Ballscrew driven setups and their speed limitation?
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Hi, I recently found this and this both showing Cartesian printers with one or two axis driven by ball screws.
I quickly did some speed calculations based on numbers I found on the forum:
Delta setup: 120khz step rate / (80 steps/mm * 16 micro steps) = 93.75 mm/sec, which seems realistic.
When I apply a 10mm pitch ball screw from the first vid I get: 120khz step rate / (320 steps/mm * 16 micro steps) = 23.4375 mm/sec, which seems too slow.
However, I'm not quite sure I calculated that right, because I've seen much faster printer e.g. deckingman reporting 300mm/sec here and many slicing profiles use 120-150mm/sec for travel moves. Even assuming that going beyond these calculated max speeds will only gradually increase positional error, 23mm/sec seems far away from usable speeds.
What am I missing?Cheers
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with 320 steps/mm do you need microstepping?
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@bearer said in Ballscrew driven setups and their speed limitation?:
with 320 steps/mm do you need microstepping?
good point, going to 4 microsteps would leave us at 80 steps/mm that would be fine, although losing the 256x interpolation would be a shame
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@sungod3k said in Ballscrew driven setups and their speed limitation?:
good point, going to 4 microsteps would leave us at 80 steps/mm that would be fine, although losing the 256x interpolation would be a shame
Using x4 microstepping would also make the machine much noisier. Except that if you can use a Duet Maestro (or eventually a Duet 3) then the drivers can interpolate to x256 from any microstep setting.
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thats good to know.
using 20mm pitch screw at 4 micro steps gets us theoretically to 187.5 mm/s. so there is way...
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@sungod3k said in Ballscrew driven setups and their speed limitation?:
When I apply a 10mm pitch ball screw from the first vid I get: 120khz step rate / (320 steps/mm * 16 micro steps) = 23.4375 mm/sec, which seems too slow.
What am I missing?
Cheers
I think you have applied micro stepping twice in your calculations. What we really need to know is the lead of the ball screw not the pitch. The lead is how far a nut would travel in revolution of the screw. The pitch is the distance between thread peaks. It just happens that with a single start screw, the lead and the pitch happen to be the same.
However, assuming your screws are single start, and the lead is indeed 10mm, then it means that 1 revolution of the screw would give 10mm of linear travel. Also assuming that you are using 1.8 degree motors then 1 revolution would be 200 full steps. So we then have 20 full steps per mm and if you use 16X micro-stepping, that then becomes 320 micro steps.
So assuming the step pulse rate is 120kHz (it's usually 200kHz for a single motor) then the calculation becomes 120,000/320 which gives 375mm/sec.
Edit. In summary, I think you've taken 320 micro steps per mm (at 16x) then multiplied that by 16 again which gives you 5,120 micro steps per mm.
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Ah right, indeed micro stepping is multiplied twice. Thanks for spotting that.
With that, the higher speeds make much more sense now and 10mm lead/pitch should be sensible.
Ill guess i have a new project now^^
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You should be looking at acceleration as well as speed. A drawback of screws in a 3d printer is that the screw is attached directly to the motor shaft and therefore needs to be spun up without any mechanical advantage.
Every gram of screw might have the same inertia as 5-10 grams of extruder weight. The exact ratio will vary dramatically but is a function of screw diameter and lead.
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@691175002 very good points.
A ball screw needs to be spun up -- acceleration will be very slow. This is especially the case when using screws with high leads like 16 mm or 20 mm. These screws are usually very large in diameter.
There is, however, a concept where the ball nut is held captive in the rotor of the motor itself, and the motor is held stationary on the axis. When it moves, the motor and nut travel along the stationary screw. This alleviates the drawback of the inertia of the screw.
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Here is a non-captive option.
https://www.robotdigg.com/product/32/Non-captive-Nema17-Linear-Stepper -
@jmjcoke oops. Yeah that's it. NON-captive. Heh.
I guess a ballscrew version of this would be much much larger than we would want. I suppose someone could adapt a ball screw to act in this way but it would be cumbersome.
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@sungod3k Those video links you posted aren't exactly ringing endorsements for using ball screws honestly. I certainly wouldn't want to spend much time in the same room as either of those printers while running. Between the noise and reduced print quality or print speed, and cost, what exactly are the advantages other than saying it can be done?
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Rotational inertia: yes thats something to watch out for. ill look into that.
@Phaedrux: Im not sure if the quality will go down, if that were the case that it would indeed not be worth it. From my amateur view of the motion system I would imagine, that the acceleration effects on the print head would be reduced. That might not give us top speeds and the rotational inertia could indeed slow down the movements too much, but that aside maybe we get more stability in the upper third of speeds.
also I just found this from dyze. theyre using 4mm pitch in their calculation and the end up cautiously max speed of 100mm/s. ill try to repeat those calculations for other diameters and pitches over the weekend.
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@sungod3k To clarify a bit what I said before. I guess I see it as a trade off between speed and quality that goes beyond what we typically already experience, in that you have to reduce acceleration enough that you may not be able to hit your top possible speeds on anything but larger objects in order to keep quality acceptable.
In a CNC or a large format printer that is using volumetric throughput rather than motion speed to reduce print times I could see it being more applicable.
Regardless, I'm very interested to see how it turns out.
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@sungod3k I think that if you are going to make that comparison, then you need to compare the print quality using ball screws with the print quality using belts at the same speed and acceleration. With ball screws you have to slow things down, with belts you have the choice.
It's not for me to say which you should use - that is your choice. But in general terms, ball screws are used on industrial machinery which can have very large moving masses because belts would need to be too big. If you believe in the myth that timing belts stretch, then you could argue that ball screws don't suffer that problem. On the other hand, belts aren't probe to having issues with back lash which ball screws can be.
But hey, this is all part of the fun of this hobby. If you want to build a machine using ball screws, go for it. I'm sure it could be capable of producing good quality prints albeit a bit slower and noisier.
