Machinery Repairman

Figure 4-30.—Common types of clamping straps.
types of clamping straps, shown in figure 4-30.
(Clamping straps are also identified as clamps or
dogs.) The U-strap is the most convenient for many
setups because it has a larger range of adjustment.
It is often necessary to use tools such as steel
parallels, V-blocks, and angle plates to support and
hold the work. Use steel parallels to elevate the work
above the table so you can better see the progress of
the drill. Use V-blocks to support round stock, and
angle plates to support work where you will drill a
hole at an angle to another surface. Figure 4-31
shows some examples of setups.
Drilling Hints
To ensure accuracy in drilling, position the work
accurately under the drill. Use the proper techniques
to prevent the drill from starting off center or from
moving out of alignment during the cut. Here are
some hints that will help you to correctly start and
complete a drilling job.
Before setting up the machine, wipe all foreign
matter from the spindle and the table. A chip in
the spindle socket will cause the drill to have a
wobbling effect which tends to make the hole
larger than the drill. Foreign matter on the
work holding device under the workpiece tilts
it in relation to the spindle, causing the hole to
be out of alignment.
4-20
Figure 4-31.—Work mounted on the table.
Center punch the work at the point to be
drilled. Position the center-punched workpiece
under the drill. Use a dead center inserted in
the spindle socket to align the center-punch
mark on the workpiece directly under the axis
of the spindle.
Bring the spindle with the inserted center down
to the center-punch mark and hold it in place
lightly while you fasten the locking clamps or
dogs. This will prevent slight movement of the
workpiece, table, or both when they are
clamped in position.
Insert a center drill (fig. 4-32) in the spindle
and make a center hole to aid in starting the
drill. This is not necessary on small drills on
which the dead center of the drill is smaller
than the center-punch mark. But on large drills
it will prevent the drill from “walking’* away
from the center-punch mark. This operation is
especially important to drill holes on curved
surfaces.
Figure 4-32.—Combined drill and countersink (center drill).