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

[pgk]

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Whilst I realise that such a thought is sacrilege in a precision situation, but given the few threads used in a lens mount would 26tpi and 1mm pitch (25.4tpi)  be more or less interchangeable with some tolerance? 0.6mm over 25 threads when a lens only uses around what - 3ish?

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[wlaidlaw]

56 minutes ago, pgk said:

Whilst I realise that such a thought is sacrilege in a precision situation, but given the few threads used in a lens mount would 26tpi and 1mm pitch (25.4tpi)  be more or less interchangeable with some tolerance? 0.6mm over 25 threads when a lens only uses around what - 3ish?

Paul, given how finely the threads are cut, I am sure they would not fit. A couple of years ago, I bought some new L39 body/deep lens caps from China. I suspected they were cut to 1mm pitch as my metric thread comb/thread file fitted in perfectly, whereas (annoyingly) it doesn't really fit 26tpi threads for cleaning them up. The lens caps which had very shallow threads cut on them, would screw on about 1½ turns but the body cap would only screw in ¾ of a turn. I had some correspondence with the seller, who finally admitted that they had cut the threads at 1mm pitch, as they only had metric change wheels for their lathe. 

Wilson

[pgk]

Too high precision. They need to be sloppier.

[TomB_tx]

Canon, for example (and I think Nikon), got the thread wrong on their early cameras / lenses. While most of the Canon cameras in the 30s & 40s used a Nikon-made focusing mount adapter to bayonet-mount lens (also made by Nikon), their budget models used “J mount” which was close to ltm, but wouldn’t fit. After the war they changed the bodies to a “universal flange” that was sloppy enough to use both their lens thread or Leica’s. Then by about 1953 they changed all bodies and lenses to ltm standard. 

I’ve run across a couple other early Japanese Leica copies where the flange seems to be metric or off tolerance so the fit is too tight.

[Pyrogallol]

I have got a chrome Minolta lens back cap that ought to fit, but it only screws on less than a quarter turn, don't know what it would have come off?

[willeica]

For what it is worth, my CLA man is still working on my Reid III. He reports the following, so far.

1. All outside top housing body shell screws are metric at m1.70 x 0.35mm.

2. The slow speed dial mechanism screws are at m1.7 except for 3 small centre holding slotted head grub screws that measure at m1.20 x 0.25mm

3. Only 2 of the screws mentioned at 2 are fitted and both have breakout on the slots. My CLA guy will replace them with the m1.40 x 0.30mm screws which are usually found on Leica LTM models with a slow speed dial.

4. The slow speed dial has to be removed as it hides a lock screw on the main body shell.

I am reporting these details here, but I don't fully understand the details. Others may wish to comment. I totally trust my CLA man who has done excellent work on any cameras that I have given to him.

William

Edited January 29, 2019 by willeica

[pgk]

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I presume that no camera manufacturer makes their own nuts and bolts and that they are therefore bought in from another manufacturer or importer. Availability no doubt plays a big part in sourcing. By the sound of it Reid were buying in metric items which may have been a deliberate policy, or simply a decision based on availability and continuity of supply. The idea that manufacturers were wedded to imperial or metric in the past because of tradition is clearly wrong.

[wlaidlaw]

B.A. screws are metric as they are based on Swiss Thury threads. 1.7mm is not a standard metric size but it is 10BA. Standard metric screw sizes are 1mm, 1,2mm, 1.6mm and 2mm, with 1.4 and 1.8mm very occasionally used for special purposes. 1.2mm threads can be either metric or 12BA. 

Wilson

[levegh]

The cameras in the Reid probably used the same threads as Reid and Sigrist's aircraft instruments. 1.70 x 0.35 is surely 10BA, while 1.2 x 0.25 is probably 3/64" x 100TPI. It is very difficult to measure such small screws especially if they are very short. Once you get below the smallest BA size, selection is probably somewhat arbitrary but it makes sense to use Imperial sizes and threads per inch based on whole numbers rather than a pitch.

Stuart

[levegh]

Wlaidlaw, If it was measured correctly, I don't think 1.2 x 0.25 is close enough to be 12BA. But I did say that it is difficult to measure them. 12BA is closer to 1.3 x 0.28.

Stuart

[ph.]

Most interesting piece of industrial history. I would not be surprised if manufacturing inertia was caused by the Brits having pioneered lathes.  Our medium ancient  Land Rover series II still has BA screws on the speedo-output from the gearbox.

p.

[pgk]

FWIW: http://www.earlyphotography.co.uk/site/lens_mounts.html#Screw Thread

[pico]

I rebuilt a 1973 Rickman-Trident, also here, and had enough English motorcycles before that I was stunned to find that, with one exception, the fasteners were USA standard. (Confounding was the steering races and bearing which were Russian not to be found in my bible of Barry's Bearings at the time.)

Oh, that motorcycle is still raced and winning at Daytona 43 years later.

Just found, once a popular image. Sharon back in the day.

