A different kind of project...help needed

[ssgross]

Friends, a long time ago, before the great reset where we lost all our photos, I posted some of my old telephone restorations.
Here is my Western Electric 320 "Explosion proof telephone" I restored back then, proudly displayed in a corner of my home office where it get's used for work. During the pandemic, I would call in to video-conference meeting from it for the audio, so my collegues would see me standing there talking on the phone. Priceless. It has the benefit also of being able to hear the ringer from down the street.


It's missing the key and the inside cable gland on the handset. Well, wouldn't ya know my luck always comes through, eventually...and I found not only a key, but a later 520 series in near mint condition...complete with its cable gland to replicate for the 320. I'm hoping to find a milsurp friend with a lathe willing to help me make this rather simple set of brass parts.

Here is a stock image (since I already disassembled mine) behind the front cover. Arrow pointing to the part I'm talking about...

Now On the backside of the bulkhead...


I cannot remove the handset without splicing the handset cord. The speaker/mic covers are welded/sealed on, and the heavy rubber knob that forms the seal was spliced in and heat-welded to the rest of the outer insulation. Nonetheless, I can dissemble enough to get accurate measurements.
It's in 4 parts. 1) outer sleeve that passes through the front of the bulkhead. 2) a nut that screws on to the outer sleeve from the inside...in the event of that internal atmosphere sparking, the explosion would expand the outer brass sleeve to seal against the bulkhead. It's a tight fit to the bulkhead, but not a press fit. the brass sleeve plus out with fingers, but is a perfect fit. 3) the solid rubber knob on the cord's insulation is compressed by 4) the inner compression nut.


I could wait a few years until I have lathe, but that goal keeps getting pushed back. Anyone interested?

Information

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[browningautorifle]

As you must be aware, I have a lathe but since it's small I can't really thread accurately as one would on a bigger model. Also you're going to need to reverse engineer and do a set of drawings for this one.

[Flying10uk]

And it has slots which normally get machined in a mill. Sometimes I file a small slot for something like a screwdriver slot but it would depend on how fussy you are and how good the person doing the filing is at filing.

[ssgross]

And it has slots which normally get machined in a mill.

I've taken measurements, and will translate to detailed drawings. I was thinking I'd do all the slots myself with a file. I'm good enough at it for my standards, and it's only brass after all.

the threads all seem to be ~30 per inch, or so it seems counting under a magnified glass and putting up against an 1897 barrel I happened to have next to my desk. Looking at standard die sizes though maybe those outermost threads are 28 per inch. This isn't terribly important. Firstly, these are now rare enough that there is no chance of ever needing to interchange the pieces - and the explosion proof phone parts store has been closed for a very, very long time. Secondly, this phone was restored back into service by me in the same way the phone company would have done it in the 40's and ever after. I'm not bragging, just saying that it will be a long time before someone considers scrapping it to salvage parts for another. The stamps on the inside give away how many times they were overhauled and put back in service...at least a half dozen on the 320. None I can find on this 520. Thirdly, none but one of the actual dimensions in any of the pieces are super critical. The only real show stopper measurement in the whole assembly is the 1.000" outer diameter of the outermost cylinder, as it must be a good clean fit with the polished circumference in the bulkhead. In fact, this is the only part of any of the pieces that measures consistent. everything else has slop - the flange on the outer sleeve is < 4 thou out of round, as is the outer nut. I couldn't even tell the outer nut was knurled until I had it out, as it sits down below the mounting plate everything else sits on (in the 520 that is).

[browningautorifle]

I'd suggest taking the parts you need fabricated to a local machine shop and they'll turn them out correctly. Otherwise it'll be guess work.

[Flying10uk]

Looking at standard die sizes though maybe those outermost threads are 28 per inch.

If you look at your first pic on the bottom row of pics, post 1, you can see that, on the outer sleeve, at the end of the threaded portion, there is an "undercut" suggesting that the thread was turned on using the lathe's "lead screw", as would be expected. This is for the "run-out" of the cutting tool so that when the cut is finished the tool is not actually cutting any metal. If this "undercut" was not there normally the very tip of thread cutting tool gets broken off.

Manual lathes capable of screw cutting normally come either with an imperial lead screw, on an imperial lathe, or a metric lead screw, on a metric lathe.

With a turned on thread you can have almost any threads per inch/pitch you want subject to the capabilities of the lathe. Some lathes have greater capabilities than others.

Cutting internal threads, using a lathe, is more difficult than external threads, partly because you can''t see what is going on as the tool disappears, under power, into the workpiece.

Some people, such as a machine shop, may decide to produce the parts and cut the threads using a CNC lathe.