[TheForge] Re: Rebound and Work

[Mike Spencer]

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.

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^

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