Choose the joint by material stack and tool access, not by habit. Clinching forms a mechanical lock with no consumable, but it needs a die on the far side and ductile sheet. Self-pierce riveting drives a rivet through the top sheet into the lower one without a pre-drilled hole, so it handles aluminium-to-steel stacks that spot welding dislikes. Resistance spot welding is usually the cheapest joint on plain steel, provided both electrodes can reach the spot.
The four processes side by side
| Process | Consumable | Access | Materials | Best fit | Main limit |
|---|---|---|---|---|---|
| Clinching | None | Punch above, die below | Ductile sheet, coated steel, aluminium | Cost-sensitive coated or dissimilar stacks where a visible indentation is acceptable | Needs die access; limited on high-strength low-ductility steel |
| Self-pierce riveting | One rivet per joint | Punch above, die below | Mixed stacks including aluminium and steel | Mixed-material bodies and fatigue-critical joints | Rivet supply and die maintenance; needs C-frame access |
| Resistance spot welding | Electrodes (wear) | Both electrodes | Plain and coated steels | High-volume steel bodies with familiar quality routines | Aluminium needs high current and electrode maintenance; no access means no weld |
| Blind riveting | One rivet per joint | One side only | Most sheet, thin stacks | Enclosures and access-limited panels at low volume | Lower strength; hole required; head visible |
What each process actually does
Clinching
Clinching presses the sheets between a punch and a die so the metal flows into a button. Nothing is added and nothing is melted. The joint is a mechanical interlock whose strength depends on the button geometry, the sheet thickness and the ductility of the stack. Because there is no heat, coating damage is small and dissimilar metals can be joined without forming a brittle intermetallic layer at a weld nugget.
The limits follow from the mechanism. Low-ductility high-strength steels crack instead of flowing, so the working window narrows or closes. The die leaves an indentation on one face, which matters on visible panels. And the tooling must reach both sides with enough stiffness to take the squeeze force.
Self-pierce riveting
A semi-tubular rivet is driven through the top sheet and flares into the lower sheet, which is held by a die. There is no pre-drilled hole. The rivet supplies the strength, so the process tolerates stacks that are awkward to weld, including aluminium on steel. Automotive bodies use it for exactly that reason.
The recurring cost is the rivet, and the logistics are real: rivet feed, selection by stack, die wear and a C-frame that can reach the joint. Setting force is high, so the fixture and the robot’s structure have to absorb it without movement between the sheets.
Resistance spot welding
Two electrodes clamp the stack and pass current until a nugget forms. It is fast, well understood and cheap per joint on steel, and the quality routine is familiar to every automotive supplier. The constraints are electrical and physical: the material must weld predictably, and both electrodes must reach the spot with the right force and alignment.
Aluminium changes the economics. It conducts heat away quickly and needs higher current, which shortens electrode life and pushes maintenance up. That is usually where a mixed-material design moves to self-pierce riveting or clinching instead. The spot welding cell guide covers guns, schedules and quality checks in more detail.
The four questions that decide it
- What is the stack? Material, thickness and coating decide which processes are physically possible. Mixed aluminium and steel points away from spot welding. Thick or low-ductility steel points away from clinching.
- Can the tool reach both faces? Clinching and self-pierce riveting need a die underneath. Spot welding needs a second electrode. Blind riveting is the only option when one face is closed.
- Is a rivet per joint acceptable? Consumable cost and feed logistics are the price of SPR. Clinching avoids them but gives up some strength and leaves an indentation.
- What must the joint survive? Static peel, cross-tension, fatigue, corrosion and appearance are different tests. A joint that passes a peel test can still fail in service if the loading is cyclic.
Where the robot changes the calculation
All three processes are repeatable when the robot presents the part consistently. What changes is the mass and reaction the robot carries. A spot gun with transformer and cables, a hydraulic clinch unit or a heavy C-frame all add payload and inertia, and the robot has to hold position against the setting force while the tool works. Fixture stiffness and part fit-up matter as much as robot repeatability.
Cycle time per joint is approach, clamp, join, release and retract. The joining step itself is often the shortest part. Reducing air-cut distance, grouping joints by tool and keeping the rivet or electrode change out of the cycle usually buys more than a faster process. The cycle time worked example shows how to add the steps honestly.
Cost drivers, not price lists
Price the whole cell, not the joint. Consumables separate the processes: electrodes wear, rivets are consumed one per joint, clinch tooling wears slowly with no added material. Tooling separates them again: a spot gun, a clinch unit and a rivet C-frame are different purchases with different maintenance. Then add rework and scrap, because a joint that fails inspection is more expensive than the consumable it saved.
For welded work, the weld cost per metre method turns geometry, consumables and productive time into a number you can defend. For mechanical joints, count the rivet, the die life and the inspection time per joint in the same way. The robotic assembly cost guide covers the cell-level budget.
Acceptance tests that catch the real failures
- Clinching: button diameter and height, neck thickness, and a peel or shear test on production coupons.
- Self-pierce riveting: rivet head flushness, interlock and remaining bottom-sheet thickness, checked by macro section at the start of each shift or stack change.
- Spot welding: nugget diameter from a destructive sample, weld schedule monitoring and electrode tip condition.
- All processes: fatigue and corrosion tests on the actual stack, because a static test does not predict a joint that sees vibration or salt.
What to put in the integrator brief
Give the integrator the stack drawing, every joint location, the access available above and below, the target cycle time, the acceptance tests and the data you want logged. Add the safety case: the robot safety standards guide explains the ISO 10218 risk assessment, and a C-frame or gun introduces pinch points that a bare arm does not. The integration checklist lists the phases from specification to handover.
Frequently asked questions
Can clinching join aluminium to steel?
In many stacks yes, because it forms a mechanical interlock without melting. Ductility, coating and die design decide the working window, and the joint needs corrosion protection. For structural aluminium-steel joints, self-pierce riveting is the more common choice.
Does clinching use consumables?
No fastener and no filler metal. The tooling wears, but nothing is added per joint, which is its main cost advantage over self-pierce riveting and spot welding.
Can a robot install blind rivets?
Yes, with a one-sided rivet tool and mandrel collection. Strength is lower than self-pierce riveting or spot welding, so blind rivets suit enclosures and access-limited panels rather than structural joints.
Which joint is cheapest?
On plain steel at high volume, spot welding usually has the lowest consumable cost. Clinching avoids consumables but needs tooling and two-sided access. Self-pierce riveting adds a rivet per joint but avoids welding aluminium. Compare the whole cell, not the joint.
Sources and further reading
- TWI: comparing clinching and self-piercing rivets with spot welding — materials and consumables comparison.
- Huang et al., Chinese Journal of Mechanical Engineering — joining-strength comparison of self-piercing riveting, clinching and resistance spot welding on Q195 steel and 5A05-O aluminium.
- Robot spot welding in India — guns, cost and quality.
- Weld cost per metre — the estimation method.
- Robotic assembly in India — tolerance, cost and payback.
Battery packs are the other place these joints decide the outcome. EV battery pack assembly automation covers tab-to-busbar joining on the pack line.
