5 ms·

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

by 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+).

YZF 7y ago
You're right that one can't compensate for lack of rigidity with software but you can have an inaccurate but rigid structure and compensate on top of that. It's pretty common practice to use some sort of software compensation to improve accuracy (at least in certain applications where it matters). Metal milling machine need to be super rigid because of the cutting forces... You can build super accurate machines (e.g. with granite) that can't be used for milling but can place a tool with a (sub-)micron accuracy. Even with metal you can take lighter cuts and trade off some rigidity for accuracy, but then it's gonna take much longer to get anything done...
- jbay808 7y ago
  An inaccurate but rigid structure, unfortunately, won't hold its calibration. Go for the jig borer, and get them while you still can. It's what Professional Instruments does, and it's what I would do if I had space for one.
  - YZF 7y ago
    It can. Basically a repeatable machine can be made accurate by calibration. It doesn't have to be accurate to start with. If it's not repeatable (i.e. won't hold its calibration) then yeah, it won't ;) But accuracy != repeatability. I've worked on precision machinery (1um accuracy) and we used calibration techniques.
    - jbay808 7y ago
      This is also my field. The issue is, the machine will be repeatible on the short term but not on the long term. It will look like it's behaving well during calibration and the next few months of operation. But if the ways and mating surfaces aren't almost-perfectly straight and flat, they'll experience accelerated wear. (If you're using hydrostatic bearings, they won't work to begin with unless the surface is accurate). Then the calibration is gone. And that's just in the static case. If your ballscrew has uneven pitch, is eccentric, or any number of other issues, you can calibrate it out. But now to move at a constant speed, your servo controller has to drive that inertia at a wobble, and everything shakes.
      - YZF 7y ago
        The machines I worked on had air bearings. No wear. Basically we're talking about something like an error of 10um over 2 meters. It's practically impossible, or cost prohibitive, to remove that error without calibration. The granite surface the air bearings ride on is locally flat but not perfectly accurate over its entire length. I agree this setup is almost perfectly straight and flat. But it's still not accurate without the calibration. Just to be clear, there's weren't milling machines...
colechristensen 7y ago
What kinds of control systems did you use? My ideas are in the realm of developing detailed physical models of machines and developing control systems out of them using precise measurement tools to both tune the model and act as feedback mechanisms. (I'm not talking about machine learning) I don't think my living room floor would support three tons of machine tools unfortunately.

by msds 7y ago

YZF 7y ago

You're right that one can't compensate for lack of rigidity with software but you can have an inaccurate but rigid structure and compensate on top of that. It's pretty common practice to use some sort of software compensation to improve accuracy (at least in certain applications where it matters). Metal milling machine need to be super rigid because of the cutting forces... You can build super accurate machines (e.g. with granite) that can't be used for milling but can place a tool with a (sub-)micron accuracy. Even with metal you can take lighter cuts and trade off some rigidity for accuracy, but then it's gonna take much longer to get anything done...

jbay808 7y ago
An inaccurate but rigid structure, unfortunately, won't hold its calibration. Go for the jig borer, and get them while you still can. It's what Professional Instruments does, and it's what I would do if I had space for one.
- YZF 7y ago
  It can. Basically a repeatable machine can be made accurate by calibration. It doesn't have to be accurate to start with. If it's not repeatable (i.e. won't hold its calibration) then yeah, it won't ;) But accuracy != repeatability. I've worked on precision machinery (1um accuracy) and we used calibration techniques.
  - jbay808 7y ago
    This is also my field. The issue is, the machine will be repeatible on the short term but not on the long term. It will look like it's behaving well during calibration and the next few months of operation. But if the ways and mating surfaces aren't almost-perfectly straight and flat, they'll experience accelerated wear. (If you're using hydrostatic bearings, they won't work to begin with unless the surface is accurate). Then the calibration is gone. And that's just in the static case. If your ballscrew has uneven pitch, is eccentric, or any number of other issues, you can calibrate it out. But now to move at a constant speed, your servo controller has to drive that inertia at a wobble, and everything shakes.
    - YZF 7y ago
      The machines I worked on had air bearings. No wear. Basically we're talking about something like an error of 10um over 2 meters. It's practically impossible, or cost prohibitive, to remove that error without calibration. The granite surface the air bearings ride on is locally flat but not perfectly accurate over its entire length. I agree this setup is almost perfectly straight and flat. But it's still not accurate without the calibration. Just to be clear, there's weren't milling machines...

colechristensen 7y ago

What kinds of control systems did you use? My ideas are in the realm of developing detailed physical models of machines and developing control systems out of them using precise measurement tools to both tune the model and act as feedback mechanisms. (I'm not talking about machine learning) I don't think my living room floor would support three tons of machine tools unfortunately.