Why did Leica use a British thread pitch and profile for the L39 mount

[wlaidlaw]

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Excuse me if this question has been answered before but none of the Leica books I have read to date (I still have three new ones to read), explains why Leica/Oskar Barnack opted to use a British Thread pitch (26 threads per inch) and Whitworth thread profile on a metric diameter thread for the L39 mount. The only possible answers I can come up with are:

1) This was already a thread mount in use for cine camera, microscope or telescope lenses and Leica adopted it as an industry standard - but even so who then originally opted for a metric diameter and British thread pitch. 

2) Leica's medium size threading machine/lathe was of British manufacture and did not have the appropriate change wheels (127 and 53 tooth gears on my 1910 vintage Wilson Smith lathe) to cut metric pitch threads. 

Interestingly Reid and Sigrist, who "imperialised" all the IIIb measurements other than the lens mount, as all their machine tools in Leicester, were graduated in inches and decimal fractions of an inch, opted to use British Association (B.A.) thread screws throughout their camera, rather than the metric fine used by Leica. B.A. was the thread system in almost universal use in the UK, for laboratory and aircraft instrumentation, photographic equipment, clock and model making. However, these are actually metric pitch screws, derived from the Swiss Thury watchmakers screw thread system and first proposed by the UK industry standardisation body, The British Association, in 1884. 

Wilson

[willeica]

23 minutes ago, wlaidlaw said:

Excuse me if this question has been answered before but none of the Leica books I have read to date (I still have three new ones to read), explains why Leica/Oskar Barnack opted to use a British Thread pitch (26 threads per inch) and Whitworth thread profile on a metric diameter thread for the L39 mount. The only possible answers I can come up with are:

1) This was already a thread mount in use for cine camera, microscope or telescope lenses and Leica adopted it as an industry standard - but even so who then originally opted for a metric diameter and British thread pitch. 

2) Leica's medium size threading machine/lathe was of British manufacture and did not have the appropriate change wheels (127 and 53 tooth gears on my 1910 vintage Wilson Smith lathe) to cut metric pitch threads. 

Interestingly Reid and Sigrist, who "imperialised" all the IIIb measurements other than the lens mount, as all their machine tools in Leicester, were graduated in inches and decimal fractions of an inch, opted to use British Association (B.A.) thread screws throughout their camera, rather than the metric fine used by Leica. B.A. was the thread system in almost universal use in the UK, for laboratory and aircraft instrumentation, photographic equipment, clock and model making. However, these are actually metric pitch screws, derived from the Swiss Thury watchmakers screw thread system and first proposed by the UK industry standardisation body, The British Association, in 1884. 

Wilson

Thanks Wilson. My CLA guy is actually looking at my Reid III just now and he told me recently that it had got metric screws. He is very experienced with metal work and machine tools and I will send this to him and ask him for his comments.

William

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[pgk]

Without doing any actual research on it my guess would be that it was an existing thread which had other relevant applications. I think that I am correct from my reading that the 1/4" and 3/8" Whitworth threads still used for tripod mounts today were originally proposed as such and most importantly were adopted by the Royal Photographic Society (RPS) as their 'standards' - in much the same way as the aperture scale we all use today was another RPS adoption. This mirrors the RMS thread which was and still is used for some microscope objectives (as adopted by the Royal Microscope Society I believe). The Whitworth 55 degree thread seems to have been empirically found to be highly fit for purpose and is still in use (14" 20tpi in the USA or close to UNC). Back in the 'early' days of photography there was little standardisation and many manufacturers used threads suitable to their application rather than anything else although certainly in the UK there were meetings between manufacturers to try to agree on standardisation - I have Wray lenses from the 1890s, 1910s and a post WWII TTH lens, all of which share the same imperial flange thread. No doubt the exact reasons for choice of the L39 are documented somewhere but I wouldn't be at all surprised if you were correct in that their machine tools may have been a factor. FWIW its getting increasingly tricky to buy brass BA nuts and bolts - I eventually tracked some down in a model shop in Chester the other day so that I could bolt a lens flange to a shutter.

