7 ms·

This talks about accuracy, which is about positioning accuracy, but not repeatability, without which accuracy is mostly worthless. They are also often giving u

This talks about accuracy, which is about positioning accuracy, but not repeatability, without which accuracy is mostly worthless. They are also often giving unidirectional accuracy numbers, which doesn't account for backlash in positioning (let alone repeatability). Think of it as (this is not perfectly right but good enough for this explanation): unidirectional accuracy - I command a position from 0. Where am I? bidirectional accuracy - I command a position from 0. Where am I? I command a position in the opposite direction. Where am I? repeatability - I command a position from 0. Where am I? I command us back to zero. Where am I? Repeat. Compare results. It is true that, given a single directional commanded position from 0, the accuracy is likely to be 0.1mm or better. But one missing factor in whether that will be true repeatedly is not just how it is driven, but what is being driven. Is it lead screws? ball screws? gears on rack + pinion? Nothing? If it's nothing, you will have repeatability issues, because both stepper motors alone, and stepper motors + belts, accumulate error through backlash. It is possible to get a lot of the backlash out using the right types of belts or gearboxes or what have you. But even at this price point, you probably want some mechanism (helical rack + pinion, ballscrew, etc) that ensures repeatability, to ensure your accuracy will be worthless.

rkagerer 3y ago
What do you think of magnetic linear motors like the one in the new Peoploly Magneto X? Does it help achieve substantial improvements in backlash? BTW I would have expected accuracy would be quoted from a fixed reference point, that would coalesce all the repeatability scenarios you laid out into a single 'worst case', error-does-not-exceed value. (Are you implying with low accuracy but high repeatability you could get good results that are, conceptually, simply offset by whatever margin? Or that you'll get jagged edges / artifacts / incomplete or overdone cuts at scales below the stated accuracy threshold eg. if you have a corner approached by cuts from different ends?)
- carterschonwald 3y ago
  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
  They still have backlash, but less so, it all depends on the masses of the gantry and the head and the speeds at which it moves. Whether it's a magnetic field or a belt doesn't matter that much, in principle there is some elasticity in the system and the height of the head above the gantry rails is a big factor in how much slop there is. In practice you'll cut at speeds low enough that these things aren't an immediate issue, though when you start to cut cardboard or paper at near maximum speeds there likely will be some artifacts. I will try to find the limits of the machine to see at what speeds these issues become apparent. Given the flimsy construction I'm amazed at how well it does, to be honest I had not expected it to be as immediately usable as it is. Cutting 18 mm plywood with a < 0.5 mm kerf and < 0.1 mm repeat accuracy - especially compared to all of the other tools I have access to - is incredibly precise. In metal it wouldn't be all that impressive, but that's not what this thing is intended to do. Anyway, the article wasn't intended as a treatment on CNC accuracy issues, there are many more that the GP hasn't touched on (such as: positioning errors due to temperature variation, which with aluminum frames can be considerable, and frames being out-of-true). It's a dancing pig: it dances, that's the amazing thing, how it holds up compared to industrial machines that cost 400x as much isn't all that relevant.
  - DannyBee 3y ago
    I do think repeatability is very important. If you try to cut a circle and it ends up not even close to because you can't circle around a point twice without it being close to the same circle, .... The rest i mostly agree with except maybe the 400x number. Seems high to get to a better level.
    - jacquesm 3y ago
      For the price point the repeatability is uncanny. Of course this machine is still reasonably fresh so we'll see how it holds up over time but on 10 complete passes over a work piece that spans 80% or so of the total work surface the last cut is dead on on top of the first. I'm actually quite surprised at this, I did not expect that to be the case. But: this is my first laser (previous CNC tooling: Lathe, Mill, plasmacutter, the latter a homebrew affair at 8x4') and the main advantage that it seems to have over the other tools that I worked with is that the gantry and the head are fairly light in comparison to what you would normally expect. Even a plasmacutter requires a movable Z in order to compensate for warp (or you will definitely have material strikes). So the head is probably < 1 Kg all together and the gantry < 10. This most likely is the biggest factor in how with such a light and - bluntly - flimsy drive mechanism it works as well as it does. It's got less backpressure than a pen plotter would have, basically just the rolling resistance of the rollers and the drag from the airhose and a thin electrical cable. I did add a segmented chain for the main gantry to ensure the cables and hose can never get tangled. As for the 400x, an industrial laser from a brand with a good rep runs between 20 and 40K, these open frame lasers sell from anywhere between 500 and a thousand $US, I mistakenly added a zero too much so you are right about that! I spent a whole day on writing that up and was super tired, I slept a bit since and it's much better now.
jfoutz 3y ago
