What is a servo press?

A servo press uses servo motors for precise force and position control during forming operations.

How do I calculate servo press tonnage?

Use our free tonnage calculator at servopress.club/tonnage-calculator.html

Reading a tonnage monitor signature - what to look for

larry_presses

Trying to learn how to read our tonnage monitor signatures. What patterns indicate problems vs normal operation?

**What the Tonnage Monitor Measures
**
The tonnage monitor measures the strain on the press frame or slide using strain gauges. It displays force as a function of slide position (or time), creating a "tonnage signature" for each stroke.

**Reading a Normal Tonnage Signature
**
A typical blanking operation signature:
Approach phase: (TDC to material contact): near-zero tonnage
Elastic compression: (material contact to punch breakthrough): rapid tonnage rise
Breakthrough — sharp tonnage drop (the "snap-through")
Return — tonnage returns to zero

A typical drawing operation signature:
Blank holder contact — initial tonnage rise
Drawing phase — gradual tonnage increase as material flows
BDC — peak tonnage
Return — gradual decrease

**Diagnosing Problems from the Signature
**
**Signature shifted earlier in stroke:**
- Die is closing too early —check slide height setting
- Material thicker than nominal

**Signature shifted later in stroke:**
- Die is not closing fully —check slide height
- Material thinner than nominal

**Peak tonnage increasing over time:**
- Die wear (punch becoming dull) —check punch edge condition
- Material hardness increasing —check material certification

**Double peak in signature:**
- Two-stage forming operation —normal for some dies
- Die misalignment causing uneven loading —check die alignment

**Setting Tonnage Limits**

Set upper limit at 110% of normal peak tonnage. Set lower limit at 80% of normal peak. Any stroke outside these limits should trigger an alarm and stop the press.

lisaQ_metrology

Tonnage signature trending is one of the most underused quality tools in stamping. We overlay the current signature against the golden sample from the first article approval. Any deviation greater than 5% triggers an investigation. This approach has caught 3 die failures in the early stages before they produced scrap. The data is already there in the press controller. You just need to use it.

toolndye_dave

We overlay tonnage signatures from the first article run with current production as our primary SPC tool. When the current signature deviates more than 5% from the reference, we investigate. This catches die wear, material variation, and setup drift before they produce bad parts. Simple but incredibly effective.

mchen_servo

The thing most people miss when reading tonnage signatures: you need to look at the shape, not just the peak value. Two strokes can hit the same peak tonnage but have completely different signatures ? one might have a clean single peak (normal blanking) and the other might have a double peak (die hitting, then breaking through, then hitting again on a secondary feature). The double peak is normal for progressive dies but if it shows up on a single-station die, something is wrong.

Practical tip: set up four monitoring channels if your monitor supports it. Two on the frame (left and right uprights) and two on the slide (front and back). Compare left vs right ? if they diverge more than 8-10%, your load is off-center and you are wearing the gibs unevenly. Compare front vs back for the same reason in the other axis.

For trending: we export CSV data every shift and run a simple Python script that flags any stroke where peak force deviates more than 3 sigma from the running average of the last 500 strokes. Catches slow die wear about 2000 strokes before it becomes visible in part quality. The script is maybe 40 lines ? happy to share if anyone wants it.

pressroom_tom

Practical tip for reading signatures that nobody taught me in school: export the raw data and plot the first derivative (rate of force change). The signature itself shows you what happened, but the derivative shows you what's about to happen.

Specific patterns I've learned to recognize over the years:

- Gradual slope increase before peak → die cutting edges dulling. The press has to work harder to initiate the cut. When the slope drops below 80% of the golden sample, you've got maybe 10-15K hits before you need to sharpen.

- Spike just before main peak → stripper hitting material before the punch. Check stripper spring preload or die shut height.

- Asymmetric signature (left channels higher than right) → off-center loading. Either the die shifted in the bolster or you've got uneven wear. We caught a cracked die shoe this way — 8% left-right imbalance that wasn't visible on the parts yet.

- Oscillation on the return stroke → nitrogen spring bounce-back or counterbalance mismatch. Not dangerous but it accelerates gib wear.

One more thing — sample rate matters. Our old monitor sampled at 1 kHz which was fine for blanking but completely missed a 3ms spike during a coining operation. Upgraded to 10 kHz and suddenly saw force spikes 40% above what we thought was our peak. That was a wake-up call for die protection settings.