Contact rivet clinching and staking: process, defects and inspection

What staking does

Staking pushes a punch into the end of the rivet shank and deforms the metal outward. The shank fills the hole in the carrier and forms a foot on the far side. The result is a mechanical lock that holds the contact in place and carries current from the shank into the blade.

The process is a cold forming operation. The metal flows rather than being cut, so the grain structure follows the deformation and the joint can be strong. If the metal is asked to flow too far in one stroke, it cracks. If it is not asked to flow far enough, the hole stays partly empty and the joint is loose.

Clinching and staking are often used interchangeably. In practice, clinching usually describes a joint formed by local deformation without a separate rivet, while staking describes forming the rivet shank. The physics is the same: displace metal, fill the hole, create a mechanical interlock.

The variables that decide the joint

The hole diameter relative to the shank is the first variable. Too tight and the shank cannot enter freely, so it buckles or tears the blade. Too loose and the rivet sits off centre, the shank cannot fill the gap, and the joint is loose from the start. A clearance of roughly 0.05 to 0.10 mm on diameter is a common starting point for a cold-formed rivet, but the right value depends on the material and the clinch form.

The shank protrusion above the blade is the second. It has to supply enough material to fill the hole and form the foot. A short protrusion gives a thin, weak foot. A long protrusion gives a tall foot that may crack or fold. A starting range of one to one and a half shank diameters is illustrative, not universal.

The punch form and the anvil support are the third. The punch decides where the metal goes. The anvil supports the head and the blade so the force forms the shank rather than bending the assembly. If the anvil does not support the blade around the hole, the blade bows and the joint loosens when the press force is released.

Clinch forms and where they fit

A roll clinch curls the shank outward into a ring. It gives a large bearing area and good pull-out strength, and it is common on thin blades. It needs enough protrusion to roll without cracking, and the roll must not interfere with the contact face or the mating part.

A star clinch splits the end into petals and spreads them. It is fast and tolerates a softer shank, but the splits are stress raisers and can produce loose fragments. It is less suitable for a joint that will see vibration or a lot of thermal cycling.

A radial or spread clinch pushes the shank outward into a controlled bulge that fills the hole. It gives good hole fill and a clean appearance, but it can distort a thin blade. A flat stake simply upsets the end into a flat foot; it is simple and cheap, with a smaller bearing area and lower pull-out strength.

Tooling and press setup

The punch should be concentric with the anvil and the hole. A small offset tilts the rivet and produces an uneven foot. Check the alignment with a setting gauge or a test piece before running production, and recheck after a tool change.

The anvil needs a relief around the hole so the deformed shank can form a foot without being crushed against a flat face. The relief depth and diameter set the foot shape. A worn anvil changes the foot geometry even when the punch is new, so the anvil belongs in the tool wear check.

Press speed matters. A fast stroke can shock the material and crack it, especially in a hard shank. A slower stroke gives the metal time to flow. For a brittle or work-hardened shank, an orbital forming head can produce the same shape with less impact and a lower crack risk.

Defects and what causes them

An under-clinched joint is loose and has a high resistance. It usually comes from a short shank, an oversize hole or insufficient press force. An over-clinched joint has a cracked head or a bowed blade, and it usually comes from too much force or a punch that is too large for the head.

An off-centre rivet comes from a misaligned punch, a hole that is not concentric or a part that moves during staking. A buckled shank comes from a tight hole, a long protrusion or a fast stroke. A cracked head comes from a hard shank, a sharp punch edge or a head that is too thin for the force.

Plating damage is easy to miss. A cracked or flaked plating layer exposes the base metal and starts a corrosion cell. If the part is silver plated, a flake at the contact face changes the resistance and can cause a hot spot. Inspect the head and the foot, not just the shank.

• Loose joint: short shank, large hole or low force. Check pull strength and staked height.
• Cracked head: hard shank, sharp punch edge or high force. Check visually and in section.
• Bowed blade: no anvil support or too much force. Check flatness after staking.
• Off-centre rivet: punch misalignment or a hole that is not concentric. Check position and runout.
• Buckled shank: tight hole, long protrusion or a fast stroke. Check the section.
• Plating flake: excess deformation or poor adhesion. Check the surface and the adhesion test.

Inspection that catches the hidden failure

Start with the dimensions that the process controls: staked height, foot diameter and head height after forming. Measure them on a sample from the start, middle and end of each run. A trend in staked height is the earliest sign of tool wear or material variation.

Then test the joint. A pull-out or push-out test measures the force needed to separate the rivet from the blade. A torque test measures rotation resistance, which matters where the contact is a terminal or where the blade can turn. Set a minimum force from the application load, with a margin for vibration and thermal cycling.

Finally, cut a cross-section. Look for hole fill, a sound fold, no cracks and a continuous head-to-shank bond. A section is destructive, so use it on a sample per lot or after a tool change. It is the only check that shows whether the joint is solid inside.

Process control for production

Write the process parameters down: hole size, shank protrusion, punch and anvil part numbers, press stroke, staked height and foot diameter. Keep the settings with the tool, not in an operator's memory. A tool that is set by feel will drift when the operator changes.

Check the first part, then at a defined interval, then after any tool change or material batch change. Record the results on the same chart as the pull test, so a shift in height and a shift in strength can be seen together. If the two move in opposite directions, the cause is probably the material or the hole, not the press.

Review the anvil and punch at a set interval, not only when a defect appears. Wear changes the foot shape gradually, and the first sign is often a slow fall in pull force rather than a visible defect. A wear limit on the tool is cheaper than a recall.

Four common mistakes

The first is setting the press by tonnage. The second is inspecting only the top of the staked head, where a clean appearance can hide a hollow or cracked joint. The third is ignoring the anvil, which wears and changes the foot geometry. The fourth is accepting a pull test on a loose rivet rather than on the assembled blade, which misses the effect of hole fit and blade support.