Sunday, April 08, 2018

Use a Bigger Hammer…


... or maybe NOT

part 4 on the ‘Hammers & Hammering’ series (c)

One of the biggest mistakes I see with new (and not so new) blacksmiths is a tendency to ‘use the biggest hammer’. (a)
Unfortunately, is is often the result of completely incorrect cultural stereotypes, mixed with what is basically ‘macho BS’.

* It does not matter how hard you HIT - if you just MISS. *

I am going to attempt to interject some logic - and basic science, into this commentary.
As you will see, there are a large number of factors effecting the ‘most effective’ production of hammer force into a specific forging operation. *Biggest* may not be *best*.

1) Input Energy
Any given individual will be limited to how much physical muscle energy they can apply into moving a hammer. This will be determined by raw body size, muscle strength, physical fitness.
- Generally, the larger the person, the greater the potential amount of muscle energy available.
- However, mere *size* may not prove more important than *fitness*.
Critically important - *Energy input is effected by other variables beyond simple muscle size.*

2) Deforming Force
Any metal bar will require a certain amount of force applied to it (Energy) before it will start to deform. - The larger the diameter of the bar = the greater the resistance to changing shape.
R ~ d
- The higher the temperature of the metal, the lower the resistance to changing shape.
R ~ T
So remember you could apply more energy (hammer force) or just increase the temperature of the bar, especially important when working thick bars.
You also see that as a bar cools, its resistance will increase. At a certain temperature point (with the hammer force consistent) you will not be overcoming the resistance, effectively accomplishing nothing.
(We are avoiding the major problem of continuing to attempt to work a bar ‘below critical’, so no longer at its ‘plastic’ state.)

3) Work
Given above, how many individual hammer stokes are applied in one ‘heat cycle’ may prove as important (if not more) than the energy of any single stroke.
W = E x f ~ R
You can see that this also needs to be related back to overcoming the bar’s resistance.

4) Energy
How is Energy (force as applied to the bar) produced?
Energy is the mass (of hammer) times the velocity (speed of the hammer) *squared* all divided by a half
E = (m x v2) x 1/2
What this should be showing you is that you actually create *significantly* more energy by moving the hammer *faster* - than by using a heavier hammer, moving slow (!). Typically, a longer swing will increase the speed of any individual hammer stoke.

5) Control
Once a mass is in motion, it will require force applied to change that motion (overcoming inertia). Both increased hammer weight and increased hammer speed are factors.
F = m x (∆ v x t)
In practice, you will find it requires more energy to change a heavy hammer, moving slow, than a lighter hammer, moving fast. This because even a small shift at the start of a swing can make a large difference in strike position over the length of that swing.

6) ‘Penetration’
Now, from above you can see the factors that effect the creation of the raw energy applied to a bar for a forging step.
Beyond the resistance to changing shape at all, there is also an ‘impact effect’ - the penetration of force into the bar. (There may be math for this, but a casual search did not yield anything not terribly complex.) A lighter hammer, moving fast, will effect the metal differently than a heavy hammer, moving slow - even though the energy involved may be identical.
The bar has ‘inertia’, that ‘resistance to changing shape’.
Consider this analogy :
You have a choice between taking a fist strike from either Bruce Lee, or Muhammad Ali.
Bruce Lee gives you a light fist, but moving extremely fast. The result is that your jaw is shoved back faster than your head can move. Result is a shattered jaw, and down you go.
Muhammad Ali gives you a heavy blow, moving (relatively) slower. Result is your whole head rocks back, sloshing your brain unconscious, and down you go.
Similarly:
A light hammer, moving fast, will impact and move the *surface* of the bar. The net result is a T shaped distortion of the bar. This effect most pronounced working the edges of tall, thin bars.
A heavy hammer, moving slow, will impact and shift the entire mass of the bar. The net result is a squashed barrel shaped distortion. This effect most pronounced on square bars.

Conclusion:

You may start to see there that there is a clear relationship between the choice of hammer (both weight and handle) and body type. Obviously, a heavy hammer is not always the ideal.

Most typically, those with ‘barrel’ body builds and heavy joints and short arms, may chose for purely mechanical reasons, to work with heavier hammers, using shorter strokes. in contrast, those with lighter builds, with thinner joints and longer arm linkages, are better suited to using lighter hammer weights and much longer strokes. (b)

You can see clearly that hammer *speed* is a greater energy producer than hammer *weight*. For this reason anyone ‘choking up’ on the hammer handle is not using the tool efficiently. (Short ‘club’ handles obviously a bad choice).

- The easiest way to increase effective hammer impact is merely to use the entire handle length.
- Simply raising the hammer higher on each stroke will significantly increase impact energy. This with less actual muscle force required than by using a heavier tool.
- Increasing the strokes per minute will also obviously both quickly and easily improve the amount accomplished per heat cycle.
- A lighter hammer is always easier to control. Do consider roughing out with a heavier tool, then switching over to a lighter tool for surface finishing.


Symbol Definitions

d     is ‘diameter’, here the measurement at right angles to direction of forming
E     is ‘energy’
f    is ‘frequency’, here the number of hammer stokes (per minute or heat cycle)
R     is ‘resistance to deforming’
T    is ‘temperature’
t     is ‘time’
v     is ‘velocity’, here the speed the hammer is moving
W    is ‘work’, here the amount accomplished (per minute or heat cycle)
~     is ‘varies with’ (note this not the correct symbol, not on my keyboard)
∆    is ‘change in’

(a) I have repeatedly heard any number of  feature demonstrators remark about ‘how much my elbow hurts’. At the same time observing them using not only hammers obviously too heavy for them to correctly control. Or using very poor technique. Usually both.

(b) One of the trends over the last decade particularly has been to ‘celebrity’ blacksmith demonstrators - with ‘named designed’ hammers. New blacksmiths have proven especially susceptible to mimicking these methods. In my experience often not understanding the clear relationship between not only effective, but actually *safe*, selection of tools and physical technique based on body type. There is a reason there are so many hammer styles - this must remain a purely personal choice!


(c) Others in this series
2018/02/getting-hammered.html
2018/03/getting-hammered-2-dynamics.html
2018/03/getting-hammered-3-setting-up.html
Earlier commentaries
2013/01/hammers-weights-styles-and-rounding-type.html
2014/07/a-bit-about-hammers.html
2016/08/getting-hammered.html
2016/08/more-on-hammers.html

 

February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

COPYRIGHT NOTICE - All posted text and images @ Darrell Markewitz.
No duplication, in whole or in part, is permitted without the author's expressed written permission.
For a detailed copyright statement : go HERE