Skip to content

Shimming and Flattening the X Tube

The hot rolled mild steel tubing used for the X axis (and the Y axis) isn't always flat enough on the wide face for perfect mounting of the rails without some extra work.

In one trial, there was variance of 0.01 inches across the wide face of the tube, with about 0.007" of variance over the width of the rail.

First, we recommend checking each side to see if one is flatter than the other. Check it with a straight edge, and look for light coming through under the blade of the straight edge.

What looks like a small bump on the edges will still add up to too much difference in rail flatness to mount the Z axis without binding the rails.

Ways to Mount the Rails Flat

Luckily, there are a few options if your X tube isn't flat.

  1. Get someone to mill the face of your tube flat.
  2. Use epoxy to level the face of your tube.
  3. Shimming the rails. (this is what we're talking about here)

Shimming the Rails

Luckily, shimming the rails isn't hard, and can be done stably with commonly available materials.

Most grocery stores will sell heavy duty aluminum foil.

This is inexpensive (though more expensive than regular aluminum foil) and is consistently 0.001" in thickness.

Aluminum soda cans have a thickness of 0.004". Soda can walls will be very sharp when cut, so proceed with caution.

The rails are 12mm wide, with a clearance of 3.5mm in the center for the M3 screws. So the width of the shim needn't be much wider than 4.25mm (12mm / 2 - 3.5mm / 2)

Shimming With Foil

Fold a length of foil in half, and insert the folded seam under the low side of the rail.

If you have a dial indicator, tighten a section of the rail and check the flatness over the short dimension of the top face. Loosen and add more shims as needed. Tightening screws before each measurement, and loosening to add more shim material.

Shimming With Soda Cans

Cut the top and bottom off the can, and make one straight cut up the side so you can unroll the can flat. Use acetone to clean off the food-safe lining from the inside, and remove any ink that will come off the outside.

Cut the can into 4-5mm wide strips, so the long edge is the circumference of the can. You should be easily be able to get eight strips from a standard can, which is enough for two strips under the entire length of each 300 mm rail.

Insert one shim under the low side of the rail. Tighten, and measure. Add additional stock if needed, and repeat measurements until the rail is mostly flat along it's short dimension of the top face.