Ltm threads per inch/cm

[FrankS]

Came across this web page http://www.peeble.com/screwmount.htm
Which states that while the screw mount diameter is metric, the thread pitch is imperial. That's not correct, is it?

 

[Particular]

I think it is correct. Leitz used whitworth threading machines, which were the standard used on microscopes.
The soviet ltm mounts were made with metric threads, which accounts for their slightly awkward fit.

 

[FrankS]

Cool. Thanks!

 

[Spavinaw]

Current standard LTM thread is 39mm diameter, 26 threads per inch. It was different before standardized.

 

[FrankS]

That 26 threads per inch, is that a metric or imperial standard? (Or whitworth?)

 

[Particular]

It's Imperial. Whitworth is the original British standard thread type. The 26 threads per inch is an uncommon pitch, but I guess that was the point.

 

[Roger Hicks]

Current standard LTM thread is 39mm diameter, 26 threads per inch. It was different before standardized.

Are you sure? As far as as I am aware, standardization was flange to film distance, using the existing thread.

Cheers,

R.

 

[Roger Hicks]

That 26 threads per inch, is that a metric or imperial standard? (Or whitworth?)

Dear Frank,

"Inch" gives you something of a clue that it ain't metric...

Cheers,

R.

 

[Dez]

The Whitworth system is rather strange. There are many different thread diameters with a 26 TPI diameter. Old British motorcycles had all Whitworth fasteners, and as I recall, every bolt from the largest to the smallest was 26 TPI.

Cheers,
Dez

 

[David Hughes]

Hi,

The Royal Microscope Society www.rms.org.uk had a hand in it and standardised things internationally a long time ago. Have a search of their site and you might find the original papers somewhere about it. They include a bit about standardisation of Whitworth as an aside.

I used to have it bookmarked but a mega crash took it and the computer from me, sorry.

I wonder if they'll think the history of Windows is as strange* in a hundred years time. I already do...

Regards, David

*That was because I remembered all the problems I had getting my data to run in the replacement computer.

 

[john_s]

The Whitworth system is rather strange. There are many different thread diameters with a 26 TPI diameter. Old British motorcycles had all Whitworth fasteners, and as I recall, every bolt from the largest to the smallest was 26 TPI.

Cheers,
Dez

The 26 thread per inch was chosen primarily for thin walled tubing, where a deeply cut thread was inappropriate in large diameter tubes. If you think about it, Imagine a standard bolt thread (say 1/2inch) enlarged up to 39mm in diameter: the thread would be cut very deep. Those of us who work on Victorian light fittings still cut British Standard Brass threads, all 26 tpi.

 

[Dez]

Sadly, one of the thread pitches not available with the standard gear set of my little Atlas 5" lathe is 26 TPI!

Cheers,
Dez

 

[oltimer]

Sadly, one of the thread pitches not available with the standard gear set of my little Atlas 5" lathe is 26 TPI!

Cheers,
Dez

To even cut metric threads on your lathe; you would need a 27 tooth lead screw gear. You can make one though

 

[Kate-the-Great]

To even cut metric threads on your lathe; you would need a 27 tooth lead screw gear. You can make one though

So LTM uses Imperial thread pitch, but Metric thread profile? That's bizarre, but I suppose I've come across enough strange engineering choices that these things shouldn't surprise me anymore. :bang:

 

[David Hughes]

Hi,

These choices are as much politics as engineering. Also the makers have a say in it. So a sensible suggestion from a small firm carries little weight compared to what the large firms are planning or making. Often a lot of those involved sat back and watched the fight and did nothing much.

I was involved in this in the 60's when there were only 6 or 7 countries involved and some countries had three large firms fighting between themselves with the regulator and courts involved. Other countries had experience of just one experimental one that the Govt used and so on. In terms of people it might be lawyers vs engineers vs accountants...

By the late 80's the internal (countries) rows were about the same scale as the international ones in the 60's. Sense didn't come into it. Sometimes even the banks who would finance things would veto the decision afterwards. And then there were the rumours about the security people being involved...

Regards, David

 

[oltimer]

Hi,

These choices are as much politics as engineering. Also the makers have a say in it. So a sensible suggestion from a small firm carries little weight compared to what the large firms are planning or making. Often a lot of those involved sat back and watched the fight and did nothing much.

I was involved in this in the 60's when there were only 6 or 7 countries involved and some countries had three large firms fighting between themselves with the regulator and courts involved. Other countries had experience of just one experimental one that the Govt used and so on. In terms of people it might be lawyers vs engineers vs accountants...

By the late 80's the internal (countries) rows were about the same scale as the international ones in the 60's. Sense didn't come into it. Sometimes even the banks who would finance things would veto the decision afterwards. And then there were the rumours about the security people being involved...

Regards, David

How true you speak David. In Canada the Gov. finally decided in the '70's they wanted everything metric. The 1st thing was the gas pumps. Our Can. gallons being larger than the US gallons. I remember tourist saying our gas was so cheap, but did not know 5 of their gallons equal 4 of ours. The Gov. never asked one machine shop about costs on purchasing equipment to do metric stuff; as they just rammed it through for their benefit. I will take one of my screw mount Leicas out to the shop to actually measure the thread pitch, as I have never ever checked one.

 

[Sparrow]

... the Whitworth thread came first so technically its all the rest that are odd ;D ... it is unusual in one respect though, in that the tops and bottoms of the thread have a radial form.

That made screws cut with it much stronger and less prone to fatigue failures ... important as the threads were part of the gauges used to standardise the British army musket production way back (I know they try to tell you it was Ford who did all this stuff but it wasn't)

 

[Dralowid]

The only reason I ever go to the States is to savour the feet and inches...

 

[Sparrow]

... or Greece for the imperial plumbing ... BSP and BSPT for all that ¾ malleable iron tubing