5 ms·

I’ve been told that magnetic linear motors have much better accuracy.

jacquesm 3y ago
In principle they do, as do servos, you'll find neither on machines like this. An industrial servo + driver (a single one) costs more than this whole rig. I've designed CNC gear for a living and I'm aware of the way things are done in industry. I think this low cost approach opens up an entirely now domain and if you're making boxes, wooden toys and decorative pieces the accuracy requirements are much less critical than they would be if you were to make parts for aerospace or such, but nobody is going to attempt to do that using a rig like this.
- kragen 3y ago
  i feel like if brother can make an inkjet printer for a hundred bucks that gets positioning repeatability well below 100 microns with an optical servo, laser engraver makers can do the same thing. my capacitive digital calipers cost like ten dollars and are also in that precision range. optical, magnetic, or capacitive servos don't need to cost thousands
  - ComputerGuru 3y ago
    You have no idea how much this question has been vexing me! I gave up on the development of a public good product because I couldn’t answer that very question (low cost braille reader). I couldn’t get it to work seamlessly without high precision and couldn’t achieve high precision at low cost. Bought a couple of cheap inkjets and stripped them for parts, found proprietary optical strips and encoders, but still couldn’t figure out how they managed to machine/manufacture the plastic/nylon/POM parts to such high precision and still make a profit. In the end, I surmised they don't make a profit off the parts (though selling at a loss is illegal in the EU?) and rely on the cartridges to make money, but the bigger part of the equation is that they probably have those parts manufactured in massive numbers and with highly tuned and optimized designs carefully matched to the manufacturing process and the application. I even put out ads trying to hire someone that’s worked as an (electro)mechanical engineer at an inkjet company to hire on a contract basis but got no responses. It’s possible those are mainly outsourced - or that the know how turned into domain knowledge that can’t be reproduced these days!
    - jacquesm 3y ago
      Making things in prototype form: easy. Making things production grade: hard. Making things economically production grade: super hard.
    - kragen 3y ago
      i don't think the nylon and delrin parts have to be high precision; the way i see it, all that matters is that the displacement between the optical sensor and the print head is constant and that the plastic tape isn't stretched so far that the printed page looks wrong, and that the print head stays more or less the same height off the paper and, more importantly, exactly the same angle backlash, variable friction, motor power variation due to voltage, belt stretch, most flexions of the frame — all of that should just be 'external disturbances' that the negative feedback system automatically corrects. only the position feedback itself (and the time of actuation of the inkjets) has to be precise, that's the magic of negative feedback as for the optical strips and encoders, i figured that a 600dpi laser printer printing on laser printer transparency film should be able to print a light/dark transition every 42μm, though it might take some fiddling to get that to actually work. supposedly 1200×1200 dpi laser printers also exist on the market for US$300. the standard way inkjet printers do this seems to be with a slit that's only slightly wider than the size of a single stripe, but a second transparency with the same 50% black pattern would also work, producing a moiré pattern (though with a viewing angle of only 25° or so due to the thicknesses of the transparent films). let me know if this is unclear, i'll make an animation or something with a quadrature cycle (as the inkjet printer sensors seem to use, according to the datasheets i've managed to find) every 84μm you get a full cycle, so you get a transition every 21μm and you know your position ±10.5μm. that's half a thou, good enough for machining a piston if you don't truncate the brightness to one bit, though, you can measure the phase within the cycle to probably within a tenth of a cycle, so you get ±4μm as for who did the mechanical engineering, i suspect that it's something like ten people in the world, half of them retired. dissecting printers from different decades i see an astounding degree of similarity from one decade to the next
      - jacquesm 3y ago
        The one exception is probably if you want to screw bolts straight in without any kind of prep work (tapping) on the hole. Then accuracy matters, too much slop and your bolt won't hold or it will strip the material, too little and you may well end up snapping the bolt, especially a thin one. Apropos machining pistons: the bigger issue with anything that needs a reliable 'Z' dimension on any kind of cutter like this (essentially a two-dimensional device) is that that third dimension is really only well specified at the point of focus. Outside of that it is more or less conical depending on the kind of cutter and the optics in case of a laser. Waterjet, plasma and laser all have different characteristics depending on what you cut with and in case of a laser the construction of the head and the kind of optics installed. Plasma also has work hardening effects that can not be ignored. The only economical way to accurately cut large pieces of thick material is by using a heavy gantry mill or an EDM machine. Both will still be very costly and this sort of use is probably outside of the hobby arena anyway. If you need that kind of work piece I would suggest outsourcing it.
- DannyBee 3y ago
  Steppers with encoders aren't that expensive. Surestep is an example. 150 bucks for a nema34 motor with plenty of torque that won't lose steps. Nema23 is like 50 bucks. That is the middle ground that won't lose steps but isn't as powerful. I agree linear motors are well overkill for this. They are mainly useful when you need very high acceleration, which isn't true here
  - jacquesm 3y ago
    The drive electronics on the budget machines have no easy way to add feedback mechanisms. So you'd somehow have to either roll your own driver electronics or do major surgery on the existing board. Since I don't actually have a feedback issue at all right now I'll just leave it as it is but if this becomes an issue I will definitely look into it. I still have a small form servo set + drivers laying around from another project so it isn't as if I'm wanting for hardware. And it would be nice to finally put that to use, even though it is probably a bit much for this machine.
    - numpad0 3y ago
      I haven't encountered other happy customers of it, but for a bolt-on closed loop upgrade I'd recommend MKS SERVO42 series. It's a hobbyist type product that comes from AliExpress no enclosures or safety protections, and slots right into RAMPS driver ports. It just needs an initial calibration, no software changes needed at the host side.
      - jacquesm 3y ago
        That's interesting, ok, I will look into this. Thank you!

