Drawing or sample
A 2D PDF with the profile and key dimensions, or a physical sample we can measure. STEP files help for mating parts.
Metal Spinning Design Guide
The rules that decide whether a spun part is cheap and repeatable or expensive and out of tolerance — written from the shop floor, not from a catalogue.
Part of our custom metal spinning service: these rules are what our toolmakers and operators work to every day. Sizing a blank for your own part? Try the spinning blank & disc weight calculator — it develops the flat blank from your finished dimensions.
Process basics
How Spinning Actually Forms a Part
Metal spinning forms a flat disc over a rotating mandrel with a roller. The blank is clamped against the mandrel face, the roller pushes the metal progressively outward along the profile, and the part takes the mandrel's shape from the inside. Nothing is cut away, so there is no scrap web and no material waste beyond the trim allowance.
Three consequences follow, and every design rule on this page comes from them. First, the inside profile is the accurate one because it is defined by the mandrel; the outside follows the material thickness. Second, the wall thins slightly as it is stretched over deep profiles. Third, sharp internal corners cannot be formed — the roller needs a radius to work against.
If your drawing dimensions the outside of a deep part and the inside is critical (or the reverse), tell us which surface is the datum. It changes the mandrel size and saves a revision round.
Blank sizing
Blank Diameter, Wall Thickness and What Happens to Both
Blank diameter is the developed length of the profile — the path length from centre to edge, measured along the finished shape — plus a trim allowance. For most parts the allowance is 10–25 mm, trimmed off after forming because the edge of a spun part is never perfectly true.
Wall thickness changes during forming. Shallow parts keep close to nominal; deep draws thin the wall by roughly 5–15% depending on material, profile depth and roller passes. We compensate by starting with a thicker blank, and we tell you the expected finished wall in the quote rather than letting you discover it in inspection.
| Parameter | Practical range | Notes |
|---|---|---|
| Blank thickness | 0.5–6 mm | 0.5–4 mm most common; thicker needs larger tonnage and slower passes |
| Finished diameter | Ø20–1,500 mm | Above 1,500 mm parts are usually made in segments |
| Part depth | up to ~1.0× diameter | Deeper profiles need multiple passes and intermediate annealing |
| Trim allowance | 10–25 mm on diameter | Removed after forming; edge then rolled, flanged or machined |
| Wall thinning | 5–15% on deep draws | Quoted as a finished-wall range, not a single number |
Geometry
Radii, Draft and Depth: Where Parts Get Expensive
The single biggest cost driver in spinning is a design that fights the process. These are the limits we work to.
- Inside radius: at least 1× material thickness in aluminium, 1.5× in stainless. Tighter radii wrinkle the wall and force extra passes.
- Draft angle: 2–5° on straight walls deeper than about 50 mm, so the part releases from the mandrel without distortion. Zero draft is fine on shallow, open shapes.
- Depth-to-diameter ratio: one pass covers roughly 0.5× the diameter in depth; beyond that we plan two or three passes with annealing between them.
- Sharp external corners: form them slightly generous and machine to final size — cheaper than trying to spin a knife edge.
- Holes and slots: cut after forming. Holes formed into the wall distort and drift off position.
None of this means a deep part is impossible. It means a deep part has a process plan, a pass count and a price that reflects both — which is exactly what you want to know before you commit to tooling.
Tolerances
Tolerance Bands We Can Hold Without Heroics
These are the bands we quote as standard. Tighter values are possible on specific features, usually by adding a machining operation — and we will say so instead of quietly accepting a drawing we cannot meet.
| Feature | Standard | Tight (with added op) |
|---|---|---|
| Outside diameter | ±0.2 mm | ±0.1 mm under Ø300 mm |
| Height / depth | ±0.3 mm | ±0.1 mm |
| Roundness | 0.3 mm | 0.15 mm |
| Concentricity (bore to OD) | 0.2 mm | 0.05 mm after machining |
| Wall thickness | ±10% of nominal | ±5% on shallow parts |
| Flange flatness | 0.3 mm | 0.1 mm after facing |
| Surface roughness | Ra 1.6 µm as spun | Ra 0.4 µm polished |
A general tolerance block of ±0.5 mm on a spinning drawing is normal and healthy. Stacking ±0.05 mm on every dimension is not — it triples cost and usually reflects a copied CNC drawing rather than a real functional need.
Cost drivers
Five Features That Quietly Add Cost
- Welded bosses and studs: welding distorts a spun wall. A machined insert or a pressed-in fastener is usually cheaper and straighter.
- Mirror polish on the inside: only worth specifying where it is functional (optical, sanitary or flow surfaces).
- Non-standard alloys: 1050, 3003, 5052 aluminium and 304/316 stainless are stock. Titanium or Inconel means mill minimums.
- Very tight flange flatness: a spun flange is never dead flat; facing it is a second operation on a second machine.
- Small quantities of a deep part: the pass count is the same whether you order 5 or 500, so the per-piece cost curve is steep at the start.
What to send
What We Need to Quote Accurately
Material and finish
Alloy, temper, thickness, and whether the surface is mill, anodised, polished or painted.
Quantity and application
Prototype, pilot batch or series volume — plus what the part has to survive. That single sentence often changes the recommended process.
Send those three things and you get a feasibility answer, a blank size, a finished-wall estimate and unit pricing — normally within one working day.
FAQ
Questions Buyers Ask Us
How do I calculate the blank diameter for a spun part?
Take the developed length of the profile — the distance from centre to edge measured along the finished cross-section, including the flange — and add 10–25 mm for trimming. We calculate this from your drawing and confirm it in the quote, because the trim allowance depends on how the edge will be finished (rolled, flanged or machined).
Does spinning reduce the wall thickness?
Yes, on deep profiles. Shallow parts stay close to nominal; deep draws typically thin by 5–15%, depending on material, depth and the number of passes. We start from a thicker blank so the finished wall lands inside your tolerance, and the quote states the expected finished range.
What is the tightest tolerance you can hold on a spun part?
±0.1 mm on diameters under 300 mm and ±0.1 mm on height, when the feature is machined after forming. As spun, plan on ±0.2 mm on diameter and ±0.3 mm on height. Concentricity of 0.05 mm is achievable on a machined bore; 0.2 mm is the as-spun figure.
Can you spin square or rectangular parts?
Only with a matching internal mandrel and a shaped roller path, and the corners will always carry a generous radius. For true square or rectangular housings, deep drawing or fabricated-and-welded construction is usually cheaper — we will tell you if that is the case rather than sell you the wrong process.
What is the minimum order quantity?
One piece for a prototype. Spinning tooling is inexpensive compared with stamping, so prototypes and pilot batches of 10–100 pieces are normal. Series production typically starts around 500 pieces, but there is no hard minimum.
Keep reading
Related Capabilities
More guides: Stainless Steel Metal Spinning · Aluminium Spinning Reflectors · Spun Tank Heads & Dished Ends · Low-Volume Metal Spinning · Metal Spinning vs Stamping · CNC Machined Aluminium Enclosures · Spun Cones & Funnels · Spinning for Lighting · Roof Vent Caps · Spun Drain Pans & Ice Tubs
Send Your Drawing for a 24-Hour Quote
Feasibility review, material advice and unit pricing — free of charge.