Servo Press Slide Position Drifts 0.3mm Over a Shift: Causes and Fix

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**Question:** We stamp automotive brackets on a 160-ton servo press. At shift start, parts are within tolerance. By end of shift, part height is 0.3mm taller than spec. We adjust the die height and it drifts again. Thermal expansion or something else?

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**Answer:**

0.3mm drift over 4 hours is worth investigating. Here is how to diagnose it.

## Separate Thermal Expansion from Mechanical Drift

**Thermal expansion test:**
1. Measure part height every 30 minutes for the first 2 hours
2. Plot the measurements
3. If the drift follows a curve that flattens out (asymptotic), it is thermal expansion
4. If the drift is linear or accelerating, it is mechanical

Typical thermal expansion for a cast iron press frame: 0.01-0.02mm per degree C per meter of height. A 1.5m press frame warming from 20C to 35C expands about 0.2-0.3mm. This matches your symptom.

## If Thermal: Solutions

1. **Warm-up cycle**: run the press at 50% SPM for 30 minutes before production. The frame reaches thermal equilibrium and drift stops.

2. **Thermal compensation**: some press controllers automatically adjust BDC position based on a temperature sensor in the frame.

3. **Adjust after warm-up**: set the die height after the warm-up cycle, not cold.

## If Mechanical: Check These

1. **Gib clearance increasing**: worn gibs allow the slide to tilt slightly under off-center load. Measure gib clearance at start and end of shift.

2. **Connecting rod bearing wear**: worn bearings allow the slide to drop slightly under load. Check crankshaft end play.

3. **Die clamping**: if the die is not fully clamped, it can shift under repeated loading. Check clamp bolt torque.

## Quick Field Test

Put a dial indicator on the slide and log BDC position for 20 consecutive strokes at start and end of shift. If the indicator reading changes, the mechanical BDC is drifting. If the indicator stays the same but parts change, the die or material is the variable.

mike_chen_eng

On the tonnage monitoring question — we run 108% high limit and 88% low limit on our HSS stamping lines. The 110% high limit is fine for mild steel but for 780MPa HSS the material variation is tighter so we run a tighter window. Caught two double-blank events last quarter that would have been die crashes without it.

mike_chen_eng

**Related guides:** For tonnage monitor setup details, see [How to Read a Servo Press Tonnage Monitor](https://servopress.club/d/94). For die protection setup that works alongside tonnage monitoring, see [Servo Press Die Protection: Setup and Parameter Guide](https://servopress.club/d/104).