How to Specify Punch Radius for Stamped Parts

Why Punch Radius Matters More Than You Think

Punch radius controls the transition between the straight wall of the stamped part and the bend or corner. It is not just a cosmetic detail. It directly affects material flow, stress concentration, and the likelihood of cracking during forming.

A small radius creates a sharp transition. This looks fine on a drawing, but the metal struggles to move without tearing. The result is a thickened corner and a high risk of a visible burr. A larger radius allows the material to flow more smoothly. The corner is less sharp, but the part is stronger and easier to stamp consistently.

The challenge is finding the balance. Engineers often get stuck between two extremes. They either choose a radius that is too small, hoping to get a sharper feature, or they choose a radius that is too large, losing the required part fit. This guide walks through a practical method for specifying punch design so the die matches the part requirement without guessing.

Step 1: Review the Part Print and Tolerances

Before touching any die design software, study the part print. Identify the critical features. Is the corner a structural feature? Does it mate with another part? Are there specific tolerance calls on the corner profile?

If the part print has a specific radius call, such as R1.5 or R2.0, the punch radius must match that specification. If the print is silent on the corner radius, you have more freedom. In that case, the radius becomes a tooling decision to manage processability and part quality.

Check the general tolerance block. If the part has a strict overall fit requirement, a larger punch radius might cause interference. The material may spring back into a shape that does not match the mating part.

Step 2: Determine Material Thickness and Hardness

The stock thickness is the primary driver for punch radius selection. The material hardness is the secondary driver. Soft, annealed materials are forgiving. Hard, work-hardened materials are not.

A common starting point for a punch radius is 5 to 10 percent of the stock thickness. This range works for many standard stamped parts. For example, on a 1.5 millimeter sheet, a radius between 0.075 and 0.15 millimeters is a reasonable starting range.

Hardness changes the equation. If the material is drawn or work-hardened, the metal is less able to stretch. It resists the bending force. A larger radius reduces the strain in the corner. It helps the material flow without cracking. If the material is very soft, like annealed copper, a smaller radius may be acceptable because the material flows easily.

Step 3: Evaluate the Part Geometry

Not all corners are the same. A 90-degree corner behaves differently from a 45-degree corner. A concave corner behaves differently from a convex one.

Concave corners, where the material is bent inward, put the outer surface of the bend in tension. This is the side where cracking usually starts. The punch radius needs to be generous enough to keep the tension within the material limit.

Convex corners, where the material is bent outward, put the inner surface in tension. The punch radius is less critical here, but it still matters for appearance. A sharp convex corner can catch on a finger or scratch the part during handling. A small radius smooths this out.

If the part has a complex 3D shape, the corner radius must be consistent with the adjacent surfaces. A mismatch creates a visual step or a stress riser. The die designer must ensure the punch profile blends smoothly into the surrounding flat or curved surfaces.

Step 4: Select the Punch Radius Using the Table

Use the table below as a reference for standard materials. It provides a baseline radius range based on stock thickness. Adjust upward for harder materials or complex geometries. Adjust downward if the part fit is extremely tight and the material is soft.

Stock Thickness	Suggested Punch Radius	Notes
0.5 mm	0.05 - 0.10 mm	Thin gauge, high springback risk
1.0 mm	0.10 - 0.20 mm	Common sheet metal range
2.0 mm	0.20 - 0.40 mm	Heavier gauges need more flow
3.0 mm	0.30 - 0.60 mm	High stress areas, check cracking

This table is a starting point. It is not a rule. The final radius depends on the specific interaction between the punch, the die, and the material. If you are using a high-speed press, the metal has less time to flow. You may need a larger radius to reduce the forming force and prevent cracking.

Step 5: Consider the Die Design and Tooling

The punch radius does not exist in a vacuum. It must work with the rest of the die design. The die surface must have a corresponding radius or a sharp edge that can accommodate the punch.

If the die surface is flat and the punch has a large radius, the metal will not fill the corner completely. You will get a gap or a thin wall. This is a common cause of part quality issues. The die must be designed to match the punch profile.

For progressive dies, the radius must be consistent across the tool. If one station has a different radius than the next, the part will have an uneven profile. This looks bad and often fails the fit check.

The tooling material also matters. A hardened punch wears differently than a softer one. A worn punch will have a larger radius than the original. This changes the part profile over time. You must consider the expected life of the tool when specifying the initial radius.

Step 6: Check for Springback and Fit

Springback is the material trying to return to its original shape after the punch is removed. The punch radius affects this. A larger radius creates a more gradual bend. The springback is usually more predictable. A small radius creates a sharp bend. The springback can be erratic.

If the part must fit into a housing, you need to measure the springback on the first article. Do not rely on the drawing alone. The drawing shows the theoretical shape. The real part has a real shape.

If the springback moves the corner away from the required position, you may need to increase the punch radius. This changes the bend angle slightly and helps the part seat correctly. If the springback moves the corner too far, you may need to reduce the radius. This is a delicate balance.

Step 7: Run a Test Article and Verify

The final step is always a physical test. Machine the die, stamp a few parts, and measure them. Do not just look at them. Use a micrometer or a profile gauge to check the corner.

Check for burrs. Run a finger over the corner. If you feel a sharp edge, the radius is too small. If the corner feels rounded and smooth, the radius is likely in the right range.

Check for cracks. Look at the outer surface of the bend. If you see a thin line or a rough spot, the material cracked. You need a larger radius.

Check the fit. Put the part into its mating part. If it does not seat correctly, the springback is the issue. You may need to adjust the punch radius or the die profile.

Common Mistakes in Punch Design

Engineers make a few consistent mistakes when specifying punch design.

The first is ignoring the material hardness. They use the same radius for soft and hard material. This leads to cracks in the hard material and a sloppy fit in the soft material.

The second is assuming the punch radius is the only factor. They ignore the die surface and the press speed. A large punch radius on a flat die surface will not form the part correctly.

The third is not checking the first article. They release the tool based on the drawing, not the physical part. This is how scrap happens.

Final Verification Step

Before releasing the die, perform a final verification. Compare the measured punch radius to the drawing. Compare the part corner to the required fit. Check the burr on the first three parts. If all three checks pass, the punch design is correct.

If any check fails, do not release the tool. Adjust the radius and re-test. It takes time, but it prevents the cost of rework. A small change in the punch radius can save a large amount of money in production.

The goal is a part that stamps cleanly, fits correctly, and lasts. The punch radius is a small feature that has a large impact on that outcome. Use the method above to specify it with confidence.