Deep Drawn Covers: Design, Materials & Sourcing Guide

A deep drawn cover is a seamless metal shell that closes, shields or locates whatever sits underneath it. Because the wall and the crown are formed from one continuous piece of metal, there is no weld seam to leak, no joint to corrode and no assembly step to go wrong. This guide covers the geometry that makes a cover drawable, the materials that draw well, the tolerances that hold in production, and what to put on a drawing so that the first quotation you receive is already the right one.

What a deep drawn cover actually is

Deep drawing starts with a flat blank cut from coil. A punch pushes that blank through a die while a blank holder presses down on the rim, and the metal flows plastically into a hollow shape. The result is a single-piece shell — a crown, a wall and (usually) a flange, all formed from one sheet. Nothing is joined, so nothing can come apart.

That distinguishes a drawn cover from the alternatives it is usually compared against. A stamped cover is bent and formed from sheet but cannot go deep without wrinkling, so it stays shallow. A machined cover is cut from solid bar, which is precise but waste-heavy and slow. A die-cast cover can carry ribs and bosses but brings porosity and a higher tooling bill. A drawn cover sits in the middle: deep, seamless, thin-walled, and economical once the volume justifies the die.

The features that define a drawn cover

Every drawn cover can be described by six parameters. Get these right on the drawing and the rest usually follows:

Where drawn covers are used

Drawn covers appear wherever something needs a thin, seamless, rigid enclosure that has to be produced in quantity. The applications we run most often:

A large share of these parts are drawn in batches that repeat for years. That is the real argument for tooling a cover properly: the die cost is paid once, then amortised across every subsequent order. You can see the range of shapes this produces on our deep drawn parts page, from simple domes to flanged, slotted and louvred covers.

Draw ratio: the number that decides whether your cover is possible

Before anything else, a tooling engineer calculates the draw ratio. It is the blank diameter divided by the punch diameter:

Draw ratio β = blank diameter ÷ punch diameter

If the ratio a cover needs exceeds what the material can survive in one hit, the part is drawn in two or more stages, with a redraw die between them. Each redraw reduces the diameter by roughly 20–25 per cent. So a cover needing a very deep wall is not impossible — it just takes more stations, and each station is a cost.

Worked example. Take a cover needing a 30 mm outside diameter with a 40 mm deep wall in 304 stainless. The blank works out at roughly 105 mm. A single draw would need a ratio of about 3.5 — far beyond what stainless can take. So it is staged: draw 1 to about 58 mm, draw 2 to about 45 mm, draw 3 to 30 mm, then trim. Three draws plus a trim station means a progressive or transfer tool. That is the honest answer to give a customer, and it is better said at quotation stage than discovered at first article.

If your cover is shallow — say a depth less than half the diameter — you are often finished in a single stroke. The economics of deep drawing get difficult only when the ratio climbs past about 2:1.

Wall thickness: where the metal actually goes

Newcomers to deep drawing usually assume the wall keeps its starting thickness. It does not. As the blank flows inward the circumference shrinks, and the surplus material has nowhere to go but into the wall — so the wall is typically thicker near the rim and thinnest at the bottom radius. Thickness variation of 10–15 per cent around a drawn wall is normal and should be expected in any design review.

If you need a wall that is genuinely controlled — a uniform, specified thickness — the answer is ironing. In a redraw with a deliberately undersized die, the material is squeezed through and thinned in a controlled way, which reduces the wall by 30–50 per cent and makes the thickness far more repeatable. Many sleeves and tube-shaped covers are ironed for exactly this reason. Where the tolerance is tighter still, the bore or outside diameter is finished by CNC afterwards, which is a normal route for us because machining sits in the same building as the presses.

Design rule: do not call out a wall thickness tolerance tighter than ±10 per cent of nominal on a plain drawn cover. It cannot be met consistently, so you will pay for sorting rather than for parts. Where a controlled wall is genuinely needed, specify it and say so on the drawing — that signals ironing or a post-machining step, and the quotation will reflect it honestly.

Radii: the design rules that prevent tearing

The radii you call out decide how the metal flows. Too small a die radius and the wall is stretched beyond its limit and tears; too large and the blank loses support and wrinkles. These are the ranges that work in production, expressed as multiples of the sheet thickness t:

The single fastest way to make a cover cheaper and more reliable is to relax a sharp internal corner to the next reasonable radius. It costs nothing in function in most designs and removes a whole class of forming risk. Our tool and die design team reviews drawings against exactly these rules before a die is ever cut, and the feedback comes back with the quotation.