In principle they do, as do servos, you'll find neither on machines like this. An industrial servo + driver (a single one) costs more than this whole rig. I've designed CNC gear for a living and I'm aware of the way things are done in industry. I think this low cost approach opens up an entirely now domain and if you're making boxes, wooden toys and decorative pieces the accuracy requirements are much less critical than they would be if you were to make parts for aerospace or such, but nobody is going to attempt to do that using a rig like this.

kragen 3y ago
i feel like if brother can make an inkjet printer for a hundred bucks that gets positioning repeatability well below 100 microns with an optical servo, laser engraver makers can do the same thing. my capacitive digital calipers cost like ten dollars and are also in that precision range. optical, magnetic, or capacitive servos don't need to cost thousands
- ComputerGuru 3y ago
  You have no idea how much this question has been vexing me! I gave up on the development of a public good product because I couldn’t answer that very question (low cost braille reader). I couldn’t get it to work seamlessly without high precision and couldn’t achieve high precision at low cost. Bought a couple of cheap inkjets and stripped them for parts, found proprietary optical strips and encoders, but still couldn’t figure out how they managed to machine/manufacture the plastic/nylon/POM parts to such high precision and still make a profit. In the end, I surmised they don't make a profit off the parts (though selling at a loss is illegal in the EU?) and rely on the cartridges to make money, but the bigger part of the equation is that they probably have those parts manufactured in massive numbers and with highly tuned and optimized designs carefully matched to the manufacturing process and the application. I even put out ads trying to hire someone that’s worked as an (electro)mechanical engineer at an inkjet company to hire on a contract basis but got no responses. It’s possible those are mainly outsourced - or that the know how turned into domain knowledge that can’t be reproduced these days!
  - jacquesm 3y ago
    Making things in prototype form: easy. Making things production grade: hard. Making things economically production grade: super hard.
  - kragen 3y ago
    i don't think the nylon and delrin parts have to be high precision; the way i see it, all that matters is that the displacement between the optical sensor and the print head is constant and that the plastic tape isn't stretched so far that the printed page looks wrong, and that the print head stays more or less the same height off the paper and, more importantly, exactly the same angle backlash, variable friction, motor power variation due to voltage, belt stretch, most flexions of the frame — all of that should just be 'external disturbances' that the negative feedback system automatically corrects. only the position feedback itself (and the time of actuation of the inkjets) has to be precise, that's the magic of negative feedback as for the optical strips and encoders, i figured that a 600dpi laser printer printing on laser printer transparency film should be able to print a light/dark transition every 42μm, though it might take some fiddling to get that to actually work. supposedly 1200×1200 dpi laser printers also exist on the market for US$300. the standard way inkjet printers do this seems to be with a slit that's only slightly wider than the size of a single stripe, but a second transparency with the same 50% black pattern would also work, producing a moiré pattern (though with a viewing angle of only 25° or so due to the thicknesses of the transparent films). let me know if this is unclear, i'll make an animation or something with a quadrature cycle (as the inkjet printer sensors seem to use, according to the datasheets i've managed to find) every 84μm you get a full cycle, so you get a transition every 21μm and you know your position ±10.5μm. that's half a thou, good enough for machining a piston if you don't truncate the brightness to one bit, though, you can measure the phase within the cycle to probably within a tenth of a cycle, so you get ±4μm as for who did the mechanical engineering, i suspect that it's something like ten people in the world, half of them retired. dissecting printers from different decades i see an astounding degree of similarity from one decade to the next
    - jacquesm 3y ago
      The one exception is probably if you want to screw bolts straight in without any kind of prep work (tapping) on the hole. Then accuracy matters, too much slop and your bolt won't hold or it will strip the material, too little and you may well end up snapping the bolt, especially a thin one. Apropos machining pistons: the bigger issue with anything that needs a reliable 'Z' dimension on any kind of cutter like this (essentially a two-dimensional device) is that that third dimension is really only well specified at the point of focus. Outside of that it is more or less conical depending on the kind of cutter and the optics in case of a laser. Waterjet, plasma and laser all have different characteristics depending on what you cut with and in case of a laser the construction of the head and the kind of optics installed. Plasma also has work hardening effects that can not be ignored. The only economical way to accurately cut large pieces of thick material is by using a heavy gantry mill or an EDM machine. Both will still be very costly and this sort of use is probably outside of the hobby arena anyway. If you need that kind of work piece I would suggest outsourcing it.
DannyBee 3y ago
Steppers with encoders aren't that expensive. Surestep is an example. 150 bucks for a nema34 motor with plenty of torque that won't lose steps. Nema23 is like 50 bucks. That is the middle ground that won't lose steps but isn't as powerful. I agree linear motors are well overkill for this. They are mainly useful when you need very high acceleration, which isn't true here
- jacquesm 3y ago
  The drive electronics on the budget machines have no easy way to add feedback mechanisms. So you'd somehow have to either roll your own driver electronics or do major surgery on the existing board. Since I don't actually have a feedback issue at all right now I'll just leave it as it is but if this becomes an issue I will definitely look into it. I still have a small form servo set + drivers laying around from another project so it isn't as if I'm wanting for hardware. And it would be nice to finally put that to use, even though it is probably a bit much for this machine.
  - numpad0 3y ago
    I haven't encountered other happy customers of it, but for a bolt-on closed loop upgrade I'd recommend MKS SERVO42 series. It's a hobbyist type product that comes from AliExpress no enclosures or safety protections, and slots right into RAMPS driver ports. It just needs an initial calibration, no software changes needed at the host side.
    - jacquesm 3y ago
      That's interesting, ok, I will look into this. Thank you!