ABC or XYZ stepper motors steps per mm

[Bruce Welford]

Hi all,

I wanted to ask if anyone else is wondering how to measure and calibrate steps per mm for the XYZ stepper motors.  I have looked and really not found a general consensus answer for a delta printer.  Not sure if related to this or not, but I seem to have a problem getting my bed to be "level."   I can calibrate with paper under the nozzle at various locations around the print bed and yet when I go to lay my first layer, the printer will begin fine, but then as it moves say along toward negative Y axis, the extruder will push down(and stop extruding) because it becomes closer to the print bed.  Then as it moves along in space it will rise up, perhaps getting too far from the print bed, then again move back down closer as it finishes the first brim ring of the first layer.  It's like the printer thinks the print bed is all wobbly or wavy.  And I can't figure out why this happens.  Any insight would be greatly appreciated.

Bruce Welford

General Manager

Rapid Response Ink, Toner & 3D Filament  

[Dr. Bas Wijnen]

Hi,

Delta calibration is tricky. There are 9 variables that all need to be set
correctly for it to be perfect. Because it is pretty much impossible to
calibrate all of them, care must be taken to make sure some are equal. To be
exact:

The limit switch positions can be pretty well calibrated, so each of them should be set right.

Rod lengths must be equal between two rods of one pair, otherwise there is no
way of correcting for it. They can also be measured accurately and they should.
Normally I assume that all are equal, but if you have measurements you should
of course use the real values.

The final three values are the radii for all the rods. Those cannot be
accurately measured, so you need to move the machine and see that the nozzle
follows the plane of the bed.

Obviously the steps per mm must also be correctly set, but that is easy: it
depends on the angle that the motor turns for one (micro)step, the number of
teeth on the gear that's on the motor and the distance between teeth on the
belt.

Trial and error is useful for determining the limit switch positions (make sure
the rods at that switch are (almost) vertical) and for the radius (if you
assume all three are equal). Don't use it for determining the rod length,
because it's possible to have a near-flat surface (so seemingly correct values)
with both rod length and radius wrong. In that case, the scale of the prints
will be off.

The above assumes that you use Franklin. For other systems, they may have
different knobs to turn (I know some define the radii as equal, but allow for
variation in the angles where the motors are on the circle; Franklin defines
those as exactly at 120 degrees from each other).

Does that answer your question?

Thanks,
Bas Wijnen

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[Ben Hubbard]

I've had issues in the past with my drive gears/sprockets not staying put. Some of my print quality issues are documented here, here, here, and here. That last one is probably the closest to the issue you're describing.

I've since resolved those issues, and I believe the ultimate solution was applying Loctite Blue to each set screw on each sprocket and  to the motor shaft, bonding the sprocket to the shaft. After two broken belts, two failed motors, and lots of failed prints, the Loctite seems to have fixed everything - my prints turn out great now.

[Bruce Welford]

Dr. Wijnen and Ben,

Thank you both for responding so quickly.  You're the superstars of the Group for sure!  Ben, this image:

depicts the exact problem that has plagued me the entire time I have been printing with my Athena II.  I have spent MANY hours trying to calibrate and still end up with this behavior.  I will be sure to inspect my set screws on the drive gears and Loctite them down.  I've never had any belt failures or skipped prints, so not sure if I am having an issue with the drive gears moving on the motor shafts, but will check to make sure.  

Dr. Wijnen, it is true that I am not using Franklin, but rather a Duet wifi.  Your detailed explanation of the 9 variables certainly points me in the right direction of where I can start to try and fix what is going on in the picture above.  So limit switch position, end effector rod length, and rod radii.  This last one, rod radii,  I need clarification on.  Is this the position of the three motors/verticle rods around the base of the printer?  Also, in regard to motor stepping, is there a way to measure each one individually as far as how many steps/mm they are actually moving, i.e. given the mechanical specifications notwithstanding, don't the motors all move slightly differently anyway just because they all are not perfect?  I know with the extruder motor,  you can mark filament at say 100mm increments, and then ask the software to extrude at 100mm increments and then compare the results.  If off, make adjustments in the software to correct so that 100mm is what extrudes.

Thank you both again so much for your guidance!!

Bruce Welford

General Manager

Rapid Response Ink, Toner & 3D Filament  

[Ben Hubbard]

To answer your question about motor stepping, I would suspect this is not an issue due to the nature of stepper motors. Each step brings the motor to a well defined angular position. This means that a single revolution will take an exact, integer number of steps. This would make any stepping error non-cumulative (error in one step will not affect the amount of error in the next step), as the first page of this article so nicely puts it.

A little more background on the loctite - I had observed my drive gears drifting inward on the motor shafts. I'd set the gears out near the end of the shaft, then by the end of a print they were snugged up against the motor, about half a centimeter of movement. I tried adjusting them and tightening the set screws more, but to no avail. It may be worth checking for similar behavior, but I can't imagine a situation where Loctite would be anything but helpful.

