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Chapter 3 Screw It or Glue It: Fastening and Joining Parts 55

FIGURE 3-3 Anatomy of bolted joints: using a clearance hole with a nut on the end (left) and
screwing into one piece that is tapped (right)

With wood and most plastics, it is best to drill a pilot hole in the piece that you’ll drive
the screw into to avoid splitting or cracking and make it easier to install the screw. A
pilot hole is just a hole that’s slightly smaller than the screw’s major diameter and
makes it easier to install.

You can find the right size pilot hole for a corresponding screw by looking it up on
McMaster. For example, for part 90031A153, a wood screw, the pilot hole size in soft
wood is 1/16 in. In metal, you must drill a pilot hole equal to the tap drill size listed for
your screw. Then use a tap that matches your screw size to create the threads in the
piece you are screwing into. This is not as complicated as it may sound.

For a well-designed joint, use Table 3-1 to determine the clearance hole size that
matches your screw. The table also lists the tap drill sizes. (The table information is
from www.stanford.edu/~jwodin/holes.html and www.efunda.com/DesignStandards/
screws/tapdrill.cfm.)