Calculating Travel per Revolution

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rjs
Posts: 10
Joined: Fri Jan 18, 2019 11:11 am

Calculating Travel per Revolution

Post by rjs » Wed Feb 06, 2019 5:59 am

When I first setup my CNC with the Buildbotics controller I tried to convert the values I used for Mach3 for the stepper motors and ended up with the wrong settings. For X & Y my old setup used 320 steps per unit (my system is set to MMs) and for the Z axis it used 400 steps per unit. All of my steppers are 1.8 degrees per step. With Buildbotics it looks for travel per rev and I was not sure how to convert. Using trial and error I came up with 5 for X & Y and 4 for Z but I would feel better If I knew the math to insure I am using the correct values.

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Doug
Posts: 58
Joined: Fri Feb 02, 2018 4:56 pm

Re: Calculating Travel per Revolution

Post by Doug » Wed Feb 06, 2019 11:37 pm

In most instances, users will get the step angle or steps per revolution from the stepper motor data sheet. if given the number of steps per revolution (or steps per 360 degrees), you can calculate the step angle by dividing 360 by the number of steps per revolution. Be aware that the step angle refers to full steps, not micro-steps. Micro-steps are an internal function of the stepper motor drivers and subdivide full steps to provide smoother motion, better accuracy of movement, and avoidance of motor resonance issues.

The travel per revolution is the distance that the machine will travel with one revolution of the stepper motor.

I'm not really familiar with Mach configuration, but it sounds like they combined the two values together by saying the machine will travel 400 steps per unit on the Z axis and 320 steps per unit on X and Y. I'm guessing that their units were "inches" rather than "millimeters". 320 steps per millimeter would be very fine pitch.

So, let's assume they meant 400 steps per inch, and lets assume that the stepper motors are 200 steps per revolution (most modern stepper motors are 200 steps or 1.8 degrees (360 / 200) per revolution). That means that 400 steps would turn the motor exactly two turns and the machine would move one inch in two revolutions of the motor. It also means that the machine would move 1/2 inch with one turn of the motor.

Entering those two values (1.8 degrees per step and 0.5 inches per revolution) into the Buildbotics Controller would produce the same result.

The X and Y axis move one inch with only 320 steps or (1.6 turns of the motor). Therefore the motor would move 0.625 inches in one turn of the motor. (1 / 1.6 = 0.625").

Entering those two values (1.8 degrees per step and 0.625 inches per revolution) into the Buildbotics Controller would produce the same result.

Be aware that you can set the units to either "Metric" or "Imperial" on the "Settings" page in the Buildbotics Controller and the values entered on the "Motor Configuration" page will be treated accordingly.

Does this help clarify things?

rjs
Posts: 10
Joined: Fri Jan 18, 2019 11:11 am

Re: Calculating Travel per Revolution

Post by rjs » Fri Feb 08, 2019 11:35 am

I am not sure I am following the math correctly but when I changed my settings from Metric to Imperial and the travel per revolution for X & Y changed from 5mm to 0.197". I then tried to move the x axis by 1", G0 X1, and it moved 1". Then using the 0.625" number you mention, G0 X1 moved about 5/16" or 3.125 which, I am not sure if this is a coincidence or not, is 1/2 of 0.625.

Is there a way to tell the Buidbotics to take a specific number of steps from the Web interface?

Thanks,

Randy

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Doug
Posts: 58
Joined: Fri Feb 02, 2018 4:56 pm

Re: Calculating Travel per Revolution

Post by Doug » Fri Feb 08, 2019 3:44 pm

There is no way to simply tell it to take a specific number of steps.

If you told the axis to move 1" with the travel per revolution set to .197 and it moved exactly 1", then .197 sounds like the correct number. By the way,0.197 inches equals 5.0038 mm and ball screws with 5mm pitch are very common. You should leave it at 5 mm. I recommend changing to IMPERIAL units on the "Settings" page and resetting the travel-per-revolution back to 5 mm.

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