It tends to be an ACME or trapezoidal screw lifting the head. It's always
under load from the considerable weight of the head, so backlash isn't a
problem. Even though the screws and nuts can suffer significant wear over the
years (especially if there isn't an automatic lubrication mechanism), the wear
over short distances will be consistent, so a properly geared screw can
produce very high precision movements. Silly as it might seem, the slight
vibration that is inherent in running the machine ensures that any slack will
be taken up.
Note that a heavy cut can lift the wheel-head within the 'play' of the screw
and nut. This is one reason for doing a spark-out pass at the end of the job,
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