Deep drawing design rules for production parts

A cup with a 2.4 draw ratio in 1.2 mm CRCA will tear at the punch nose on the first stroke, and no press in the shop will change that. Deep drawing design rules for production parts begin with material flow, not force: the ratio of blank to cup, the radius the metal has to turn over, and how much of the wall is allowed to thin.

Most drawing problems arrive as a tooling problem when they are a drawing problem. A 0.5 mm punch radius on a 1.5 mm cup bottom cannot be drawn at any speed, and a die entry radius under 4t will mark and tear the wall of every part in the batch. Fix those on the drawing and the press settings become routine.

Which deep drawing design rules decide the draw ratio?

Draw ratio is blank diameter divided by cup diameter. Each material has a first-draw limit, and above that limit the part needs a redraw with an anneal in between.

Three consequences follow. A stainless cup that looks like a mild steel cup on the drawing is probably a two-stage tool, and that changes the tooling cost by a large multiple. An aluminium part with a deep draw and a small punch radius will mark on the punch face unless the tool is polished. And a brass or copper part needs an anneal between draws, which is an extra operation, extra handling and an oxide removal step.

Square and rectangular shells are worse than the table suggests, because the corners control the draw while the flat sides do not draw at all. Add 15–20% to the effective draw depth of a rectangular shell when checking against the limit.

How do you size the blank and the press?

The blank for a plain cylindrical cup with no flange comes from the surface area of the cup. Diameter equals the square root of (d squared plus four times d times h). For an 80 mm cup 60 mm deep, that is the square root of 25,600, or 160 mm.

The ratio is 160 ÷ 80, or 2.0, which sits at the top of the mild steel band. On 1.2 mm DD-grade steel that is a single-stroke job with a well-polished die, but there is no margin. If the design can accept a 55 mm depth, the blank drops to about 155 mm and the ratio to 1.93, which is a safer tool to build.

Draw force is roughly pi times cup diameter times thickness times tensile strength, with a factor of 0.7–0.9 for friction and blank-holder load. For the 80 mm cup in 1.2 mm steel at 350 MPa UTS and a 0.85 factor, the force is about 90 kN, or 9.1 tonne-force.

Add blank-holder force at 25%, about 22 kN, and the stroke needs roughly 112 kN. A press rated at 40 T gives better than three times margin, which is where you want a drawing job: cushion pressure, cushion return and the breakthrough shock all sit on top of the calculated figure. The tonnage selection guide covers the press-size checks beyond force, including shut height and bed area.

Why does the flange wrinkle while the wall tears?

Wrinkling and tearing are the two ends of the same control loop, and the blank holder sits between them.

The blank holder stops the flange from buckling as its circumference shrinks. Too little force and the flange buckles into waves that are ironed into the wall, leaving a lumpy ring. Too much force and the material cannot flow inward, so the punch stretches the wall until it tears at the nose. On mild steel the window is forgiving. On stainless it is narrow, and on thin aluminium it is narrower still.

Four levers move the window:

• Blank-holder pressure at 1.5–3 MPa, set by the cushion, not by feel.
• Die entry radius, which sets how gradually the material turns; 6–10t for a first draw, 4–6t for a redraw.
• Lubricant, which reduces friction on the die face and the blank holder but must be removed before coating.
• Draw beads, which control flow on a complex shape where one region would otherwise pull faster than another.

Record the pressure and the lubricant on the tool card. A repeat order with the same die and a different lubricant is a different process.

What radii and clearance should the drawing call out?

Radii and clearance are the two numbers that most often get lost between the design and the tool room.

Punch nose radius should be at least 2t, and 4t is better for a first draw in steel. A cup bottom drawn with a 0.5 mm radius in 1.5 mm steel concentrates strain at one line, and the part tears there even at a 1.6 draw ratio. A generous radius costs nothing on a housing that will never be seen from inside.

Die entry radius should be 6–10t for a first draw and 4–6t for redraws. Tighter than that and the material is sheared over a sharp edge, which leaves score marks and shortens die life.

Clearance between punch and die is 1.1–1.2t per side for steel, and 1.05–1.1t for brass and copper. Too little clearance irons the wall and raises the punch load; too much leaves the wall loose and the diameter inconsistent around the circumference.

Springback matters less in a deep draw than in a bend, because the wall is plastically strained in every direction, but the finished diameter still opens by 0.1–0.3% on a shallow cup. For a housing that has to fit a moulded insert, measure the diameter on the first article and set the punch size from that, not from the nominal.

How much wall thinning is acceptable?