These are excellent points. I think, these are sort of problems after you "level up". Lasers and 3d printers both have pretty light toolheads and no resistance. So you can get away with being pretty half assed with your stepper control, and still get results. You're not wrong. But there's a big mountain of stuff to learn when starting with CNC stuff. Getting a cheap machine, figuring it out, tinkering with it, improving repeatability, these things are all part of how you get better. This is sort of a hobbyist diy mindset. Even if it's not a very good machine, you can still get results out of fusion360, or scad or whatever toolchain you work through. Now, you're absolutely right, if you've got some cash burning a hole in your pocket, you can skip a lot of that machine tinkering hassle. pro level gear is absolutely magical. I'm more of a dabbler kinda guy, try it out, learn about it, if it seems cool 10x my investment in the hobby. Anybody who wants to turn a laser cutter into a business is going to know all the stuff you've addressed. A hobbyist, they're going to need to not die from toxic gas from tanned leather. they're going to need to work out if they want to make stuff or if they want to tinker with the machine (both are totally valid). To be super clear, I agree with all of your points. And it would be good to indicate the suffering you'll go through with a cheap machine. But, I'd argue a cheap machine can still be really fun. It all depends on what you're after.
- jacquesm 3y ago
  It's all about the work piece. If you have that kind of requirement you most likely aren't going to be making it out of wood anyway, after all wood is an organic material and things like moisture content and temperature and moisture related shrinkage and expansion are going to undo pretty much all of your efforts to achieve 100ths of mm worth of precision. It would cost a fortune and you'd end up with workpieces that are no better than what these cheap machines produce. If you were to improve on this I'd invest in a better laser head long before I'd start to worry about the final bits of precision because for that you are using the wrong material to begin with. Woodwork to within 0.1 mm is insanely precise. You won't be making watches with this, but a mechanical clock with wooden gears is well within the realm of the possible and your accuracy will be much better than that of the best woodworker using non-CNC tooling.
  - 3y ago
jacquesm 3y ago
None of these cheap machines have rack + pinion or ballscrews, that's simply not available at this budget and if it is you'll end up with a machine so small that it is probably worthless. But you probably already knew that. You could retrofit it onto an existing machine but by the time you're done it would cost more than the original and the improvement would be too small to notice. While we're at it let's throw in servos and focus compensation as well as a movable Z axis... But now the machine is priced out of hobbyist territory. An aluminum open frame machine like this is not aimed at industry and so should come with lesser expectations. It doesn't even compensate for thermal expansion of the frame. The belts are usually quite good in quality, contrary to your assertion belt drives do not accumulate backlash (though they will have some it is more or less constant as long as you don't lose steps, which normally should not happen), have a Kevlar component in them to remove a lot of the stretch issues that you'd have with cheaper belts and either the gantry is moved with two steppers or there is a cross gantry shaft which operates a passive gear (without a motor) on the other side. Obviously this isn't perfect, the shaft is long enough that it will see some torsion so when moving fast one side will lag a bit and when you come to a sudden stop you'll see some overshoot. But even at very high speeds and long series of repetitions (100's) I've yet to see any backlash or 'slop' that is visible or measurable with the tools that I currently have at my disposal. This is funny because I totally expected to find a measurable positioning error but given a nice micrometer I find the positioning error when the machine comes to a stop after many 100's of meters of travel to be < 0.1 mm and positioning error from the origin to any point on the machine to be well within the acceptable. The thing that you will notice is that because of the open construction of the frames that the machines aren't going to be square 'out of the box', and you'll spend quite a bit of time getting them to be just so. I don't have access to an interferometer but if I can get my hands on one for bit by borrowing one somewhere I'll do some measurements on it but for now my simple tests suffice to show that the machine is quite usable and produces output that is dimensionally accurate to the point that it makes zero sense to farm out jobs to professional laser cutting services. If I were cutting metal (which you won't be doing with a diode laser for obvious reasons) it would be a different matter, but even there in sheet cutting the tolerances on larger work are different than they are with for instance a mill or a lathe. You won't be making any press-fit shafts with a machine like this, nor will you be cutting gears with 1 mm teeth. For that kind of work it just isn't the right tool, and lasercutting isn't the right process. If you want that kind of precision in sheetmetal you would probably either use a mill (but then your workpieces will likely be small) or you'd etch your workpiece after a photographic process to create a mask. When working in wood, cardboard or textile the precision that these cheap machines offer is ample.

