[TheForge] Re: Rebound and Work

[Steve Smith]

Mike, I think you've centered in on the confusing part. The two 
situations are quite different, anvil vs. glowing bit of steel. I think 
you make plenty of sense. Change in kinetic energy (scalar) is what 
counts and is what is converted to heat or smush.

How about this one--does a dead blow hammer do any good in forging? My 
bet is no, you just make the shot in the head warmer, but look how good 
I did on the last one.

Steve

Mike Spencer wrote:
> jonned> I remember seeing some old advertising for some brand of cast
> jonned> anvil that addressed the rebound issue. Their copy stressed
> jonned> that the cast anvil with minimum rebound allowed for more of
> jonned> the force from the hammer blow to penetrate the metal being
> jonned> worked on....
> 
> sos> Well, we're confused here. I started out thinking (and still
> sos> think) that if the hammer rebounds, you've applied more force to
> sos> the work than if the hammer stops dead.
> 
> Confusion may arise because there are two different cases here:
> rebound off the bare anvil face and rebound off a hot workpiece.
> 
> Drop a steel billiard-sized ball onto an anvil from 10 feet up.  If it
> bounces back to nearly 10 feet, then the anvil isn't soaking up and
> dissipating much energy.  If it only bounces to 2 feet, the anvil
> itself is soaking up a *lot* of energy and dissipating it as heat and
> sound.  You want your anvil to be like the first case.  This is just a
> formalization of the ad hoc test of lightly dinging an anvil face with
> a hammer to test for rebound.
> 
> Now put a piece of hot iron on your anvil -- the one that will bounce
> the steel ball back to, say, 10' 6" -- and drop the ball on it from
> 10' as before.  The fraction of the ball's energy that the anvil
> "wastes" should be the same as before but the ball will only bounce
> to, say, 18" if the iron is orange or even less if the iron is at a
> blinding white heat.  The amount of rebound or bounce off the hot
> metal is (roughly?)  proportional the fraction of the ball's energy
> that is *not* going into the hot workpiece and doing useful work to
> shape it.
> 
> Repeating the quote:
> 
> sos> ...if the hammer rebounds, you've applied more force to the work
> sos> than if the hammer stops dead.
> 
> All else being equal, you'll get *some* rebound of the hammer off the
> hot workpiece no matter how hard or lightly you hit it unless the
> workpiece is nearly at a liquid/slush temp. You just can't detect the
> rebound of a ridiculously light blow.  If you hit the workpiece
> harder, you'll put more work into the workpiece and you'll get more
> rebound, too, but  the proportion of work that is wasted in rebound
> will be the same. [1]   MV^2 of the rebounding hammer is energy that
> you put into the hammer with your arm and which it still has after it
> has left the workpiece.
> 
> 
> Pontifically,
> - Mike
> 
> 
> [1] Unless there's a non-linearity here that I fail to comprehend.
>