Hammer Physics Math (Myth?) Manual Manipulation Something Or Other | Page 2 | Problem Solving | Forum

A hammer flies downward with a certain mass and speed, and thus a certain kinetic energy. It hits the workpiece and rebounds. In the rebound, the hammer has the same mass and a reduced speed, and so has a reduced kinetic energy. The energy delivered to the workpiece (and to the anvil, the stand, the floor, the smith's arm, etc.) is the difference between the "before" and "after" kinetic energies of the hammer. If a hammer hits a workpiece without deforming it much, the hammer will rebound with a high speed, thus delivering only a small amount of energy to the workpiece. The only way a hammer can deliver *all* its energy to the workpiece is if it comes to a dead stop on impact -- with NO rebound.

Our only disagreement so far seems to be in our meaning of "deliver". When I say "deliver" I mean "deliver and leave behind as energy of deformation". Where we part ways is in respect to the question of whether a heavy hammer deforms hot steel more effectively than a light hammer does -- when initially moving at the same speed. A scientific test would be to drop hammers of different weights from the same height onto identical workpieces heated to the same temperature. My experience is that a heavier hammer rebounds less from a hot, deformable workpiece than a light hammer dropping from the same height. This is related to the fact that a light hammer or a light blow tends to move just the surface material, while a heavy hammer or heavy blow will move material to a much greater depth.

I was demonstrating this to some students just today: when spreading a bar of 440 stainless to make a blade, a lot of weak blows managed eventually to spread the bar, but resulted in "mushrooming", where the top and bottom surfaces moved a lot more than the middle of the bar did, so a concave crease formed along the edge. With a heavy hammer -- or with much heavier (that is, higher speed) blows with a light hammer, the middle of the bar swelled out more than either the top or bottom surfaces, resulting in a convex instead of concave edge.

The bottom line seems to be very similar regardless of the reasoning behind it: "use the smallest hammer that is heavy enough to get the job done," and "use a hammer light enough that you can control it well and swing it all day without hurting yourself".

[QUOTE=Scott Rash;17946]Ok Billy, I’m going to disagree with you, but only slightly.
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I also disagree with an earlier comment that a heavier hammer will deliver more of its energy than a lighter hammer. By definition, each hammer delivers ALL of its stored kinetic energy to the workpiece. We know this because the hammer stops traveling downward! Some of the energy is lost to deformation of the workpiece, ‘cause we’re hitting a heated ductile material. But steel isn’t completely plastic at forging temperature, there’s still elastic behavior. So a good portion of the energy delivered to the workpiece is transferred back into the hammer, resulting in rebound.

What I actually do in practice is use the smallest hammer that is heavy enough to get the job done. If I’m presented with thicker stock, I switch to a bigger hammer. Then I switch back as soon as possible to avoid fatigue, and also because I have better control. No need to use a 4# hammer on ¼” stock.

Another possibly relevant example, akin to the concave edge vs convex edge example: in my experience, when a piece of hot metal is squeezed in a hydraulic press, it bulges out in the middle more than at the top and bottom. Maybe it's because the top and bottom dies cool the hot metal, making it stiffer at the top and bottom surfaces. Maybe it's because there is a lot of "stiction" between the dies and the metal. Maybe it's something I haven't thought of. Again, if it matters to anyone it wouldn't be very difficult to devise an experiment to settle the question.

Scott, I suppose I should confess that I've got a MA in physics and a couple of years of further graduate studies in physics and electrical engineering. Of course that doesn't mean I'm necessarily right about anything, but it lets you know where I'm coming from. Just like the term "deliver", any word like "energy", "force", "speed", "velocity", or "work" probably doesn't mean exactly the same thing to both of us.

[QUOTE=Scott Rash;17953]You bring up some great points, Steve, especially about the net force "delivered" or left behind. I'd been reading some physics blogs that pointed out that all of the force is transferred at impact - a de facto thing because the hammer stops travelling forward. But could there be a difference in "work" and rebound with hammers of different mass? I'm still pretty dubious. I want to say force is force, period. Does the larger hammer rebound less simply because there's more hammer mass to move? Dang this is hard to think through, especially finding any hidden assumptions.

I'm assuming that the area of contact between workpiece and hammer is the same in both cases, so the force radiates out from the same place. Thus the physics of crystal dislocation and slippage really should be the same if the force is the same.