ALUMINIUM DIAPHRAGMS.

[emgcr]

Many thanks for the excellent photos---good luck on the lathe and I am sure you will produce very worthwhile items. Look forward to hearing them.

[edisonphonoworks]

I have a Victor no. 2 with a glass diaphragm and it sounds very good. It may be one of very few around.

[emgcr]

Hello all, here are a couple of pictures of the Overstall diaphragm making equipment. It comprises of-Aluminium on a roll (2 thicknesses.)

Alastair, do you happen to have any record of the actual aluminium specification/heat treatment etc used by George ? It would seem to be important to posterity to try to find and publish this information thereby saving the necessity of re-inventing the wheel.

[estott]

Wood has been tried,often called "violin spruce". I would imagine that thickness would be an issue- to thick and it lacks response, too thin and it splits. The ones I've seen appear to have been formed with thick areas in the center and at the edge.

[old country chemist]

Hello Estott-well, violin spruce-sounds interesting!.Probably would require sealing with Canada Balsam, or a suitable varnish before use, but may contain enough pinene resinoids to seal it. I also thought of Cedarwood-had a feeling that had been tried many. many years ago! In answer to Graham re. the heat treatment, I think it was hit and miss-the friction of the small fibre type block as it was held up to the spinning die with the aluminium attached, was all the heat that was produced. I imagine that the faster the lathe turned then the more hear produced, and I know that George did use quite a high speed on his lathe when turning some diaphragms-but not always. I remember once, when I was watching, that he used quite a slow speed-about 200 rpm, as he was experimenting himself. I do not know the outcome of the prepared diaphragms from that experiment, but the trickiest thing of all, was the use of a suitable parting tool to removed the diaphragm from the remaining aluminium held in place by the spring clip on the former. Even George, with all his experience, might only have made 5 out of 10 decent ones. I tried using a Stanley knife to part the diaphragm from the former-not a good idea. Perhaps my inexperience was to blame.The critical moment is when the last part of the diaphragm comes away from the former. I well remember at a diaphragm making session, when George was making them, that several spun off into the air like small UFO'S! Graham, you are the chap who will make a really excellent job of whatever you put your mind to. So if you do not mind me suggesting this, I will let you have all the equipment when I see you later this year, then you can try it all out. I will, in the meantime, call upon my local Austin Ten engineer friend, and try spinning a few more. Great stuff this gramophone hobby!

[emgcr]

Yes, I am afraid these things are never as easy as they look !

With reference to the heat treatment, what I was refering to was the heat condition (reference ductility etc) that the aluminium foil stock is supplied in by the manufacturer. Many different grades can be produced in all sorts of annealed or non-annealed conditions according to the particular alloy and end-user requirements. It seems to me that the two vital considerations in this instance are:

1. What aluminium alloy will reproduce first class sound waves ?

2. Is such material consistent with easy manufacturing properties ?

Before even starting on the lathe, there is an all important research period required speaking to the aluminium foil manufacturers to establish what the best product to use is likely to be. Even then, a certain amount of experimentation is likely to be necessary---all very time consuming as the original knowledge seems to have been lost. Do you happen to know the specification and source of the two existing foil rolls ?

A certain amount of work hardening is likely to take place in any spinning process which may not be present to the same degree should the item be pressed, for instance. There are competing requirements here---ductility is desirable for ease of forming whereas a fully annealed membrane is perhaps unlikely to produce a good resonant sound.

[emgcr]

Alastair, many thanks for sending the two Overstall aluminium foil offcuts, the thickness of which measure:

1. 0.00345"

2. 0.00400"

ie. approximately three and a half thou' and four thou' respectively.

This is very much in line with our experience and expectations isn't it but we still do not know the actual alloy or state of anneal etc.

I think experimentation is going to be the only way forward as, even if the original specifications were ever to come to light, there is still the likelyhood of age hardening over the last 80 years to contend with which could be misleading.

[Frankia]

This is a fascinating thread. I'm excited to hear that Paul Morris (who I don't know), produced diaphragms which sounded good. Did he use the rolls of aluminium in the photograph?

[emgcr]

I believe so but am not 100% sure. I shall find out and also ask him about his manufacturing experience.

[Frankia]

That should be interesting. Look forward to the next instalment!