Civil Engineering Reference
In-Depth Information
Fig. 1.7. You can use a dividing head to drill a traction engine wheel for the spoke
holes. If you tilt the milling machine head at 45 degrees and the dividing head at 45
degrees you can mill and drill quite large diameters even on a smallish mill.
MILLING MACHINE ELECTRICS
With single-phase electrics you plug the mains lead from the milling machine
straight into the mains supply in your house or workshop. The motor is fed
from the live, neutral and earth leads. Mills built for industrial use almost al-
ways have three-phase motors. Three-phase motors need three live wires.
Each wire will be on a different phase. Each phase is like an incoming wave
on the seashore, with three waves, one after the other, making up the three
phases required. Each phase is the same voltage and current as the preced-
ing one, but each phase takes up a third of each cycle so three sequential
phases add up to each complete cycle.
A Bridgeport turret milling machine will almost certainly have three-phase
electrics and, unless you are lucky enough to have a three-phase supply to
your workshop, it will require a single to three phase converter.
Some three-phase motors will run from a single-phase supply via a con-
verter or an inverter. To install a converter or inverter the converter/inverter
maker's instructions should be followed.
Inverters
Some change-over tags may be fitted inside a motor to change it from 440V
three-phase down to 240V three-phase. In this case, the motor information
plate will often say 240V/440V. If the motor plate says this, the motor is almost
certainly capable of running on an inverter; it is just a matter of changing over
a few connections inside the motor. This is usually an easy job involving mov-
ing some brass or copper links from one terminal to another. Instructions are
usually on, or inside, the motor's cover. The three-phase inverter or convert-
er setup will give a quieter and smoother running motor compared to a motor
running on a single-phase supply. The inverter will need a little bit of program-
ming to set it up, but if you follow the manufacturer's instructions it will only be
a five-minute job to get the motor and inverter combination working properly.
Although the inverter runs at a constant voltage the frequency can be var-
ied. This variation changes the motor speed as the inverter frequency is
