The Rivet That Wasn’t Setting Properly
We built a pneumatic press for riveting: Ø4 mm semi-tubular rivets into aluminum brackets. The press delivered 800 N at 6 bar. On the bench, the rivets set fine. On the line, about 10% of the rivets were loose. The problem: we sized the press for the average riveting force, not the peak. The semi-tubular rivet needs a force curve — it starts easy, then peaks when the rivet buckles and spreads. The press’s force (800 N) was just barely enough for the peak. When the air pressure dipped (from another cylinder cycling), the press didn’t reach peak force, and the rivet didn’t set. We upsized the cylinder to deliver 1,500 N at 6 bar (and added a pressure regulator with a gauge). The rivets set every time. The mistake was not accounting for the force curve and pressure variation.
Pneumatic press and riveting process design isn’t just the average force. The riveting process has a force curve, and the press must handle the peak. This article covers the design.
The Riveting Force Curve
A semi-tubular riveting process has three phases:
- Initial contact: The rivet touches the hole. Low force (just alignment).
- Upset (buckling): The rivet tail buckles and spreads. This is the peak force. It happens at about 70% of the stroke.
- Forming: The rivet head forms over. Lower force, final shaping.
The peak force (during buckling) is 2–3× the average. If the press delivers only the average, it stalls at the peak. The rivet doesn’t fully spread.
Step 1: Calculate the Peak Riveting Force
For a semi-tubular rivet, the peak force depends on the rivet diameter and material. A rough estimate:
F_peak ≈ 300 × d² (for aluminum rivet into aluminum)
Where d is the rivet diameter in mm. For a Ø4 mm rivet: F_peak = 300 × 16 = 4,800 N. Wait, that seems high. Let me recalculate. For a Ø4 mm semi-tubular aluminum rivet, the actual peak force is about 800–1,200 N. The exact value depends on the rivet type (semi-tubular, blind, solid) and material. Use the rivet manufacturer’s data sheet — they specify the setting force.
Rule of thumb: for a Ø4 mm aluminum semi-tubular rivet, the peak is about 1,000 N. Add a safety factor (1.5×): F_required = 1,500 N. The press must deliver this at the operating pressure (with margin for pressure drop).
Step 2: Cylinder Sizing for Peak Force
The press cylinder delivers force based on bore and pressure (article 53):
F = P × π × d² / 4
At 6 bar (0.6 N/mm²):
- Ø50 mm bore: F = 0.6 × π × 2500/4 = 0.6 × 1963 = 1,178 N.
- Ø63 mm bore: F = 0.6 × π × 3969/4 = 0.6 × 3116 = 1,870 N.
- Ø80 mm bore: F = 0.6 × π × 6400/4 = 0.6 × 5026 = 3,016 N.
For F_required = 1,500 N (peak with margin), a Ø63 mm cylinder (1,870 N at 6 bar) handles it. The Ø50 mm (1,178 N) is too close to the peak — when the pressure dips, it stalls.
The press sizing rule: Size the cylinder for the peak riveting force (not the average), with margin. The rivet that wasn’t setting had a Ø50 mm cylinder (1,178 N) against a peak of 1,000 N — too close. Upsize to Ø63 mm (1,870 N). Add a regulator so the pressure stays at 6 bar regardless of other machines cycling.
Step 3: Stroke and Ram Design
The press stroke must be enough to reach the rivet and form it. The stroke is typically 20–50 mm (the rivet tail length plus clearance).
The ram (press tool) holds the rivet set (the forming tool). The set contacts the rivet tail. Use a floating set (self-aligning) so the set hits the rivet square, not at an angle.
Step 4: Pressure Regulator and Gauge
The press needs a dedicated regulator (not the main line pressure). If the main line drops (another cylinder cycles), the press force drops. A dedicated regulator with a gauge holds the pressure at 6 bar.
Add a pressure switch: if the pressure drops below 5.5 bar, the press won’t cycle (it faults). This prevents under-riveted parts.
Process Monitoring (Force-Stroke Curve)
For critical riveting, monitor the force-stroke curve. A load cell (or pressure sensor) measures the force as the ram extends. The curve should match the expected profile. If it’s too low (weak rivet) or too high (cracked rivet), the part is rejected.
This is “press fit monitoring” — standard for safety-critical joints (automotive). For general riveting, a pressure switch is enough.
| Application | Monitoring | Best For |
|---|---|---|
| General riveting | Pressure switch (minimum force) | Consumer products, light assembly |
| Critical joints | Force-stroke curve monitoring | Automotive, safety parts |
| High-volume | Force sensor + PLC logging | Data traceability |
Two-Hand Control (Safety)
A press that the operator loads manually needs two-hand control. The operator presses two buttons simultaneously (both hands on the buttons) to activate the press. This prevents the operator from having a hand under the ram.
The two-hand control is a safety circuit (article 64) — a safety relay monitors both buttons. If one is released, the press stops.
Rivet Feed (Automatic)
For high volume, the rivets are fed automatically (a vibratory bowl feeder, article 47). The pick-and-place (or the press itself) picks the rivet and places it in the hole. The operator only loads the part.
For low volume, the operator places the rivet manually. Two-hand control is required.
A Press Design Checklist
- What rivet? (Diameter, material, type.)
- What is the peak setting force? (From the rivet datasheet.)
- Safety factor (1.5×)?
- Cylinder bore sized for F_peak at operating pressure?
- Dedicated pressure regulator with gauge?
- Pressure switch (fault if low)?
- Stroke enough for the rivet tail?
- Floating ram set (self-aligning)?
- Two-hand control (for manual loading)?
- Force monitoring? (Pressure switch or force-stroke curve?)
- Rivet feed? (Manual or automatic bowl?)
- Is the press guarded? (Fence or light curtain?)
The Bottom Line
Pneumatic press and riveting design is the peak force, not the average. The rivets that weren’t setting had a cylinder sized for the average force — too close to the peak. Size for the peak (with 1.5× margin), add a dedicated regulator with a gauge, and use a pressure switch. For critical joints, monitor the force-stroke curve. The press that sets every rivet wasn’t the biggest one — it had enough margin for the peak force.
