Coming back to add something I've learned since my last post here — the spec that separates the good servo presses from the great ones is something most spec sheets don't even list: position loop bandwidth.
This is the servo system's ability to follow a commanded position trajectory accurately at speed. Higher bandwidth = the slide follows your programmed motion profile more precisely, especially during rapid direction changes and at the transition from fast approach to slow forming speed.
Why it matters practically: on a low-bandwidth system (say 50 Hz position loop), when you command a sharp deceleration from 500mm/s to 50mm/s at the start of forming, the slide overshoots by 0.5-1mm before settling. That overshoot hits your die at forming speed instead of approach speed. On a high-bandwidth system (150+ Hz), overshoot is under 0.1mm.
You won't find this on a brochure. Ask the manufacturer: "What is the position loop bandwidth with a full-mass slide at rated speed?" If they can't answer or give you a vague response, that tells you something.
Other specs I now ask for that aren't standard:
- Slide velocity ripple at low speed (important for draw quality — should be under 2% at 10mm/s)
- Regenerative braking capacity (affects how fast you can decelerate without tripping an overvoltage fault)
- Time from stop to full tonnage (some presses need 50-100ms to build force from a dead stop, which limits your dwell-then-form profiles)
- Maximum slide acceleration (tons/second ramp rate) — this determines how quickly you can transition between motion segments
sarahJ_mfg's point about SPM at actual stroke is critical. I'd add: also ask for SPM at your actual tonnage AND stroke. A press might do 40 SPM at full stroke unloaded but only 25 SPM at full stroke AND full tonnage because the motor can't deliver peak torque at high speed simultaneously.