Sheet metal stamping design guide: 10 rules that lower cost

Why stamping DFM starts before the drawing

A stamped part is a tooling project with a part attached. The die decides the strip layout, the number of stations, the scrap, the burr direction and the cycle time. If the design ignores those constraints, the supplier either quotes a complicated die or quotes a simple die and struggles to hold the part.

The rules below are not cosmetic. Each one maps to a cost line in the quote: tooling, material, press time, maintenance or inspection. A part that follows them is not automatically cheap, but it is predictable.

The ten rules

1. 01

   Keep holes at least one thickness from an edge

   A hole too close to the edge tears or leaves a weak wall. As a starting point, keep the hole diameter at or above the sheet thickness and the edge distance at least one thickness.

2. 02

   Use a bend radius at or above one thickness

   A tight inside radius concentrates strain on the outside of the bend. For mild steel, one thickness is a common starting point. For high-strength or stainless sheet, go larger and ask for a springback check.

3. 03

   Keep bend lines away from holes and slots

   A hole inside the bend zone distorts. Keep at least two thicknesses from the bend line, or add a relief notch if the design needs to be closer.

4. 04

   Design the strip before the part outline

   The part must be carried through every station. Ask the supplier to show the strip layout. If the part cannot be nested without a weak carrier, the tool will be more expensive.

5. 05

   Use standard material thicknesses

   A non-standard gauge adds lead time and may force a special coil order. Choose a standard thickness unless the function genuinely needs the extra 0.1 mm.

6. 06

   Put tolerances where the assembly needs them

   General tolerances cover most dimensions. Mark the critical-to-quality features and leave the rest to the process. Tightening forty dimensions does not make the part better, it makes the inspection pack longer.

7. 07

   Think about burr direction

   The burr side is set by the die. If the burr will sit against a mating face, a seal or a sliding surface, say so on the drawing.

8. 08

   Avoid sharp internal corners

   Sharp corners concentrate stress and wear the die. Add a radius. The radius also makes the punch easier to maintain.

9. 09

   Design for the finish you actually need

   Pre-plated coil avoids a post-plating step but limits the bend radius and the die contact. Post-plating adds handling and masking. Choose early.

10. 10

    Decide the inspection plan with the drawing

    If a feature is critical, say how it will be measured. A dimension that cannot be reached with a gauge becomes a CMM job and a cost.

Tooling economics: when a die makes sense

Laser cutting and bending have almost no tooling cost and suit prototypes, low volumes and large flat parts. A stamping die adds a fixed cost but spreads it across the volume and drops the cycle time. The crossover depends on the part, the material and the number of operations.

A useful way to compare is total cost at the expected volume, not piece price at the first quantity. Add the die, the trial parts, the maintenance reserve and the scrap. Then compare with the laser route at the same volume. Many programmes use laser for the first build and a die for production.

What a stamping die can hold

A progressive die can hold a tight tolerance on a stable feature. It struggles when the tolerance crosses a bend, when the material varies, or when the feature is far from the datum. The drawing should use a functional datum and a realistic general tolerance.

For a typical electrical stamping, the critical calls are the contact gap, the hole position for a terminal, the overall length and the flatness. The material thickness tolerance also matters because it shifts the bend allowance and the springback. Ask for the mill tolerance, not just the nominal gauge.

Material choices that change the tool

Copper and brass are common for current-carrying stampings because they combine conductivity with formability. Copper is soft and can gall on the die, so lubrication and tool maintenance matter. Brass machines and forms well but has lower conductivity. Aluminium is light and conductive but can gall and needs a different surface treatment. Stainless steel work-hardens and reduces tool life.

Pre-plated material can remove a plating step, but the coating must survive the bend. Tin plating can crack at a tight radius. If the part needs a tight bend and a plated finish, test the coating adhesion on the first article.

Defects to watch in the first trial

The first trial is where the die and the drawing meet. Look for slugs, galling, miss-hits, burr height, bend angle variation and cracks at the outside of a tight bend. A crack that is visible now will get worse in production.

Ask for the strip layout, the first article report and a sample from each cavity if the tool is multi-cavity. For a progressive die, ask which station makes which feature so a non-conformance can be traced.

What to send with a stamping RFQ

Send the drawing, the native CAD file, the annual volume, the release size, the material grade and thickness, the finish, the inspection level and the delivery destination. Add the critical dimensions and the burr direction if they matter.

If the part is not yet designed for stamping, ask for a DFM review before the quote. The supplier should mark the features that will need a tool change, a secondary operation or a tolerance relaxation. That review is cheaper than a redesign after the die is built.

• Current drawing and revision, plus native CAD
• Material grade, temper and thickness tolerance
• Annual volume and release quantity
• Finish and any masked areas
• Critical dimensions and burr direction
• Inspection level and PPAP requirement
• Delivery destination and Incoterm