What size drill should I use for a"3/8-32"tap

you can do 5/16"

Clearance Hole Drills Size of Screw Tap Drill Close Fit Free Fit No. or Dia. Decimal (inch) Threads Per Inch Drill Size Decimal (inch) Drill Size Decimal (inch) Drill Size Decimal (inch) #0 0.06 80 3/64 0.0469 52 0.0635 50 0.07 #1 0.073 64 53 0.0595 48 0.076 46 0.081 #1 0.073 72 53 0.0595 48 0.076 46 0.081 #2 0.086 56 50 0.07 43 0.089 41 0.096 #2 0.086 64 50 0.07 43 0.089 41 0.096 #3 0.099 48 47 0.0785 37 0.104 35 0.11 #3 0.099 56 45 0.082 37 0.104 35 0.11 #4 0.112 36 44 0.086 32 0.116 30 0.1285 #4 0.112 40 43 0.089 32 0.116 30 0.1285 #4 0.112 48 42 0.0935 32 0.116 30 0.1285 #5 0.125 40 38 0.1015 30 0.1285 29 0.136 #5 0.125 44 37 0.104 30 0.1285 29 0.136 #6 0.138 32 36 0.1065 27 0.144 25 0.1495 #6 0.138 40 33 0.113 27 0.144 25 0.1495 #8 0.164 32 29 0.136 18 0.1695 16 0.177 #8 0.164 36 29 0.136 18 0.1695 16 0.177 #10 0.19 24 25 0.1495 9 0.196

7 0.201 #10 0.19 32 21 0.159 9 0.196 7 0.201 #12 0.216 24 16 0.177 2 0.221 I 0.228 #12 0.216 28 14 0.182 2 0.221 I 0.228 #14 0.242 20 10 0.1935 D 0.246 F 0.257 #14 0.242 24 7 0.201 D 0.246 F 0.257 1/4 0.25 20 7 0.201 F 0.257 H 0.266 1/4 0.25 28 3 0.213 F 0.257 H 0.266 5/16 0.3125 18 F 0.257 P 0.323 Q 0.332 5/16 0.3125 24 I 0.272 P 0.323 Q 0.332 3/8 0.375 16 5/16 0.3125 W 0.386 X 0.397 3/8 0.375 24 Q 0.332 W 0.386 X 0.397 7/16 0.4375 14 U 0.368 29/64 0.4531 15/32 0.4687 7/16 0.4375 20 25/64 0.3906 29/64 0.4531 15/32 0.4687 1/2 0.5 13 27/64 0.4219 33/64 0.5156 17/32 0.5312 1/2 0.5 20 29/64 0.4531 33/64 0.5156 17/32 0.5312 Top of Page

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.

No way. Wouldn't even clear the minor diameter.

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.

Wolfgang

Gem> What size drill should I use for a"3/8-32"tap,and where I can get both > items.Thanks

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 was derived?

Thanks,

Wes S

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 screws.

The derivation is some high-school math based on the UN thread geometry specifications.

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 truncation height.

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_.

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.

Good luck! Ace

get both

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".

Len

Thanks for the explanation. I'll try this formula for oddball fine pitched taps that I can't find on a tap drill chart.

Wes S