Backlash (engineering)
[citation needed] Reasons for specifying a requirement for backlash include allowing for lubrication, manufacturing errors, deflection under load, and thermal expansion.
Factors affecting the amount of backlash required in a gear train include errors in profile, pitch, tooth thickness, helix angle and center distance, and run-out.
The material in the machine where: Standard practice is to make allowance for half the backlash in the tooth thickness of each gear.
For a 14.5° pressure angle the extra distance the cutting tool is moved in equals the amount of backlash desired.
The best example here is an analog radio tuner dial where one may make precise tuning movements both forwards and backwards.
Most machine slides for many decades, and many even today, have been simple (but accurate) cast-iron linear bearing surfaces, such as a dovetail- or box-slide, with an Acme leadscrew drive.
On manual (non-CNC) machine tools, a machinist's means for compensating for backlash is to approach all precise positions using the same direction of travel, that is, if they have been dialing left, and next want to move to a rightward point, they will move rightward past it, then dial leftward back to it; the setups, tool approaches, and toolpaths must in that case be designed within this constraint.
Unlike in the radio dial example, the spring tension idea is not useful here, because machine tools taking a cut put too much force against the screw.
These screw-adjusted split-nut-on-an-Acme-leadscrew designs cannot eliminate all backlash on a machine slide unless they are adjusted so tight that the travel starts to bind.
], because hydraulic anti-backlash split nuts, and newer forms of leadscrew than Acme/trapezoidal -- such as recirculating ball screws -- effectively eliminate the backlash.
The simplest CNCs, such as microlathes or manual-to-CNC conversions, which use nut-and-Acme-screw drives can be programmed to correct for the total backlash on each axis, so that the machine's control system will automatically move the extra distance required to take up the slack when it changes directions.
This programmatic "backlash compensation" is a cheap solution, but professional grade CNCs use the more expensive backlash-eliminating drives mentioned above.
The dynamic response of backlash itself, essentially a delay, makes the position loop less stable and thus more prone to oscillation.
