Wanted to follow up on this since we've now had the servo running for about a year on that mixed line I mentioned.
The flexibility argument proved out even more than I expected. We've added 6 new part numbers to that press since install, and 3 of them needed motion profiles that a knuckle press physically couldn't do ??specifically a mid-stroke pause for in-die tapping on one part, and a slow approach + fast return for two deep draw parts where we needed to control material flow rate.
But I'll be honest about the downsides too:
1. Energy consumption at high SPM is higher than the knuckle press it replaced. The knuckle stores energy in the flywheel and the linkage geometry does the work. Our servo is pulling 15-20% more kW at 45+ SPM on simple blanking. Below 30 SPM or on complex profiles, the servo wins on energy.
2. The servo is more sensitive to parameter drift. Our knuckle press ran for 8 years with basically zero tuning changes. The servo needed gain adjustments twice in the first year ??once after a motor bearing replacement, once after we changed die weight significantly. Not a huge deal but it's a different maintenance mindset.
3. pete_retro's retrofit idea is legit. We actually considered it. The reason we went new was our existing knuckle frame was a 1998 Minster with some crown fatigue showing. If the frame had been solid, the retrofit math would have won.
Bottom line: if you're running fewer than 3 different part families on that press, and they're all BDC-dwell operations, save the money and get the knuckle. If there's any chance your part mix will diversify, get the servo. The flexibility premium pays for itself the first time you avoid buying a second press.