What is a good and easy way to compute and/or measure the force of a hammer blow, not using any fancy or specialized equipment?
If the hammer is swung by hand through an arc, it is not obvious to me how to measure the speed the hammer will be when it strikes the metal.
Also, when the hammer strikes the metal, the heavier it is, the more force there will be persistent and the less rebound. Also, the smith may use what is called a "dead blow" hammer to reduce the rebound. Thus, measuring the force is not just a question of the instantaneous force of the hammer, but how much it "presses" after that first instant, its impetus so to speak.
Now, one idea I had was to use a teeter-totter. You could place a heavy weight on one end of the teeter-totter and then hit the other other with the hammer and see how far it moved. Of course, what will happen is that when the hammer hits the pad, the teeter totter will accelerate, reach a peak, then decelerate, and the profile of this curve of acceleration will be the measurement of the instantaneous force of the hammer over time. Perhaps this could be measured by an accelerometer, but it is hard to see how to make the measurement with no special instrument.
Best Answer
Of course the force changes during the impact - so to get close to an answer, you need both the time of the impact and the magnitude of the momentum transfer.
As user77567 pointed out, a fairly simple way to measure momentum transfer is with a ballistic pendulum. This would be a heavy steel ball (much heavier than the hammer) hung from a long wire. When you strike the ball, the hammer will bounce back (since it is much lighter) and the ball will swing through an arc $\alpha$ before returning to the equilibrium position.
If ball has mass $M$ and hammer mass $m<<M$, then conservation of momentum tells us that
$$M\cdot v_{ball}= m\cdot \Delta v_{hammer}$$
For an elastic collision with $m<<M$, $\Delta v_{hammer} \approx 2 v_{initial}$
If the wire has length $\ell$ and moves through a distance $d$, so $\alpha = \tan^{-1} \frac{d}{\ell}$, conservation of energy tells us that for small deflections, the height $h$ that the ball rises after the impact is
$$h = \ell (1-\cos\alpha)\approx \frac{d^2}{2\ell}$$
Conservation of energy then tells us
$$M\cdot g\cdot h = \frac12 M v^2$$
and it follows that
$$v = d\sqrt{\frac{g}{\ell}}$$
Of course we could have got the same result directly from the equation of motion for a simple harmonic oscillator (pendulum).
The remaining interesting question is the impact time. This can be measured with simple electronic components. If you connect a resistor and a charge capacitor in parallel, with a "switch" formed by the contact between the hammer and the ball, then you can compute the impact time by observing the fraction of discharge of the capacitor due to the "closing of the switch" when the hammer hits the ball. Sufficiently thin and flexible wires should allow this measurement without disturbing the mechanics. Use a digital multimeter with sufficiently high impedance (at least 10 M). If the capacitor leaks slowly after you first charge it (say with a battery), you can observe the voltage dropping and hit the ball with the hammer just as the voltage hits a "round" value - this allows you to minimize the ffect of drift.
To make the measurement of impact time repeatable you could make the hammer part of a second pendulum that hits the ball from different heights: you can then plot the relationship between impact velocity and impact time, and this will allow you to get the time when you hit the ball really hard (when you might not get a good repeatable measurement of the time or velocity).
I hope this is enough to get you going...