By Joseph A. McGeough
Several tools involve a violent propulsion to deliver a telling blow. These have been named percussive tools, and their principal representatives are the axe and hammer. Under these two names are found an immense number of variations. The percussive group may also be called dynamic because of the swift motion and the large short-term forces they develop. This means that mass and velocity and, hence, kinetic energy and momentum are factors related to the force generated or transmitted. The distribution of weight between the head and handle and the mechanical properties of the head (i.e., its suitability for a cutting edge or its lack of elasticity) must also be recognized in the design of a percussive tool. Obviously, these various influences were not formally considered during the age long trial-and-error evolution of a now successful tool, but recognition of them aids in identifying the evolutionary stages of the tool.
Percussive tools generally have handles that allow them to be swung; that is, their rapid motion endows them with kinetic energy. The attainable energy of a blow depends upon a number of factors, including the weight of the tool head, the angle through which it is swung while gaining speed, the radius of the swing (handle length plus part or all of the arm length), and the muscle behind it all. There is a permissible energy level for a given task and tool, set by either the nature of the task or the material of the tool. Thus, a blacksmith flattening a 1-inch (2.54-cm) iron bar needs a heavy, fairly long-handled hammer, whereas a light and short-handled hammer, used with wrist action, is appropriate for forging a small soft gold wire. A hafted flint axe is an effective tool, but it may be destroyed if swung too hard or if twisted while in the cut. Bronze and steel axes can, and do, take longer handles than the stone axe and, being of tougher material, will not break under use that would fracture a stone head.
The physics of percussive tools takes into consideration the centre of gravity and what is technically called the centre of percussion—i.e., a unique point associated with a rotation, in this case the arc through which the tool is swung before delivering its blow and coming to rest. The tool’s centre of gravity is readily found because it is the balance point, or location along the handle at which the tool can be picked up loosely and still remain in the horizontal position. The centre of percussion is the ideal point at which striking should occur on the tool head to minimize the sting of the handle in the operator’s hand as well as to deliver a blow with maximum force; this point is farther out than the centre of gravity and should be as close to the centre of the head as possible. This last condition is best met with a light handle and heavy tool head, which places the centre of gravity close to the head and the centre of percussion in an optimum location in the cutting edge.
It is apparent that the sheer weight of the head is of paramount importance in promoting a proper balance, or hang, to the tool. On this basis alone, the shift from stone axe heads to metal was a step in the proper direction because metal heads of the same size as those of stone are about three times as heavy. With the heavier head, the centre of gravity of the hafted tool is closer to the head, and the centre of percussion is more likely to be properly located.
With the mallet and chisel still other interrelations are involved. When working stone, a brittle material that responds to a sharp tool point by breaking into small chips, the sculptor strikes many light blows to remove material. As a consequence, mallets have short handles and the amplitude of swing is small, allowing a succession of rapid blows without undue fatigue. To provide energy and momentum, the mallet head is heavy. Being made of wood, it does not rebound in the manner of a metal head but stays on the chisel, which transmits the blow to the cutting edge and focuses it into a small area of stone to be chipped off. The net effect of the proper combination of all elements—the properties of wood, chisel, and stone, the weight of the head (perhaps even heightened by a lead-filled cavity), and the short handle—is to waste the least energy. The wooden head is of course expendable, particularly if it is of a one-piece club like construction, for it becomes badly battered from contact with the metal chisel. A more refined mallet consists of a separate head and handle, the head having a working face of end-grain wood.
Working metal with a chisel requires that heavy blows be struck to enable the chisel to dig into the metal and lift out a chip. A steel hammer with a hardened face is used, and in this operation it is the soft end of the chisel that is battered and needs periodic dressing.