Fine blanking: when a sheared edge needs to be smooth

What fine blanking is

Fine blanking is a shearing process that produces a smooth edge in a single press stroke. Conventional blanking starts a crack at the cutting edge and lets it propagate, which leaves a shear zone at the top, a fractured zone below it and a burr at the bottom. Fine blanking suppresses the crack and forces the material to shear through the full thickness.

The result is an edge that is mostly clean, with a narrow tear zone and a small burr. For many parts that edge can be used without milling, shaving or grinding. That is the economic case: the process replaces a blanking step plus a secondary edge operation with one stroke, provided the volume and the part shape suit it.

The four features that make it work

Fine blanking is not a conventional die run slowly. The tool has a specific set of features that hold the material under pressure while the punch shears it.

1. 01

   V-ring

   A raised ridge on the blanking punch or the die presses into the material and locks it, stopping the material from flowing inward and suppressing the crack.

2. 02

   Counterpunch

   A counterpunch on the opposite side applies a counterforce. It keeps the part flat and builds hydrostatic pressure in the shear zone.

3. 03

   Small clearance

   The punch-to-die clearance is roughly 0.5 per cent of thickness per side, compared with 5 to 10 per cent for conventional blanking.

4. 04

   Triple-action press

   The press provides the blanking force, the V-ring force and the counterforce independently. A standard single-action press cannot run the tool.

What the edge actually looks like

A fine blanked edge has three zones. The top is a narrow rollover where the punch enters. The middle is a clean shear zone with a low roughness, often around Ra 1 micrometre or better depending on the material and the tool. The bottom is a small tear zone and a burr. The clean shear zone is typically about 90 per cent of the thickness, but the exact share depends on the material, the clearance, the V-ring and the counterforce.

The edge is not the same as a machined edge. There is a slight draft and a direction. If the part has to seal, slide or mesh, the drawing should state which face is the functional one and which direction the punch enters. That lets the toolmaker put the clean zone where it is needed.

Tool and press requirements

A fine blanking tool is heavier than a conventional blanking tool. The V-ring and the counterpunch add force, and the small clearance means the punch and die must be guided very accurately. The die set needs strong guides, a rigid frame and a way to adjust the counterforce. Tool steel grades and coatings are chosen for wear, because the small clearance is less forgiving of a worn punch.

The press must be a triple-action type or a hydraulic press with a separate counterforce. Tonnage is higher than conventional blanking for the same part because the V-ring and the counterforce add load. That limits the part size and the material thickness on a given press. A supplier should be able to state the press tonnage, the counterforce and the V-ring force used for the part.

Tool maintenance is more demanding than for a conventional die. The small clearance leaves little room for wear, and a punch that has been reground may need the die reworked to keep the clearance. The V-ring also wears and changes the hold-down pattern. Ask for the regrind allowance, the clearance setting and the wear limit in the tool handover, so the edge quality does not drift between services.

Materials and part limits

Fine blanking needs a material that can shear cleanly. Low and medium carbon steels in the annealed or spheroidised condition are the standard choice. Aluminium and copper alloys can be fine blanked, and some stainless grades work with the right tool and settings. High-strength and work-hardened materials are harder because they resist the shear and wear the tool.

Thickness typically runs from about 0.5 mm to 6 mm, with some presses going thicker. The part must be flat, because the process does not form a deep shape in the same stroke. Holes and cut-outs are possible, and the small clearance allows holes that are small relative to the thickness, but the punch still needs enough strength to survive. A very narrow web between holes is a tool life risk, not just a design risk.

When the economics work

Fine blanking has a higher tool cost and a slower stroke than conventional blanking, so it needs volume to pay back. The saving comes from removing a secondary operation. If a conventional blank needs a shave, a mill or a deburr, the fine blanking route can be cheaper even at a moderate volume, because it eliminates a handling step, a second tool and the risk of a rework batch.

The comparison should be made on the finished part, not the blank. Add the cost of the secondary operation, the inspection of the edge, the scrap and the extra handling. Then compare that with the fine blanking tool and piece price at the same annual volume. For a part with a functional edge, the gap is often smaller than the tool quote suggests.

Volume still matters. A fine blanking tool is more expensive to build and slower to run, so a very low-volume part may not justify it even with the secondary saving. The break-even is usually earlier than for a progressive die because the process replaces a whole operation, but it is not automatic. Price the complete route for both options, including the inspection of the edge on the conventional part.

Design rules for a fine blanked part

Design the part as a flat profile with a clean edge. Keep the outline simple, avoid sharp internal corners and give every corner a radius. A sharp corner concentrates stress in the die and produces a weak point in the part. Holes should be round where possible, because a round punch is stronger and easier to maintain than a shaped one.

Edge distance and web width matter more than in conventional blanking because the material is held under pressure. A narrow web can collapse or tear. As a starting point, keep the web between holes and the edge at or above the material thickness, and ask for a review below that. The V-ring also needs space on the scrap side, so the strip layout has to be designed with the process in mind.

• Keep the part flat, with no deep forms in the same stroke
• Use radii at every internal corner
• Keep holes round and webs at or above one thickness where possible
• State which face is functional and which way the punch enters
• Allow space for the V-ring on the scrap side of the strip
• Call out the edge zone and burr height that the function allows

Inspecting a fine blanked edge

The inspection plan should measure the edge, not just the outline. Record the clean shear share, the burr height and the roughness on the functional face. Check the flatness, because the counterforce and the V-ring can leave a slight bow if the tool is not set correctly. Check the hole size and position against the datum scheme.

On the first article, ask for a cross-section through the edge. A photograph or a micrograph shows the rollover, the shear zone and the tear zone, which is more useful than a single burr height number. If the part is plated after blanking, check that the plating covers the shear zone and does not build up on the burr.