My son spent another week in Tebis training last week...
http://en.wikipedia.org/wiki/Tebis

So, I asked him what he learned. He's now so far into tooling design I
can hardly follow it. he's now lead CAM programmer for his company.

But he did tell me this little fact. For a tap drill size you subract
the recipricle of the lead from the nominal size. ie. for a 3/8 x 16
thread subrtract 1/16 from 3/8 and use tap drill size 5/16.

For metric taps subtract the lead. ie. for M10 x 2.0, 10 -2 = 8 mm tap
drill size.

I had always just looked at my handy tap chart.

On Fri, 06 Sep 2013 15:23:33 -0500, Karl Townsend
wrote:

My son spent another week in Tebis training last week...
http://en.wikipedia.org/wiki/Tebis

So, I asked him what he learned. He's now so far into tooling design I
can hardly follow it. he's now lead CAM programmer for his company.

But he did tell me this little fact. For a tap drill size you subract
the recipricle of the lead from the nominal size. ie. for a 3/8 x 16
thread subrtract 1/16 from 3/8 and use tap drill size 5/16.

For metric taps subtract the lead. ie. for M10 x 2.0, 10 -2 = 8 mm tap
drill size.

I had always just looked at my handy tap chart.

Cool tip! Saved and remembered (hopefully.) Thanks, Karl.


--
It is common sense to take a method and try it. If it fails,
admit it frankly and try another. But above all, try something.
-- Franklin D. Roosevelt

On 9/6/2013 8:10 PM, Larry Jaques wrote:
On Fri, 06 Sep 2013 15:23:33 -0500, Karl Townsend
wrote:

My son spent another week in Tebis training last week...
http://en.wikipedia.org/wiki/Tebis

So, I asked him what he learned. He's now so far into tooling design I
can hardly follow it. he's now lead CAM programmer for his company.

But he did tell me this little fact. For a tap drill size you subract
the recipricle of the lead from the nominal size. ie. for a 3/8 x 16
thread subrtract 1/16 from 3/8 and use tap drill size 5/16.

For metric taps subtract the lead. ie. for M10 x 2.0, 10 -2 = 8 mm tap
drill size.

I had always just looked at my handy tap chart.

Cool tip! Saved and remembered (hopefully.) Thanks, Karl.


--
It is common sense to take a method and try it. If it fails,
admit it frankly and try another. But above all, try something.
-- Franklin D. Roosevelt


that cool tip follows from 6 degree thread pitch - 1/2 base times height
- it's in all the handbooks.

On 9/7/2013 1:20 PM, . wrote:

that cool tip follows from 6 degree thread pitch - 1/2 base times height
...

You mean 60 degree, of course. And "... 1/2 base times height" is the
area of a triangular thread cross section & has nothing to do with
diameters.

The height of the triangle is P/2 * SQRT(3)(where P = pitch), which is
the difference between the major & minor _radii_ of a _triangular_ form
thread. The difference in diameters is then P*SQRT(3) which is quite a
bit larger than P (1.7 times).

But the standard thread form is not a triangle - the crest and root are
both truncated. So by a happy cancellation of inaccuracies (the pitch
being to small a difference for triangular threads and the standard
thread not being triangular), the pitch is a close-enough difference
between major & minor diameters.

I suppose that if one went through the geometry of the truncations, the
difference would come out being (almost)equal to the pitch. But it
certainly does not come from the height of a 60 degree triangle whose
base is equal to the pitch.

Bob

"Bob Engelhardt" wrote in message
...
On 9/7/2013 1:20 PM, . wrote:

that cool tip follows from 6 degree thread pitch - 1/2 base times height
...

You mean 60 degree, of course. And "... 1/2 base times height" is the
area of a triangular thread cross section & has nothing to do with
diameters.

The height of the triangle is P/2 * SQRT(3)(where P = pitch), which is the
difference between the major & minor _radii_ of a _triangular_ form
thread. The difference in diameters is then P*SQRT(3) which is quite a
bit larger than P (1.7 times).

But the standard thread form is not a triangle - the crest and root are
both truncated. So by a happy cancellation of inaccuracies (the pitch
being to small a difference for triangular threads and the standard thread
not being triangular), the pitch is a close-enough difference between
major & minor diameters.