rkagerer 3y ago

What do you think of magnetic linear motors like the one in the new Peoploly Magneto X? Does it help achieve substantial improvements in backlash? BTW I would have expected accuracy would be quoted from a fixed reference point, that would coalesce all the repeatability scenarios you laid out into a single 'worst case', error-does-not-exceed value. (Are you implying with low accuracy but high repeatability you could get good results that are, conceptually, simply offset by whatever margin? Or that you'll get jagged edges / artifacts / incomplete or overdone cuts at scales below the stated accuracy threshold eg. if you have a corner approached by cuts from different ends?)

carterschonwald 3y ago
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
They still have backlash, but less so, it all depends on the masses of the gantry and the head and the speeds at which it moves. Whether it's a magnetic field or a belt doesn't matter that much, in principle there is some elasticity in the system and the height of the head above the gantry rails is a big factor in how much slop there is. In practice you'll cut at speeds low enough that these things aren't an immediate issue, though when you start to cut cardboard or paper at near maximum speeds there likely will be some artifacts. I will try to find the limits of the machine to see at what speeds these issues become apparent. Given the flimsy construction I'm amazed at how well it does, to be honest I had not expected it to be as immediately usable as it is. Cutting 18 mm plywood with a < 0.5 mm kerf and < 0.1 mm repeat accuracy - especially compared to all of the other tools I have access to - is incredibly precise. In metal it wouldn't be all that impressive, but that's not what this thing is intended to do. Anyway, the article wasn't intended as a treatment on CNC accuracy issues, there are many more that the GP hasn't touched on (such as: positioning errors due to temperature variation, which with aluminum frames can be considerable, and frames being out-of-true). It's a dancing pig: it dances, that's the amazing thing, how it holds up compared to industrial machines that cost 400x as much isn't all that relevant.
- DannyBee 3y ago
  I do think repeatability is very important. If you try to cut a circle and it ends up not even close to because you can't circle around a point twice without it being close to the same circle, .... The rest i mostly agree with except maybe the 400x number. Seems high to get to a better level.
  - jacquesm 3y ago
    For the price point the repeatability is uncanny. Of course this machine is still reasonably fresh so we'll see how it holds up over time but on 10 complete passes over a work piece that spans 80% or so of the total work surface the last cut is dead on on top of the first. I'm actually quite surprised at this, I did not expect that to be the case. But: this is my first laser (previous CNC tooling: Lathe, Mill, plasmacutter, the latter a homebrew affair at 8x4') and the main advantage that it seems to have over the other tools that I worked with is that the gantry and the head are fairly light in comparison to what you would normally expect. Even a plasmacutter requires a movable Z in order to compensate for warp (or you will definitely have material strikes). So the head is probably < 1 Kg all together and the gantry < 10. This most likely is the biggest factor in how with such a light and - bluntly - flimsy drive mechanism it works as well as it does. It's got less backpressure than a pen plotter would have, basically just the rolling resistance of the rollers and the drag from the airhose and a thin electrical cable. I did add a segmented chain for the main gantry to ensure the cables and hose can never get tangled. As for the 400x, an industrial laser from a brand with a good rep runs between 20 and 40K, these open frame lasers sell from anywhere between 500 and a thousand $US, I mistakenly added a zero too much so you are right about that! I spent a whole day on writing that up and was super tired, I slept a bit since and it's much better now.

