Stamping Design Guidelines: Tolerances, Bend Radius, Burr and Hole Design
Most stamping problems are designed in, not manufactured in. A drawing that ignores bend radius, hole spacing, burr direction or tolerance stack will be expensive to produce and difficult to inspect.
These guidelines are practical starting points for design engineers. Final values depend on material, thickness and process, and should be confirmed during DFM review.
1. Tolerances
General stamping tolerances are often around ±0.05 mm. Precision stamping can hold ±0.01-0.02 mm on critical dimensions, with die cavity accuracy around ±0.004 mm. Do not apply the tightest tolerance to every dimension; mark only the critical ones.
2. Bend radius
Minimum inside bend radius is usually expressed as a multiple of material thickness (t):
- Soft materials such as copper and brass: about 1t.
- Low-carbon steel: about 1t to 1.5t.
- Stainless steel 301/304: about 2t to 3t, depending on temper.
- Beryllium copper: about 1t to 2t, depending on temper and bend direction.
Sharp corners increase stress and cracking risk. A larger radius usually improves formability and reduces springback variation.
3. Hole design
A common rule is minimum hole diameter ≥ 1t for mild materials, and ≥ 1.2t to 1.5t for high-strength or thick materials. Keep holes at least 2t plus the bend radius away from a bend to avoid distortion.
4. Burr
Burr forms on the die side of a sheared edge. Burr height depends on die clearance, punch/die wear, material and lubrication. Specify a maximum burr height if it matters, and state which side is acceptable.
5. Bend relief and slots
Add bend relief where a bend meets a nearby edge to prevent tearing. Avoid narrow slots and tabs that are difficult to strip and prone to distortion.
6. Tolerance stack
Each bend and feature adds variation. In a multi-bend part, the tolerance on the final dimension is the sum of the contributing tolerances. Design with this stack in mind, and use a datum scheme that matches how the part is inspected.
7. Material grain and springback
Bending parallel to the rolling direction can reduce cracking risk, while bending across the grain may increase springback. Springback is compensated in the die, but the compensation depends on material and temper, so changes in material require a die review.
Quick guideline table
| Feature | Typical guideline |
|---|---|
| General tolerance | ±0.05 mm |
| Precision tolerance | ±0.01-0.02 mm on critical dimensions |
| Minimum bend radius | 1t soft material; 2-3t for 301/304 stainless |
| Minimum hole | ≥ 1t mild; 1.2-1.5t high-strength |
| Hole to bend distance | ≥ 2t + bend radius |
| Burr | Specify max height and acceptable side |
FAQ
Can you hold ±0.01 mm on every dimension?
Not usually. Precision tolerances are applied to selected critical dimensions. Applying them to all dimensions increases cost and may not be measurable in production.
Should I specify the burr side?
Yes, if burr direction matters for assembly or appearance. The burr normally forms on the die side, so the drawing should state which side is acceptable.
How do I reduce springback?
Use a larger bend radius, select a material with lower springback, and let the die maker compensate. Over-bending and bottoming are common die-level solutions.
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