Deep Drawn Covers and Stamped Metal Caps

What the part has to do

Covers and caps look simple on a drawing and often are not. In the parts we run, three requirements usually arrive together: the outside diameter has to fit the bore it goes into, the closed end has to stay flat or keep a defined dome, and the wall has to be strong enough to carry a thread, a bearing or a press fit. Where the drawing shows a separate cap sitting on a cover, tell us which joint is intended – press fit, swaging, riveting or welding. Forming the shells is our part; the joint method is confirmed against the drawing before a tool is cut.

Cover, cap, or one drawn piece?

What the drawing shows	How it is made	What to watch
Shallow cover, flange only	Blank and form, one or two stations	Flange flatness after forming and trimming
Cover with a defined corner radius and a wall height	Drawn from a blank, over as many stages as the height requires	Draw ratio, and wall thinning at the radius
Cap that fits over a joint, a port or a bearing	Drawn shell, trimmed, then sized on the outside diameter	Springback on the outside diameter; the sizing stage
Cover and cap as one part	Drawn in one piece, then pierced and flanged	The material has to carry the depth and the flange at once

What decides whether it can be drawn

• Draw ratio. Blank diameter divided by finished diameter. Every material has a limit it will survive in one draw without tearing; past that the part needs another station. The per-material single-draw figures we work to are set out on the deep drawing guide.
• Corner radius against wall thickness. A tight radius against a thin wall is where covers tear. Opening the radius, or adding a draw, is almost always cheaper than fighting it.
• Blank size and material utilisation. A cover with a wide flange is mostly blank. Nested blanks and a strip layout that suits the press decide how much of the coil ends up in the part rather than in the scrap bin.
• Flange width for clamping and trim. The blank holder has to have something to hold and the trim has to have something to cut, so the flange cannot be as narrow as the drawing would like it to be.
• Flat or domed closed end. A bottom that has to stay flat normally gets a coining or sizing stage; a domed bottom forms with the draw itself.

Materials we draw

Material	Typical use on a cover or cap	Note
Low carbon steel, DC01–DC06	General covers, caps, housings	DC04 and the softer grades draw deepest; DC01 is for shallow forming
Zinc-coated steel, DX53D+Z	Covers that need corrosion protection without a paint line	The coating has to survive the draw, so keep the radii generous
Stainless 304 / 304 DDQ / 316	Covers for wet, medical and food-adjacent equipment	Work hardens quickly, so anneal between draws
Stainless 430	Caps where the part also has a magnetic duty	Ferritic and magnetic; less ductile than 304
Aluminium 1100-O / 3003-O / 6061-O	Light covers, caps and heat shields	Soft tempers draw well and the harder ones crack
Copper and brass	Conductive caps, terminals, decorative covers	Excellent formability; specify the temper

The grades and thicknesses the plant runs regularly are listed on the materials page. If your grade is not among them, send the specification with the drawing rather than picking a substitute.

Our means

Item	Range
Presses	25 t to 350 t – single station, progressive die and transfer
Maximum draw diameter	Ø250 mm
Wall thickness	0.25 mm to 1.00 mm; 0.50 mm is the most common figure in the housing configurations we publish
Tooling	Designed and built in-house, in the same plant as the presses
Secondary operations	In-house CNC facing, drilling and tapping; deburring
Surface and heat treatment	Through qualified suppliers, with certificates held against the batch
Metrology	In-house lab: 2.5D optical measuring projector, digital height gauge, Ra / Rz roughness testing
Documentation	ISO 9001:2015 processes, first article inspection, PPAP-compliant documentation, IMDS data for automotive part numbers

What we need to quote

Four things decide the answer on a cover or a cap: the 2D drawing with tolerances, the material and its temper, the surface the part has to end up with, and the annual volume. If the joint between cap and cover is inside our scope, say so on the drawing – the joint method can change the tooling for both shells. The route from sample to series is described under prototyping and first article.

Frequently asked questions

How thin can the wall of a drawn cover be?

The housing configurations we publish run from 0.25 mm to 1.00 mm, with 0.50 mm the most common. What limits it in practice is not the sheet you start from but the wall at the corner after forming, because material thins where it is stretched around the radius. Send the drawing and we will say where in the part the wall will come out thinner than the blank.

Can a drawn cover carry a thread?

A drawn wall is usually too thin to cut a reliable thread into directly, so the thread normally goes into a thicker collar, an inserted bush, or a machined feature added after drawing. Designing that in from the start is cheaper than asking the formed wall to do it.

What tolerance can you hold on the outside diameter?

The outside diameter is one of the more controllable dimensions, because the final draw or sizing stage sets it. How tight it goes depends on the diameter, the height and the grade. We do not quote a blanket figure: the dimensions that matter to function are the ones worth marking on the drawing, and we confirm each of them against the forming route. The reasoning is set out under housing tolerances.

Can the cap and the cover be made in one tool?

Yes, and for series volumes it is often the better route: two shells in one progressive tool, nested in the same strip, with one press stroke producing both. It costs more to build and less to run, and it removes a second setup and a second material order.

Written by the Balford technical team, Zhuji, Zhejiang. Last updated 8 October 2026.