Deep Drawing: The Complete Technical Guide

If you take one thing from this page, take the distinction above. Everything else — draw ratio, blank holder pressure, number of draws, whether you need an anneal — follows from the fact that material has to flow.

1. How the process works

Four elements do the work:

• Punch — pushes the centre of the blank down into the die
• Die — the cavity the material is drawn into, with a rounded entry radius
• Blank holder (or draw ring) — clamps the flange so it cannot rise and wrinkle
• Die radius — the rounded lip the material bends over as it flows

In sequence: the blank is placed, the blank holder closes, the punch descends, the flange material is pulled inward and over the die radius, and the part is formed in one continuous action. The material does not stretch uniformly — it is thickest where it started (near the flange) and thinnest where it has been stretched over the die radius and down the wall.

What happens to the material

• The flange is compressed circumferentially as its circumference shrinks. This is why it thickens, and why it wrinkles if unsupported.
• The die radius is where bending and unbending happen, and where thinning is worst.
• The wall is under tension and thins progressively as it is drawn.
• The bottom stays close to the original blank thickness and is effectively undeformed in the centre.

This gradient is not a defect — it is the physics of the process. It becomes a problem only when a drawing specifies a wall thickness without saying where it should be measured.

2. The four calculations that decide everything

Before a deep drawing job is quoted properly, four numbers are established. They decide whether the part needs one die or five, and whether it needs an anneal.

2.1 Draw ratio

Typical single-draw limits:

2.2 Blank diameter

Getting the blank wrong is expensive in both directions. Too small and the part tears or comes up short. Too large and you pay for material you trim off — and material is usually the single biggest line in a deep drawing quotation.

2.3 Draw force and press tonnage

Then add the blank holder force. Quoting a press on draw force alone is one of the classic ways a deep drawing job goes wrong: the press has enough tonnage to pull the part but not enough to hold the flange, and the part wrinkles.

2.4 Die clearance

Clearance controls how much ironing effect the draw has. Tighter clearance deliberately thins the wall (see section 5); looser clearance lets the wall stay closer to nominal but risks wrinkling.

3. Blank holder: why it exists

When the punch pulls the flange inward, the flange circumference has to shrink. Something has to go somewhere, and in a thin sheet the excess material buckles out of plane. That is a wrinkle.

The blank holder prevents it by pressing the flange flat while still allowing it to slide inward. Its force is a balance:

• Too low → wrinkles in the flange, and those wrinkles travel into the wall
• Too high → material cannot flow, the wall stretches instead, and the part tears at the die radius

On a deep drawing tool the blank holder pressure is adjustable, because the correct setting depends on the actual material lot, its thickness tolerance, and the lubricant. This is also why deep drawing tooling needs tryout time that press forming does not.

4. Single draw, redraw, and when an anneal is needed

When the required ratio exceeds the material's single-draw limit, the part is formed in stages. Each intermediate shape is called a redraw.

• First draw takes the blank down to roughly the material's limiting ratio
• Subsequent draws typically achieve a smaller ratio each, commonly around 1.2 – 1.4
• Each draw work-hardens the material, so the next draw has less formability available
• When the material has hardened too far, an intermediate anneal restores ductility before drawing continues

This is why a tall part is not simply "deeper" than a shallow one — it can be a different manufacturing route with a different tool count and a different price.

Redrawing vs reverse redrawing

A reverse redraw flips the part over between stages. It can reduce the number of operations for certain geometries and helps control wall thickness, but it needs a more sophisticated tool. Choosing between straightforward and reverse redrawing is a tooling design decision made while the die is still on the drawing board, not after tryout.

5. Ironing: when you want the wall thinner on purpose

Ironing is a deliberate thinning operation. The part is pushed through a die whose clearance is less than the wall thickness, so the wall is squeezed thinner as it passes.

This is one of the most commercially useful techniques in deep drawing, because it solves three problems at once:

• Outside diameter is set by the die bore
• Inside diameter is set by the punch
• Surface finish is formed against polished tooling, not cut by an insert

The consequence is that a deep drawn and ironed housing can hold both diameters in tolerance without a turning operation. For a part that would otherwise be machined from bar, that can remove most of the machining cost — and for a solenoid housing, a formed bore can be smoother than a machined one, which matters for armature guidance.

6. Materials and formability

Deep drawing demands formability above almost everything else: high elongation, low yield-to-tensile ratio, and a high normal anisotropy (r-value, the resistance to thinning).

Two practical points:

• Thickness tolerance of the incoming coil matters. A strip that runs thick in one area changes blank holder behaviour and can produce inconsistent parts.
• Lubricant is not optional. It controls friction at the die radius, which is where tearing starts. Changing lubricant without re-tuning the blank holder will change the process.

7. Defects and remedies

These are the failures that come up in every deep drawing job. Each has a cause and a tooling-side remedy — which is why DFM review before tooling is cheaper than fixing after.

