Ironing tolerances and surface finish: what a thinned wall can hold

Two questions decide whether a part can be ironed instead of turned: how tight the diameters and wall have to be, and how good the surfaces have to look. This page explains where the tolerance in an ironed part actually comes from, and why an ironed surface behaves differently from a turned one.

Where the tolerance comes from

In an ironed part there are three formed characteristics - outside diameter, inside diameter and wall thickness - and they are produced by two tools:

• The ironing ring (die). Its bore is slightly smaller than the incoming cup, so the outside diameter of the finished part is a copy of the die bore, less the elastic recovery of the material.
• The punch. Its outside diameter sets the bore of the part, again less elastic recovery.
• The gap between them. The radial gap between punch and die bore is the finished wall thickness - which is why wall thickness in an ironed part is a tool dimension rather than a process accident.

Three consequences follow. First, the tolerance on the two diameters depends on the accuracy of the tooling rather than on springback alone. Second, the wall thickness tolerances of the incoming blank matter less, because ironing is what sets the wall. Third, concentricity between the bore and the outside diameter is a property of the tool, not of how well the part was gripped.

Where a drawing needs tighter diameters than ironing alone delivers, a short sizing pass - a light second ironing or a calibration pass - is added after the main reduction.

Typical figures for an ironed part

These are typical figures, not a specification. The drawing decides: tolerances that are not stated on the drawing follow ISO 2768-m by default, and the characteristics that genuinely matter are flagged during the DFM review so that nothing is discovered at first article.

How an ironed surface differs from a turned one

• No feed marks. Turning leaves a helical tool mark whose pitch is set by the feed rate. Ironing leaves the finish of the die and punch, so there is no feed pattern to see or to measure.
• No striations from the draw. A drawn surface carries the marks of material flowing over the die radius. Ironing happens after that reduction, on a straight wall, between two polished surfaces.
• Both faces improve. On a turned part only the machined face improves; on an ironed part the outside and the inside are both burnished in the same pass.
• Cold work is beneficial - to a point. Ironing raises yield strength and wear resistance. Past a certain reduction the material needs annealing, and on soft-magnetic parts the annealing step is what sets the magnetic properties.

The surfaces are inspected in Balford's own lab on a portable surface roughness tester, alongside the dimensional checks: outside and three-point bore micrometers for the diameters and wall, a concentricity gauge for runout, and a 2.5D optical projector for profile and form.