Figure 14-11.--Standard universal dividing head driving mechanism connected to the dividing head, and showing the location of
change gears's A, B, C, and D.
GEAR TRAIN RATIO
When a helix is milled on a workpiece, the
We said earlier in this chapter that the main purpose
workpiece must be made to rotate at the same time it is
of gearing is to transmit motion between two or more
shafts. In most cases these shafts are in fixed positions
fed into the revolving cutter. This is done by gearing the
with little or no adjustments available. Therefore, it is
dividing head to the milling machine table screw. To
important for you to know the center-to-center (C-C)
achieve a given lead, you must select gears with a ratio
distance between the gear and the pinion.
that will cause the work to rotate at a given speed while
it advances a given distance toward the cutter. This
If you know the tooth elements of a helical gear,
distance will be the lead of the helical gear. The lead of
you can say that when the real pitch radius of the gear
the helix is determined by the size and the placement of
(RPRg) is added to the real pitch radius of the pinion
(RPRp), you can determine the C-C distance of the two
Gears X and Y are set up to mill a left-handed helix.
gears (gear and pinion).
You can set a right-handed helix by removing gear Y
The ratio of the NT on the gear and the pinion is
and reversing gear X.
equal to the ratio of the PD of the gear and the pinion.
Before you can determine which gears are required
This will allow you to solve for the necessary elements
to obtain a given lead, you must know the lead of the
of both gear and the pinion by knowing only the C-C
milling machine. The lead is the distance the milling
distance and the ratio of the gear and the pinion.