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Need to drill a
pilot hole in metal so you can tap it to accept, say, a 3/8-16
bolt (a 3/8” diameter bolt having 16 threads per inch)? A
tap-drill size chart, which is carried by most millwrights, will
tell you to use a 5/16” diameter drill bit for the job.
Don’t have a chart
handy? Here’s a mathematical formula that will yield the same
answer—courtesy of veteran millwright Dale Shoemaker, a
coordinator at the UBC’s International Training Center:
TAP DRILL SIZE =
FASTENER DIAMETER – (1 / THREADS PER IN.)
Using this formula,
the pilot-hole diameter for our 3/8-16 bolt = 3/8 – 1/16 =
5/16”, which is consistent with the chart. In many cases,
converting to decimals and then back to 64ths of an inch will be
easier than subtracting fractions (for example 3/8 – 1/16 = .375
- .0625 = .3125; .3125 x 64 = 20, which indicates that the
answer is 20/64” or 5/16”). If your answer doesn’t equal an
exact drill-bit diameter, round it off to the next larger 1/64”
and bore a slightly oversize pilot hole.
The metric tap-drill
formula is even easier:
METRIC TAP DRILL
SIZE (in mm) = FASTENER DIAMETER – THREAD PITCH
In other words, the
tap-drill size for an M14 x 2 metric bolt (bolt diameter = 14
mm, thread pitch = 2) is 14 – 2 = 12 mm. If you don’t have
metric drill bits, Shoemaker says to divide the answer by 25.4
and then convert it to 64ths to yield the size of the required
standard drill bit (31/64” in this case). |
New homes must be built for growing populations,
and old homes must be renovated or replaced with
new, more efficient ones.
It takes a lot of
skilled carpenters to create or renovate these
structures expertly and efficiently. Residential
carpentry is a challenging career because of the
new building products, new styles, new tools,
new technology and hard work which goes into today’s housing
industry.
Housing contractors are always seeking
good residential carpenters.