I suppose that if one went through the geometry of the truncations, the
difference would come out being (almost)equal to the pitch. But it
certainly does not come from the height of a 60 degree triangle whose base
is equal to the pitch.



Quicker to simply look at a chart than to convert say 1/13" or 1/11" into a
decimal format...

I've had most of the common tap drill sizes memorized nearly forever anyways
although frequently I'll use a slightly larger size, depending on the actual
job

"PrecisionmachinisT" wrote in message
news:cq2dnU9ZJ5D_ibHPnZ2dnUVZ_vGdnZ2d@scnresearch. com...


I've had most of the common tap drill sizes memorized nearly forever
anyways although frequently I'll use a slightly larger size, depending on
the actual job

Same here, a larger hole makes hand tapping so much easier and cuts
down on tap breakage.

A common internal thread, drilled so that it results in 50% of full thread
will break the external thread, before the internal thread will strip. A
common internal thread drilled out so that it contains 100% of full thread
is only 5% stronger than a 75% height of thread, yet it requires 3 times the
power to tap.

http://www.kennametal.com/kennametal...rill-size.html

Best Regards
Tom.

On 9/6/2013 11:10 PM, Larry Jaques wrote:
On Fri, 06 Sep 2013 15:23:33 -0500, Karl Townsend
wrote:

My son spent another week in Tebis training last week...
http://en.wikipedia.org/wiki/Tebis

So, I asked him what he learned. He's now so far into tooling design I
can hardly follow it. he's now lead CAM programmer for his company.

But he did tell me this little fact. For a tap drill size you subract
the recipricle of the lead from the nominal size. ie. for a 3/8 x 16
thread subrtract 1/16 from 3/8 and use tap drill size 5/16.

For metric taps subtract the lead. ie. for M10 x 2.0, 10 -2 = 8 mm tap
drill size.

I had always just looked at my handy tap chart.

Cool tip! Saved and remembered (hopefully.) Thanks, Karl.


--
It is common sense to take a method and try it. If it fails,
admit it frankly and try another. But above all, try something.
-- Franklin D. Roosevelt




Do you have "CRS" disease? (can't remember ****) Welcome to the
---whatever it is!

On Sunday, September 8, 2013 2:35:56 AM UTC-4, Howard Beal wrote:


A common internal thread, drilled so that it results in 50% of full thread

will break the external thread, before the internal thread will strip.



Best Regards

Tom.

If I recall correctly that is true for holes that the engagement length is at least as long as the diameter.

Dan

" fired this volley in
:


If I recall correctly that is true for holes that the engagement
length is at least as long as the diameter.


Rule of thumb is five full threads of engagment. For common coarse
threads, that's _about_ one major diameter. (i.e. 1/4-20... 5 threads
equals 1/4")

LS

wrote in message
...
On Sunday, September 8, 2013 2:35:56 AM UTC-4, Howard Beal wrote:
A common internal thread, drilled so that it results in 50% of full
thread
will break the external thread, before the internal thread will
strip.
Tom.

If I recall correctly that is true for holes that the engagement
length is at least as long as the diameter.
Dan

I distantly remember from college that the minimum engagement length
was a good rule of thumb to apply if you didn't have data on the
material properties. Later I heard it in auto and aircraft contexts.

A Grade 8 screw in an aluminum casting won't fail the same way as a
Grade 2 in a steel nut, and coarse threads are more likely to break at
the thread root, fine ones more likely to strip. If we knew the
properties we were supposed to calculate how much the male element
would stretch and IIRC design so the threads fail by stripping
progressively, beginning at the hole surface where bolt stretch is
greatest.

Herreshoff designed his rigging turnbuckles such that the barrel and
eyebolts stretched identically. When tested to failure all parts
deformed before any broke.

The college class was for chemists and perhaps not as extensive in
mechanical failure modes as one for MEs, but it went far into
alloying, grain structure and corrosion.

jsw

On 9/8/2013 7:39 AM, Lloyd E. Sponenburgh wrote:
Rule of thumb is five full threads of engagment....

The ROT that I read here, a long time ago*, was 4 threads for steel and
6 for aluminum.

Bob

* - i.e., too long ago to remember the author and so to determine
credibility