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My CoreXYU Printer project

Posted by lars.arvidson 
Re: My CoreXYU Printer project
October 22, 2017 09:09AM
PS - someone recently posted here [forums.reprap.org] about compensating for leadscrew wobble by pre-processing the GCode using a Python script. I've considered supporting this type of compensation in RepRapFirmware. RRF already does XYZ skew compensation, so compensation for leadscrew wobble would be trivial to do if the wobble is constant in amplitude with increasing Z. But I suspect that will often not be the case.


Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]

Large delta printer, and other 3D printer blog postings: [miscsolutions.wordpress.com]

Disclosure: I have a financial interest in sales of the Panel Due, Mini IR height sensor, and Duet WiFi/Duet Ethernet [www.duet3d.com].
Re: My CoreXYU Printer project
October 22, 2017 11:33AM
Thanks for your feedback guys! Lots of things to consider…

I'll have to weigh the bed, I do imagine it will be a few kilos...

Quote
dc42
2. Lifting force. Delta printers only need to lift an effector, which is normally light in weight. You will need to lift the mass of the bed. The required torque needed per motor is:

required_torque = (bed_mass * g)/(pulley_radius * number_of_motors)

The radius of a 16 tooth GT2 pulley is (2 * 16)/(2 * pi) which is close to 5mm.

To ensure good microstepping positioning, the required torque should be no more than about 5% of the motor's holding torque (perhaps 10% at a push) - and that is after you have derated the published holding torque by 0.8 to allow for the fact that the motors will get too hot if you run them above about 80% of rated current, and by a further 0.71 to allow for using microstepping.

I do believe there’s a typo in the required_torque formal, it should be:
required_torque = (bed_mass * g * pulley_radius) / number_of_motors

If I understand you correctly I can only use 0,05 * 0,8 * 0,71 = 2,84% of the steppers holding torque if I want microsteppings to work well?

Quote
dc42
PS - someone recently posted here [forums.reprap.org] about compensating for leadscrew wobble by pre-processing the GCode using a Python script. I've considered supporting this type of compensation in RepRapFirmware. RRF already does XYZ skew compensation, so compensation for leadscrew wobble would be trivial to do if the wobble is constant in amplitude with increasing Z. But I suspect that will often not be the case.

Interesting. Might be hard to calibrate as the auto levelling might change the wobble slightly each time?
Re: My CoreXYU Printer project
October 22, 2017 01:37PM
I can't imagine how you'd account for all the variables in trying to compensate wobble caused by lead screws. Besides the variability of the bend in the screws, there's the axial alignment with the motor, and the variable flexibility of the screws, guide rails, and printer's frame, all of which vary as a function of Z. The problem becomes much harder when you factor in multiple screws and multiple guide rails.

Maybe the nuts could be loosely coupled to the plate they lift in a way that allows them to slide around laterally a couple mm. Then the screws can't generate any lateral forces except for the sliding friction. The nuts have to be prevented from rotating, yet allowed to slide easily in any direction. If a nut were mounted on something like two stacked, very short, linear guides, one aligned in X and the other in Y, both with over-sized holes through them to give the screw some room to move, the bent, misaligned screws would be free to wobble all they want without putting any lateral force on the bed support. Maybe there's an off-the-shelf solution available for this, but probably not cheap. I have purchased very short linear guides on ebay for about $10 each. Drilling a hole through the guide rail would be a problem, so you'd have to cantilever to the sides a little to make it work. It would be sort of bulky, heavy, and expensive if there are multiple screws, but you might then get away with using threaded rods instead of lead screws.

Or you could just build a rigid frame and use fully supported linear guides in Z that would force the screws to flex instead of shifting the bed laterally.

Or use belts that don't create any lateral forces that need to be resisted or compensated. Then you might not even need the rigid frame or fully supported guide rails.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: My CoreXYU Printer project
October 23, 2017 01:10AM
What about connecting the screw nut to the bed via a caster ball wheel?
To keep the nut from rotating you'd also need torque arms with an elbow joint. ( traxxas rod ends? )
Re: My CoreXYU Printer project
October 23, 2017 03:14AM
Quote
the_digital_dentist
I can't imagine how you'd account for all the variables in trying to compensate wobble caused by lead screws. Besides the variability of the bend in the screws, there's the axial alignment with the motor, and the variable flexibility of the screws, guide rails, and printer's frame, all of which vary as a function of Z. The problem becomes much harder when you factor in multiple screws and multiple guide rails.

