Core XY - 1.8 or 0.9 steppers for X & Y
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@zapta 16T on the Dbot.
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@phaedrux said in Core XY - 1.8 or 0.9 steppers for X & Y:
16T
Thanks. I think I will switch to 16T as well.
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@zapta That takes me up to 200 steps per mm on X and Y compared to a more frequently seen 80, 100, or 160. I don't know if that level of resolution is helpful or hurtful. For detailed pieces with lots of curve segments maybe it helps? That's 0.005mm of travel per step. Downside is increased step generation load and lower top speeds. Maybe other things I'm not aware of.
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@phaedrux, my motivation is mostly having smooth movement. I don't mind if the print quality will not improve (I am already very happy with it) and don't plan to print very fast, 100-150mm/s is more than enough for me.
A stupid question, if I reduce the steppers' pulleys from 20T to 16T, it's ok to stay with 20T idlers, right? All I need is to move their centers a little bit to have the belts square (?).
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@zapta idler size will depend on whether the critical belt lengths stay parallel or not.
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With regard to 16Teeth pulleys vs 20Teeth, there are a couple of other things which might, or might not have an impact.
Firstly the smaller pulley will have a smaller bend radius for the belt. That may or may not be an issue, although in general terms, a larger bend radius is preferable to a smaller one.
Secondly, the smaller pulley will have less teeth engaged with the belt and so less grip but that really shouldn't be a problem unless one tries to accelerate a large mass at a high rate.
The third thing, which again might not have any significant impact, is the steps per mm in terms of whole steps. It is sometimes useful to consider this as it isn't best practice to rely on microstepping for positional accuracy. A 20 tooth pulley of 2mm pitch means 40mm of travel per revolution. This means either 5 or 10 full steps per mm depending on the step angle of the motor. Or we can say that 1 full step is equal to either 0.2 or 0.1mm. A 16 tooth pulley means 32 mm of travel per revolution so either 6.25 or 12.5 full steps per mm. Or 1 full step equals either 0.16 or 0.08 mm.
Taking a 10mm cube as an example, using a 20 tooth pulley would mean either 50 or 100 full steps per side. Using a 16 tooth pulley would mean either 62.5 of 125 full steps per side. So in that example, the 16 tooth pulley with a 1.8 degree motor would rely on microstepping for positional accuracy (because 10mm equates to 62.5 full steps).
Obviously, it depends on the length of the move but if one were to say that the best positional accuracy would be achieved by using full steps (and I don't know for sure if that is a valid point) then using a 16 teeth pulley the length of the move would need to be divisible by either 0.16 or 0.08 whereas with a 20 tooth pulley the length of the move would need to be divisible by either 0.2 or 0.1.
So my take on that is that the best resolution and accuracy would likely be achieved by using 20 tooth pulleys and 0.9 degree motors (or 2:1 gearing and 1.8 degree motors).
I favour the gearing approach for a number of reasons but that's going too far off topic.
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I don't have a CoreXY printer, but I doubt that changing from 20 to 16 tooth pulleys would make a significant difference. You would probably need to change the motor position slightly to keep variable-length belt segments parallel, because of the smaller pulley diameter. So I suggest you keep the 20-tooth pulleys for now.
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the belts have to be parallel in terms of height, but can vary in x and y position for not critical segments. they dont even need to be the same length.
see: Myth Busting
https://drmrehorst.blogspot.com/2018/08/corexy-mechanism-layout-and-belt.htmlso as long as the belt is not rubbing against anything you can just replace the 20 tooth pulley with a 16 tooth pulley as long as the critical belt segments are parallel.
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"as long as the critical segments are parallel".
but they will never be if you switch from 20T to 16T without moving the pulleys holes **.
you just can't modify the number of tooth of the pulleys without critical segments to lose parallelism.
because pulleys on both sides of the belt. it would be possible if every pulley or idler was running on the same side of the belt, but it's not the case, whatever belt arrangement you use, there will always be pulleys or idler on both side of the belt on critical segments.** Which is actually impossible because the center of the new hole for the 16T pulley would be inside the already existing hole of the 20T pulley. that is almost impossible to cleanly do afterwards.
I'd stick to 0.9° steppers under 24V, and follow duet's guide to choosing steppers and especially the recommended inductance value.
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@tpra said in Core XY - 1.8 or 0.9 steppers for X & Y:
but they will never be if you switch from 20T to 16T without moving the pulleys holes **.
Yes, that what I meant by re-dimensioning the parts. I am leaning now toward 16T for the steppers for smoother movement and 20T idlers for less stress on the belt.
BTW, I replaced the two steppers with 0.9deg, still with 20T pulleys, and the resonance/vibration noise I experienced is gone.
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Taking a 10mm cube as an example, using a 20 tooth pulley would mean either 50 or 100 full steps per side. Using a 16 tooth pulley would mean either 62.5 of 125 full steps per side. So in that example, the 16 tooth pulley with a 1.8 degree motor would rely on microstepping for positional accuracy (because 10mm equates to 62.5 full steps).
@deckingman, I think that even in the first example it relies on microstepping because the corner ends are not necessarily on full step boundary. This also applies to the Z axis, even if you use 'magic numbers' your layers can be on a fraction on a full step.
(would be nice if the Duet would be full step aware, e.g. round to full step on idling, allowing to round to full step on homing, etc. This has several advantages such as less stepper noise on idling, less chance of step mismatch between two Z motors, etc).
