The surface quality of a stamped metal component is determined by more than just the sheet material. The cutting edge geometry, burr height and roughness values are the direct result of tool selection, cutting clearance and process setting. In this article, we explain how roughness and finishing work in stamping, which Ra values are achievable and when fine blanking or post-processing is the better choice.

What roughness means in stamping

Roughness in stamped parts is about the quality of the cutting edge, not the surface of the sheet material itself. That cutting edge consists of four zones: the rollover zone at the top, the burnished zone, the fracture zone and the burr at the bottom. Each zone has its own surface structure and its own functional significance. When assessing pressed metal parts it is important to view these zones separately instead of assessing the entire cutting edge as a whole.

The shear zone is the smoothest part of the cutting edge. With good tooling and the correct cutting clearance, this zone accounts for forty to sixty percent of the material thickness. This zone has a low roughness, comparable to milled steel, and is functionally usable as a mating surface. The fracture zone is rougher and inclined in character. This zone is inevitable in conventional stamping, but the ratio relative to the shear zone can be controlled via tool selection and process setting.

The four factors that determine the level of finish

The quality of the cutting edge is the result of four interrelated factors. Those factors are controllable, but require knowledge of the process.

Cutting clearance is the first and most decisive factor. Too much clearance enlarges the fracture zone and results in more burr. Too little clearance increases the pressing force and accelerates tool wear. For cold-rolled steel, the optimal clearance is between five and ten percent of the material thickness. For aluminium and copper, this is slightly lower.

Tool quality subsequently determines how stable the cutting edge geometry remains throughout the production run. A worn punch produces a wider fracture zone and larger burrs. At Eurotechniek, the condition of the tools is monitored based on stroke count and periodic quality control on reference parts, so that the degree of finish remains consistent.

Material type also plays a direct role. Soft materials such as aluminium 1050 yield a larger fracture zone than hard materials such as DC01 cold-rolled steel. Stainless steel produces a remarkably smooth shear zone with correctly adjusted tooling, but places higher demands on cutting forces and tool maintenance.

Press speed and lubrication are the fourth factor. Higher press speeds cause heat generation at the cutting edge. That affects the surface structure of the shearing zone. Good lubrication reduces friction and improves the quality of the shearing zone, especially in thin materials.

What the Ra value tells you about blanked edges

Surface roughness is expressed in Ra, the average roughness value. In conventionally blanked cutting edges, the Ra of the shear zone typically lies between 0.8 and 3.2 micrometres. The fracture zone is rougher and often exceeds 6.3 micrometres. These two zones lie next to each other on the same cutting edge, which means that a single Ra value for the entire cutting edge tells you very little.

An example: for a 2 mm DC01 mounting plate with a round through-hole, we set the cutting clearance to 0.18 mm per side. The resulting shear zone covers 55 percent of the material thickness with an Ra of approximately 1.6 micrometres. That is sufficient for a pivoting fit, but not for a hydraulically tight surface. If your application requires a lower Ra, then finishing or fineblanking is the way forward.

Fine blanking as an alternative for higher quality requirements

Fine blanking is a process variation in which the shear zone runs across virtually the entire material thickness. This is achieved by using a toothed V-ring impingement around the contour, combined with a counterforce beneath the material. The result is a cut edge with an Ra of 0.4 to 1.6 micrometres over the full height, virtually square and without a fracture zone.

Fine blanking is used for components that have higher functional requirements for the cutting edge. Think of gear teeth for transmissions, locating holes for bearings and hydraulic valves. Tooling costs are higher than in conventional blanking, and the presses require triple-action force. At Eurotechniek, we advise customers on the tipping point between conventional blanking with post-processing and fine blanking, based on batch size and quality requirements.

An automotive customer supplied a painted guide bracket for years via milling. Following analysis, fineblanking with an Ra of 0.8 micrometres on the fitting holes proved to meet the assembly specification. The cost price per piece fell by 35 percent with a batch size of 25,000 pieces per year.

Braamhoogte and what it means in practice

Burrs are an inevitable by-product of blanking. The height of the burr varies depending on material thickness, cutting clearance and tool condition. With well-maintained tooling and correct process settings, the burr height for cold-rolled steel from 1 to 3 mm in thickness is below 0.1 mm. That is functionally acceptable for most applications.

Burrs on the wrong side of a part cause problems during assembly or at contact surfaces. Stamping always produces a burr on the side where the punch pushes through. If the functional side needs to be burr-free, you orient the tool so that the burr is on the non-functional side. That is a design measure that costs nothing and prevents a lot of trouble.

Is burr-free a strict requirement? Then there are three options: burr-free punching with special tool technology, mechanical deburring via rumbling or brush deburring, or fine blanking. Eurotechniek offers all three and advises based on the specific functional requirement and economic feasibility per batch size.

Post-processing and surface treatment after stamping

Pressed parts are in many cases post-processed or provided with a coating. This post-processing influences the final surface quality of the part as a whole. Galvanising, phosphating, powder coating and anodising are common treatments that each have their own effect on dimensions and roughness.

Zinc plating adds a layer of five to fifteen micrometres to all surfaces, including the cutting edges. That is relevant if cutting edges act as fitting surfaces. Anodising on aluminium yields layers of five to twenty micrometres and affects the dimensions of holes. At Eurotechniek, we take the planned post-treatment into account in the tool compensation, so that the final dimension after coating falls within the specified tolerance.

An example: an aluminium front panel with a material thickness of 3 mm is given a hard anodised layer of 25 micrometres. The holes are punched 0.05 mm larger in the tool than the final dimension on the drawing, in order to compensate for the layer thickness. Without that correction, the screw will no longer fit through the hole.

Frequently asked questions about roughness and finishing in die cutting

What Ra value can you expect on the cutting edge of a conventionally blanked component?

The shear zone of a conventional blanked part has a Ra of approximately 0.8 to 3.2 micrometres, depending on the material type, cutting clearance and tool condition. The fracture zone has a higher roughness and is not suitable as a functional surface without post-processing. If you need a Ra below 1.6 micrometres over the entire cutting edge height, fine blanking or post-processing is the preferred choice.

Can the burr height be specified on a drawing?

Yes. Burr height is a measurable dimension and can be included in a drawing or quality plan. A common limit for cold-rolled steel with a thickness of 1 to 3 mm is a maximum burr height of 0.1 mm. Stricter requirements can be achieved using modified tooling or deburring as a secondary operation. Please indicate the functional side on the drawing, and we will orient the tooling accordingly.

Does the choice of material affect the finish quality of the cutting edge?

Yes, and that influence is significant. Aluminium and mild steel types result in a larger fracture zone and more burr than harder steel types with the same cutting clearance. Stainless steel yields a remarkably smooth shear zone with correctly adjusted tooling, but requires higher cutting forces and more frequent tool inspection. The material choice partly determines which process window is achievable for the desired degree of finish.

The standard of finish begins with the design

The quality of a blanked cutting edge is no accident and no afterthought. It is the result of deliberate choices in material, tooling strategy, process parameters and any subsequent finishing. Those who make these choices early in the design process avoid extra costs and disappointments during the first series production.

Eurotechniek helps think things through right from the drawing stage. Get in touch and submit your specifications.