Metal Spinning Diameter and Wall Thickness Calculator
A spun disc keeps its volume: thin the wall and the diameter grows, thicken it and the diameter shrinks. Fill in any three of the four values below and the one you leave empty is worked out for you.
Figures from this tool are for reference only. The blank is the disc you start from, the part is what comes off the machine after the wall has been thinned: the tool holds the volume of metal constant — π D² / 4 × t on both sides — and nothing else, with no trim allowance, tooling or pass schedule. Because the tooling, the actual material and the roller path can all shift the result, confirm the numbers with your own trial run before releasing a blank size, a tolerance or a quotation.
How the numbers are worked out
One idea drives the whole calculator: the metal keeps its volume, so the area of the disc and its thickness can only trade against each other.
One relationship
The volume of a disc is its area multiplied by its thickness, and that volume does not change while the wall is spun thinner:
D0² t0 = D1² t1
A thinner wall therefore always means a wider disc, and a thicker wall means a narrower one. Nothing else on the page changes the answer.
Rearranged for whichever value you need
- Part diameter
D1 = D0 × √( t0 / t1 ) - Part thickness
t1 = t0 × ( D0 / D1 )² - Blank diameter
D0 = D1 × √( t1 / t0 ) - Blank thickness
t0 = t1 × ( D1 / D0 )²
Worked example
A 500 mm disc of 2.0 mm sheet is spun down to a 1.2 mm wall. Everything else follows from the volume:
D1 = 500 × √( 2.0 / 1.2 ) = 645.50 mm
The diameter grows by 29%, the wall ends up 40% thinner, and both sides still carry 392 699 mm³ of metal. A reduction that heavy is usually split over several passes.
This is the calculation for wall thinning, where the metal is free to spread. When a part is drawn over a former, the shape of the former decides where the metal goes and the wall thickness no longer sets the diameter.
Diameter and wall thickness questions
It can. Conventional spinning holds the wall close to the blank thickness. In flow forming and shear spinning the wall is thinned on purpose, and the metal that leaves the wall spreads outwards — which is why the diameter grows as the wall gets thinner.
Volume is conserved: the metal has nowhere else to go. Blank and part satisfy D0 × D0 × t0 = D1 × D1 × t1, so D1 = D0 × sqrt(t0 / t1). A 500 mm × 2.0 mm blank thinned to a 1.2 mm wall comes out about 645 mm across.
Yes. Enter the part diameter and wall thickness you want and either the blank diameter or the blank thickness, then leave the fourth value empty — the tool works it out.
It depends on the metal. We spin aluminium, carbon steel and copper down to 0.4 mm, and stainless steel down to 0.5 mm. Thinner than that the wall tends to wrinkle or tear under the roller, so the finished part usually has to be planned thicker.
Not on its own. When the metal is drawn over a former, the shape of the former decides where the metal goes, so the wall thickness does not set the diameter. This calculator is for wall thinning, where the metal spreads freely and the volume is all that changes.
