Page 149 - Robot Builder's Bonanza

118 WORKING WITH METAL

hold. Once locked, you can hold the pliers normally, without continually exerting pressure. To
unlock, squeeze or pull the release lever.
If using a drill press, your model may require you to change the position of belts in order
to alter the speed. Set the speed to the lowest available.

Tapping Holes
When using thicker metal (1/16″ or more), you can tap the holes you drill so they have
threads for machine screws. To use a tap you must first drill the appropriate- size hole. You
then manually “screw” the tap into the hole; this conforms the hole with the threads needed
for the screw.
You don’t need a fancy 10- in- 1 tap- and- die set. You can get by with just a couple of tap
sizes, along with their corresponding drill bits. I use 4-40 and 6-32 screws almost exclusively
in my robot designs, so I really only need the 4-40 and 6-32 taps. If you require holes for other
screw sizes, you can purchase taps for these on an as- needed basis.
In order to thread the tap into the hole, you need a T- wrench for the tap (see Figure 11-9).
The tap is secured by a chuck, as shown, or a setscrew in the wrench. Follow these basic steps
to tap holes for threading with machine screws:

1. Drill the hole with the appropriate- size bit. The hole is always smaller than the tap. For
example, to tap a hole for a 4-40 screw, use a 3/32 (or #43) drill bit.
2. Add a drop of oil at the location of the hole, and proceed to drill.
3. After drilling is complete, check the hole for burrs and remove them as detailed in the
next section, “Finishing Metal.”
4. Mount the tap into the T- wrench. Add a few drops of cutting oil to the entire length of
the tap.
5. Keeping the tap as perpendicular to the surface of the metal as possible, slowly thread
the tap into the hole. Apply steady but firm downward pressure on the wrench. You
should soon feel the tap “bite” into the metal.
6. Continue threading the tap into the metal until about 1/4″ of the tap is fully through
the other side.
7. Clean off any metal debris, add a drop or two more of cutting oil, and carefully back
the tap out of the hole. Clean the newly threaded hole when done.
See Appendix C, “Mechanical Reference,” for a handy chart comparing tap sizes and
FYI
appropriate drill bits, for both imperial (inch) and metric.

Figure 11-9 A T- wrench and tap are used to
thread holes for mechanical fasteners. Be sure to
drill the hole to the proper size for the tap you are
using.

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