Confused by 26, 32, 40 TPI Threads

I want to buy some boiler and pipe fittings and they are usually listed for example as 1/4 X 40, 5/16 X 32, 3/8 X 26 TPI.

What is the Standard or Series of these threads?

Having compiled a Table of these (available if you ask) together with Model Engineer Threads and Cycle threads it seems they are none of those.

Examples:-

• Model engineer threads are 55 deg
• Cycle threads are 60 deg
• Pipe fitting listed as 1/4 X 40 but ME has no 40 TPI thread
• Pipe fitting listed as 9/16 X 26 but ME has no thread for 9/16 dia.

Another question would be "what is the Form of the Model engineers thread series".

I do have the Model Engineer (ME Handbook) but it doesn't explain these questions.

By the way I have a full set of ME taps and dies but concern they might not be compatible with the listed fittings.

I am no expert but I have always assumed that the boiler fittings were ME threads (or Brass thread for 26 TPI) with a Whitworth thread angle. Someone with far more experience than me will soon enlighten us.

Tracy Tools list 1/4 x 40 ME taps and dies, for 9/16 x 26 they have Brass.

Thor

Edited By Thor on 25/08/2015 05:20:59

As far as I know Brass thread uses the Whitworth thread angle.

Thor

Thanks Thor, I now know what a Brass Thread is.

I should mention that I have sent the same question to the people selling the pipe fittings.

It still leaves the question unanswered:-

• Pipe fitting listed as 1/4 (plus 3/16) X 40 but ME has no 40 TPI thread (at least according to my Model Engineer Handbook)
  

Perhaps the Trade has generated these in 40 TPI but they aren't listed by in Tubal Cain's book.

I suppose somewhere there must be a definition of what "Model Engineer" threads are but I have never seen one.

Can anyone help, please?

To the best of my knowledge:

The ME series is de facto, not de jure

It was, I think, "introduced" by the Model Engineer magazine [but I know not when], and uses Whitworth-form threads.

'Constant Pitch' is a convenient basis for manufacture, and therefore appropriate to miscellaneous diameters.

The use of 26, 32, or 40 tpi depends mostly on the thickness of the component being threaded ... although 32 and 40 are also convenient for feedscrews.

MichaelG.

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P.S. ... If anyone can Identify the "first publication" of the ME Series, I would be grateful.

For anyone who want to know all about model engineer's threads, including discussion of metric and BA alternatives I recommend them to refer to Chapter 24 of The Model Engineers Workshop Manual by George Thomas.

Colin Usher has a useful page

MichaelG.

This is quite a good site and shows the form of ME, Cycle, CEI and brass along with a lot of others

Ah! A most useful reply, thank you.

I have the George Thomas book open in front of me now. On page 266 there is a Table II listing the 'ME' series (amongst other sizes).

1/4 (plus 3/16) X 40 are listed as being ME threads.

The origin of the ME series is given as "... established by a committee of the SM & EE in 1912...)...

There are no details of (for example) core diameters but I suppose these could be worked out using the Whitworth Form dimensions. (It - the book - doesn't actually say that the ME series is Whitworth Form)

**LINK**

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Hint: That nice Mr. Usher has done the hard work ... check the links on his page.

This is very useful indeed. It points to the inside of a hardcopy book "Guide to World Screw Threads" By P A Sidders.

This gives the origins of the ME series and two tables of sizes and thread pitches. One Table also gives data such as Major and Minor diameter and Tapping Drill sizes, but not for the 60 TPI threads.

Strangely, again, it does not state that the series is based on the Whitworth Form nor does it give the thread angle.

Many thanks Michael, I'll try to get a copy of that book.

Sometimes the ME threads that fit into the other common series are not listed, presumably to help folk avoid buying two of the same tap and die.

Neil

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In 1912 ... before the World went mad ... Whitworth was the form for vee screw threads.

MichaelG.

Interestingly, Table II in the George Thomas books (where he *s the ME series threads) differs from that in the "Guide to World Screw Threads".

I don't have time immediately to itemise the differences but will do it later today or tomorrow.

Please bear in mind that I'm not trying to research ME threads (or threads in a broader sense) I'm just trying to make sure that when I buy pipe fittings that I know what thread it is and to avoid buying another set of taps and dies. Pretty important when the threads will be used on a 1/2 sized traction engine with a boiler running at 125 psi.

If I may be allowed to give a plug for my book "Metalworkers Data Book" it gives thread forms, pitches, core diameters, tapping drill sizes, % depth of thread using the quoted drill sizes, dimensions given in both metric and Imperial, etc. and for the following thread types

Metric course
Metric fine
Metric Spark plugs
Metric conduit
BA
UNC
UNF
BSW
BSF
ME
British standard brass
British standard cycle
British standard pipe parallel
British standard pipe taper
National pipe taper
Metric threads recommended for model engineering
The book gives outline drawings for their thread forms .

The book also has details for many other subjects,see link above for details

Myhobbystore have it listed here

Harold

According to Machinerys Screw thread book,1965 edition the model engineers thread was established in 1912 by a meeting of the SMEE and the editor of the Model Engineer,it lists a series of fine Whit form threads from 1/8 to 1/2 inch with a variation of pitch 40 ,32 and 26 .. then mentions that combinations of threads from 1/8 to 9/16 with pitches varying i.e. 60 ,40 and 32. Instrument makers also used lots of specials to suit brass tubes and eyepiece fitting,and were sometimes available off the shelf or to special order,taps were more commonly used,male threads tended to be chased with whit form chasers,though special 40 tpi dies were used on capstan lathes for higher volumes, i.e. 1000 batches (carbon cut thread dies would last for thousands of pieces) Brass thread apparently did not have any recognised standards, it seemed to stem from the millions of gas lamp fittings in Victorian times,they are 26 tpi and whit form. So take it that all the model engineer threads ,brass thread and bsp pipe are to whit form.When I was apprenticed (1958) as an instrument maker all the threads apart from BA were whit form,plus the occasional Acme and Square thread, and so was a lot of other engineering. Outside of work ,cycle threads (60 degree form)were encountered on push bikes and some motor cycles,all nuts were whit size,then came cars with unified threads and a/f nuts,then came further confusion with metric, I can never understand why cycle threads were 60 degree form used from early time instead of whit form. I can understand why there is now confusion ,as when I was trained you just knew that the common fine threads were Whit form.

My first lathe was a quite old Carl Hurth, it was made in Germany but all the threads were Whitworth.

Best regards,

Swarf, Mostly!