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The die cushion is a critical component in servo press deep drawing operations. Proper setup directly impacts part quality and tool life.
## Die Cushion Function
The die cushion provides controlled blank holder force during the drawing stroke. In servo presses, the cushion can be hydraulic, pneumatic, or servo-driven. Servo cushions offer the most flexibility — force and position can be programmed independently throughout the stroke.
## Force Setting
Blank holder force should be set to the minimum that prevents wrinkling. Too much force causes tearing; too little causes wrinkling. Start at 20-30% of blanking force and adjust based on part quality.
For servo cushions, program a **force ramp** — start at 80% of target force at initial contact, ramp to 100% as the draw progresses. This reduces shock loads and improves material flow.
## Stroke Setting
Cushion stroke must be greater than draw depth plus 10-15mm clearance. Set the cushion return speed to match the press slide return speed to avoid impact at the top of stroke.
## Pressure Pins
Distribute pressure pins evenly around the blank holder. Uneven pin distribution causes uneven blank holder force and part distortion. Use a pressure-sensitive film (Fujifilm Prescale) to verify even force distribution.
## Troubleshooting
- **Wrinkling in flange:** Increase blank holder force 10% at a time
- **Tearing at punch radius:** Decrease blank holder force, check lubrication
- **Uneven draw depth:** Check pressure pin heights (should be equal ±0.05mm)
- **Cushion bounce:** Reduce cushion return speed, check nitrogen pressure