Technical whitepaper · WP-02 · 13 pages

Drawn Housing Tolerances

How to dimension a deep drawn housing so that it can be inspected, and what the process can and cannot hold. Written for the engineer writing the call-outs and the inspector who has to prove them.

Download the 13-page PDF

What is in it

01 Why a drawn part resists dimensioning - the wall tapers, the part springs, nothing is a cylinder
02 The datum scheme - three datums that stop two inspectors measuring two different parts
03 The seven call-outs - what each means, what it costs, what buys it tighter
04 What the process does to the wall - thinning, work hardening and what ironing changes
05 Springback - why the diameter is never exactly what the die says
06 Inspection methods and their limits - what each instrument actually proves
07 Writing the tolerance block - a worked set of call-outs for one housing
08 First article and PPAP - where the tolerance block meets the evidence
09 Disputes and how to prevent them - the four arguments that repeat
10 The tolerance checklist - what to settle before the drawing is released

Why a drawn part resists dimensioning

A machined part is dimensioned against surfaces that exist. A drawn housing is dimensioned against surfaces that were formed by flow, that taper, that spring back, and that are measured on a part which is not a cylinder. Four things move: the wall tapers, so the diameter at the rim is not the diameter at the bottom; the part springs back when the punch withdraws, by an amount that varies with material and wall; the wall thins where it bends, and not uniformly around the circumference; and the rim is never level. The practical consequence is that a diameter is not a property of the part - it is a property of the part, at a height, measured from a datum, in a stated plane. An unqualified diameter call-out on a drawn housing is not a loose requirement, it is an unmeasurable one, and it will be argued about at first article.

The datum scheme

A housing is located before it is measured. Three datums do that job and they are the same three whether the part is checked on a surface plate or on a CMM. Datum A is the seating face: the bottom of a drawn housing is a formed surface that seats against something in the assembly, whereas the rim is a trimmed edge, and referencing to the rim puts the trim tolerance into every other measurement on the part. Datum B is the inside diameter: on a housing that receives a component the inside diameter is functional and the outside is often clearance, so making the inside the axis datum means the tolerance stack is measured from the surface that matters. Datum C is a pierced feature - a hole, a flat or a notch - which stops rotation, without which any non-symmetric feature is measured in a different position every time.

The seven call-outs, and what buys each one tighter

Inside diameter lands at IT9 to IT11 as drawn and is bought tighter with a sizing or ironing pass. Outside diameter lands at IT10 to IT12 and is bought tighter with a sizing ring. Wall thickness varies by about plus or minus 5 percent of nominal and is tightened by ironing. Roundness is typically 0.03 to 0.10 mm and improves with a sizing pass. Coaxiality between inside and outside diameter is typically 0.05 to 0.15 mm and needs a tool designed for it rather than one that happens to achieve it. Height holds plus or minus 0.2 to 0.5 mm with a trim fixture referenced to the seating face. Bottom flatness is 0.10 to 0.30 mm and needs coining or machining to go tighter. Before tightening any of them, ask what the assembly does with that surface: on most housings only one of the seven is genuinely functional and the rest are drawing habit, paid for at every operation.

What the process does to the wall, and why springback is not a defect

A drawn wall is thinnest just above the bottom radius, which is where the blank was pulled over the die shoulder and the material had to stretch rather than flow. It is not a defect and it is predictable: the deeper the draw and the tighter the radius, the deeper the dip. An ironing pass pulls the wall through a ring and equalises thickness along the length, which is what it is for, but it does not remove the dip entirely and it costs a die. A wall tolerance written without knowing this will be argued about on the first sectioned sample, so state a nominal wall, a tolerance, and the zone the tolerance applies to. Springback is separate: when the punch withdraws, the wall relaxes elastically and the part is slightly larger than the die that made it, and because the wall thickness varies around the circumference, the recovery does too. On a 40 mm housing a tenth of a percent is 0.04 mm, most of an IT9 band - which is why a sizing pass exists.

Inspection methods and their limits

Every method proves something slightly different, and a call-out that no available method can resolve is not a requirement, it is a negotiation. A bore gauge gives diameter at a specific height quickly, in production, and says nothing about roundness or taper. An air gauge is faster but needs a dedicated plug per diameter and is sensitive to oil and burrs. A contact CMM gives diameter, roundness, coaxiality and position against a stated datum, but it is slow and the probe force can deflect a thin wall if the part is not supported. Optical methods read rim profile, hole position and earing, but cannot see inside a deep housing. Ultrasonic thickness gives a point reading of wall thickness non-destructively; a sectioned sample gives the whole profile, which is what the argument is actually about. Matching the method to the call-out in the control plan is what makes a PPAP defensible.

The four arguments that repeat

These come back on programme after programme, and none of them is a quality failure - in every case the part was within the tolerance that was written, and the tolerance that was written was not specific enough to be measured. "Your diameter is out" means two inspectors measured at two different heights on a tapered wall, and is prevented by the sentence: diameter measured 20 mm above datum A. "The wall is thin" means one reading came from just above the radius and the other from near the rim, and is prevented by naming the zone. "It is not round" means the part was clamped in a fixture while being measured, and is prevented by stating that it is measured free standing. "The height varies" is earing plus a trim referenced to the outside, and is prevented by referencing the trim to the seating face. The cost of each lands on the programme as rework, sorting or a concession.

Frequently asked questions

What tolerance can a deep drawn housing hold as drawn?

Typically IT9 to IT11 on the inside diameter, 0.03 to 0.10 mm roundness, 0.05 to 0.15 mm coaxiality between inside and outside diameter, about plus or minus 5 percent of nominal on wall thickness, plus or minus 0.2 to 0.5 mm on height, and 0.10 to 0.30 mm on bottom flatness. Each of those is bought tighter by a named operation - sizing, ironing, a trim fixture or coining - and that operation has to be in the price from the first quotation.

Why does the diameter change depending on where I measure it?

Because the wall tapers and then springs back when it leaves the die. The diameter at the rim is not the diameter at the bottom. This is why a diameter call-out on a drawn housing is incomplete without a measuring height and a datum, and why the same part can read in tolerance and out of tolerance on two different benches.

Is the PDF free?

Yes, ungated. The link goes straight to the file. WP-01 Deep Draw Design Handbook covers the process itself; this document assumes the part is drawable and deals only with how it is specified and proven.

Download

WP-02 Drawn Housing Tolerances (PDF, 13 pages). Companion documents: WP-01 Deep Draw Design Handbook and WP-03 Sourcing Drawn and Stamped Parts.

Send a drawing with a tolerance you are not sure about

We will name the call-outs the process cannot hold as written, what each would cost to hold, and what the schedule looks like either way. Send it through rfq.balford.net, or read all three whitepapers.