Popular Science Monthly/Volume 9/May 1876/Hammers and Percussion
|HAMMERS AND PERCUSSION.|
THE only mechanical tools for external use with which man is provided by Nature are: the hammer, a compound vise, and a scratching or scraping tool; these are all in the hand. As a vise, the hand is worthy of a very lengthened notice; as a hammer alone it is now our concern. While upon a substance softer than itself the fist can deal an appreciable blow, with one harder than itself the reaction of the substance transfers the blow to the flesh and bone of Nature's hammer. Hence early arose the necessity of an artificial hammer of stone or other hard substance.
Among the contrivances which have come down to us from the ages before history was written, or the use of metals known, are found stones shaped, as we may suppose, by the action of water, and so rounded as to lit the hand. These stones are called by antiquarians "mauls," and they were probably held in the hand and struck against objects which otherwise could not have been broken. The maul is the original form of the hammer. This maul might occasionally have proved too heavy, but more frequently too light. For that tapping-action which in our minor wants is often more requisite than blows, our prehistoric ancestors seem to have devised an ingenious appliance consisting of a stone specially prepared for this somewhat delicate operation. (Fig. 1.)
This is supposed to be one of these tapping-hammers, held between a finger and the thumb; the original bears traces of wear, as if it had been employed in striking against a cylindrical or sharp surface.
When, now, we pass from this light to very heavy work, it will be obvious that to hold a stone in the hollow of the hand, and to strike an object with it so that the reaction of the blow shall be mainly met by the muscular action of the back of the hand, and the thinnest section of the wrist, would be not only fatiguing, but might be injurious to the delicate network of muscles there found, and so damage this part of the hand. It may have been from such effects that even in the Stone age there are traces of mauls which have double ends and are held by the middle. A blow given by such is counteracted not only by the increased mass of material, but also by the changed position of the hand and wrist in relation to the direction of the blow. When held in the hollow of the hand, the reaction was met by (say) a. depth of tissue of about three-quarters of an inch, but, when held as the maul now alluded to must have been held, this reaction is met by a depth of tissue of about three inches. Hence, while mechanically (owing to the mass of stone) and muscularly (owing to the position of the hand in reference to the direction of the blow) the maul in this second stage is a decided improvement upon its primitive form, we cannot but admit that experience would soon suggest that even thus there was wanting sufficient energy to overcome reactions, and that the double-headed maul might be improved. The men of the Stone age early perceived the advantage of having a handle of some kind for their mauls, and doubtless their first expedient consisted in lashing withes around such mauls as were found suitable, as the blacksmith at the present day lashes withes round the heads of his cutting and punching tools and swages. Evidences of a further advance toward a perfect hammer are to be seen in stone mauls with holes through them suitable for handles; and these holes are in some instances coned, and as well adapted for hammer-handles as the best-made metal tools of our day.
Before inquiring into the reasons which may have led to the adoption of the various materials and forms of hammers now in use, it will be well to consider the hammer in, and of, and by itself. We are so apt to look upon it as a rude implement, necessarily associated with a superior class of finishing-tools, that the materials, forms, and scientific principles involved in its construction and use, not only as an adjunct to other tools, but as a sole independent and final tool, are much over-looked.
In some handicrafts, and those too involving a high class of finished work, the hammer is the only tool employed. That great artistic skill in the use of the hammer as a finishing-tool can be acquired, is manifest from the many beautiful specimens of répoussé work to be seen in silversmiths' shops. The details of the ornamentation are not only minute, but they so harmonize as to give elegance and expression to the whole, exclusive of the form of the articles themselves. The variety of shape is mainly produced by changes in the form of the "pane" of the hammer and in the weight of it. These changes of "pane" are sometimes effected by separating the pane from the hammer, and then the separated piece is called a "punch."
The famous shield of Achilles, in the "Iliad" of Homer, is described as the result of hammer-work; and, though this shield may not have been actually fashioned, nevertheless the description gives an idea of what a hammer was in early times poetically supposed to be capable of accomplishing. The scenes wrought upon the shield of Achilles are—1. The earth, sea, and heavenly bodies. 2. In a city at peace there are (a.) Marriage festivities; (b.) Judicial suit or trial. 3. In a city at war there are (a.) A scene before the ramparts; (b.) An ambush and surprise; (c.) A bloody fight. 4. The ploughing of a field. 5. The harvest and the meal in preparation. 6. The vintage, with music and a march. 1. A herd of cattle attacked by lions. S. Sheep at pasture, and their folds. 9. A dance. 10. The great ocean-river encompassing the whole, as, in the mind of Homer, it encompassed the earth. For examples of the use of hammers in the production of works of great variety and extent on a large scale, see the ancient hammered wrought-iron gates, hinges, and panels, in the architectural room in the South Kensington Museum; also the suits of mail and chain-armor in the Tower of London; also the formation of gold-leaf, the springs of carriages, and the stiffening of saw-plates.
