CNC backlash & how to solve it

What is CNC backlash

CNC backlash is a mechanical looseness and play between components and the transmission system of a CNC router. Most notably gearbox, belt, rack and pinion, or ballscrew. The backlash would create circles that are not fully round with jagged edges or oval shape and in doing point to point machining after a couple of runs machine would lose its coordinates and need to be homed again. This is extremely troubling for nested machining  for example, cabinet panels or type of work that need precision repetition.

How to identify backlash in CNC router

1- Test rack & pinion

Mechanical backlash normally happens when you have loose rack and pinion engagement. To test for this first power up your CNC or plasma table so servo or stepper motors are holding position. Then try and pushing X & Y axis back and forth by hand. If there is any play that indicates rack and pinion is not fully engaged. This could be due to wear and tear of the pinion so teeth are not sharp or maybe mounting bolts are loosened. Tighten rack and pinion engagement by moving the mounting plate up and firmly tighten all the bolts.

2- CNC transmission (Belt / Gearbox)

If the transmission belt is loose then it will skip steps and not wrap around reducing pulley. Simply tighten belt tension to solve this problem.

Another transmission system would be a planetary gearbox and if the gearbox used in your CNC router has a high arcmin twist angle then you might get slipping and backlash. It is costly to repair the gearbox and the best option might be replacing it with <1 arcmin type of gearbox.

3- Software adjustment to fix backlash

There are many types of CNC controllers available in the market. depending on your controller there might be an option to compensate for this backlash. Mach3, for instance, has the option to compensate for the backlash but it is not a recommended repair. You can search online for backlash compensation for your control system