Specify material and thread engagement.
Hole size = Basic Major Diameter - 1.08253 * (Fraction of Full Thread /
Number of Threads per Inch)
Assume 0.75 fraction of full thread engagement.
Hole = 3/8 - 1.08253 * (.75/32) = 0.350
A letter "S" drill is the closest standard size.
Another poster posted tap drill dia. = major diameter - pitch.
which is what I've always used when a chart wasn't available. So far
it hasn't let me down even with 7/32-40.
Where did you get that method and is there an explanation on how it
No, that formula gives too small a hole, and doesn't allow for any
choice of thread engagement. The minor diameter of the internal threads
will be out of spec (too small). You will have more torque on the tap,
perhaps even from smearing instead of cutting metal depending on the
tap, and you are more likely to break the tap. The internal thread
crests may be ragged or rough, or may interfere with close-tolerance
The derivation is some high-school math based on the UN thread geometry
The magic factor comes from:
sqrt(3)/(2*0.8) = 1.08253
Which comes from:
sqrt(3)/2 = Height of a full sharp UN thread in proportion to pitch
Divided by 0.8, which is the specific UN thread form internal thread
This factor is further reduced by the proportion you choose for thread
engagement, as a fraction of a full thread, such 0.75. You can vary the
engagement if the full thread strength is not needed, to make tapping
easier/faster/cheaper/safer from breakage.
The formula (but not the derivation above) is in _Machinery's Handbook_.
For 60 degree included angle threads (UN and Metric) the formula for
tap drill size is:
tap drill dia. = major diameter - pitch.
For example, 3/8-16 UNC, the tap drill size in the table below is given
as 5/16" dia.
For #10-32UNF thread the table below gives .159" dia., ie.
.190"-1/32"=.159 (within a thou or so).
Consequently for a 3/8-32 tap you would use a 11/32" dia. tap drill. A
slight adjustment may be made for stainless steels and tool steels:
For these you would drill the holes a few thoudandths larger to ease
the tapping operation.
11/32 " dia. (0.34375) tap drill should do the trick. Try McMaster-car.com
for a source. Incidentally, I was looking at a 1/2-20 tap this morning on
their site and they listed the size of tap drill required.
There is a chart of Model Engineer (M.E.) taps, tap drill, and
clearance drill sizes at:
for 3/8-32 ME the tap drill is an 'S', the clearance is a 'W'.
There are a number of sources for those 115 piece drill bit sets
that include fractional, letter, and number size bits. Just
'Google!' "115 piece drill bit set".
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