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

Servo Press for Automotive Structural Parts: UHSS Forming Challenges

admin

Ultra-high strength steel (UHSS, 980-1500 MPa) is increasingly used in automotive structural parts —B-pillars, door beams, bumper reinforcements. Forming UHSS on a servo press requires specific techniques to manage springback, edge cracking, and die wear.

## Why UHSS Is Challenging

UHSS has:
- High yield strength (700-1200 MPa) —large springback
- Low elongation (5-12%) —prone to edge cracking
- High work hardening rate —forming force increases rapidly
- High surface hardness —accelerated die wear

## Servo Press Advantages for UHSS

**Controlled forming speed:**
UHSS is sensitive to forming speed. Too fast causes adiabatic heating and edge cracking. Optimal forming speed: 30-80 mm/s (vs 150-300 mm/s for mild steel).

**Dwell at BDC:**
200-400ms dwell allows stress relaxation, reducing springback by 40-60% compared to no dwell.

**Programmable overbend:**
Servo press can precisely overbend by 1-5mm to compensate for springback, then return to exact BDC position.

## Motion Profile for DP980 B-Pillar

```
TDC —20mm above material: 250mm/s
20mm —5mm above: 80mm/s
5mm above —BDC: 40mm/s
BDC dwell: 300ms
BDC —TDC: 200mm/s
```

## Die Design Considerations

- Die clearance: 105-108% of material thickness (tighter than mild steel)
- Die radius: minimum 3? material thickness to prevent edge cracking
- Die material: D2 or PM tool steel (not H13 —too soft for UHSS)
- Die coating: TiAlN or CrN coating extends die life 3-5? on UHSS

## Springback Measurement and Compensation

Measure springback on first article:
1. Form part at nominal BDC
2. Measure angle/geometry after springback
3. Calculate compensation (typically 3-8? for DP980)
4. Adjust BDC position or add overbend to motion profile
5. Iterate until part meets specification

yuki_rnd

UHSS forming above 1180 MPa is where servo presses become essential, not optional. The forming speed window is very narrow. Too fast and you get edge cracking. Too slow and the material work-hardens before the form is complete. Our research on 1500 MPa martensitic steel showed the optimal forming speed is 30-50 mm/s with a 200ms dwell. You cannot achieve this on a mechanical press.

mike_chen_eng

On UHSS springback: we found that measuring springback at room temperature vs. immediately after forming gives different results for DP980. The steel continues to relax for 2-4 hours after forming. Measure springback 24 hours after forming for stable data when setting up compensation.

dave_toolmaker

Good point on the servo press selection criteria. One thing I'd add: check the machine's actual cycle time under load, not just the rated speed. We had a press that was spec'd at 60 SPM but only hit that on light forming — under full tonnage on a deep draw, it dropped to 35 SPM. The integrator's test was done with a dummy load, not actual production dies.

Also worth verifying: the repeatability spec under thermal load. A press that's ±0.05mm cold might drift to ±0.15mm after 4 hours of continuous production if the thermal compensation isn't tuned right.