Civil Engineering Reference
In-Depth Information
5
Setting Work True and Finding the
Datum Position
The milling machine is the ideal machine for working accurately straight onto
metalwiththeminimumornomarkingout.Whilethemachine'sdialswillallow
you to work to a reasonable accuracy, I highly recommend that you fit some
sort of digital readout to at least the X and Y axes of your milling machine.
These will not only increase accuracy, they should also help you to eliminate
errors, such as where you wind the handwheel on an extra turn.
DIGITAL READOUTS
Readouts are quite easy to fit (
see
Chapter 2
) and can be purchased at a
reasonable cost. It is not quite as important to have a readout on the Z axis
where a simple up and down movement is all that is required. It is however
quite easy to fit a cheap vernier type scale to the Z axis if you wish.
A readout gives you an exact reading of the position of a particular axis
and is unaffected by backlash in the machine slides. It can be set to zero at
any point on the machine's travel. Instant conversion from imperial to metric
is normally available at the touch of a button.
Absolute or incremental mode
Most digital readouts can work in absolute or incremental mode. Absolute
mode means that every position is taken from one absolute point, usually
where you set the initial datum point. Incremental mode means you can move
to the first position and set the readout to zero, then move to the next position
incrementally. Set the readout to zero again and then move incrementally to
the next position and so on.
A useful technique I have used in industry is a combination of absolute and
incremental. Although this was done more often on a CNC mill, the technique
is easily applicable to manual milling machines. A typical use for this tech-
