How to design drawn cups and housings

A drawn cup fails in one of two places: the wall tears at the punch radius on the first stroke, or the rim comes back wavy and 4 mm too tall. Both are decided by numbers that are easy to calculate before a tool exists and expensive to discover after. Knowing how to design drawn cups and housings is mostly the arithmetic that precedes the tool: blank diameter from the finished geometry, draw ratio from the material, draw force from the tonnage, and the radii that keep the wall from thinning past its limit.

The guidance below is for the design engineer sizing a housing, a cover, a motor can, a lamp body or a filter cup, and for the buyer who has to judge whether a quote for tooling is realistic. It applies to cups drawn on mechanical presses from 20 T to 300 T, which is the range Ganesh runs.

How to design drawn cups and housings: blank first, then tool

A drawn cup is made from a flat blank that is the same surface area as the finished part. That gives the blank diameter for a simple flat-bottomed cylinder:

D = √(d² + 4dh)

where d is the finished outside diameter and h is the depth, both in mm. The formula ignores the corner radius, the flange and the thinning in the wall, so add 3–5% for trim allowance. The trim is not waste; it is what lets the die cut a clean rim after the material has flowed unevenly.

A worked example. An 80 mm diameter housing, 60 mm deep, in 1.2 mm CRCA:

D = √(80² + 4 × 80 × 60) = √(6,400 + 19,200) = √25,600 = 160 mm

So the blank is 160 mm across, plus trim. Now the second number: the draw ratio, which is the cup diameter divided by the blank diameter.

m = 80 / 160 = 0.50

That sits exactly on the limit for a single draw in ordinary mild steel. The part can probably be made in one hit with a good draw-quality material and plenty of lubricant, and it will tear in one hit with a commercial-quality sheet and a tight punch radius. The material specification is what decides, not the press.

Draw ratio limits by material

The first-draw limit is expressed as a blank-to-diameter ratio, which is the reciprocal of m. A ratio of 2.0 means the blank can be twice the cup diameter.

Read the table as a planning tool, not a guarantee. Every value assumes a good blank holder, a polished die, correct lubrication and a punch radius in the stated band. Change one and the limit moves.

If the ratio is beyond the single-draw column, the part needs a redraw: draw to an intermediate diameter, then draw again to final size, usually with an inter-stage anneal for stainless and for any part taken past two stages.

The radii that keep the wall intact

Three radii decide whether the material flows or tears.

The punch nose radius should be 4–6t. A sharp nose concentrates strain at the point where the wall leaves the punch, and that is the classic tear location. On a 1.2 mm part, 5–7 mm is the working range.

The die entry radius should be 6–10t for a first draw, and smaller on redraws. A tight die radius raises the force needed to pull the flange through and burnishes the wall, which is fine for appearance but bad for tool life.

The bottom corner radius, where the wall meets the base, should be at least 1.5t and preferably 3t. This is the radius most often specified too small, because it looks better in a rendered model. A 0.5t corner on a 1.2 mm cup is a crack initiation site and will need die maintenance far more often.

Wall thinning follows from the radii and the draw ratio. A well-drawn mild steel cup thins 10–15% at the mid-wall and 20–25% at the die radius on the first draw. If the drawing calls for a minimum wall of 1.0 mm from a 1.2 mm blank, that is a 17% thinning allowance, which is achievable but leaves no margin. Specify the starting thickness from the minimum wall plus the thinning you can accept, and say so on the drawing.

Draw force, blank holder force and press size

Press selection is a force calculation, not a diameter rule. The draw force is approximately:

F = π × d × t × σ_b × k

where d is the cup diameter, t the thickness, σ_b the tensile strength and k a factor between 0.6 and 1.0 that accounts for friction and the die radius. For the 80 mm cup in 1.2 mm CRCA at 400 MPa with k = 1.0:

F = 3.1416 × 80 × 1.2 × 400 = 120,637 N ≈ 12.3 tf

The blank holder adds its own load. At 2 MPa over a blank annulus of 15,080 mm², that is 30 kN, or about 3.1 tf. Total is around 15.4 tf. A press should not run above about 70% of its rated tonnage for a drawing operation, so a 20 T press gives 14 tf of usable force and is already short. A 40 T press or larger is the sensible starting point, and the 60–100 T band is where most production housings sit.

Blanking the 160 mm circle is a separate load. At 300 MPa shear strength the blanking force is:

π × 160 × 1.2 × 300 = 181 kN ≈ 18.4 tf

That is why a combined blank-and-draw tool needs a press rated well above the draw force alone. On a 250 T punching press the blank is a light load; the tool has to be built for the combined tonnage at the same stroke.

The press tonnage calculator does this arithmetic for a given diameter, thickness and material, which is quicker than a spreadsheet when a design is still moving.

The defects, and what each one tells you

Defects are diagnostic. Read the pattern before changing the tool.

