What has to be true for a housing to be drawable?
The draw ratio per stage has to sit inside what the grade allows, the base radius has to be generous enough to let material flow, and the wall either has to stay close to the blank thickness or be thinned deliberately by ironing. Everything else follows from those three.
The four numbers that decide a programme
- Draw ratio per stage, by grade - 304 is the reference, 310S the most demanding.
- Blank size, which sets the material cost and the coil width.
- Draw force, which sets the press tonnage. Balford runs presses from 25 t to 350 t and draws up to Ø250 mm.
- Die clearance, which controls thinning and the surface of the drawn wall.
Rules of thumb worth following
- Round the base radius up, not down: it is the cheapest way to remove cracking risk.
- Put steps and flanges where the wall still has thickness to carry them.
- If the wall has to be thin, call it out as a thinning operation instead of drawing a thin blank.
- Keep the number of tightly toleranced features small, and put them where they can be machined after forming.
Grade, wall thickness and tolerance are confirmed against the customer drawing. Batch heat treatment, slow wire cutting, passivation, electropolishing, plating, phosphating and coating are placed with qualified external processors and disclosed on the inspection plan. The quality system is ISO 9001:2015 with controls aligned to IATF 16949 requirements.