There's a misinterpretation of the question here. I'm asking about Strokes Per Minute as reference to travel speed of the blade, for one stroke. Not attempting to use the machine in a continuous cycle mode. So the length of a "part" has no bearing on this. I think that would be obvious from the description in post #1 ?
travel speed of the blade is calculated by the stroke length X the rpm of the mechanical shaft, and a few other things as the speed of the blade isn't linear as there is acceleration at the beginning and end of the stoke and all depends on the diameter of the pivot at the top and connecting rods length.
as for the minimum speed, that is all depending on tonnage required to shear a specific thickness. this is where hydraulics excell over mechanical shears.
but for easy calculations, force = mass x velocity. so if the velocity is reduced by 50% so will the force. so if its a 30 ton press and you need 17 tons to shear it, you can only reduce it by a little but, also motors loose there torque/hp when dropped down on a vfd and that isn't a linear line either.
better question is why though? if it works now, why complicate it and chance to break it or burn the motor windings up as it will pull more amperage also.