# Calibrating Belt Stretch

The Shapeoko 3 uses GT2 belts for all axes. This particular line of Gates belts is designed for power transmission and precise positioning and easily exceeds in specification the typical operational loads for the machine.

For accurate cuts, it is necessary to calibrate the number of steps in Grbl to match the physical dimensions of the belts as tensioned on your machine. Doing so will require precision measurement tools such as a dial indicator or pair of good quality calipers.

To simplify the math and reduce rounding errors, you may wish to set your initial values for # of steps to round numbers. The Shapeoko 3’s defaults are well-suited for this:

- $100=40 (originally 20) (x, step/mm)
- $101=40 (originally 20) (y, step/mm)
- $102=40 (originally 20) (z, step/mm)

Mount a piece of stock which is dimensionally stable and mill 3 holes in it so as to make a right triangle, sized to be close to the maximum size which your calipers can measure (again, a nice round value here will make the math simpler).

Measure the three holes in a consistent fashion, e.g., left edge to left edge, front edge to front edge. Then compare the measurement to the distance which is expected. Convert it into a ratio by dividing the expected distance by the measured distance, multiply the number of steps by this ratio, and update the number of steps in Grbl with the new value. If the left edge to left edge of two holes which were expected to be 75mm apart measures as 74.97mm, one would calculate:

75mm ÷ 74.97mm = 1.00040016 × 40 = 40.0160064

And then enter $100=40.0160064 into Grbl to calibrate the X-axis. This measurement would then be repeated for each axis.

For the Z-axis, since one cannot do holes on a 3-axis machine, the best thing to do is to mill steps and measure their height.

c.f., http://community.carbide3d.com/uploads/default/54/373405639a1b73fa.pdf