8. Design rules for a manufacturable deep drawn part

1. Keep the depth-to-diameter ratio realistic. Below about 0.5 you probably do not need deep drawing at all. Above roughly 2, plan for multiple draws and possibly anneals.
2. Do not specify a sharp corner at the bottom. Punch radius has a minimum, and it grows with material thickness.
3. Give the wall a generous die radius. Too tight a radius is the single most common cause of tearing.
4. Specify wall thickness with a measurement location. Say where on the wall it is measured. This removes the most common first-article dispute.
5. Allow a trim allowance on the top edge. The drawn edge is uneven because of earing and normal process variation.
6. Prefer a simpler profile over a stepped or tapered one if the function allows it. Each change of diameter can add a draw.
7. Define which diameter is functional. If one diameter can be free, the tool has more tolerance to work with.
8. Do not put a fine thread on a drawn wall unless the wall has been ironed — thread engagement needs controlled thickness.
9. Think about the blank layout. Nesting affects material cost, which is usually the largest part of the unit price.
10. State the cosmetic requirement. Draw marks are inherent; if the outside wall is visible, say so early so a finishing route can be planned.

9. Tolerances and surface finish

• Diameters repeat well because they are set by tooling. A drawn diameter holds tolerance more consistently than a formed one.
• Wall thickness is a gradient, not a constant. Tolerance must be tied to a measurement position.
• Wall height is the least precise dimension, because the top edge is trimmed.
• Inside finish can be excellent — it is formed against the punch rather than cut, which is why drawn bores suit armature guidance and sealing surfaces.
• Outside finish will carry draw marks unless a finishing operation is added.

10. Deep drawing vs other processes

The most common question is whether deep drawing is the right process at all. Our side-by-side comparisons:

• Deep drawing vs press forming — the decision that comes first
• Deep drawn stamping vs progressive die stamping
• Cold extrusion vs deep drawing
• Deep drawing, spinning and rounding
• Hydroforming for deep drawing

11. What Balford can run

• Press capacity to 350 t
• Maximum draw diameter Ø250 mm
• Single-station, progressive die and transfer press routes, so the process is chosen by geometry and volume rather than by what happens to be free
• In-house tooling design and build. Our tool room runs surface and cylindrical grinding, centre lathe, CNC lathe, CNC milling, medium and fast wire EDM, tapping, drilling, precision surface grinding, large surface grinding, die spot welding, air die grinding, TIG welding, EDM hole drilling and demagnetising. Slow-wire EDM is the one operation we subcontract
• Production heat treatment (hardening, tempering, annealing, demagnetising) is outsourced; magnetic annealing know-how is in-house, and the tool room keeps a small furnace for emergency die work
• ISO 9001:2015 and ISO 14001:2015 certified; PPAP Level 3 documentation can be provided for new programmes
• Inspection includes 2.5D projector, 3D measuring system, portable roughness tester, Rockwell hardness tester, salt spray chamber, digital height gauges, internal and external micrometers, bore gauges, go / no-go gauges, flash measuring and concentricity instruments
• In-house design and build means the loop between "it tears" and "it runs" is measured in hours, not weeks of freight and coordination

Where we are honest about limits

We would rather tell you before you send a drawing than after. Our capability boundaries page sets out what we do and do not do, including the operations we outsource and the qualifications we do not hold.

12. Frequently asked questions

What is the difference between deep drawing and drawing?

In sheet metal terminology, "drawing" is the general operation of pulling material into a die. "Deep drawing" specifically describes the case where the depth is large relative to the diameter — conventionally more than about half the diameter — so that material has to flow a significant distance and a blank holder becomes necessary.

At what depth does a part need deep drawing instead of press forming?

A common working threshold is a depth-to-diameter ratio of about 0.5. Below that, forming is usually cheaper. Above it, the wall has to be formed by material flow, and the tooling, press tonnage and cost structure change.

Does deep drawing make the wall thinner?

Yes, in the areas that are stretched. The wall is close to nominal at the bottom, thins progressively up the wall, and thins most where it passes over the die radius. If a uniform wall is required, ironing is used to bring it to a controlled thickness.

Can deep drawing hold both an inside and an outside diameter?

Yes, and this is one of its main advantages. With ironing, the die sets the outside diameter and the punch sets the inside diameter, so both are controlled by tooling. That is why a drawn and ironed housing can replace a machined one.

How many draws will my part need?

It depends on the draw ratio against the specific material's single-draw limit. A part within the limit needs one draw; beyond it, redraws are added and an intermediate anneal may be required. This is one of the first things a DFM review establishes.

Is deep drawing expensive?

The entry cost is higher than press forming because the tooling is more complex and needs tryout. The unit cost at volume is usually lower, especially when drawing removes a machining operation. The crossover depends mainly on how much secondary machining the part would otherwise need.

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