Thining Allowances for Various Dishends

This is a matter of design calculation. If you are in the shops, your designer would have indicated the minimum acceptable thickness in the drawing. Pls. refer the drawing.

Whatever the thinning happened to the head, the design is based on the min. thickness after complete fabrication of the head. Therefore, when we specify the components of any vessel, the head must be specified by its min. thickness, and that min. thickness shall be used in design calculations and no way to use the nominal thickness of plate used in fabrication of that head. The max. thinning is expected to occurs the knuckle radius.

Example: Vessel shell thickness : 20 mm & Ellipsoidal head 2:1 thickness : 20 mm (min.)

I normally use rule of thumb of two gauge sizes bigger before dishing. If raising (spinning) the metal thickens, but we always specify a couple of extra gauge sizes because there are often thin spots in a spinning.

Of course, I dish by hand over an anvil, and raise by hand over a form. Machines would make my life a little easier I suspect.

Yusef1

medieval body armour maker

Normaly the thinning of dished heads, (Klöpper, korfbogen, ellisoidal etc ) is approx. in between 10 and 15% of the thickness of the original plate before forming.
The difference in thinning is caused by the radius of the knuckle and the way they are produced, by cold pressing or hot rolling .
The thinnest part of the formed head is at the start of the knuckle part at the straight end side.
But for ordering you specified the minimum required thickness after forming as you get it from your stress calculatie .