Motor housings and end caps look simple on a drawing and behave badly in assembly when three things drift: wall thickness, concentricity and stack height. Each is produced by the drawing process, not by inspection, so each has to be designed into the tool.
How do you control wall thickness on a deep drawn motor housing?
Wall thickness on a deep drawn motor housing is controlled by the blank diameter, the draw ratio, the die clearance, the blank holder force and the lubrication, and by whether the wall is intentionally thinned through ironing. Because material thins where it is stretched and thickens where it is compressed, a nominal wall of 1.0 mm typically measures thicker near the cup bottom and thinner in the mid-wall. Holding a uniform wall therefore requires either an ironing stage that deliberately reduces and equalises the wall, or a drawing specified as a reference dimension with the critical thickness called out at a defined location. Concentricity and stack height are held by tooling alignment, piloting and trimming rather than by sorting.
Ironing: when the wall must be uniform
Where a motor housing carries a stator, a bearing seat or a magnet, the inner wall is often an ironed surface: a punch pushes the drawn cup through a die with controlled clearance, deliberately thinning the wall and improving both thickness uniformity and surface finish. Ironing also raises the material's strength through cold work, which is useful for thin-wall housings and less useful where subsequent forming or welding is required.
Concentricity and roundness
Concentricity between the bore and the outside diameter is set by die and punch alignment, by the accuracy of the blank and the strip feed, and by how the part is trimmed. Because it is a tooling characteristic, it degrades with wear and after sharpening unless the tool is realigned. The practical inspection for it is a concentricity gauge or a vision measuring system on sampled parts, with the measurement recorded per lot.
Stack height and end caps
Motor stack height is the sum of the housing length, the end cap and any spacer or shim. Each contributes its own tolerance, so the assembly allowance should be checked before the tool is cut — a frequent cause of "the parts are in tolerance but the motor will not close". Where the assembly is sensitive, the practical answer is to define one feature as the datum, control the stack on that basis, and inspect the assembled height on a sample rather than sorting parts afterwards.
What to put on the drawing
- Wall thickness and the location where it is measured
- Whether ironing is required, or whether a reference wall with local critical thickness is acceptable
- Concentricity and roundness requirements with the datum defined
- Stack height and the assembly allowance across the mating parts
- Surface finish inside the bore where a bearing or magnet sits
- Annual volume — ironing adds a stage and only pays off at volume
Key point
Motor housings fail in assembly, not in inspection. Ask for the DFM review to include a stack-up, and the thickness and concentricity questions get answered before the tool is cut.
Frequently asked questions
Why is the drawn wall not uniform?
Because drawing stretches and compresses material: walls thin where the material is pulled and thicken where it is compressed. Uniformity is achieved through ironing or accepted as a reference condition.
Does ironing make the part stronger?
Yes — cold working during ironing raises strength and improves surface finish, which is why it suits thin-wall housings. It can reduce subsequent formability and weldability.
How is concentricity inspected?
With a concentricity gauge or a vision measuring system on sampled parts, recorded per lot, with the datum defined on the drawing.