Subject: Re: hole size for metric taps

From: Steve Austin <[email-masked]>
Date: Wed, 24 Apr 1996 10:59:27 -0700

honore de Gronckel wrote:
> 
> The tap drill size for metric threads is, following DIN 13, given by the
> nominal diameter minus the pitch (both in mm).
> 
> The most commonly used metric threads are:
> M2x0.4, M2.5x0.45, M3x0.5, M4x0.7, M5x0.8, M6x1, M8x1.25, M10x1.5, M12x1.75,
> M16 x2
> 
> The complete table for standard threads up to M16 is (drill size in mm included
> between brackets):
> M1x0.25   (0.75)    M1.1x0.25 (0.85)    M1.2x0.25 (0.95)     M1.4x0.30 (1.10)
> M1.6x0.35 (1.25)    M1.7x0.35 (1.30)    M1.8x0.35 (1.45)     M2x0.40   (1.60)
> M2.2x0.45 (1.75)    M2.3x0.40 (1.90)    M2.5x0.45 (2.05)     M2.6x0.45 (2.10)
> M3x0.5    (2.5)     M3.5x0.6  (2.9)     M4x0.7    (3.3)      M4.5x0.75 (3.7)
> M5x0.8    (4.2)     M6x1      (5.0)     M7x1      (6.0)      M8x1.25   (6.8)
> M9x1.25   (7.8)     M10x1.5   (8.5)     M11x1.5   (9.5)      M12x1.75  (10.2)
> M14x2     (12.0)    M16x2     (14.0)
> If a finer thread is required DIN recommends the use of one the following
> pitches (in mm): 0.2, 0.25, 0.35, 0.5, 0.75, 1, 1.25, 1.5
> The tap drill size for such threads is also given by the nominal diameter minus
> the pitch.
> This formula gives a tap driil size for a 92% thread (using a UN thread
form).  Most tap drill charts will give a drill size for 70% to 75% 
thread.  In tool steel I use about a 60% thread, and for really narlly
stuff I'll go down to 45% thread.  It really helps to figure out the size
for yourself rather than use a chart.  This is the formula to do so for
UN threads.

O.D. - (lead * 1.732 * .625 * percent of thread)

For example a M6x1.0 would be
6.0 - ( 1.0 * 1.732 * .625 * .75) = 5.155
5.155 / 25.4 = .2043 
For an English thread divide the pitch into 1 for the lead.
For example a 5/16 - 18 works out like this
.3125 - ((1/18 * 1.732 * .625 * percent of thread)) = .2674

If you're using a calculator 1.732 is the square root of 3.

Now if we look up 5/16 -18 in out tap drill chart we find --- 1/4???!
Most of the tap drill charts that are still around were done for older
thread systems.  Since the early 70's, we in the U.S. have been using
the UN thread form and all of the standard bolts and machine screws
that you buy are made to this form (metric included).  Thats why its
good practice to figure out the tap drill size for yourself.
Also, I think most tap drill charts were the product of the W.P.A. 
Like the trig tables, there are errors in them and they kind of self-
propogate.  Get yourself a cheap scientific calculator (chips do awful
things to plastic) and familiarize yourself with it.  It will replace
a whole raft tables and charts.  The scientific calculator is perhaps
the most important addition to the machinist's toolbox in the last 100
years.

---

From: [email-masked] (Jim Hanrahan)
Date: Wed, 1 May 1996 18:30:33 GMT

In article <[email-masked]> Steve Austin <[email-masked]> writes:
   <Note: I hope I quoted this right - Jim>

>.3125 - ((1/18 * 1.732 * .625 * percent of thread)) = .2674

  That looks like 75% for UN (Unified National). I have started looking into 
this, and the same equation for American National (old form) from Machinery's 
Handbook is:

 O.D. - (lead * 1.732 * .750 * percent of thread)

I used a spread sheet to generate tables, an example for 5/16-18 is (I can 
post the full tables, or email, if requested):

Nominal Basic Major Threads  American Unified
Diameter Diameter   Per Inch National National Drill Size

   5/16  0.3125      18      49%     59%    0.2770    J
           0.3125      18      56%     67%    0.2720    I
           0.3125      18      64%     77%    0.2660    H
           0.3125      18      65%     78%    0.2656  17/64
           0.3125      18      71%     86%    0.2610    G
           0.3125      18      77%     92%    0.2570    F

So the "F" drill typically recommended in tables is based on the old AN thread 
form, and 77% thread. Remember, the "F" drill might make a hole slightly 
larger than 0.2570 and the percent thread would be less.

Does anyone have info on the strength of using 92% versus 77% UN?
The old rule of thumb (for AN?) was something like; For lengths of engagement 
of at least 1.5 times the nominal diameter, the strength of 65% (AN) is almost 
the same as 75% (and is much easier to tap). I have used 60-65% (calculated 
using the AN formula) for stainless steel and it made the tapping easier.

> Since the early 70's, we in the U.S. have been using

What percent thread do you recommend using UN?

--------------------------------------------------------------
  Jim Hanrahan, PE, CP-IA         Naval Surface Warfare Center
  [email-masked]      Annapolis, Maryland, USA