There has been a fantastic article written by one of our customers who purchased the Vertical Linear Digital Scale for his milling machine to eliminate backlash.
“On my machine, even with the spindle bearings correctly adjusted, there is around a millimetre (40 thou in imperial) of backlash, I assume this slack must be in the rack and pinion, the effect of this is that the quill clamp always needs to be slightly tightened putting additional pressure on the rack and pinion making it harder to move the quill. Failure to clamp the quill can result in a chattering cutter that can drift up and down when machining. I have seen people fit springs to hold the quill rack up against the pinion but spring rates change as the spring is stretched so the lift force gets stronger as the spring gets longer, again causing more pressure, hence wear, on the rack