EMG style 2 spring conversion soundbox

[soundgen]

Well done Alex.
Regarding George Overstall's formers. Unfortunately they are still in storage in Hong Kong and with travel being restricted, I am not sure when I will be next allowed to go there. Never mind trying to visit the UK. I will return them next time I am in the UK.

There were two formers with a different profile on each. Neither one was like any EMG or Expert profile. The formers were made from soft brass and unfortunately were scarred with a spiral. I assumed someone had tried to release the diaphragm from the former using a blade and had cut into the former. I filled the scarring with car body filler and was then able to make a few.
The Aluminum roll was very soft and the resulting diaphragms were disappointing. The best results were from soft drink cans although they did retain a slight curved memory from the shape of the can.
The process was to wrap the former in the sheet aluminum and clamp it using a clamping ring. It was them fitted to a lathe and using soft tools, plastic and wooden, push the sheet into the former profile. It was finished off with soft rags. Trying to cut the diaphragm to the correct size and release it was tricky. One out of ten was successful.
I was never happy with the final results. I always meant to make new formers and come up with a better way to cut and release the finished diaphragm. One day.

I'll try and dig out some pictures.

the reason your diaphragm was poor because as you say the aluminium roll is soft , after pressing the diaphragm you need to "spin" it , spinning aluminium toughens it up ! you may be able to do this with your hands rotating the two formers while applying pressure . if this doesn't work maybe your foil is too thin , what is the thickness , can you measure it?

[kirtley2012]

Thanks Chunny

Interesting to hear about drinks cans, because guess what this diaphragm is made from!, In order to remove the spring the aluminium should ideally be annealed to soften it and this removes the spring, and makes it a bit easier to work, the working into a diaphragm does work harden it a bit, it is tricky as I did melt a few, but in my experience, it made an excellent diaphragm, I shall do a video of this soundbox tonight if I get a chance

I looked up the grade of aluminium for drinks cans and saw several reports saying it is 3004, which is an alloy of Aluminium, manganese and magnesium, but 97.8% Aluminium, not the 1000 grade 99%+ pure that I believe the EMG and Expert diaphragms were made from, but not as inferior as might be assumed

[chunnybh]

Alastair, these might be your pictures pre damage. The spiral damage was barely visible but showed up in the diaphragm when pressed.

Soundgen, the diaphragms were "spun" using rags. They got quite hot and did work better. This also helped remove the can memory. Sorry can't remember the thickness. There were two different rolls.

Alex:

In order to remove the spring the aluminium should ideally be annealed to soften it and this removes the spring, and makes it a bit easier to work, the working into a diaphragm does work harden it a bit, it is tricky as I did melt a few, but in my experience, it made an excellent diaphragm

As you say, the annealing softens the aluminum. Too soft I found. The best result I found was heating the can sheet flat in an oven at 250C and letting it cool overnight. This removed the can memory and the resulting "spun" diaphragms were excellent. I found It was the profile on the formers that mainly let them down and that was the reason I gave up on them. It would be interesting to have an original diaphragm checked for exact composition.
One thing to note is of course all aluminum age hardens so all our soundboxes do not sound as they did originally.

[physicist]

I have also been experimenting with forming diaphragms from aluminium cans. It is easy to flatten the sides of the can into a sheet. The metal appears to already be work hardened by the can forming process. The can thickness is almost identical to the Expert diaphragm. I 3D printed a pair of dies for cold pressing (shape based upon Expert soundbox diaphragms). I found that the hardened aluminium split at the most curved region rather than stretching. Heat treatment to anneal before pressing fixed that and I'm not convinced that the end result is noticeably softer than the original Expert diaphragm. It is also just possible to form the un-annealed can material if the forming process is divided into stages rather than trying to form in one go. Using the un-annealed sheet should result in a harder product but I still don't think I can tell the difference. I'm also experimenting with titanium. This is about twice the density, significantly harder, and springier (which may be a bad thing). It can be formed in much the same way.

[kirtley2012]

Very interesting experiment Physicist!

Here is the soundbox in action

https://youtu.be/IhNauqBQJb8

[old country chemist]

Well! It looks like a lot of experimenti9ng has been going on with at least three of you-physicist, chunny and Alex.
The diaphragm question is-would Titanium, being twice as dense as Aluminium, would it not be suitable. I was told that diaphragms ought to be as light as possible, with a certain stiffness. physicist-it will be most interesting to hear how you manage in producing diaphragms from titanium. If you do, you may have struck "gold" as it were! I for one would be delighted to hear the results, and I wish you well in your experimenting. The quest for the finest acoustic reproduction is now on!
Edison used, I see in some of his reproducers, copper diaphragms-is that worth a try.
I used to own a 1924 Beltona "Peridulce gramophone. It had a large "Pianini" style soundbox fitted with an aluminium diaphragm, made by Captain Barnett. Has anyone thought of producing a large diaphragm of that type to see how it would perform. George Overstall used to say,(and I think he was wrong), that a small gramophone required a large diaphragm soundbox, and something like an EMG oversize would require a small diaphragm. Has anyone come up against this type of thinking before?

[physicist]

The diaphragm question is-would Titanium, being twice as dense as Aluminium, would it not be suitable. I was told that diaphragms ought to be as light as possible, with a certain stiffness. physicist-it will be most interesting to hear how you manage in producing diaphragms from titanium. If you do, you may have struck "gold" as it were! I for one would be delighted to hear the results, and I wish you well in your experimenting. The quest for the finest acoustic reproduction is now on!

Edison used, I see in some of his reproducers, copper diaphragms-is that worth a try.

The titanium sheet (commercially pure grade), that I have, is 0.1mm thick which is about the same as both aluminium cans and the original EMG/Expert diaphragms. I have been etching the titanium to get the mass down to something like the same mass as the aluminium (50 micron thick sheet proved to be difficult to find). Commercially pure titanium forms well. Grade 5 TiAl6V4 alloy (used for aircraft and some medical joint implants) is of similar density but much stiffer. This would, however, be much more difficult to form.

Copper is about 3 times denser than Aluminium and of similar hardness, an equivalent sized diaphragm in copper would be much heavier. I don't think that would be a good idea.

[BassetHoundTrio]

Wow, Chunny!! Wonderful to see - I wonder about using the bottom of an aluminum can would achieve.

Here they are.

[physicist]

I wonder about using the bottom of an aluminum can would achieve.

I have tried that, unsuccessfully. The base is significantly thicker than the sides and formed into a dome. This makes it too rigid to flex in the way most old diaphragms are designed to function. It could only work well if it was edge mounted like a modern loudspeaker cone with a flexible roll surround to allow the entire diaphragm to move like a piston. Even then, I suspect the mass might be too much.

[BassetHoundTrio]

I seem to recall someone using aluminum from a can with great results - perhaps the sides are thinner and that was used?