Regarding rod radius, duet's documentation for command M665 describes the radius as "the horizontal distance spanned by each of the rods when the effector is centred... measured between bearing centres." When calibrating my printer using Franklin, this value allows the print head to translate horizontally when moving outward from the center. A radius that is too large causes my print head to lift up off the build plate as it moves away from the center of the plate.

[Matt]

Bruce, 

I see you are using corrugated cardboard to "float" your glass. I would recommend using a firmer substance like several layers of a non-corrugated cardboard or cork piece. Also are the cardboard sections at the towers, ie under the limit switches, or in between towers? They need to be directly under the limit switches, or at the tower locations. 

[Bruce Welford]

Ben,

You have given me a lot to work on and I really appreciate the links.  I will get on this and report back once I get a chance to work on this(don't get a lot of free time between work and home stuff.)

Matt, 

Thank you for your observation regarding the build plate.  I was just using that image(not my printer) to illustrate the problem I have been having.  Here is a picture of my printer:

I've gotten away from using the blue tape and have found a glue stick to be a lot easier.  And the printer is off the folding table and on a more firm table.

Thank you all so much for your help!!

Bruce Welford

General Manager

Rapid Response Ink Toner & 3D Filament

[Matt]

Bruce, 

We have had similar problems on our Athenas. What I think is happening is the birch plywood is not perfectly flat, but has inconsistent thicknesses. When the clamps press the glass into the bed, if there is a slight raised section in the plywood somewhere under the glass, the glass flexes around the hill and the glass is no longer a true plane. 

We lift the glass above the plywood using spacers under the glass at the clamp locations. The clamp locations also need to be under the limit switches (yes where it is harder to screw them down.) Then with the glass at a true plane and it can be set parallel to the limit switch plane. 

You can see a print bed of ours below. The glass cut shape does not matter (hex vs circular)

[Bruce Welford]

Matt,

I never dreamed the glass would be tilted.  You've given me something else to do to improve on this problem. 

Thank you again,

Bruce

[Bruce Welford]

Update,

I am happy to thank all of you for your help in getting my printer calibrated.  I have to say that I was able to adjust the steps per mm(in Reprapfirmware) using the method in this Youtube video, starting at 35:40:

https://youtu.be/VXBr3MTs50c

I am not sure if Franklin offers a similar ratio adjustment for the steps per mm, but I did find that my carriages did not all travel the same 100 mm test distance when measured with digital calipers as explained in the video.

Overall after changing out my carriage bearings(the ones I had were completely moving around on the shafts creating tremendous slop), tightening my belts, and yes Ben, utilizing all of the benefits of the auto-calibration and mesh bed compensation that the Duetwifi2 board offers per your suggestion, I finally have had first layer success.  I don't know if Franklin can accommodate a bed probe, but I did design a bracket to hold a BLTouch on the side of the end effector(openscad file attached).

 Again, thank you all so much.  You people are the best, I wish I could contribute more to the group.

Bruce Welford

General Manager

Rapid Response Ink, Toner & 3D Filament

[Bruce Welford]

For some reason the group chat won't let me attach the openscad file for the bracket.  Well here are the openscad lines:

difference() {

    cube([3,35,18]);

    translate([0,5.63,3.13])

    rotate([0,90,0])

    cylinder(r=1.33,h=3,center = false,$fn=100);

    translate([0,29.43,3.13])

    rotate([0,90,0])

    cylinder(r=1.33,h=3,center = false,$fn=100);

}

difference() {    

    translate([0,0,15])

    cube([14.25,35,3]);

translate([8,5.125,15])

    cube([3.4,24.75,3]);

    translate([2.5,10.5,15]) 

    cube([14.25,14,3]);

}

[Ben Hubbard]

Hi Bruce!

Thanks for sharing - glad you've got things nailed down. To answer your questions - yes, Franklin can adjust steps/mm in its calibration

It also looks like Franklin can accommodate a probe, though I have not experimented with that. There's a spot to tell Franklin where the probe is connected, as well as some settings regarding motion during/after probing.

Does the bracket simply screw into the end effector? Did you have to modify the end effector in any way?

[Bruce Welford]

Ben,

Sorry, yes the bracket just screws into the end effector with very small diameter "wood" screws.  I pre-drilled small holes into the rim of the end effector where I attached the bracket for the BLTouch.

Chad,

The Duet interacts with RepRap firmware.  It was very easy to follow the configuration wizard to set up my AthenaII printer and control it through the web interface with the Duet.   Since I never used Franklin, I didn't have to make the transition.  I am not familiar with the MKS board; I am guessing that it must be able to interact with something like Marlin.  If it would be helpful, I would be very happy to share the configuration files that I have in the RepRap firmware, just let me know.

Bruce Welford

General Manager

Rapid Response Ink, Toner & 3D Filament

[Bruce Welford]

Chad,

Here are the configuration files that I believe are the most relevant to configuring the AthenaII in other firmware.  They can be opened with a text editor except for the config.json which can be opened by loading the file here: https://configtool.reprapfirmware.org/Start and pick "use existing configuration."

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