LinuxCNC StepConf and Shapeoko X & Y Calcs

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drmacro
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Joined: Wed Aug 15, 2012 11:29 pm

LinuxCNC StepConf and Shapeoko X & Y Calcs

Post by drmacro » Mon Aug 20, 2012 11:07 am

Hi,

I'm curious how the calculations for the X & Y axis are made to determine the leadscrew pitch given the .08" pitch of the Shapeoko belt, 18 tooth gear on the motor and typical 200 step motor.

In the details shown here: http://www.shapeoko.com/wiki/index.php/EMC2#StepConfig the pitch is set to .69444375. This doesn't appear to move .1" when I put a dial indicator on the axis and tell it to move .1". (I'm using the test button in step conf to move .1")

Also, how was the value of 16 determined for micro stepping?

Regards,
Mac

cvoinescu
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Re: LinuxCNC StepConf and Shapeoko X & Y Calcs

Post by cvoinescu » Mon Aug 20, 2012 12:26 pm

The cleanest way is to replace "leadscrew" with "belt" in that dialog, and then everything makes sense.

Motor steps per revolution is given by the motor construction: 200 steps for 1.8 degrees per step, 400 for 0.9 degrees per step.

The driver microstepping is given by driver design and configuration. Allegro A4988 drivers (as on the Pololu carrier boards) support 1x, 2x, 4x, 8x and 16x microstepping. They are usually configured using jumpers on the driver board (buildlog stepper shield, RAMPS etc). Texas Instruments DRV8811 drivers (as on the Grblshield) support 1x, 2x, 4x and 8x microstepping. On the unmodified Grblshield, they are configurable only for all drivers at the same time, using a single set of jumpers. The Z-axis microstepping modification allows the Z axis to be set to 2x microstepping while the X and Y axes are set to 8x. Grblshields from full and premium Inventable kits are already modified. Gecko controllers support, if I recall correctly, 10x microstepping. And so on.

Pulley teeth is 18 : 1 (the actual number of teeth on the pulley).

Set the leadscrew pitch to your belt pitch, which is 12.5 teeth per inch for MXL.

The Z axis settings are 1 : 1 motor:leadscrew (direct coupling) and a leadscrew pitch of 20.32 turns per inch (M8 thread). Set the microstepping as discussed above. If you have independently configurable microstepping for your Z axis, 2x is usually fine enough.

By the way, the .69444375 number as shown is correct: it is 12.5 / 18, because they set the pulley ratio to 1 : 1. To move the axis one inch, the motor needs to turn .69444375 turns. It is the same thing, but I think my way is easier to understand.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

alex_hh
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Joined: Sun May 12, 2013 1:28 pm

Re: LinuxCNC StepConf and Shapeoko X & Y Calcs

Post by alex_hh » Sun May 12, 2013 1:40 pm

Hello to the members of this discussion board.

Since several days I own an eShapeOko - the european version on ShapeOko. For stepper motor controlling I'm going to use a TB6560 3 Axis Motor driver PCB (made in China) and
three Nema 17 stepper (1,8°, 1.68A)
As software I decided to use LinuxCNC, the configuration is made in mm - not in inch!
This fundamental difference to the discussion here brings me to ask you for some help regarding the axis configuration.
For inches the wizzard ask for Leadscrew Pitch rev/in. In the mm setup it ask for mm/rev.
This difference forced me to recalculate the above mentioned 0.6944475 rev/Inch into
33.16112875mm/rev. (! Inch = 25.4 mm)
Is this correct?

It would be great to get some hints.

Thx and all the best
Alexander

hammy
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Re: LinuxCNC StepConf and Shapeoko X & Y Calcs

Post by hammy » Sun May 12, 2013 5:31 pm

Hi ,
The MXL belt has a pitch of 2.032mm so with an 18 tooth belt 1 rev will be 36.576mm.

I have just put together eShapeoko running Linuxcnc and all seems OK with the above values

Ken

alex_hh
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Joined: Sun May 12, 2013 1:28 pm

Re: LinuxCNC StepConf and Shapeoko X & Y Calcs

Post by alex_hh » Sun May 12, 2013 7:54 pm

Hello Ken,

many thanks for your feedback, I will try your pitch!
What pitch do you use for z axis - 1.25mm/rev.?
This is the standard pitch of M8...

Alexander

edwardrford
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Re: LinuxCNC StepConf and Shapeoko X & Y Calcs

Post by edwardrford » Sun May 12, 2013 8:27 pm

Hi Alexander,

yes, the standard M8 threaded rod = 1.25mm per full revolution.

-Edward
Shapeoko 1 #0 - a couple of upgrades.
Shapeoko 2 #0 - a couple of upgrades.
Shapeoko 3 #2 - Stock

northbear
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Re: LinuxCNC StepConf and Shapeoko X & Y Calcs

Post by northbear » Mon May 13, 2013 5:07 pm

cvoinescu wrote:By the way, the .69444375 number as shown is correct: it is 12.5 / 18, because they set the pulley ratio to 1 : 1. To move the axis one inch, the motor needs to turn .69444375 turns. It is the same thing, but I think my way is easier to understand.
Working through this in preparation for testing my electronics, shouldn't the exact value be .694444444444444 (4 repeating) ? I am not sure where the "375" at then end comes from. Using the method cvoinescu describes should give the exact value to the program as it is giving it the fraction.

Of course it will only make a difference if the machine was impossibly large, I was just doing the math to compare to my pulleys.

BTW, my pulleys are 18 tooth GT2 3mm pitch which I calculated gives me a lead screw pitch of 1/((3*25.4)*18) = 0.470370370370370 inches. Another lovely repeating decimal :lol:
My buildlog is here

cvoinescu
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Re: LinuxCNC StepConf and Shapeoko X & Y Calcs

Post by cvoinescu » Mon May 13, 2013 8:02 pm

northbear wrote:Working through this in preparation for testing my electronics, shouldn't the exact value be .694444444444444 (4 repeating) ? I am not sure where the "375" at then end comes from.
Yes, it should be a repeating 4. I don't know where the 375 comes from. Probably some approximation at an intermediate step, who knows.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

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