Quote Originally Posted by tonyfoale View Post
It would not be too difficult to make some form of stamp holder with a vertical extension to hold some form of weighty block at an appropriate height, on a slide. Add a trigger (for example) release and hey presto - even indents every time. A little experimentation prior to building would soon get the mass and height of the block determined.
I have tried that years ago - it produces multiple imprints because punch and hammer rebound. And I have also learned that I cannot suppress rebound even if I build the hammer as deadblow: my stamp, the item to be stamped, and the support upon which the item rests are all elastic, and will cause rebound. Once you look at the physics, you'll see why: dropping a hammer plus stamp of mass m a distance h stores energy
E=m.g.h
in it (g=9.81m/s is the acceleration of earth). When the stamp then penetrates a distance x, leaving its imprint behind, the (dynamic) force F it exerts is
F=Edx2/dt2
And in this formula all the problems with non-repeatable penetration depth are represented but not visible, since much of what goes into it is hidden: how the item deforms as the stamp starts to penetrate is a function of x, how much it is accelerated downwards due to the force applied to it, the mass and elastic properties of the support it rests on, you name it, all contribute to x, and as a result it muddies up the penetration depth x to make it as good as unpredictable. Since the stamp's downward moving velocity v is
v=dx/dt
it follows that, for repeatable penetration depth, the first derivative of x, the velocity with which the stamp moves downward must be close to zero, and thus the force, dependent upon the second derivative of x with respect to time t, exerted by the stamp on the item must be static in nature, not dynamic. And static forces require a press, not a hammer blow.
How potent a press? I have progressed so far that I can say for a 4mm number stamp I need a force in the vicinity of 5 ton for a decent impression in mild steel, i.e. an arbor press is ruled out as not powerful enough. The next question I need to decide is whether I want to measure and control the force acting on the stamp, using a strain gauge, or do I want to measure and control its travel into the item using a DTI? On that I am currently undecided, what is more robust? and what is more repeatable, to a measuring error of, say, 20 micrometer? After all, my eyes are quite capable of perceiving such a step, and that is the measure I need to apply.