Designing the edge: flange, curl and trim

A drawn cover does not come off the press with a clean, dimensionally accurate rim. The material at the top of the wall has flowed unevenly, so the edge is wavy and the height is approximate. That is why trimming is a required step, not an optional one. Trimming in a following station brings the height to a real tolerance — typically ±0.15 mm, or ±0.10 mm on a well-controlled progressive tool.

Three ways to finish the edge, and when each makes sense:

• Plain trim — the edge is cut square. Cheapest, and correct when the cover is not handled by the rim and does not need to seal at the edge.
• Flange — a horizontal band formed outward. This is what gives you a mounting face and a sealing land. It also adds stiffness, which is often why a flange appears on covers that do not strictly need one.
• Curl (rolled edge) — the edge is rolled over. It removes the burr from where fingers go, adds rigidity, and gives a finished appearance. Common on covers the end user sees or touches, and on covers that must not cut a cable or a seal.

Where a cover has to close against a gasket or seal, the flange face is the sealing surface. Call out its flatness — 0.15 mm is a reasonable general figure, 0.08 mm where a thin gasket has to seal reliably. And say explicitly which face is the sealing face; when the drawing is silent, that information is guessed, and guesses about sealing faces are expensive.

Material selection for drawn covers

Drawability is not the same thing as strength. A material that draws beautifully may be too soft for the finished part; a material with excellent mechanical properties may fight the die at every stroke. The table below is the honest trade-off.

Two practical notes. First, temper matters as much as grade: a 5052 in the soft O condition draws completely differently from the same alloy in H32. Always state the temper. Second, material selection should be settled before tooling, because the die is cut for a specific material behaviour — changing grade after the die exists usually means a new die, not a tweak. We source coil against mill certificates through our own material sourcing channel so the grade on your drawing is the grade that ships.

Tolerances you can actually hold

Tolerance is where drawings most often become unbuildable. The table below separates what is routine from what is possible with extra care — and what should simply not be specified on a drawn part.

Unless your drawing says otherwise, unmarked dimensions fall back to a general tolerance standard — DIN 6930-2 or ISO 2768-m are the usual choices for stamped and drawn parts, and German-speaking customers expect one of them to be named. Name it explicitly rather than leaving it to the shop’s default, because the default is not the same everywhere.

Seven defects and the design changes that prevent them

Every forming defect has a cause, and most causes are decisions made on the drawing. Knowing them in advance is far cheaper than discovering them at first article.

Note what most of these fixes are: they are process decisions, not drawing changes. That is the reason to bring a supplier into the conversation while the part is still a sketch. A manufacturability review at quotation stage costs nothing; a tool re-cut after approval costs weeks.

Process route: one hit, progressive or transfer

How a cover is drawn has as much influence on cost as what it is drawn from. The choice follows volume, and volume should be stated honestly — overstating it to get a lower unit price is the most common way buyers end up with a tool they cannot pay off.

A pragmatic rule: below a few thousand pieces a year, a simple tool and a higher unit price usually beats a sophisticated tool you never fully amortise. Above tens of thousands, the progressive tool pays for itself quickly and the unit price collapses. What matters is that your quotation is built on the volume you will actually order.

Secondary operations

Very few covers ship straight off the press. Typical follow-up work, all of which can be planned into the same production flow rather than farmed out:

• Trimming — sets the real height and a square edge
• Piercing and slotting — cable exits, fixing holes, louvres, vents
• Flanging and curling — mounting face, sealing land, safe edge
• Threading or tapping — for screw assembly into the cover
• Deburring and cleaning — essential before any plating step
• Welding — only where a drawn shape genuinely cannot provide the feature; laser and resistance welding are both common
• Surface treatment — plating, anodising, passivation, coating

The value of keeping these in one plant is not sentimentality about vertical integration. It is that a dimension changed at the drawing stage propagates through the whole chain without a week of emails between vendors, and that responsibility for the finished part sits in one place when something needs fixing.

Surface finishing

One thing to decide early: if the cover is visible to the end user, say so. Draw marks are unavoidable on a formed wall, and a bright chrome or mirror finish will show every one of them. Options are to accept a satin or textured finish, to polish after forming, or to move the visible surface to a machined face. All three are workable — but only if the requirement is known before the die is cut.

