Re: Centralizing Acme inserts

On Sat, 10 Jun 2006 17:23:29 +0200, J. Nielsen
<morgan@xxxxxxxxxxxxx> wrote:

On Fri, 09 Jun 2006 17:12:49 -0500, F. George McDuffee
<gmcduffee@xxxxxxxxxxxxxxxxxxxxxx> wrote:

A tangential question on acme threads. Does anyone know why the
included angle is 29 degrees?

Nope!

A quick question to JN in Denmark -- how much problems do you
have keeping the 29 degree acme tools/products separate from the
30 degree iso/din trapezoidal tools/products.

I can't recall doing any 29 degree profiles. 30 degree seems pretty
much the norm over here. (and an occasional 40 degree) When I mentioned
acme it was to avoid confusion - we refer to the profile as trapeze
thread.

Does anybody know why Whitworth threads have a 55 degree included
angle? [It took a while but I was able to dig this out of a very
old engineering text.]

Do tell... Whitworth is my hero due to his work on small bore rifling.
(back in an era when .45 cal was considered small)
=========================
The Whitworth screw thread system including standardized thread
form, diameters and pitch per diameter was originally published
in 1841. Obviously, Mr. Whitworth had been conducting work on
this for several years previously. At that time there was no
good and easy way to calculate stresses, and no good data on
materials. Thread forms, pitches and screw sizes were whatever
the blacksmith felt like making that day, (and what he had tools
for.)

The Whitworth threads were empirical before they were imperial,
in that he collected samples of every thread that had been in use
for a while from every part of England (I think Scotland, Wales,
Ireland, etc. were excluded) assuming that if it had been in use
over time, it was most likely an acceptable thread. He then
"averaged" the screw thread specifications. Thus the 55 degree
included angle of the Whitworth is the average of the included
angles of long-use threads in use in England from about
1835-1840.

The relationship of pitch to diameter was established in the same
way. A complication was that the lead screw pitch for lathes
still had not become more-or-less standard, and there were lathes
with 4,5,6,7,8,9 etc. (and some fractional) threads per inch and
each of these would result in a different sequence of [easily
cut] thread pitches.

It appears the only reason we don't have Whitworth pitch
specifications such as ¼ inch diameter X 26.75 t.p.i. is that
these were difficult to cut with the lead screws and gears in
common use at that time. He did however compile the pitches and
diameters in long-term use and picked the most common one for a
given diameter after establishing a mathematical relationship or
ratio of t.p.i to diameter. [I don't think the concept of helix
angle had yet been introduced.

Considering the state of engineering at that time, this was
perhaps the only practical approach. Given the materials [highly
variable cast iron, brass etc.] in use at the time, the coarse
thread series was entirely functional.

From a theoretical standpoint the rounded root of the Whitworth
thread has much to recommend it, as it avoids a stress riser.
The UN series of inch threads includes a similar rounded root
thread for high stress aerospace use. In actual use, a 60 degree
HSS [and the early carbon steel] thread tools tended to rapidly
wear from a sharp corner to a more rounded root and rolled
threads also have a rounded root.

I do not know if anyone with access to a computer with finite
element software has evaluated the 55-degree Whitworth v. the
60-degree thread profile for use in the soft and variable
materials in use at that time. There may be a slight theoretical
advantage.

Now if I can just find out where that 29 degree included angle
for the U.S. Acme came from ?



Unka George
(George McDuffee)

There is something to be said for government by a great aristocracy
which has furnished leaders to the nation in peace and war for generations;
even a democrat like myself must admit this.

But there is absolutely nothing to be said for government by a plutocracy,
for government by men very powerful in certain lines and gifted with the "money touch,"
but with ideals which in their essence are merely those of so many glorified pawnbrokers.

Theodore Roosevelt (1858-1919), U.S. Republican (later Progressive) politician, president. Letter, 15 Nov. 1913.
.

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