Edited January 25, 2019 by pgk

[wlaidlaw]

Paul, 

I believe at some point in the 1960's or 70's the tripod bush was changed from 1/4" and 3/8" Whitworth to 1/4" and 3/8" UNC. These threads are slightly different with Whitworth using a 55º included angle, against UNC using a 60º. If the male to female mating is not very tight, you can often persuade a Whit nut to screw onto a UNC thread but not always. This is particularly relevant when working on older British cars, which use British Standard Fine (BSF) and Whitworth coarse for everything and as you say, it is getting difficult to find new nuts and bolts. I always try never to reuse nuts and bolts where possible. A prime cause of breakdowns on older cars. The motor racing industry world wide generally used UNC and UNF threads for everything except the engines, which are normally metric. This aways meant I had to carry around two complete sets of spanners, socket sets and hex keys when going to some event. The reason for this is that they use US aircraft specification bolts and nuts, which are all non-metric. My modern Morgan three wheeler is the other way round with UNC/UNF bolts and nuts on the American S&S engine but metric on all the chassis, body and transmission (Mazda MX5), so I still have to have two sets of spanners and hex keys. 

Wilson

[pgk]

34 minutes ago, wlaidlaw said:

If the male to female mating is not very tight, you can often persuade a Whit nut to screw onto a UNC thread but not always.

Indeed. Sloppy tolerances have their advantages in some circumstances. I'm not sure whether tripod threads have changed but I do find differences in that 3/8" 'adapters' vary from being extremely tight to loose - could be different threads or just plain poor machining.

I want to use a number of barrel lenses in front of a Copal 3 shutter and discovered that it uses M58 threads which are the same as standard 58mm filter threads. Unfortunately the front thread is too deeply recessed to take a filter though, so I couldn't fit an adapter easily - until I discovered RAFCamera in Russia who make exactly the adapter I needed which was basically a very deep filter. now all I need to do is find a machine shop able to measure the old imperial barrel lens flange threads and make adapters to 58mm filter threads. Several think that they may be able to do so - its just a matter of measuring the imperial threads. Unfortunately none of the really old flange threads are standardised - I have 4 lenses from one maker all with differering threads!

One of the lenses is a 12cm Summar - this had an M36 x 0.75mm metric thread and mine came with an adapter to M40 x 0.75mm. Fortunately the adapter also acts as a locking ring so I'm up and running with this one without having to have a custom mount built.

Edited January 25, 2019 by pgk

[luigi bertolotti]

Tricky and someway intriguing question … one had to verify on similar mechanical couplings from German concerns in that era..

for my feeling (for what is Worth…) i tend to hipotesis 2) more than 1) … the decision to convert the original fixed lens Leica to an interchangable Mount camera, taken (probably) after the intro (and success) of the Leica I, surely originated a number of problems due to the  constraint of not modifyng the basic body structure, with its precious shutter subassembly well fit into etc... all in all, I think that the decision to have a 39mm diameter was an engineering-driven decision , not related to existing products.  On the contrary, machinery of Britsh construction and standards can have a sense even in Germany of the 20's... (and btw, my father told me that the lathes in his workshop in the '30s were strictly based on Imperial standards… … though in the years of Mussolini's "perfida Albione" )

 

[wlaidlaw]

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Luigi, 

One has to recall that the UK were in the 19th and earlier 20th century, scientific instrument makers to the world. Alas now sadly mostly gone. When I was at school and later university in the 50's and 60's, all our chemistry and physics equipment was British made by the likes of Griffin and Tatlock, Reid and Sigrist, Wheatstone and Griffin and George etc. The fine and precise 26 t.p.i. threads on the L39 mount, may well have been cut on an instrument maker's lathe. At the turn of the last century, many of these would have been either British (Drummond, Colchester, Adept etc - pre WW2, there were over 200 small precision lathe makers in the UK) or Swiss (Lorch-Schmidt, etc). It is only between WW1 and WW2 that German instrument makers lathes came to be highly regarded, I would guess driven by foreign exchange difficulties and hyper inflation, making for a healthy domestic market and difficulty in importing. 

Wilson

[zeitz]

I don't know what I'm talking about, so you may want to ignore this heretical statement.  How do we know the thread diameter is 39mm?  When measured with a calipers, the diameter of any male thread is always less than the nominal diameter.  My belief is that the nominal male diameter is that of the material that is used as the starting point, before the threads are cut.  Why could not the L39 thread really be 1 17/32" x 26 tpi with Whitworth profile?