Wall thinning is the honest measure of whether a draw is under control. Measure the wall at roughly 25%, 50% and 75% of cup height on a sectioned first article, and compare with the blank thickness.

A thinning of 10% at the punch nose is normal for a well-set tool. At 15% the part is close to the limit, and the next tool adjustment or a slightly harder coil will tear it. Above 20% the wall is thin enough to affect strength, and on a pressure or fluid-carrying part it is a leak risk.

Record the values against the coil lot. A change of supplier or temper shows up as a shift in thinning before it shows up as a reject, and catching it at first-off is much cheaper than sorting a batch.

What changes when the shell is rectangular?

A rectangular shell is not a cylinder with flat sides. The four corners draw, and the flats mostly bend and stretch a little, so the corners control the process and everything else follows.

Three rules keep a rectangular part drawable. Corner radius should be at least 4t, and 6t is safer on a deep shell; a 2 mm corner in 1.5 mm steel will need a redraw. The corner radius should be consistent, because one tight corner and three generous ones means the tight corner tears while the others are barely worked. And the effective draw depth is measured at the corner, where the material has furthest to travel.

Flat sides that are much longer than the drawn depth usually need a draw bead near the edge to stop the side bowing inward. On a housing with a large flat face, that bead leaves a slight groove, which is either a design feature or a painting problem depending on where it sits. Show the bead allowance on the drawing, or accept the supplier's standard position.

What tooling route suits the volume?

Three tool routes cover most drawn parts, and the annual volume decides which one is worth building.

A single-stage tool does one draw per stroke and suits moderate volumes, large parts and parts that may still change. It is the cheapest tool to build and the easiest to modify. A progressive die combines blanking, drawing, piercing and trimming in one strip, and is economical when the volume and the part size allow a strip layout. A transfer tool moves the part between stations without a strip, which suits large shells that cannot be carried on a coil.

Ganesh builds press tools, progressive dies and checking fixtures in-house. Tooling is quoted as a separate line from piece price, so the annual volume and the amortisation can be compared directly. The tooling cost versus piece price guide sets out how the crossover moves with part size and tool complexity.

What Ganesh runs for drawn parts

Ganesh deep draws on mechanical presses from 20 T to 300 T, with a 250 T punching press for blanks, louvres and perforations, and an in-house tool room that designs and builds the draw tools, progressive dies and checking fixtures. Tool tryout happens in-house, so the first article is measured against the same drawing the customer released rather than a tool room sketch.

The custom part range covers 0.8–40 mm across mild steel, CRCA, stainless, aluminium, brass and copper, with ±0.5 mm typical tolerance and tighter features by agreement. First article inspection and PPAP documentation are available, and the plants hold ISO 9001:2015 and ISO 14001:2015. The stamping route supplies customer programmes including Mahindra, Mahindra Powerol, Minda and Godrej & Boyce. The stamping capability page lists the press range, and the stamping and press tool hub collects the related design rules.

What the press drawing has to settle

• Material grade and temper, with the standard referenced.
• Blank thickness and the tolerance band.
• Draw depth, cup diameter and the inside radius at the bottom.
• Wall thickness where it matters, and the maximum thinning permitted.
• Flange width and the trim allowance, or a note that the rim is trimmed.
• Any feature that must stay concentric with the cup axis, and the datum used to check it.
• Annual volume, release size and whether the tool is customer-owned.
• Whether the surface is visible, and therefore whether marks from the blank holder are acceptable.

Use the press tonnage calculator for a first estimate of force, then send the drawing through the quotation route. A cup with a 12 mm flange and a 90 mm depth can usually be quoted without a tooling study; the same cup with a 0.8 mm bottom radius needs a conversation first. The drawn cups and housings guide takes the same material into cup and housing features, tolerances and the quote route; this article stays with the general design rules, draw forces and blank design. Where the two overlap on draw ratios, this page is the design-rules owner.

When deep drawing is the wrong route

Drawing needs a tool, and a tool needs volume. Below roughly 500 pieces a year, a fabricated shell from laser-cut and rolled sheet, or a spun form, is usually cheaper even with more welding. A prototype should not carry a draw die unless the design is already frozen.

Drawing also stops where the press stops. At 300 T, the largest blank that runs is a function of bed size and shut height as much as force, and a very large shallow panel is better press-formed or folded. Ironing to a precise wall, or drawing a very thin aluminium shell with a 2.5 ratio, needs a specialist tool and a longer tryout; we quote those only after a feasibility check on the actual grade. And if the part must hold a wall tolerance tighter than ±0.05 mm, plan a machining or sizing operation after the draw, because the draw alone will not hold it across a die life.