Tearing at the punch radius. Punch radius too small, draw ratio too high, insufficient lubrication, or a blank that is under-annealed. Check the punch radius first, then the material certificate.

Wrinkling in the flange. Blank holder force too low, or a blank holder that is not sitting flat because the press ram is not parallel. A wrinkle that appears on one side only is usually a levelling problem, not a material problem.

Earing. The rim comes back with four or six high points. This is planar anisotropy in the sheet, and it is a property of the coil, not the tool. Allow 3–5% extra height and trim. If earing is severe, ask for a material with a higher normal anisotropy r-value.

Galling and scoring. Metal picks up on the die radius and scratches the wall. Stainless is the usual offender. Polish the die, change the lubricant, or coat the die surface.

Orange peel. A rough, dimpled surface on aluminium and on coarse-grained steel. It means the grain size is too large for the strain, and no press adjustment removes it. Change the material specification.

Diameter growth after stripping. Springback opens the cup slightly, typically 0.1–0.3% on mild steel and more on stainless. A 100 mm cup can come out 0.2–0.3 mm larger than the punch. This matters when the housing has to fit a casting or a lid, and it is the reason the springback guide belongs next to the drawing. For a fit-critical diameter, plan a sizing pass or specify the tolerance on the die.

Tooling, tryout and what a realistic quote looks like

A single-stage draw tool is a punch, a die, a blank holder and a stripper, in a die set sized to the press. A progressive tool adds the blanking station, the draw stations and any piercing or trimming, and it costs several times more, but it removes the handling between presses and holds the relationship between features far better.

Tooling is quoted separately from the piece price, and it is not free at any volume. The honest question is how many pieces the tool will run before the design changes, and whether a second draw station is worth its cost now or later. A tool designed with the second stage added as a bolt-on is a false economy; the die shoe and the press window have to be sized for the final sequence from the start.

Tryout is part of the tool, not an optional extra. Expect the first parts off a new draw tool to need radius correction, blank holder pressure adjustment and a blank size adjustment, and expect the blank diameter to settle within 1–2 mm of the calculated value once earing and trim are measured.

For parts below a few thousand pieces a year, a drawn tool is often the wrong purchase. A laser-cut and press-bent housing with a welded seam avoids the tool, tolerates design change, and can be coated on the same route. The tooling cost versus piece price article sets out the crossover in cash terms.

What Ganesh runs

Ganesh stamps on mechanical presses from 20 T to 300 T, with a 250 T punching press for blanking and piercing and an in-house tool room for press tools, progressive dies, welding fixtures and checking fixtures. The tool room is the reason a draw sequence can be corrected in days rather than weeks: the same team that builds the die does the tryout and the radius changes.

Blanks are laser-cut on the 12 kW fibre machine for prototypes and low volume, so a design can be proven before a blanking tool exists. At production volume the blank comes off the press. Housings then go through deburring, welding where a fitting is added, seven-tank pretreatment, oven cure after coating, and a dry film thickness plus cross-hatch check on the finished part.

First article inspection and PPAP are available, and the typical sheet metal tolerance is ±0.5 mm with tighter control on critical features by agreement. The stamping capability page lists the press range and tool room scope, and custom parts is the route for a housing that is not tied to a specific product family.

What belongs on the drawing

• Material grade including the draw quality (commercial, DD or EDD), thickness and grain direction if it matters.
• Finished outside or inside diameter, and which one is the controlled dimension.
• Depth, measured from a defined face, with the flange called out separately.
• Punch radius, die radius and bottom corner radius, or a note allowing the toolmaker to select them.
• Minimum wall thickness after drawing, with the measurement height.
• Concentricity of the cup to the flange or to the base.
• Flatness of the base, because a drawn base is rarely flat without a coining or sizing station.
• Any piercings and their position relative to the cup axis, not to the blank edge.
• Finish specification and whether the inside is coated.
• Volume per year and expected tool life, because both change the tool design.

When drawing is the wrong route

Deep drawing is not a universal answer. Parts with a ratio beyond about 2.2 in mild steel need three or more stages, and at that point a welded or spun assembly is often cheaper to tool. Wall ironing, flow forming and hydroforming are separate processes we do not run, and a part designed for a wall thickness that varies along the height is a wall-ironing part, not a drawn cup.

Press capacity is a hard limit. A 320 T draw is outside the 300 T press, and a part that needs a stroke longer than the press window is outside it regardless of tonnage. Very large panels with shallow draws often go to the press brake instead, because the tool is cheaper and the volume does not justify a die.

We also do not do in-house heat treatment, so an inter-stage anneal for a stainless part is arranged outside and has to be planned into the route and the lead time. Where that is not acceptable, the answer is a shallower cup or a welded housing.

The deep drawing design rules article goes further into staging, and the stamping hub collects the rest of the cluster on tooling, tonnage and inspection.

Send the cup drawing with the material grade, the volume and the finish to request a quote, and the draw sequence can be checked against the press and the tool room before the tool is committed.