[levegh]

BA and Thury threads are shallower (47.5 degrees, I think) because they were usually cut with thread plates. For the same reason, the threads are rounded. Whitworth and BSF threads are rounded because they were designed that way. All three of those threads have fewer stress risers than their 'competitors,' but the inexorable advance of progress means that we end up with a slightly inferior system. I believe that Whitworth's 55 degree thread was that way because, when he researched the desirable introduction of standardised threads, he averaged the angles of existing screw threads. BA and Thury threads were based on calculations using metric measurements but apart from the different thread angle, I think that only 0BA, 1BA and 2BA are near to metric pitches. Metric and US sixty degree threads are based on earlier sixty degree threads. Early engineering workshops especially out in the backwoods of America and Europe would have found that angle easier when machining lathe tools because even a mathematical duffer can make up a pattern to grind the tools: you just have to draw an equilateral triangle. Whitworth form survives in pipe thread, ME threads and Cycle thread. The latter two are almost obsolete but the pipe threads are not and, as well as their presence on US pipe threads, I have found them on motorcycles made in Italy and Germany. There was a period post war when one popular British car had UNC/UNF, Whit/BSF and metric thread. That was rather confusing. And to confuse even more, I have a lot of dies and thread plates with sizes and pitches that I cannot find in any book.

Stuart

[pgk]

What actually astounds me is the extremely high quality of engineering that was practised in the 19th century. Before that is, the time when starting a lathe meant more than flicking an electric switch. 

[pico]

1 hour ago, pgk said:

What actually astounds me is the extremely high quality of engineering that was practised in the 19th century. Before that is, the time when starting a lathe meant more than flicking an electric switch. 

Yes, the era of steam powerhouses. We had remnants of one in town until recently. A steam engine on the ground floor with two  stories of pulleys to drive machines in leased spaces. Fascinating!

[wlaidlaw]

1 hour ago, pgk said:

What actually astounds me is the extremely high quality of engineering that was practised in the 19th century. Before that is, the time when starting a lathe meant more than flicking an electric switch. 

My father bought an engineering and foundry in 1962, when he got bored after three months of retirement. The lathes in there, mostly came from the Caledonian Railway workshops in Keith, which closed in 1923 and included one huge one, dated 1879, for turning axles and wheels. All 6 lathes except the one modern Stanley one, were overhead shaft/line driven with fast and loose pulleys and either leather or Balata belts. The shafting was driven by a Brush Co. 20 HP motor dating from 1925. The older guys preferred the belt driven lathes to the direct drive Stanley, as if they got a bit over-ambitious, the belt would slip rather than the Stanley breaking something. When my father bought the company, there were no belt guards at all, which were very rapidly installed, as my father said he did not want to have to cart any more employees to hospital with mangled limbs. My Wilson Smith 10"  throw lathe is a conversion from an overhead shaft model and to change speeds you still have to move flat belts between stepped pulleys. 

Wilson

[adan]

Laney (Leica Collectors Guide, edition 1992) says (p.38):

Quote

Third-party optical companies had seen the potential of a Leica with interchangeable lenses several years before Leica introduced their own 39mm, 1mm-pitch screw mount. Hugo Meyer's British distributor, A. O. Roth, was advertising "a multi-exposure focal plane camera" with a Meyer Plasmat f/1.5 lens in a screw mount in 1928. The camera was clearly a [converted] Leica.... Another conversion offered at the time retained the Elmar lens in a screw-mount, and partnered it with a 4-inch f/5.6 Ross [London] Teleros lens in a collapsible mount...

The first Leica made for interchangeable lenses, employing the 39mm x 1mm threaded mount ... appeared in 1930

It would appear that Leicas have a "British" screw thread simply because it was - the British - who first put mounting threads on a Leica....  

Edited January 25, 2019 by adan

[wlaidlaw]

7 minutes ago, adan said:

Laney (Leica Collectors Guide, edition 1992) says (p.38):

It would appear that Leicas have a "British" screw thread simply because it was - the British - who first put mounting threads on a Leica....  

I know that Dennis Laney is close to god status where Leica historica is concerned but I think he must have been on the 100º proof hard stuff, when he wrote that. 