What belongs on your drawing

Most quotation delays trace back to a drawing that is missing one of these. Including all of them in the first enquiry is the single easiest way to shorten the loop:

1. Material, grade and temper — not just “stainless”, but 304 or 316, with the condition
2. Thickness with tolerance — the coil thickness, not the finished wall
3. All functional dimensions with tolerance — and only those; tolerancing everything is the same as tolerancing nothing useful
4. The general tolerance standard — for example DIN 6930-2 or ISO 2768-m
5. Which surfaces are cosmetic and which are sealing or bearing faces
6. Flange width, curl or trim specification
7. Hole and slot positions, diameters, and their edge-to-edge distances
8. Concentricity or runout where rotation or sealing matters
9. Surface finish specification, including any plating thickness
10. Annual volume and typical batch size — this drives the tooling decision
11. Any standard or documentation requirement — PPAP, material certificate, first-article report
12. Packaging and labelling requirements for export

Drawn covers compared with the alternatives

We also run custom metal stamping and CNC machining alongside drawing, so the recommendation is not always going to be “draw it”. A shallow cover at low volume is frequently cheaper as a stamping, and a cover with deep internal pockets is frequently cheaper machined. Where the answer is drawing, we will say so — and the review that reaches that conclusion comes with the quotation either way.

What actually drives the cost

• Net versus gross material — the blank you cut versus the cover you ship. A deep cover with a small flange can waste a third of the coil; a nested blank layout recovers a surprising amount.
• Number of draw stages — each extra draw is another die station, another press stroke and another chance for variation.
• Tooling complexity — single-stage versus progressive is the largest single line item in most quotations.
• Tolerance strategy — tight tolerances on features that do not need them buy inspection hours, not quality.
• Surface treatment area — plating and coating are quoted per square decimetre or per kilogram, so a large cover is not proportionally cheap.
• Inspection and documentation — full CMM reports and PPAP packages are real work, and they are worth it when the part is safety-critical and wasteful when it is a cable shield.
• Coil grade and availability — a common grade delivered from stock beats an exotic one bought to order, in both cost and lead time.

Sourcing: what to ask a supplier

If you are comparing deep drawing suppliers, the same eight questions separate the capable from the trading companies:

1. Do you design and build the dies yourselves? If tooling is outsourced, your engineering changes queue behind someone else’s schedule.
2. What press tonnage do you have, and how many? Tonnage limits the size and depth of cover you can be offered.
3. Can you handle multi-stage draws with intermediate annealing? This is the difference between “we can draw covers” and “we can draw this cover”.
4. Can you hold a wall thickness to a specified band? Only if you have ironing capability.
5. How do you inspect? Ask specifically about CMM, vision systems and salt-spray testing — and ask for a sample report.
6. Do you provide material certificates with each shipment? Non-negotiable for anything regulated.
7. What does your first-article process look like? A supplier who measures and signs off a first article will not surprise you at mass production.
8. What is your reorder behaviour? The real test is whether order five is identical to order one.

A supplier who answers all eight specifically, with numbers, is worth a sample order. A supplier who answers in adjectives is not.

Drawn covers in our catalogue

Every product page below carries its own worked example of the principles above — the draw ratio that makes it possible, the radii that keep it from tearing, and the tolerances we hold in production. Start with the geometry closest to your part, then follow the guide sections above for the design rules.

