The Press That Bent the Part: Force Monitoring, Not Just Limit Switches
A pneumatic press assembling a plastic part. The press applied 500 N for 2 seconds. Some parts came out bent. The press’s limit switch said it reached the bottom. The force monitor wasn’t there. The press was hitting harder on some parts than others. This is about force monitoring on press operations, not about the press being defective.
What happened
The press used a limit switch to detect the bottom position. It extended until the switch activated, then retracted. But if the part was misloaded (1 mm off-center), the press still reached the switch — it just pushed the part sideways instead of down. The part bent. The switch didn’t know the difference between a correctly seated part and a misloaded one. It only knew “bottom reached.”
The force monitoring
A force sensor (load cell) on the press measures the force over the stroke. A correct assembly has a known force signature: ramp up, hold at target, ramp down. A misloaded part has a different signature: force spikes early (hitting the wrong feature), or never reaches target. The PLC compares the measured force to the expected window. If it’s outside, the press stops and alarms.
| Signatures | What it means |
|---|---|
| Force ramps to 500 N, holds, ramps down | Correct assembly |
| Force spikes at 200 N early, then drops | Part misloaded, press hitting wrong feature |
| Force never reaches 500 N | Part missing or wrong part |
| Force exceeds 700 N | Part jammed, press overloading |
What we did
1. Added a load cell. We mounted a load cell between the press ram and the tooling. The cell measures force to ±5 N. The PLC logs the force curve for every press cycle.
2. Set the force window. We trained the system with 10 good parts. The force curve was 480-520 N at the hold. We set the window: 450-550 N. If the measured force is outside, the press alarms and rejects the part.
3. Added a position window. The press must reach the bottom position within a tolerance (±0.5 mm). If the press stops at a different position (because the part was misloaded), the PLC catches it. The force and position together confirm a good assembly.
The data trail
Every press cycle logs the force curve to the PLC. If a part comes back from the field, we can look up its force curve and see if the assembly was within spec. This is traceability. The press that bent parts had no record. Now it does.
The press rule: A limit switch only says “bottom reached.” It doesn’t say “correct part, correct force, correct position.” Add a load cell and a force window. If the force is outside the expected range, reject the part. The press that bent parts wasn’t applying too much force — it was applying the right force to the wrong thing.
