The quality of edges and corners in stamped metal parts is no accident, but the direct result of tool selection, cutting clearance and design. An excessive burr disrupts assembly, a too-sharp internal corner accelerates tool wear, and an unspecified edge leads to disputes during quality control. In this article, we explain how the stamping process forms edges, when post-processing is necessary and how to correctly specify edge quality on the drawing.
What the stamping process does to an edge
By the metal stamping a punch cuts through a metal strip. That cutting process takes place in two phases. First, the punch penetrates the material and deforms it plastically. Then the material breaks along the cutting line.
The result is a cut edge with four zones: a rollover zone at the top, a burnish zone in the middle, a fracture zone below that and a burr at the bottom. Each zone has characteristics that depend on the material, the tool condition and the stamping process.
The burr is inevitable. Its size depends on the clearance between punch and die, the material hardness and the wear condition of the tool. With correctly set cutting clearance and a sharp tool, the burr is minimal. With a worn punch or too large a cutting clearance, the burr becomes larger and less predictable.
A practical example: when blanking a 2 mm thick galvanised steel mounting strip, a tool with too much clearance produced a 0.3 mm burr. That sounds small, but it was enough to disrupt the automated assembly. After adjusting the cutting clearance to the correct value for this material, the burr height dropped to less than 0.05 mm.
When a sharp edge is acceptable
Not every sharp edge is a problem. In many structural applications, a cutting edge is functional without additional processing. A mounting plate clamped into a housing, an earthing strip making contact via the cutting face, a support profile that is never touched directly: in all these cases, the edge as it comes off the press is sufficient.
The question is always: what does the edge do in the application? An edge that comes into contact with a seal, a cable or a human hand requires a different assessment than an edge that is built-in and never reached. Together with the customer, at Eurotechniek we draw up the programme of requirements for every edge. That prevents unnecessary finishing operations and associated costs.
The ISO 13715 standard describes how edges on technical drawings are specified. Anyone who does not specify an edge leaves room for interpretation. This leads to discussions during quality control or supplier audits. Clear edge designation on the drawing is not a luxury, but a requirement for reliable manufacturing.
Sharp corners in the design
Internal corners in stamped parts are a specific point of attention. Stamping a sharp internal corner concentrates stress at a single point in the tool and in the material. That increases the risk of cracking in the workpiece and of premature wear on the punch edge.
The rule of thumb in the manufacturing industry is that an inside radius smaller than the material thickness is risky. This certainly applies to hard or brittle materials such as spring steel or austenitic stainless steel. With softer materials, such as low-carbon steel or copper, more is possible, but always within limits that depend on the specific alloy and thickness.
When designing a chassis component in 1.5 mm stainless steel, the customer requested a sharp internal corner with a 0.1 mm radius. After consultation, it appeared that the functional requirement could tolerate a radius of 0.4 mm. That adjustment demonstrably extended tool life and prevented microscopic cracks in the corner that would only have become visible later.
Specifying edge quality tolerances
An edge specification without a tolerance is incomplete. The common metrics for edge quality are burr height, width of the shear zone, squareness of the cut surface and surface condition of the fracture zone. These properties are measurable and recordable in a quality plan.
Burr height is specified most frequently. Usual requirements in the industry are between 0.05 mm and 0.15 mm for materials up to 3 mm thick. For thin foil or precision parts for the electronics industry, stricter requirements apply, sometimes less than 0.02 mm. These requirements call for special tool steel, precise grinding of the cutting edges and shorter maintenance intervals.
The shear zone width is relevant for parts where the smooth zone needs to function as a sliding surface or sealing surface. A shear zone of at least sixty percent of the material thickness is achievable with fine blanking technology. Standard blanking does not achieve that percentage, but yields a completely acceptable result for most applications.
Edge finishing: when necessary and when not
Deburring costs money and time. It is useful if the application requires it, but not as standard for every component. At Eurotechniek, we assess per project whether deburring or chamfering is necessary, based on function and safety requirements.
Deburring can be mechanical, chemical or electrochemical. Barrel tumbling is the most widely used method for large batches. It provides a consistent, slightly rounded edge without individual assessment per part. The disadvantage is that it also reduces the sharpness of functional edges that need to remain sharp, such as cutting edges or guide edges.
Laser deburring or electrochemical deburring offers more control over which edges are treated. That is relevant for complex components with both functional and non-functional edges. The choice of method forms part of the production schedule that we establish together with the customer before the first batch starts.
Safety aspects of untreated edges
Edges on stamped metal parts are a potential safety hazard. This applies both to the person handling the part and to surrounding materials in the end application. Regulations vary by sector. In consumer electronics, stricter requirements apply than in industrial mechanical engineering.
The European Machinery Directive and standards such as ISO 12100 require a risk assessment for parts with sharp edges that are accessible to operators or users. This assessment determines whether an edge is acceptable, needs to be rounded off, or must be shielded. Eurotechniek assists customers in compiling the necessary documentation for these types of processes.
A concrete case: a component for a consumer appliance originally had an unspecified edge on the outer contour. During product validation, drop testing revealed that the edge could catch on the housing and pose a safety risk. By incorporating a 0.3 mm radius into the mould, the problem was structurally resolved without additional post-processing.
Frequently asked questions about edges and corners in stamped parts
How small can an inside radius be in stamped steel?
The minimum inner radius depends on the material thickness and the material grade. As a rule of thumb, an inner radius equal to the material thickness is achievable without an increased risk of cracking. For hard materials such as spring steel, we recommend a radius of at least 1.5 times the material thickness. Smaller radii are sometimes possible but require validation via test pieces and noticeably increase tool wear.
Is a burr always unacceptable?
No. A burr is a normal consequence of the stamping process and is completely acceptable in many applications. The assessment depends on the function of the component, the assembly method and the safety requirements. What is always necessary, however, is a clear specification on the drawing so that the manufacturer and customer have the same expectation. An unspecified edge leads to discussion; a specified edge is verifiable.
What is the difference between standard blanking and fine blanking for edge quality?
With standard blanking, the shear zone is typically thirty to fifty percent of the material thickness. The rest of the cut edge is a rougher fracture zone. In fine blanking, the material is held under high pressure while it is being cut. This yields a shear zone of ninety percent or more of the material thickness, a virtually perpendicular cut edge and minimal burr. Fine blanking is more expensive in terms of tooling and cycle time, but delivers an edge quality that in many cases makes post-processing unnecessary.
Edge quality starts with the design
Thinking about edges and corners during the design stage prevents problems in production and with the end user. The choice of radius, the specification of the burr height and the decision on post-processing are all part of a well-thought-out design. These choices determine tool life, production stability and the safety of the final product.
Contact Eurotechniek for a technical assessment of your design or for information about our range of precision stamped components.