThb9Z1jk
So, what is going on here? Are these "pros" just being deceptive? Perhaps trying to hide their trade secrets? Does their "base" mirror have some kind of optical trick in it? Is there some other kind of trick? Or what? Perhaps they calibrated the repeat-o-meter on a laboratory grade plate they keep on their truck, out of sight, and then go more by the readings of that secretly calibrated repeat-o-meter while the autocollimator was just for show. You know, dazzle the dumb hicks in those shops with the magic, high tech, expensive, optical instrument, but actually work with the plain-Jane repeat-o-meter.
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