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

Coin and medal stamping - high pressure forming setup

carlos_mx

We do coin and medal stamping and the press has been great for it. High pressure, precise dwell at BDC. Here's our setup for anyone interested.

**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

toolndye_dave

Medal stamping is even more demanding than coin blanking because of the relief detail. The servo press dwell at BDC is essential —it allows the metal to flow into the fine details of the die engraving. Without dwell, you get incomplete fill on the highest relief areas. We typically use 200-500ms dwell depending on the detail depth.

lisaQ_metrology

Surface finish on coins and medals is measured in Ra (roughness average). Target is typically Ra 0.1-0.2 micrometers. The servo press's vibration-free forming helps achieve this —any vibration during the coining stroke shows up as surface texture on the finished piece. We measure Ra on every 50th piece as part of our quality protocol.

carlos_mx

we do coin blanking at 120 SPM on our 200T. the key is getting the dwell time right at BDC - too short and you get incomplete fill.

pressroom_tom

We ran into a similar challenge doing commemorative medal runs on our 300T Komatsu servo. One thing that made a huge difference for us was ramping the force in the last 2mm of travel instead of hitting full tonnage instantly. On brass blanks especially, a controlled ramp gives the material time to flow into the fine relief details without cracking at the edges.

Dwell time is critical like you said — we settled on 350ms for most coin work, but for deep relief medals we go up to 600ms. The trick is watching the force-displacement curve in real time. If you see a plateau during dwell, the metal has filled the cavity. If force is still climbing, you need more time or more tonnage.

One gotcha: die temperature matters more than people think. We preheat our coining dies to 60-80C. Cold dies on brass = incomplete fill on the high points every time. Learned that one the hard way after scrapping about 200 pieces on a Monday morning cold start.

raj_pm

One thing worth mentioning for anyone getting into coin/medal work — die temperature management is critical and often overlooked. On brass at 600+ MPa, the die face temperature climbs fast. We measured 85°C after 200 cycles on a commemorative run. That thermal expansion shifts your BDC by 0.008-0.012mm, which doesn't sound like much but it's enough to change the fill on fine relief details.

Our fix was simple: thermocouple in the die shoe, 2mm below the working surface, feeding back to the PLC. When temp exceeds 55°C we automatically insert a 3-second pause every 10th cycle. Keeps the die under 60°C and the fill consistency is night and day. Before that we were getting incomplete fill on the high-relief areas after about 150 cycles, then it would come back after a break. Classic thermal drift.

Also for anyone doing silver — watch your lube. Silver is way more sensitive to lubricant contamination than brass or cupronickel. We switched to a synthetic ester-based lube (Fuchs Renoform MCO 3) specifically for precious metal coining. The old chlorinated stuff was leaving micro-staining that only showed up under UV inspection. Took us three rejected batches to figure that one out.