Metal Spinning Blank Size Calculator
Enter the finished spun shape and sheet thickness to estimate the equal-area blank diameter. The result adds trim stock, weight, scrap, and a simple machine-swing clearance check.
Recommended cut diameter (in)11.74
- Theoretical blank diameter (in)
- 11.49
- Finished surface area (in^2)
- 103.67
- Cut blank weight (lb)
- 1.93
- Theoretical finished-part weight (lb)
- 1.85
- Trim-ring weight (lb)
- 0.08
- Trim scrap
- 4.21%
- Machine swing clearance
- 0.261 in clearance.
Assumes conventional spinning with approximately constant sheet thickness and equal finished surface area.
How to use this calculator
- Choose inches or millimeters, then pick the finished shape closest to the spun part.
- Enter mid-surface mouth diameter, base diameter, and height where the selected shape uses them.
- Enter sheet thickness, radial trim allowance, material, and available machine swing.
- Use the recommended cut diameter as a trial-blank starting point, then confirm it on the actual tooling.
How the blank diameter is calculated
Conventional metal spinning usually starts by estimating a circular blank with the same area as the finished mid-surface. This is an area-conservation estimate, not a plastic-flow simulation. It assumes the sheet thickness stays approximately constant and that the part can be represented by one simple axisymmetric shape.
The calculator first finds the ideal finished surface area A. For a cylindrical cup, it uses the base plus the straight wall:
A = πD² / 4 + πDH
For a cone or frustum, it uses the small flat end plus the conical wall. The slant height is s = sqrt(H² + ((D - d) / 2)²), so:
A = πd² / 4 + π(D + d)s / 2
For a hemisphere, it uses the curved half-sphere area:
A = πD² / 2
Once the area is known, the equal-area blank diameter is:
B = sqrt(4A / π)
The recommended cut diameter is C = B + 2T, where T is the radial trim allowance. Weight is then estimated from circular blank area, sheet thickness, and the selected density preset. In inch mode the weight is reported in pounds; in millimeter mode it is reported in grams.
What moves the result
Height usually changes a cup blank more than small changes in mouth diameter because wall area accumulates around the full circumference. For frustums, the slant height matters: a tall narrow cone can need much more blank than its mouth diameter suggests. Trim allowance also grows quickly because it is added to the diameter before calculating cut blank area and scrap.
What this calculator leaves out
The formulas omit corner radii, rolled rims, beads, flanges, steps, re-entrant profiles, local thinning, work hardening, springback, earing, anisotropy, tool nose radius, and operator sequence. Machine swing is only a diameter clearance check; it does not prove the machine has enough rigidity, power, speed range, tooling, guarding, or workholding. Treat the answer as the first trial blank size for conventional spinning, then adjust it with shop results.
Worked example
For a 6 inch cylindrical cup that is 4 inches deep, the finished area is π × 6² / 4 + π × 6 × 4 = 103.67 in². The equal-area blank is sqrt(4 × 103.67 / π) = 11.49 in.
With 0.125 inch radial trim allowance, the recommended cut diameter is 11.49 + 2 × 0.125 = 11.74 in. In 0.063 inch mild steel, the cut blank weighs about 1.93 lb, the ideal finished part weighs about 1.85 lb, and the trim ring is about 4.21% of the cut blank weight. A 12 inch machine swing leaves about 0.26 inch of diameter clearance.
Common questions
How is metal-spinning blank diameter calculated?
A common first estimate sets the circular blank area equal to the ideal finished surface area. After calculating that area from the selected cup, cone, frustum, or hemisphere shape, the blank diameter is the diameter of a circle with the same area. Trim allowance is then added to both sides of the diameter.
Should dimensions be measured inside, outside, or at mid-thickness?
Use mid-surface dimensions when possible because the area-conservation method follows the sheet centerline. For thin sheet the difference between inside, outside, and mid-surface dimensions may be small, but it becomes noticeable as thickness grows relative to diameter. If you only have outside dimensions, adjust them toward the mid-thickness surface before relying on the estimate.
What blank diameter makes a hemisphere?
For a hemisphere, the ideal surface area is half of a sphere, or πD² / 2. Setting that equal to a circular blank gives a theoretical blank diameter of D times the square root of 2. Add twice the radial trim allowance to get the cut diameter.
Does this formula apply to shear spinning?
No. Shear spinning intentionally thins the sheet according to the wall angle, so a constant-thickness area method is not the right model. This calculator is for conventional spinning estimates where thickness is assumed to remain roughly constant.
Why might the real blank size differ from the calculated size?
Real parts include radii, rims, flanges, beads, draw-in, thinning, anisotropic flow, and trimming behavior that simple geometry does not include. Material temper, lubrication, tooling, speed, and operator sequence also change where stock moves. Use the calculator result as a trial size, then tune it from measured blanks and finished parts.
How much lathe swing is needed for the blank?
At minimum, the machine must clear the recommended cut diameter while the blank rotates. The swing row subtracts the cut diameter from the available swing and reports either clearance or the extra diameter capacity required. Rigidity, power, guarding, tool clearance, and safe speed still need separate checks.