Configuration with Timing Belt

I just finished constructing my first 3-axis CNC machine and noticed that the distance scale is way off. The X and Y axis are using GT2 timing belts and the Z axis is using a lead screw. The X-Y motion is way too big for the G-code and it looked like I needed to adjust the SCALE variable in my ini file. When I recude the number, the stepper move much slower and at low SCALE number it makes almost a pulsing sound and does not move. If I change the number back to the 8000 default the machine moves smoothly, although at the wrong scale for the G-code.

I am using a GT2 timing belt with a 16 tooth pulley with 2mm pitch. Based on the LinuxCNC documentation with 1/8 microstepping I believe the SCALE number should be 50. Is that correct? Are there other values I need to change to get this to work? I haven't been able to find complete examples of people using timing belts related to the ini config file.

when you say timing belt do you mean you are using the timing belt like a gear rack?

The scale setting is simply the amount of motor steps required to move one unit ( probably 1 mm in your case )
how far will the axis move with one rotation of the stepper motor?

Chris M

If what you describe is correct 50 steps/ mm sounds like the right scale value. You may want to consider adding more gear reduction, to get better resolution and torque.

[quote="spaceghost"
I am using a GT2 timing belt with a 16 tooth pulley with 2mm pitch. Based on the LinuxCNC documentation with 1/8 microstepping I believe the SCALE number should be 50. Is that correct? Are there other values I need to change to get this to work? I haven't been able to find complete examples of people using timing belts related to the ini config file.[/quote]

Are you using two pulleys both with 16 teeth or two different sized pulleys?
I use a pulley system with a14 and a18 tooth and getting the positions wrong in step config produces what you describe.

What I am using is a single gear so there is no gear ratios in play. I am new to the CNC thing and tried to pick the preferred terminology but might have picked the wrong one.

While I continued searching online I did find a post somewhere else that gave me some hints to what I was supposed to do. Here is what I tried:

Drive Microstepping: 4
Pully Teeth: 1
Leadscrew pitch: 32

which gives 25 steps/mm. After testing this it seemed to give reasonable results. My math to get these number make (some) sense to me but I am more interested now if I did this correct and would it be correct for other situation. The problem is compounded by the fact that the Stepconf UI makes no reference to a belt system so I am left to guess which values go in which field and what formula I need to get the numbers to put in. I looks like I might have this working but it would be nice if it was easier to find a more complete expert example (or at least easier to find if it already exists).

well that makes some sense.
A 16 tooth 2mm pitch sprocket will move the axis approximately 32mm every revolution.
200 step motor times 4 microstepping mode = 800 steps per revolution of the motor.
800/32 = 25 steps per 1 mm movement.
I would bet in practice you may need to adjust that a small factional amount due to pitch/diameter tolerance but test and see.

When stepconf was built that style of axis movement was not that popular.
glad you figured it out!

Chris M

From linuxcnc.org/docs/2.4/html/config_ini_config.html#sub:[AXIS]-section

2.2.9.3 Stepper

SCALE
= 4000 (hal) Specifies the number of pulses that corresponds to a move of one machine unit as set in the [TRAJ] section. For stepper systems, this is the number of step pulses issued per machine unit. For a linear axis one machine unit will be equal to the setting of LINEAR_UNITS. For an angular axis one unit is equal to the setting in ANGULAR_UNITS. For servo systems, this is the number of feedback pulses per machine unit. A second number, if specified, is ignored.
For example, on a 1.8 degree stepper motor with half-stepping, and 10 revs/inch gearing, and desired machine units of inch, we have

Rick G