1 Topic by Rincewind

  • Rincewind
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Topic: Asymmetric steps for threadless ball screw in new firmware

Apparently with the threadless ball screw there is the issue that movement is not symmetrical, i.e. the amount of steps per mm is not the same upwards and downwards. The latest firmware contains a small patch that allows the definition of different number of steps for the negative movement direction (for the Z axis, that would be TOWARDS the extruder, so moving the bed upwards).

Get the latest firmware from https://github.com/ozadr1an/Solidoodle- … in_v1_beta and set  DEFAULT_AXIS_STEPS_PER_UNIT to the amount needed for a downwards movement (positive movement in the printer reference system), and uncomment 

#define AXIS_STEPS_NEGATIVE           {88,88,2268,138}

by changing the value for the Z axis (the third) to the amount needed for an upward movement (negative movement).

Any comment is appreciated, as I don't have a threadless ball screw and I don't know if this is useful at all.

2 Reply by ITman496

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Re: Asymmetric steps for threadless ball screw in new firmware

Huh.. I've heard in a few places that it's different up then down.  Why is that?  What mechanically causes that?

3 Reply by Rincewind

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Re: Asymmetric steps for threadless ball screw in new firmware

ITman496 wrote:

Huh.. I've heard in a few places that it's different up then down.  Why is that?  What mechanically causes that?

I have no clue and I am also curious to know, but for the moment I'll take that for granted. But if someone knows, any input would be appreciated.

4 Reply by Claghorn

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Re: Asymmetric steps for threadless ball screw in new firmware

ITman496 wrote:

Huh.. I've heard in a few places that it's different up then down.  Why is that?  What mechanically causes that?

I can't even figure out why the thing goes up or down at all without straining my brain. It is welcome to go up or down different amounts :-).

Claghorn's Website

5 Reply by ITman496

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Re: Asymmetric steps for threadless ball screw in new firmware

Well I can see that the bearings are tilted so that when you rotate the rod, it 'rolls uphill' as it spirals.  I just don't understand why it's different up and down.  Is the mount flexing?

6 Reply by 2n2r5

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Re: Asymmetric steps for threadless ball screw in new firmware

I will give this a shot hopefully tonight. Thanks for the great work.

SD3 w/ mods:
Glass bed with QU-BD heat pad upgrade, threadless ballscrew w/ 8mm smooth rod, spectra line belt replacement, lawsy MK5 extruder, Lawsy replacement carriage, E3D hotend, Ramps 1.4 w/ reprap discount controller, DRV8825 drivers, 12v 30A PS, Acrylic case, Overkill Y-idlers, Filament alarm, Extruder fan + more.

2n2r5's Website

7 Reply by BlackestofOz

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Re: Asymmetric steps for threadless ball screw in new firmware

I've been running the threadless ballscrew almost as soon as I got my machine, and noticed the asymmetrical motion almost immediately. I made sure that there was no fault/hysteresis between the smooth rod and the coupler and that they were co-linear/no wobble. Mechanically, the threadless ballscrew relies on friction for linear actuation. If the threadless ballnut were made of a material that didn't deform at all under pressure (an ideal rigid body), the movement would be the same up and down. On a printed ABS threadless ballnut, the pressure of the linear rod pushing out and against the bearings deforms the part itself slightly when gravity isn't working with you. If you had an aluminum or steel variant, the asymmetric motion wouldn't really be noticeable.