Spinning metal - searching info advice

**rendoman** · Oct 21, 2019, 09:45 PM

Hi all!
One of my future project will be a sort of horn (5" diameter mouth, 1" throat, aprox lenght 5") for a couple of planar speaker mid-tweeter.
I'm thinking about other ways to do the job. I would avoid machining from aluminum, wood would be maybe better.
I remember some old YouTube videos about metal spinning with normal "home size" lathes, it seems possible with few tools and a good mold. I have no experience in this field, do you think could it be possible to have a nice internal surface with aluminum (2mm or close)
Any advice welcome
thanks!

**Frank S** · Oct 22, 2019, 12:07 AM

It should be possible. I've done some spinning a long time ago. I don't remember ever trying to do anything with quite that small of throat though.
What I think I would do would be to make my mold then start out with a small sacrificial disk say 3" in diameter see how it turned out You can always make a small funnel out of it later. Then go to a larger disk say 4" take note of how much further up the mold the disk wound up and go from there by the time you are ready to make the actual part you will have a fair understanding of how to do it.
On thing don't try to achieve the final shape in only a single push up the mold. Work a small short distance up the mold from the throat then run your spoon across the face all the way to the outer rim each time you push a litter further up the mold the metal needs to be allowed to flow fluidly as you work it into the final shape taking care not to try and take too large of a push at a time to prevent the edged from wobbling fiercely. Be mindful that you are forcing the metal to stretch and shrink at the same time. Once you have your completed shape simply part off the excess .

**rendoman** · Oct 22, 2019, 10:14 AM

Thanks for the advices!
I'm thinking about other ways to build an horn, not an easy task considering the shape

**thehomeengineer** · Oct 22, 2019, 02:59 PM

Hi rendoman
I have spun several items for my steam engines mainly brass copper and steel. I will post the tool I made to fit in a standard lathe tool holder on my Colchester student lathe. 2 mm Aluminium is quite thick for a manual lathe to spin and I would be tempted to anneal before spinning. (I hope I am not telling you how to suck eggs) To anneal the aluminium you simply rub hand soap over the aluminium and heat until the soap on the surface goes black leave to cool and the aluminium will flow a lot easier. This may have to be repeated as the aluminium work hardens during the process and if not annealed potentially could crack. You will know when it has work hardened as the spinning will not flow as easy. This is only my opinion and I am sure there are more qualified people in this subject than me, but without giving it ago you will never know.
I will post tool later.
The Home Engineer

**jdurand** · Oct 22, 2019, 05:44 PM

I did some on my mini lathe but not that bit. I needed to reduce the end part of a coper pipe to an accurate size. Worked fine.

**rendoman** · Oct 22, 2019, 07:07 PM

Thank you for advices! It seems not an easy job to do, I havn't considered the possibililty to break the aluminum. I'm curious to see your tools and setup also!
I'm thinking about using plaster for a prototype, having a couple of horn finished and polished it would make a good mold (and a new polishing for sure). I'm so undecided about.

The big horn is the object I would like to duplicate