Measure over the top of the pattern from base to base for a rough guide to diameter. Bead the lip last thing seems most sensible.

Martin

Hi Roger,

On the spinnings i have done in the past i just use a piece of string
from the centre radially to the point where the end of the spinning is required
and add about 1/2 inch or so extra to work out the radius,to some extent the
limitation will be the size of the lathe ,with a Myford the maximum over the
bed would be just below 7 inch ,i have a larger Warco lathe so can just
get 13 inch. The other consideration is the available power ,presumably
your dome will be formed unbroken, ie no hole in the centre and as such
the clamping will be entirely from the tailstock ,the extra pressure
required robs the power available for spinning.

Some photos in an album "Spinning " show some examples, i bought
a spare faceplate for the Warco and just use it for this purpose it has
an extra disc added and an adapter to fix the form bolted on ,for steel and
stainless steel you should use steel formers wood is useless as the
pressure is too high.I usually spin at about 500 to 800 rpm higher would be
better but needs a lot more power .For aluminium a stout broom handle
well greased can be used ,for steel and stainless i use this sprung
roller in the toolpost.

Heating the metal while spinning just adds another level of complication
and danger .Spinning using the 3 jaw chuck is not a good idea as the
pressures encountered may well disloge the former.

You do need to have the edge trimmed and running true before starting
to spin, use a thick pair of gloves and a full face shield and try not to stand in
line with the work.

Google search term Metal spinning size calculation lots of helpful
advice there.

John

Roger,

I make fuel tanks for my Lister stationary engines out of immersion tank copper, as said above I measure the diameter add the radius and then the lip and finally add a half inch, when the disc is finished I then trim the edge to size.

With copper I have to heat it to soften it as it work hardens and can split. My former is made from plywood mounted on a steel support which I hold in my three jaw chuck, the other part of the former is also plywood and I use a running centre in the tail stock to apply the pressure. My tool is a 1/2 in bar with the nose radius'd and I use tallow as my lubricant. As said dont stand inline to the metal as it could spin out unexpectedly I assume you have a large lathe as 1mm is quite thick for my CL500M lathe.

Martin P

Roger,

There is a book which you can get from TEE Publishing called Metal Spinning which gives the the basics etc on how to do it.

No connection just bought the book.

Martin P