Wilson

[adan]

Walk me through your reasoning.

[wlaidlaw]

It was Max Berek and Oskar Barnack between them who developed the Leica model 1, first with changeable lenses and then from the 1C Standard, interchangeable lenses. I don't think the British had anything to with it at all and the two foregoing gentlemen were 100% German. 

Wilson

[Pyrogallol]

Thanks to everyone contributing to this very interesting and informative “thread”.

[adan]

I don't know if I'm allowed to post copies (copyright), but Laney shows photographs of:

- Leica I #17154 (1929) converted to screw mount by Ross, with their conversion of the Elmar into a screw-mount (thicker thread band front to back than Leitz's, ~3/8ths inch depth) and their own 4" lens.

- The 1928 advertisement from Hugo Meyer & Co. (in English, specs in inches, pounds and ounces, price in "the other" pounds (£39 0s, 0d, complete with three film spools), showing an obvious Leica 1 with their f/1.5 lens attached

As ever, two ounces of documentary evidence outweigh a million tons (or tonnes) of armchair musings.

As to Berek and Barnack, we do (?) remember that for the first year of their interchangeable Leica 1, Model C (1930), the lenses had to be individually matched to the bodies, with the last 3 digits of the camera S/N engraved on each lens. Because "Berek and Barnack between them" - in their engineering wisdom - had never gotten around to actually standardizing the manufactured flange-to-film-plane distance perfectly. Required another year to set that firmly at 28.8mm and allow free swapping of lenses between cameras. (the early "variable" cameras could be updated to the standard by the factory, of course).

Ah, yes, "German engineering."

[luigi bertolotti]

7 hours ago, zeitz said:

I....  My belief is that the nominal male diameter is that of the material that is used as the starting point, before the threads are cut.  ...

https://en.wikipedia.org/wiki/Screw_thread (see "diameter" chapter… ) and https://www.gewinde-normen.de/en/index.html (with a page on Camera threads…)

Edited January 25, 2019 by luigi bertolotti

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[wlaidlaw]

4 hours ago, adan said:

I don't know if I'm allowed to post copies (copyright), but Laney shows photographs of:

- Leica I #17154 (1929) converted to screw mount by Ross, with their conversion of the Elmar into a screw-mount (thicker thread band front to back than Leitz's, ~3/8ths inch depth) and their own 4" lens.

- The 1928 advertisement from Hugo Meyer & Co. (in English, specs in inches, pounds and ounces, price in "the other" pounds (£39 0s, 0d, complete with three film spools), showing an obvious Leica 1 with their f/1.5 lens attached

As ever, two ounces of documentary evidence outweigh a million tons (or tonnes) of armchair musings.

As to Berek and Barnack, we do (?) remember that for the first year of their interchangeable Leica 1, Model C (1930), the lenses had to be individually matched to the bodies, with the last 3 digits of the camera S/N engraved on each lens. Because "Berek and Barnack between them" - in their engineering wisdom - had never gotten around to actually standardizing the manufactured flange-to-film-plane distance perfectly. Required another year to set that firmly at 28.8mm and allow free swapping of lenses between cameras. (the early "variable" cameras could be updated to the standard by the factory, of course).

Ah, yes, "German engineering."

Andy,

I have re-read the Laney book and there does not seem to be any implication or evidence that Leica were influenced by these third party modifications. I suspect that the usual chauvinistic, conservative  and insular attitudes, I have frequently encountered in industry, could have meant that Leica might have paid little attention to what these third parties were doing. These attitudes are usually summed up under the acronyms NIH = Not Invented Here and WKB = We Know Best.

My father had an enlarger which used a  2" diameter thread for its lens mount. They did supply an adapter from the 2" (pitch unknown but very fine) to L39 for using Leica lenses for enlarging. My father habitually used a 2" mount Ross Resolux 9cm Lens for 6 x 6, a 2" mount a 3¼" Supar Wray for 4 x 4 and an L39 50mm Elmar for 35mm, with the adapter. The enlarger used an Edison screw 500 watt projector bulb and the black crackle housing got hot enough to burn you if you touched it by mistake. 

Maybe once the archives are better organised at the new museum, we can find the real reason for 26 t.p.i. 😀

Wilson