Yes, I suspect that would be the case. Varying the compensation linearly with Z would be simple to do, but anything more than that would be a nightmare to calibrate. The real killer would I think be multiple leadscrews, because leadscrew wobble might cause bed rotation.


Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]

Large delta printer, and other 3D printer blog postings: [miscsolutions.wordpress.com]

Disclosure: I have a financial interest in sales of the Panel Due, Mini IR height sensor, and Duet WiFi/Duet Ethernet [www.duet3d.com].
Re: My CoreXYU Printer project
October 24, 2017 08:06AM
Anyway, "wobble" is mostly erratic and the result of a poor design/build. It is not that difficult to eliminate or at least bring it to an acceptable level.


"You failed to maintain your weapon, son" (HARRY BROWN )
Re: My CoreXYU Printer project
October 26, 2017 10:22AM
I have decided to try a mount with a bearing above the coupler to stabilize the leadscrew a bit. Having the stepper on top and the leadscrew “hang” down would have been better than have the stepper in its current configuration (fixed at the bottom and floating at the top) but as I want to keep it out of the enclosure that is not possible. The long leadscrew might bend a bit under the weight (or be slightly bent from the beginning) so I might try a thicker leadscrew.


Edited 1 time(s). Last edit at 10/26/2017 10:26AM by lars.arvidson.
Re: My CoreXYU Printer project
October 26, 2017 06:12PM
Looks like what you need is end supports (bearing mounts). Also reduces whip.
The screw passes through a bearing, and for ballscrews anchors the driven end with the flex coupling and motor on the outside.

In my set up, motor is on the top, the spacer houses the flex coupling. Fixed bearing mount directly below, then a foating (free in Z) at the base.

Re: My CoreXYU Printer project
October 26, 2017 09:21PM
Quote
prot0typ1cal
Looks like what you need is end supports (bearing mounts).
[attachment 99211 Hlid-side.jpg]

Your set up looks really nice but do fixing the screw in both ends really help with this problem? I would have thought that it would make it even worse…
Part of my problem might be that my leadscrews are not perfectly straight. I have 1605s on the way (same as you I believe), should arrive in the beginning of next week.
Re: My CoreXYU Printer project
October 26, 2017 11:33PM
Absolutely it helps. Almost all industrial CNC machinery configure ballscrews in this manner.
Yea, they have to be aligned to prevent binding, however it almost guarantees straight, wobble free motion.
Think you'll be way happy with the 1605's.
Re: My CoreXYU Printer project
October 27, 2017 05:02AM
Hobby 3D printers usually only fix one end of the screw because the guide rails and frames of the printers are typically quite flexible. If you fix both ends of a bent screw, it will force the guide rails and frame to flex, displacing the extruder carriage or bed plate creating Z wobble. Industrial positioners fix both ends of the screws (and probably use straight screws) because the manufacturer doesn't know what speed the thing is going to be spinning. If only one end is fixed and you spin the screw fast, it whips. In 3D printers, speeds are low so you can get away with fixing one end of the screw because it isn't going to whip. Industrial positioners are generally more rigid assemblies and use fully supported guide rails or linear guides and a little bend in the screw isn't going to result in much lateral deflection of the bearing blocks or flex in the frame.

Edited 1 time(s). Last edit at 10/27/2017 09:47AM by the_digital_dentist.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: My CoreXYU Printer project
October 27, 2017 09:32AM
Quote
lars.arvidson
I have decided to try a mount with a bearing above the coupler to stabilize the leadscrew a bit. Having the stepper on top and the leadscrew “hang” down would have been better than have the stepper in its current configuration (fixed at the bottom and floating at the top) but as I want to keep it out of the enclosure that is not possible. The long leadscrew might bend a bit under the weight (or be slightly bent from the beginning) so I might try a thicker leadscrew.

In fact this is the correct way to mount a leadscrew. 99% of the hobby 3D printers are wrong in that respect.
Best is to have the motor at top. The leadscrew will work in tension, no buckling then. No need for a second ball bearing, make alignment difficult and not needed because the speed is low.

Edited 1 time(s). Last edit at 10/27/2017 09:32AM by MKSA.

"You failed to maintain your weapon, son" (HARRY BROWN )
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