Ironing instead of turning: removing a machining operation from a double-tolerance part

If a part has to hold a tolerance on the inside diameter and on the outside diameter, the drawing usually ends with a lathe operation - or two. Ironing is the alternative: the wall is thinned between a die and a punch sized to the two diameters, so the part leaves the press with both of them formed.

This page compares ironing against the four routes that normally need turning: round bar, cold extrusion from bar, casting and forging.

The four machining routes, and what ironing changes

Route	Operations before the part is finished	What ironing removes
Round bar, turned	Saw, face, turn OD, bore ID, part off, deburr	The whole turning operation. Material between the bar and the finished wall becomes chips; an ironed part starts as strip at close to finished thickness
Cold extrusion + turning	Extrude near-net from bar, then turn the bore and usually the outside	One complete turning pass - the bore and the outside are set by the ironing punch and die
Casting + turning	Cast, then turn every functional surface, plus porosity risk	Most of the turning, and the porosity question - ironing works wrought material
Forging + turning	Forge, then turn the bore, the outside and the faces, removing the forging allowance	The forging die, the stock allowance and most of the turning

Where the cost actually moves

Material. A turned part pays for the bar and then turns most of it into chips. An ironed part is drawn from strip, and the strip thickness is close to the finished wall - so material utilisation is fundamentally different, not marginally better. On a thin-walled housing the chips from the bar route can weigh more than the finished part.
Machine time. One press pass replaces a turning cycle. On a housing that needs both diameters turned, that is often two operations removed rather than one.
Setups and fixturing. Every re-fixture is a chance to introduce runout between the bore and the outside diameter. Ironing forms both against the same tooling axis, so concentricity is a property of the tool rather than of how well the part was clamped.
Surface. A turned surface carries feed marks. An ironed surface is a copy of a polished die and punch, so outside and inside roughness both improve without a finishing operation.
Grain flow. Turning cuts through the material's grain structure; ironing deforms it along the wall, which is why ironed housings show higher yield strength and better wear behaviour in service.

When turning still wins

Volume is low. Ironing needs an ironing ring and a punch made for the part; below a few tens of thousands of parts a year, turning from bar can be cheaper overall.
The part is not cylindrical. Ironing thins a wall; it does not create complex external geometry.
There are machined features. Threaded ports, cross holes, sealing faces and tight-tolerance bores still need machining. Ironing removes the turning of the wall, not every operation.
The material will not take the reduction. Low-ductility or very high-strength grades may need several passes and annealing, which erodes the saving.

Frequently asked questions

Can ironing really replace turning?

For parts whose critical features are the inside diameter, the outside diameter and the wall thickness, yes. Those three are formed by the ironing punch and die, so the part arrives with them in tolerance. Machining is still needed where the drawing calls for a thread, a cross hole, a sealing face or a bore tighter than forming can hold.

How much material is saved compared with bar-stock turning?

It depends on the wall thickness and the bar diameter, but the comparison is structural rather than marginal: a turned part starts from solid bar and converts the difference into chips, while a drawn and ironed part starts from strip whose thickness is close to the finished wall. On thin-walled housings the chip weight can exceed the finished part weight.

Does ironing replace cold extrusion?

It can replace the machining that normally follows cold extrusion, and in some cases the extrusion itself. Cold extrusion from bar produces a near-net outside shape but usually leaves the bore and often the outside to be turned; a drawn preform that is then ironed sets both surfaces in the press. See the comparison of cold extrusion and deep drawing for solenoid valve housings.

What about concentricity between the bore and the outside diameter?

It is better by construction. The punch and the ironing ring are made on the same tooling axis, so concentricity is determined by the tool rather than by how accurately a turned part was re-fixtured between the outside operation and the boring operation.

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