The nature of the work to be done by hammers calls for very great differences, not only in the form, material, and weight of the hammer-head, but also in the appendages to these. There are the material and form of the handles, the angle at which these handles should intersect the axial line of the hammer-head, the position of the centre of gravity with respect to the intersection of this axial line, the length and elasticity of the handle. If the centre of gravity is not in the central line or longitudinal axis of the hammer-head, then there is a tendency to bring the hammer down on the edge of the face and not on the face. If this defective construction were great, the muscles of the wrist might not be strong enough to counteract the tendency. If the defective construction is slight, then the work is often marked with angular indents. Arrangements, too, may be required for modifying the intensity of the blow, while retaining the effects resulting from a heavy hammer where a light one would be inefficient.
It is curious to see how in the same trade the hammers are for different purposes made of different materials. The engineer, for example, uses hammers faced with steel hardened, the stone-breaker (or mineralogist) hammers faced with steel softened (or rather not hardened). Again, in another part of his progressive work, the steel hammer with which the engineer commenced his operations gives place to a bronze or copper one, and this is sometimes displaced by one of lead alloyed with tin, and the handle entirely discarded.
The plumber dismisses all these, and for direct action upon the material employed in his trade he uses a hammer of wood, discarding not only the material but also the form of hammers used in allied crafts. Indeed, one of his hammers (Fig. 7) serves a double purpose, for, if at one moment it is a hammer, at the next it is used as a swage. Fig. 9 is his ordinary hammer, but when carrying on his allied trade of a glazier, not content with this, even the handle (Fig. 10) is finished in an unusual manner, probably for convenience in holding putty, which he often carries "dabbed" on the handle. In some cases, as in the working of copper vessels which have been silver-plated or gilt, the coats of the precious metals are so thin that, although the weight of a hammer-head is required, yet-even the wooden hammer of the plumber, or the still softer leaden hammer of the engineer, is equally unsuitable, and therefore the workers in these metals cover the face of their hammers at times with one or more layers of cloth.
The veneering hammer is compound, one end being formed of metal and the other of wood. The metal end is used as a squeezing-hammer (if such a term may be employed), and the wooden end as a tapping-hammer, to ascertain by the sound produced where the veneering is adhering and where it is not.
The stone-mason seems to claim a universal choice. As to material, he has and frequently uses hammers made of wood, of iron (steel-faced), and of an alloy of lead.
In some cases the hammer and the anvil mutually change places, the hammer of wood, the anvil of metal, or the converse. Nor is the
wood always of the same character. As varied as are the characters of the woods themselves, so varied are those chosen by different crafts for the employment of each craft.
Hammers with and without handles are in use—hammers of various weights, from half an ounce to ten pounds, and from fifteen to fifty-six pounds are now employed as hand-hammers. The angles of attachment of handles to heads are various: the position of the centre of gravity of the head in reference to the line of penetration of the handle is various; the faces have various convexities; the panes have all ranges and forms, from the hemispherical end of the engineer's hammer, and the sharpened end of the pick and tomahawk, to the curved sharpened edge of the adze, or the straight convex edge of the hatchet and axe; the panes make all angles with the plane in which the hammer moves.
Fig. 15.—Cooper's Claw-Hammer.
Fig. 16.—Ship-Carpenter's Claw-Hammer.
Fig. 16 is a ship-carpenter's hammer-head with claw. It differs from ordinary claw-hammers in that the handle is not strapped. In some American claw-hammers the strapping is carried up the back and
front of the hammer. Why this change has been made is not very apparent, for by it one strap—that nearest the claw—is in tension, while the other is in compression. With the straps on the sides, as in Figs. 18, 19, the tension is equal on both. Fig. 15 is a cooper's claw-hammer,
not strapped. In these cases, if much power is required when the claw is used, it should be applied by pressure on the face-end of the hammer as well as upon the handle.
Before considering the elements upon a combination of which the powers of hand-hammers depend, it will be well to remark upon the circumstances under which this power is actually developed. The development takes place at the instant of contact of the moving hammer with the struck body. Such contacts as those of hammers
belong to that department of mechanical philosophy called "impact." Impact is pressure of short duration
so short that, compared with the time in which the velocity of the impinging body is being acquired, it is inappreciable; or, if the comparison be between spaces passed through by the hammer-head before impact and during impact, then, generally speaking, the disproportion is the same, and the space passed through after impact is almost inappreciable when compared with the space passed through before impact.
It may assist in realizing the source as well as the magnitude of the power of a hammer, if the dynamical effect of impact be compared with what may be called the statical effect of pressure. Let any one attempt to drive a nail vertically into an horizontal piece of timber by the statical effect of the simple pressure of a load placed gently on the head, as weights are laid in scale-pans. Let the depth to which the nail is thus moved be measured. Again, let the same nail, under the same circumstances, be driven to the same depth by the impact of a hammer-head, then it may for our present purpose be said that the load placed on the nail is a representative statical measure of the impact of the hammer.
