Tap drill question

[outsider347]

anyone suggest the tap drill size for 3/8-40 tap
tks

 

[Dave Smith]

anyone suggest the tap drill size for 3/8-40 tap
tks

How deep is the hole for threading?--Dave

 

[outsider347]

3/8 deep brass

 

[Dave Smith]

I would think .350 or 9mm would work ok---maybe some other member can verify it also---------you can always test it in a scrap piece--the machinery handbook only went up to 36tpi in 3/8" tap----Dave

 

[JimDawson]

Drill size = Outside Diameter - (1.08254 x % Depth / TPI)

1.08254 is a constant. This works for any American Unified Thread Form and ISO Metric Threads

Use 1.299 for American National Thread Form

So in this case:
0.350 = .375 - (1.299 * 0.75 / 40)

 

[RandyM]

My Machinery's Handbook is listing .348-.354 for a .375-40 UNS.

 

[hman]

Drill size = Outside Diameter - (1.08254 x % Depth / TPI)

1.08254 is a constant. This works for any American Unified Thread Form and ISO Metric Threads

Use 1.299 for American National Thread Form

So in this case:
0.350 = .375 - (1.299 * 0.75 / 40)

My Machinery's Handbook is listing .348-.354 for a .375-40 UNS.

Both of you have come up with authoritative info that's nicely consistent with the very simple "rule of thumb" for common threads ...
Drill size = Outside Diameter - 1/TPI (or for metric, Drill Size = Outside Diameter - Pitch).

.350 = .375 - 1/40 = .375 - .025


Jim - Note that in your formula, the 1.299 factor is almost exactly the reciprocal of the % depth value, so they cancel. 1.299 is within 3% of 1.333! Guess that's why the simpler "rule of thumb" works so well.

Randy - that .350 is smack dab in the middle of the range you found.

I appreciate the trouble you've both gone to, and you've confirmed my confidence in the simplification.

 

[Video_man]

0.350 not being a standard drill bit, the choices seem to be:

"S" drill, 0.348 in; 9.0mm drill, 0.354 in., or "T" drill, 0.358 in.

 

[OldMachinist]

Metric drills are available in tenths of a millimeter so 8.9mm(.3504") is a standard drill.

 

[Dave Smith]

0.350 not being a standard drill bit, the choices seem to be:

"S" drill, 0.348 in; 9.0mm drill, 0.354 in., or "T" drill, 0.358 in.

my input would be the 9mm or second choice "S"--Dave