• Stamped Electric Motor Cover Manufacturer
• Canned Motor Cover – Deep Drawn Enclosure
• Cold Drawn Motor Cover – Precision Housing
• Metal Motor Covers – Custom Stamping
• Sheet metal stamping electric motor cover
• Metal stamping DC motor cover
• Stamping DC motor cover
• Deep drawn electric motor end cover
• Deep drawing electric motor end cover
• Car starter electric motor cover
• Deep drawn enclosure
• Deep drawn cover
• Deep Drawn Fuel Pump Cover with Fittings
• Deep Drawn Cans, Caps and Sleeves
• Deep Drawn Square Cover – Custom Tooling
• Wiper motor cover
• Multi-Dome Deep Drawn Cover Plate
• White Perforated Smoke Detector Cover
• Zinc-Plated Military Case with Hinged Cover
• Black Powder-Coated Electronic Frame Cover
• Drawn Rectangular Cover with Round Neck Port
• Brass Eyelets and Bushing Pins Assortment
• Drawn Cap with Hex Center Hole
• Stainless Deep Drawn Square Cover
• Brass Saddle Block and Ring Bushing Set
• Stainless Curved Saddle Mounting Bracket
• Stamped Steel Shallow Drawn Motor Cover
• Deep Drawn Square Cover with Flanged Rim Zinc Plated
• Anodized Aluminum CNC Machined U-Shaped Cover Case
• Aluminum CNC Machined Impeller Wheels and Covers
• Aluminum CNC Machined U-Shaped Channel Covers
• Steel Deep Drawn Square Flanged Cover with Web
• Steel Deep Drawn Round Cover with Mounting Clips
• Stainless Steel Deep Drawn Curved Cover Shell
• Assorted Deep Drawn Covers and Housings Zinc Plated
• Steel Deep Drawn Angled Cover with Mounting Holes
• Steel Deep Drawn Motor Bell Cover with Vent Holes
• Assorted Deep Drawn Shells Covers and Trays
• Steel Deep Drawn Stepped Cover with Side Hole
• Black Painted Steel Deep Drawn Square Cover
• Steel Deep Drawn Vented Cover with Louver Slots
• Steel Deep Drawn Cover Pan with Mounting Tabs
• Steel Deep Drawn Cover Panel with Vent Slots
• Aluminum Deep Drawn Dome Cover with Flange Base
• Assorted Stamped Brackets Covers and Trays
• Galvanized Steel Deep Drawn Junction Box Covers
• Black Steel Deep Drawn Sump Cover Shell
• Assorted Deep Drawn Cup Cover Disc and Tray
• Assorted Deep Drawn Cups and Vented Covers
• Steel Deep Drawn Dome Cover with Square Flange

Frequently asked questions

What is a deep drawn cover?

It is a seamless metal shell — typically a crown, a wall and a flange — formed by pushing a flat blank into a die with a punch. Because the wall and crown come from one piece of metal, a drawn cover has no weld seam, which is why it is chosen for enclosures that must not leak or corrode at a joint.

How deep can a cover be drawn?

It depends on the material rather than on an absolute limit. Draw ratios beyond 2:1 are routine for us on compliant materials, reached through multiple stages with optional intermediate annealing. If you have a target depth, send the dimensions and material and you will get an honest answer about how many stages it takes — at quotation stage, before any tooling is cut.

Will the wall thickness be constant?

No, and any supplier who promises a perfectly uniform wall on a plain drawn part is either ironing or misunderstanding the question. In a normal draw the wall thickens toward the rim and thins at the bottom radius, typically by 10–15 per cent. Where a controlled wall is required we use ironing, which reduces the wall by 30–50 per cent in a controlled way and holds a much tighter band.

Which aluminium should I specify for a drawn cover?

For maximum formability, 1050 or 1100 in a soft temper. Where you need more strength, 3003 is a good compromise and 5052 will still draw at moderate ratios. Do not specify 6061-T6 for a drawn cover — it is too hard to form reliably and should be machined instead.

Why is stainless harder to draw than steel?

Austenitic stainless work-hardens rapidly as it deforms, so the material gets stronger and less ductile with every stage. Combined with a higher tendency to gall against the die, that means lower draw ratios, more frequent lubrication attention and often an intermediate anneal on deep parts. The reward is a part with no coating to chip and excellent corrosion resistance.

Can you add holes, slots and vents to a drawn cover?

Yes, and they should always be pierced after drawing rather than before. Holes cut before forming will distort as the material flows. Piercing in a following station holds position to roughly ±0.10 mm and keeps the edge clean.

What is the minimum order quantity?

Tooling is the real threshold, not a nominal MOQ. Because the die is a one-off cost, the sensible question is how many parts you will buy over the tool’s life. Tell us your realistic annual volume and we will recommend the tooling route that fits it — including telling you when a simple tool is the better commercial answer even though it means a higher unit price.

What document format do you need for a quotation?

A 2D drawing in PDF or DWG with material, thickness and tolerances is enough to quote. A 3D model (STEP or IGES) helps us verify the geometry and is useful for the die design, but it is not a prerequisite. If all you have is a sample part, send dimensions and photos and we will work from those.

How long does tooling and first article take?

It depends on how many draw stages and stations the part needs, and that is exactly what the manufacturability review establishes. Because die design, wire-cut EDM and proving all happen in our own toolroom, engineering changes take hours rather than weeks — and the tooling timeline starts from a date we control rather than from a subcontractor’s queue.