 

Edited January 31, 2019 by pico

[Susie]

Hi Guys,

I have seen this reference to the 26TPI thread before and I wonder where it originated.  If the thread was measured using a standard Imperial thread gauge I can easily see why, over the short length of thread available on the mount, 1mm pitch could be mistaken for 26TPI..  Bearing in mind that the difference is only about  0.0009" per turn (0.02mm ) in the three turns needed to screw the lens in I doubt the difference would be noticed, especially if the clearance was slightly generous. The thread form is likely to have as much influence as the pitch when dealing with these fine threads over a very few turns.  This might be why your Chinese adaptors did not fit, Wilson.

In the recent book published by the LHSA the thread is given as 39mm x 1mm pitch, but I don't know where that figure was obtained. 

In the report of the post war survey of the Leitz works, the machines mentioned for screwcutting the multi-start focus helixes were made by Lorch - a modified version of their standard thread chasing machines - not using changewheels, but a master hob using an automatic indexing mechanism to produce the multi-starts.  I would assume that similar chasing lathes would be used for most of the threads needed in production.

Even today the Whitworth thread form is used extensively in the motor industry as the deeper thread of the 55 degree gives better results in alloy components.  But to show it isn't all one way, between 1923 and 1955 the threads used by Morris and MG were metric form (the machines in use came from the  French Hochkiss engine factory) but cut on Whitworth size hex stock.

Edited February 6, 2019 by Susie

[Martin K]

I have a copy of a draft DIN standard 15735 dated December 1984 in which the Leica thread featured. I attach one of the diagrams in the standard which provides the link to a thread of 1mm pitch. Whether this DIN standard was eventually published as a national standard I do not know, but it is interesting nevertheless that the pitch was stated as 1mm, whereas many publications consider it to have been 26 threads per inch. Maybe the tolerance stated in the draft standard, i.e. 4C, took care of the small differences as well as the rather small maximum length of the screw thread of 4mm. I would guess that Leica was a member of the technical committee that developed this draft DIN standard, hence the dimensions should be correct.

Martin

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[wlaidlaw]

A 1mm thread pitch is equal to 25.4 threads per inch. Leica tend to use quite short threads for their lenses (about 2½ turns from memory - I will check tomorrow) but other makers like Ross and Wray used much longer L39 threads for their enlarging lenses of about 4 turns or more. These threads are a very fine fit male to female with very little perceptible slack or rocking. I think a discrepancy from 26 tpi to 25.4 tpi would bind over 4 turns. 

I don't think the 26 tpi figure was arrived at by accident or mis-measuring, because the natural assumption would be that a 39mm diameter thread would have a metric pitch not imperial. That is the reason I asked the question in the first place, hoping there would be a definitive answer but so far, only guesswork. Interestingly the British inch is defined in centimetres as exactly 2.54 cm and the defined standard (1932/33) was taken with reference to the platinum metre in Paris. Prior to 1933 the American Inch was longer at 2.5400051 cm but the 2.54 measurement was adopted in 1933. It is rumoured that Trump wants to return to the larger inches 🙂

Wilson

[TomB_tx]

The DIN standard Martin shows from 1984 is, of course, long after Leitz sold the company and Leica had ceased using ltm bodies, so they may well have had no input to the standard. The references contemporary with Leica ltm production are quite consistent that it was an E39 26 tpi thread, not an M39.

[Susie]

The thread given in Martin's diagram also shows that it is made to a 4c standard. This will give the  tolerance, but unfortunately I don't have the ISO standards information ( and don't want to buy it!) 

I agree with Wilson that over four turns a difference of nearly 0.004" will cause it to bind, but just because Wray and Ross use 26TPI doesn't mean that Leitz did too.  As I said before, it would be interesting to know where the first mention of Leitz using 26TPI came from. It appears it certainly wasn't from Leitz itself.

The 1957 Federal standards (USA) gives the tripod screws as using a UNC thread form - both for the home market and adaptors to the European standards.  T

Just a thought, if the Leica thread is 1mm x 60 degree form and the Ross/Wray is 26TPI x 55 degree that might give enough play for the two to work together.

Susie

Edited February 7, 2019 by Susie

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[Martin K]

The draft DIN standard was developed by a technical committee representative of the German photo industry, hence I believe Leica/Leitz may well have been present. It would be interesting to measure the thread of Leica enlarger lenses of more recent date, as the same thread was used for these by Leica/Leitz long after they stopped the production of TLM bodies.

[wlaidlaw]

Where is Jim Lager (LHSA) when you need him? Jim is probably the person most likely to know the real answer to this. He has quite a few of the drawings for early Leica cameras and I am sure that the thread pitch will be shown on those drawings. 

Wilson

PS I have asked my contact at LHSA, Richard Rejino, to ask Jim Lager the next time he is in contact, if Jim can throw any light on this subject. I will also ask Malcolm Taylor, when I am in contact with him to get a progress report on my 250FF Reporter and motor drive IIIa. 

Edited February 7, 2019 by wlaidlaw