5 ms·

The black matrix glass in an LCD is typically accurate to a few microns since it's made in a photolithography process. If you couple that with a microscope/cam

by YZF 7y ago

The black matrix glass in an LCD is typically accurate to a few microns since it's made in a photolithography process. If you couple that with a microscope/camera (e.g. mounted where your tool usually goes) you can use that to calibrate your CNC mill across its work area with some machine vision software. This is probably going to be a lot cheaper and simpler than using interferometers and for any practical purpose should have better accuracy than what you can expect to ever get out of a regular CNC mill. I'm not sure if you can just buy those somewhere or you'd take a display apart or something along those lines...

madengr 7y ago
Glass scales for DRO are relatively inexpensive. The Bridgeport EZ-Trak I bought at work uses them in the servo system. So getting static accuracy is probably not too difficult, but the dynamic accuracy is a whole different can of worms. I can’t remember who wrote the white paper, but it described “rounding a square”, and measuring how much tangential material was left over at the flats, as a test of dynamic accuracy.
- jbay808 7y ago
  Even static accuracy is not so easy. The scales will measure the axis position. But when you move the table over to the other end of travel, the shifting weight tilts the knee, and now your cuts that were supposed to be parallel are not. None of the scales measure this rotation. Then, as the machine warms up, differential thermal expansion starts to twist things further...
colechristensen 7y ago
My interests are more in general purpose tools for increasing accuracy, and perhaps using them to increase the accuracy of inaccurate tools through feedback systems. That is, throw out chasing perfection of rigidity, dimension, and flatness of a mill (etc.) Instead keep adding measurement and feedback until the precision of the machine parts doesn't matter. (or, continue to amuse myself and annoy my partner by making messes and spending my disposable income trying)
- stan_rogers 7y ago
  It is, in my opinion, no coincidence that backlash and battiness both start with a B. Keeping up with (semi) predictable flexture would be... well, not a doddle as such, but... if you didn't also have to deal with backlash. Sure, there's a point where everything is, to a first approximation, made of rubber, but it's a lot easier to move that point by modifying or improving the tool than trying to trim the end of a diving board with a laser-assisted pole-mounted pruning saw.
  - colechristensen 7y ago
    My thinking goes, the end of the road is always "everything is rubber" when it comes to precision, so I might as well start off with that reality and see where it can take me. On the other hand: wear. Even the best machines all experience wear all around and there is a lot of work keeping up with it or recovering from it. Keeping precision despite drift from wear and being able to trivially quantify it are really valuable.
    - jbay808 7y ago
      The very best machines don't really ever wear out. The surfaces mate over the full area so even sliding bearings experience hydrodynamic contact. Or, they're built with fluid bearings to begin with. (Assuming they're handled properly and kept clean, of course).
- msds 7y ago
  Having tried this, with some fancy measurement tools and a pretty custom and high-performing control systems, I've concluded that it's a route to madness. Fancy software and feedback turn out to be a very hard way to emulate large, straight pieces of cast iron, and there's no substitute for rigidity once you actually start cutting - vibration is a huge issue, and very tricky to actively damp. Essentially every machine tool that does have crazy accurate realtime feedback (diamond turning for optics, ultra precision milling, etc) starts with a rigid and long-term stable frame and adds the $$$ controls. Granted, my switch over to "more rigid more better" may have gone a bit too far - I now own a 10ee (3200lbs), and am looking for a good jig borer (2400lbs+).

by YZF 7y ago

madengr 7y ago

Glass scales for DRO are relatively inexpensive. The Bridgeport EZ-Trak I bought at work uses them in the servo system. So getting static accuracy is probably not too difficult, but the dynamic accuracy is a whole different can of worms. I can’t remember who wrote the white paper, but it described “rounding a square”, and measuring how much tangential material was left over at the flats, as a test of dynamic accuracy.

jbay808 7y ago
Even static accuracy is not so easy. The scales will measure the axis position. But when you move the table over to the other end of travel, the shifting weight tilts the knee, and now your cuts that were supposed to be parallel are not. None of the scales measure this rotation. Then, as the machine warms up, differential thermal expansion starts to twist things further...

colechristensen 7y ago

My interests are more in general purpose tools for increasing accuracy, and perhaps using them to increase the accuracy of inaccurate tools through feedback systems. That is, throw out chasing perfection of rigidity, dimension, and flatness of a mill (etc.) Instead keep adding measurement and feedback until the precision of the machine parts doesn't matter. (or, continue to amuse myself and annoy my partner by making messes and spending my disposable income trying)

stan_rogers 7y ago
It is, in my opinion, no coincidence that backlash and battiness both start with a B. Keeping up with (semi) predictable flexture would be... well, not a doddle as such, but... if you didn't also have to deal with backlash. Sure, there's a point where everything is, to a first approximation, made of rubber, but it's a lot easier to move that point by modifying or improving the tool than trying to trim the end of a diving board with a laser-assisted pole-mounted pruning saw.
- colechristensen 7y ago
  My thinking goes, the end of the road is always "everything is rubber" when it comes to precision, so I might as well start off with that reality and see where it can take me. On the other hand: wear. Even the best machines all experience wear all around and there is a lot of work keeping up with it or recovering from it. Keeping precision despite drift from wear and being able to trivially quantify it are really valuable.
  - jbay808 7y ago
    The very best machines don't really ever wear out. The surfaces mate over the full area so even sliding bearings experience hydrodynamic contact. Or, they're built with fluid bearings to begin with. (Assuming they're handled properly and kept clean, of course).
msds 7y ago
Having tried this, with some fancy measurement tools and a pretty custom and high-performing control systems, I've concluded that it's a route to madness. Fancy software and feedback turn out to be a very hard way to emulate large, straight pieces of cast iron, and there's no substitute for rigidity once you actually start cutting - vibration is a huge issue, and very tricky to actively damp. Essentially every machine tool that does have crazy accurate realtime feedback (diamond turning for optics, ultra precision milling, etc) starts with a rigid and long-term stable frame and adds the $$$ controls. Granted, my switch over to "more rigid more better" may have gone a bit too far - I now own a 10ee (3200lbs), and am looking for a good jig borer (2400lbs+).