Three suppliers quote a drawn sheet metal part. The quotes come back 40% apart and nobody can explain why. Almost always the reason is on the drawing: it describes a part, but not enough of one to be quoted twice the same way. The fix is not a longer drawing. It is naming the five things a drawing operation actually depends on.
How do you specify a drawn sheet metal part?
A drawn sheet metal part is specified by five things: the material and its temper, the wall thickness together with the height at which it is measured, the inside radii, the tolerances that actually have to be held, and the surface finish. The drawing also has to say whether the wall is drawn at roughly the blank thickness or thinned on purpose, because a 0.4 mm wall from a 0.8 mm blank is an ironing operation and not a general tolerance. Sheet thickness, blank size and the number of drawing stages all follow from those choices. Without a measurement height, a drawn wall cannot even be inspected: the wall of a drawn part is a gradient, thicker near the open end and thinnest just above the punch nose. Naming these items is what makes two suppliers quote the same part instead of two different parts.
The five things a drawing has to state
- Material and temper - a grade alone is not enough. Two coils of the same grade at different tempers form differently, and the deep draw is where that shows first.
- Wall thickness, plus the height at which it is measured - the wall thins where it is stretched over the die radius. A nominal figure with no measurement point is not a specification.
- Inside radii - the punch nose radius and the die radius, or the inside radius of the finished part. This is what decides whether one draw is enough.
- The tolerances that have to be held - marked as functional where they matter, and deliberately loose everywhere they do not.
- Surface finish and any post-processing - a drawn surface is usually too rough to seal, so if a face has to seal, say so and the process plan adds a machining or coining pass.
Why wall thickness needs a measurement point
A drawn wall is not a rolled wall. The material that becomes the side wall started in the flange, so it has already been stretched by the time it arrives. The wall just above the punch nose radius is the thinnest point on the part, and the wall near the open end is the thickest. If the drawing says 1.2 mm and does not say where, every supplier is quoting a slightly different component, and the disagreement only surfaces at first article. Name the height, mark it functional, and the argument disappears.
The tolerances that decide the process
- A blanket tolerance across the whole drawing is not a specification, it is a wish. One tight callout among twenty loose ones is normal; twenty tight callouts moves a single-hit tool into a multi-stage tool with an intermediate anneal, which is a different tool, a different lead time and a different price.
- Concentricity and runout between the bore and the outside diameter decide whether the part assembles, so they belong on the drawing even when they look severe.
- Wall thickness belongs to the functional group. A wall that has to hold pressure and a wall that only has to keep two halves apart are not the same requirement.
- Burr and cleanliness limits matter on parts that feed a sensor element or a sealing surface, and they are the callouts drawings most often leave unstated.
What to send with the drawing
- The finished part drawing, with every radius, angle and critical dimension stated.
- The material grade and temper or condition, and whether the material is customer-fixed.
- The annual volume and expected batch sizes, because both move the tooling route.
- Any specification the part must satisfy: a surface treatment standard, a compliance declaration, or a customer standard.
- Whether the part is cosmetic or functional. It changes fixturing, packaging and price, and it is the cheapest question to answer up front.
Key point
A drawn sheet metal part is specified by material and temper, wall thickness with its measurement point, radii, the tolerances that matter, and finish. Everything else - blank size, stage count, tooling route - is a consequence. Put those five on the drawing and suppliers stop quoting different parts.
Frequently asked questions
What has to be on a drawing for a drawn sheet metal part?
Material and temper, wall thickness with the height at which it is measured, inside radii, the tolerances that must be held, and the surface finish or post-processing. Those five items decide the blank, the number of drawing stages and the tooling route.
Why does the wall thickness need a measurement point?
Because a drawn wall is a gradient, not a constant. The wall just above the punch nose radius is the thinnest point on the part and the wall near the open end is the thickest, so a nominal figure with no height cannot be inspected or quoted consistently.
Is a blanket tolerance acceptable on a drawn part?
No. A blanket tolerance forces the supplier to assume the worst case everywhere, which usually pushes a single-hit tool into a multi-stage tool and raises both price and lead time. Mark the few functional dimensions tight and leave the rest loose on purpose.
What is the difference between a drawn wall and an ironed wall?
A drawn wall stays close to the blank thickness because the material is bent and pulled without deliberate thinning. An ironed wall is thinned on purpose in a separate stage, so a 0.4 mm wall from a 0.8 mm blank has to be written as ironed rather than left to a general tolerance.