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Coin and medal stamping is one of the most demanding servo press applications ??extremely high forming pressures, tight dimensional tolerances, and complex surface detail requirements.
## Process Characteristics
- Material: brass (CuZn30), cupronickel, aluminum, silver
- Thickness: 1.5-3.5mm blanks
- Forming pressure: 600-1200 MPa (much higher than structural stamping)
- Surface finish requirement: mirror finish on die, Ra < 0.1?m
- Dimensional tolerance: ?0.01mm on diameter, ?0.005mm on thickness
## Press Selection
Coin stamping requires:
- **High rigidity**: frame deflection must be <0.01mm at full load
- **Precise BDC control**: ?0.005mm repeatability
- **High tonnage in small stroke**: typically 50-200 ton in 2-5mm forming stroke
- **Slow forming speed**: 1-10mm/s in forming zone
Knuckle-joint servo presses are preferred for coin stamping because they provide maximum mechanical advantage at BDC.
## Motion Profile for Coin Stamping
```
TDC ??5mm above blank: 200mm/s (fast approach)
5mm above ??1mm above: 20mm/s (slow approach)
1mm above ??BDC: 3-8mm/s (forming speed)
BDC dwell: 200-500ms (critical for surface detail)
BDC ??TDC: 150mm/s (return)
```
The BDC dwell is critical ??it allows material to flow into die detail under sustained pressure.
## Die Requirements
- Die material: D2 or M2 tool steel, hardened to 60-62 HRC
- Die surface: polished to Ra < 0.05?m
- Die temperature: maintain at 20-25?C (temperature affects blank thickness)
- Die life: typically 500,000-2,000,000 strikes before re-polishing
## Quality Control
- Measure blank weight before stamping (weight variation = thickness variation)
- Check diameter and thickness every 1000 pieces
- Inspect surface under 10x magnification for flow lines or cold shuts