Ironing in deep drawing: how wall thinning holds both the inside and the outside diameter

Ironing - also called wall thinning or wall ironing - is the operation that turns an ordinary deep drawn cup into a precision housing. A drawn cup usually has a wall that thickens toward the rim and a surface that carries the marks of the draw. Ironing pushes that cup through a ring die whose bore is slightly smaller than the cup's outside diameter, so the wall is squeezed thinner between a polished punch on the inside and a polished die on the outside.

The consequence is the reason engineers specify it: the die sets the outside diameter, the punch sets the inside diameter, and both surfaces are burnished in the same pass. A part that would otherwise need a drawn shell plus turning on both diameters comes off the press with both diameters already in tolerance.

What ironing actually does to the part

Why this replaces a machining operation

When a part needs both an inside diameter and an outside diameter in tolerance, the traditional routes all end in a lathe. Ironing is the route that ends at the press.

Where the saving comes from. It is not only the turning cycle time. It is the bar or forging stock that never becomes chips, the second machine and second setup that never happen, the re-fixturing that can no longer introduce runout, and the handling between the press and the lathe that disappears. On a part with a 0.8 mm wall that starts from bar, the chip loss alone is usually larger than the finished part.

Ironing is not free: it needs an ironing die, a polished punch, correct lubricant and, where the total reduction is large, more than one pass with annealing between them. The comparison should be made on total cost per part including tooling amortisation - which is exactly what the DFM review does.

Where Balford runs ironing: solenoid valve housings

The classic application is the solenoid valve housing. The armature has to slide inside the bore, the housing has to fit the coil and the valve body, and the wall is thin so the magnetic circuit is efficient. That is a part where inside diameter, outside diameter, wall thickness and surface finish all matter at once - and where ironing removes the turning operation that would otherwise be needed.

Two materials dominate:

• DT4E electrolytic pure iron - the soft-magnetic grade used for solenoid and sensor housings, where coercivity and permeability are part of the specification. Ironing forms the wall to thickness without introducing the surface damage that would hurt the magnetic path, and magnetic annealing follows forming.
• DC04 - the deep drawing steel used where the part is structural rather than magnetic, and where the drawing and ironing sequence is set by the wall reduction rather than by magnetics.

Both are formed on presses up to 350 t with tooling designed and built in Balford's own tool room, so the ironing ring, the punch and the preceding draw stages are made and tried out together rather than separately.

The same argument applies beyond solenoids, to any part where both diameters carry a tolerance: hydraulic and pneumatic cylinder parts, sensor sleeves, bushing shells, actuator housings and drawn tubes.

Process parameters that decide success

• Reduction per pass. Wall reduction per ironing pass is typically in the range of 20-35% of the wall thickness for steel; the exact limit depends on grade, lubrication and die geometry. Larger total reductions are done in several passes.
• Inter-pass annealing. Because ironing is cold work, the material work-hardens. Where the total reduction or the final properties require it, the part is annealed between passes to restore ductility. On soft-magnetic parts the annealing step is also what sets the magnetic properties.
• Lubrication. Ironing is a high-pressure, high-surface-contact operation. The lubricant film is what separates a burnished surface from a scored one, and it is chosen for the grade and the reduction rather than for convenience.
• Die and punch finish. The surface of the finished part is a copy of the tooling surface. Polished carbide or hardened steel rings are what produce the finish and the absence of draw marks.
• Wall thickness as a characteristic. Wall thickness is measured and recorded, not assumed. If the drawing needs a specific wall, it is specified as an ironed wall - otherwise the wall stays at blank thickness and the part is heavier than intended.

Honest limits

• Volume. Ironing needs a die and a punch made for the part. Below a few tens of thousands of parts per year, turning from bar can still be cheaper overall.
• Geometry. Ironing suits cylindrical or near-cylindrical walls. Complicated external features are better formed before ironing or added afterwards.
• Wall reduction is not unlimited. Very large reductions need several passes and annealing, which is where the cost advantage narrows.
• Material. Ductility sets the limit. Very high-strength or low-ductility grades may not accept the reduction that the drawing implies.

All four are checked during the DFM review, before tooling is cut.