I have my VFD on my hammer set to ramp up the speed very quickly. It is at full speed in less than 2 seconds. I can see it working for controlling the speed of the hammer. The only problem I see is when I'm running the hammer all in one operation I find myself starting slow and rapidly increasing the force of the blow. That's something a VFD cant do. You basically set your speed and that's what you have until you adjust the speed on the VFD. I wouldn't want to be trying to adjust the speed while trying to hammer something home! I think if you are just drawing out stock it would be okay but if you were using top tooling I believe it would hinder the performance.