jfoutz 3y ago

These are excellent points. I think, these are sort of problems after you "level up". Lasers and 3d printers both have pretty light toolheads and no resistance. So you can get away with being pretty half assed with your stepper control, and still get results. You're not wrong. But there's a big mountain of stuff to learn when starting with CNC stuff. Getting a cheap machine, figuring it out, tinkering with it, improving repeatability, these things are all part of how you get better. This is sort of a hobbyist diy mindset. Even if it's not a very good machine, you can still get results out of fusion360, or scad or whatever toolchain you work through. Now, you're absolutely right, if you've got some cash burning a hole in your pocket, you can skip a lot of that machine tinkering hassle. pro level gear is absolutely magical. I'm more of a dabbler kinda guy, try it out, learn about it, if it seems cool 10x my investment in the hobby. Anybody who wants to turn a laser cutter into a business is going to know all the stuff you've addressed. A hobbyist, they're going to need to not die from toxic gas from tanned leather. they're going to need to work out if they want to make stuff or if they want to tinker with the machine (both are totally valid). To be super clear, I agree with all of your points. And it would be good to indicate the suffering you'll go through with a cheap machine. But, I'd argue a cheap machine can still be really fun. It all depends on what you're after.

jacquesm 3y ago
It's all about the work piece. If you have that kind of requirement you most likely aren't going to be making it out of wood anyway, after all wood is an organic material and things like moisture content and temperature and moisture related shrinkage and expansion are going to undo pretty much all of your efforts to achieve 100ths of mm worth of precision. It would cost a fortune and you'd end up with workpieces that are no better than what these cheap machines produce. If you were to improve on this I'd invest in a better laser head long before I'd start to worry about the final bits of precision because for that you are using the wrong material to begin with. Woodwork to within 0.1 mm is insanely precise. You won't be making watches with this, but a mechanical clock with wooden gears is well within the realm of the possible and your accuracy will be much better than that of the best woodworker using non-CNC tooling.
- 3y ago

jacquesm 3y ago

None of these cheap machines have rack + pinion or ballscrews, that's simply not available at this budget and if it is you'll end up with a machine so small that it is probably worthless. But you probably already knew that. You could retrofit it onto an existing machine but by the time you're done it would cost more than the original and the improvement would be too small to notice. While we're at it let's throw in servos and focus compensation as well as a movable Z axis... But now the machine is priced out of hobbyist territory. An aluminum open frame machine like this is not aimed at industry and so should come with lesser expectations. It doesn't even compensate for thermal expansion of the frame. The belts are usually quite good in quality, contrary to your assertion belt drives do not accumulate backlash (though they will have some it is more or less constant as long as you don't lose steps, which normally should not happen), have a Kevlar component in them to remove a lot of the stretch issues that you'd have with cheaper belts and either the gantry is moved with two steppers or there is a cross gantry shaft which operates a passive gear (without a motor) on the other side. Obviously this isn't perfect, the shaft is long enough that it will see some torsion so when moving fast one side will lag a bit and when you come to a sudden stop you'll see some overshoot. But even at very high speeds and long series of repetitions (100's) I've yet to see any backlash or 'slop' that is visible or measurable with the tools that I currently have at my disposal. This is funny because I totally expected to find a measurable positioning error but given a nice micrometer I find the positioning error when the machine comes to a stop after many 100's of meters of travel to be < 0.1 mm and positioning error from the origin to any point on the machine to be well within the acceptable. The thing that you will notice is that because of the open construction of the frames that the machines aren't going to be square 'out of the box', and you'll spend quite a bit of time getting them to be just so. I don't have access to an interferometer but if I can get my hands on one for bit by borrowing one somewhere I'll do some measurements on it but for now my simple tests suffice to show that the machine is quite usable and produces output that is dimensionally accurate to the point that it makes zero sense to farm out jobs to professional laser cutting services. If I were cutting metal (which you won't be doing with a diode laser for obvious reasons) it would be a different matter, but even there in sheet cutting the tolerances on larger work are different than they are with for instance a mill or a lathe. You won't be making any press-fit shafts with a machine like this, nor will you be cutting gears with 1 mm teeth. For that kind of work it just isn't the right tool, and lasercutting isn't the right process. If you want that kind of precision in sheetmetal you would probably either use a mill (but then your workpieces will likely be small) or you'd etch your workpiece after a photographic process to create a mask. When working in wood, cardboard or textile the precision that these cheap machines offer is ample.