Now, although in any given case the work in a hammer consequent on its mass and velocity may be very great, yet utilizing the whole of the work produced in the expenditure of the accumulated power in the hammer depends upon the resistance met with at the instant of impact. The more perfect this resistance is, the greater will be the value of the work done; hence the practice of using massive anvils, firmly fixed, and the necessity for staying all vibrations in the body struck. Let any one attempt to drive a nail in a board not firmly supported, and then by the use of the same means drive a similar nail into the same board supported, and he will appreciate the importance of resistance to the progress of a hammer's motion if the full effect of a blow be desired.
The only exception to this is to be found in the blows given to minerals which are to be cleft, and not crushed. In their case it is desired to give only such a blow as shall accomplish the cleaving; any surplusage of energy, if expended on the material, would, of course, produce fractures over and above the required cleavage. Provision must be made for the dissipation of this superfluous energy, and it is done by placing the mineral in an elastic holding, the nature of the required elasticity being determined by experience, as different substances require different elasticities in the supports by which they are held for cleavage. Illustrations of the principle here enunciated are seen in the breaking of stones on the highway, where the elasticity is transferred from the mineral support to the handle of the hammer; also in the flaking of flints, where the elasticity is obtained by holding the mineral in the hand and supporting it on the knees. The splitting of the diamond is a case where these principles and considerations claim the greatest care.
The anvil used by the diamond-splitter is of wood, in shape not unlike a ninepin, but tapered at the lower end so as to be placed upright in a coned hole in a small block of lead. On the head of the ninepin is a flat, on which, by means of cement, the diamond to be split can be firmly fixed. Placed here so that the plane of intended cleavage shall be vertical when the wooden anvil is in the lead block, a deep scratch is made by a second diamond, in which scratch the edge of the splitter's chisel is to be planted. The diamond-splitter's chisel is very like an old razor. This chisel the workman holds in his left hand, in his right he holds that which is his hammer. The hammer is a plain steel rod, about eight inches in length, and tapering from about half an inch diameter in the middle to three-quarters of an inch at the end. The very construction of this peculiar hammer gives the operator a large range for precise and graduated blows; within certain limits he can most carefully arrange that the path of the centre of percussion, the place of impact, the line bisecting the angle of his razor-like chisel, and the expected plane of cleavage of the diamond, shall coincide; hence, with great coolness and the absence of all hesitation, he gives a blow, upon the effect of which many hundreds of pounds may depend.
In dealing with hammers—including under that term for the present purpose axes, hatchets, adzes, and picks—the following question claims consideration: What power or energy is in a hammer of known weight, moving at a known velocity, if brought to a state of rest by impact on a block? Another question also suggests itself: Can this impact effect of a hammer be converted into simple pressure, and be stated as a load or weight placed, where the impact was requisite, to produce the same effect as the impact did? If the mode of solving the first question can be made clear, then the answer to the second can be easily obtained. The measurable elements which affect the result are a variation in the mass of the hammer-head, and a variation in the length of the handle. By a varied mass there is a varied weight in the hammer; by a varied length of handle there will, with the same muscular effort, be a varied velocity in this mass, and upon a combination of mass and velocity depends the produced energy. Now, if a mass of metal, moving at a known velocity, strike an object, the energy of that blow results entirely from the conditions at the moment of impact. For example, the work in the hammer, H, as it strikes the nail, N (Fig. 20), does not depend upon its velocity through the arc,
Q N, but only upon the velocity when commencing contact with the nail. Hence, so long as the material which gives the blow and the mass of it are the same, it is not of any consequence how the velocity was accumulated. It may result from centrifugal or rectilinear action; it may result from muscular effort, or from steam-pressure, or from gravity.
It may now be obvious that, other elements remaining unchanged, whatever accelerates the velocity of a hammer increases, according to very clear rules, the energy or power of the same hammer. Hence the tendency of contrivances, as manifested in the addition to steam as well as handicraft hammers; for example, in the early lift-hammers, those which are by many still considered to produce the most perfect of hammered work, the "wiper" was so shaped as to throw the hammer very high. The ascent was checked by a powerful spring, and thus the ascensional energy was reversed and added to the accelerating force of gravity downward; and so not only was the intensity of the blows increased, but their frequency also. This spring took the place of that muscular energy which brought the hammer down with intensified effect.
Hence, also, in steam-hammers, all muscular effect to intensify the blow is transferred to the steam, and all consequences of centrifugal action, whether from hand or tilt hammers at the ends of arms, are removed. Further, in steam-hammers nowadays, the steam operates to check as well as to intensify the blow. This checking action is called "cushioning," and it seems to do what an elastic handle does in a sledge-hammer: it relieves the rigid fabric or erection from jar or destruction. "Cushioning" is brought into play by admitting steam for the purpose of checking the intensity of the blow due to the action of gravity alone, or of steam combining with gravity upon the hammer. Hence the perfect control over large steam or air worked hammers, and the rapidity with which the intensity of the blow may be changed. Such control as this over a sledge-hammer is beyond our bodily powers. We may intensify the blow, but we cannot, except just experimentally, and for the purpose of display, bring the restraining power of the muscles to diminish the energy of the descending hammer.—Journal of the Society of Arts.
- Abstract of three lectures before the London Society of Arts.