Popular Science Monthly/Volume 10/December 1876/More Concerning Mechanical Tools
|MORE CONCERNING MECHANICAL TOOLS.|
By Rev. ARTHUR RIGG, M. A.
CUTTING edges are sometimes doubled, and thus the chisel passes into another group of tools shears. The most common of these is the ordinary household scissors opened and closed by hand; when required for heavier work, then one handle is fixed in a . vise, and both hands can be employed upon the other lengthened arm (see Figs. 1 and 2). At other times this double chisel opens with a spring, and then the workman only employs himself in closing such upon their work (Fig. 3). Compound lever power is sometimes introduced, and, as an example of this, here is a pair of very light shears called the "little giant" (Fig. 4), the mechanical contrivances in which are so adjusted that we can, smoothly and without jar, cut an iron rod one inch wide by one-quarter inch thick. The lightness of the tool and the ease in cutting are very noticeable. It is an American contrivance, and the bar of iron is cut with the same ease as though it were of lead. This results to some extent from both jaws approaching each other. The arrangement of levers, cams, and stays, is worthy of examination after the lecture.
The use of the chisel, however skillfully handled, is not satisfactory over a surface wider than itself, although widened and made two-handed, as Fig. 5, and although the gouge has succeeded, or rather been planned to precede it, there is still a tendency almost unconquerable for the tool to follow the leadings of the fibres rather than cut through them at a very slight obliquity.
The only guidance either the axe, the adze, the pick, the gouge, or the chisel receives, is from the skill of the workman. Hence these tools produce such different work in different hands. However much it may be desirable to encourage skill in the workman, it is quite as desirable to furnish him with implements which shall make the least
demand upon the exercise of this skill—which shall, in fact, so assist the skill in one or more directions as to permit all its care in some other direction. The assistance which the chisel needs is such as shall not only prevent it running deeper into the timber than is desired, but shall enable it to be used with equal facility upon a broad as upon a narrow surface.
Given a rough piece of timber, nine inches wide and five feet long, to be smoothed by tools guided only by the handicraft skill of the workman, setting aside the adze as dangerous and unsuitable, the
probability is, that the tools selected would be gouges and chisels of various breadths and curvatures. The order of use would probably be, first, the narrow and deeply-curved gouges, ⌢, these to be followed by the shallower and broader, ⌢, these again to be followed by the chisels, using in the first place a chisel wider than a b. Let us consider what these tools would respectively accomplish if the timber is rough, as from the axe or pit-saw. The small gouge would corrugate the surface, ⌣⌣⌣⌣ the second gouge would enlarge the corrugation to this, ⌣⌣⌣⌣ and the chisel might render
these more irregular. Such considerations as these, combined doubtless with others, led to the designing of what may be generally called the "guide principle," and this has been extended to various branches of artisan labor. At present we are only concerned with the application of this principle to gouges and chisels. This guide principle may consist of a guide as to the depth of cut, or as to the form of the surface, or as to the direction of travel, or as to the correction of cross or longitudinal irregularities of surface.
The guide as to depth of cut is that which probably first presented itself as an important appendage to a chisel, and it has led to a form of tool of a very useful construction, although of limited range. The instrument is called a "spoke-shave." In this case the tool is that in Fig. 5 with the guide principle introduced, the depth of cut being determined by the nearness of the edge to a parallel wooden handle.
This tool, owing to the position of the application of the power, viz., the hands, and the tendency of resistance by the work to turn the whole tool in the hand, is not of general use; where, however, the curvature of surface varies, the parings to be removed are light, and the workman can have convenient access, the tool is one capable of doing good work, and, in some respects, possesses advantages over the plane, to which it probably formed an introduction.
The plane, in its most simple form, consists of a chisel inserted at an angle into a box, generally of wood, and with the cutting edge of the chisel projecting through the bottom of the box. If the actions of a workman be noted as he is smoothing wood with a chisel alone, it will be seen that he holds the bevel edge on the wood, and so elevates or lowers the handle as to secure a proper and efficient cut. Then he advances the tool in a line at right angles to its cross-section. If, now, instead of thus continuing to hold the tool, the chisel was so fixed in a movable piece of wood as to be at the same angle as the workman required, then, if the mouth were broad enough, and the instrument were propelled along the wood, a shaving would be removed very nearly the same as that obtained from the chisel alone.
In the arrangement thus sketched the workman would be relieved from the care needed to keep the tool at a constant angle with the surface of the timber. There is, however, a fixity of tool here, and consequently an optional or needful adjustment called for by any varying condition of the problem cannot be had. When operated upon by hand alone, if an obstacle to the progress of the tool is presented, as, for instance, a twist or curl in the fibre or grain of the plank—the presence of a knot—then the workman by hand can adjust the handle, and so vary the inclination of the cutting edge as the circumstances of the case require. Not so if the tool is securely fixed in a box as described.
While, therefore, one gain has been had, one loss has been encountered. Can the gain be made to more than counterbalance the loss? This can only be answered by observing the defects of the primitive plane, as hitherto described, and noting what hopeful elements it contains.
The front of the sole of the box will clearly prevent the penetration of the incased chisel into the wood, because it cannot now be drawn to follow the fibre, should it lead inward. Suppose, however, that in the progress of the work such a place has been reached as would have so drawn the chisel inward. What will happen? Either the strength of the indrawing fibre will be so great that the workman will be unable to propel the tool, or, if not thus impeded, he must by extra effort separate the fibre and so release the tool. This separation, however, will not be by the process of cutting, but by that of tearing, and shavings so torn off will have left their marks in the roughnesses which attend the tearing asunder of fibrous woods. Thus the tool will defeat the very object for which it was designed.
Now, what is it which so forcibly draws, or tends to draw, the tool downward below the surface of the timber? The forces in operation are the hand of the workman and the tenacity of the fibre. If the tenacity is greater than the power, the workman must stop. That the tool cannot follow the direction of the fibre is clear, because the front part of the wooden sole forbids the penetration; but that it may be brought to a standstill, or must tear off the fibre, is also very clear. The mechanician has therefore to consider how to defeat these tendencies, which, as now sketched, result from a collision between the indrawing strength of the fibre and the power of the man to crosscut the fibre by the tool, or else to tear it asunder and leave the surface rough.
Since the tool, as now contrived, cannot efficiently cross-cut the resisting fibre, and since that fibre has to be removed, the object must be either to prevent such an accumulation of fibres as will stop the progress of the tool, or to destroy the fibre piecemeal as it is operative for hinderance. Both plans have been adopted. A consideration of the former may prove introductory to the latter, which appears in almost all attempts to perfect this tool and its appended contrivance.
As the tool progresses, and the fibres become more and more impeding, it will be clear that a portion of this impediment results from a condensation of the fibre in the mouth of the wooden box. The more numerous the fibres admitted here, the greater will be the condensation. This state of affairs can be partially obviated by a narrowing of the mouth of the plane; such an act, of course, requires that the introduced chisel should enter less deeply into the timber being operated upon. Although thus abated, the cause is not removed, and even if so far abated as to prove no real impediment to the workman, yet the quantity of material removed on each occasion will be so small that the tool becomes one for finishing work only, and not for those various operations to which its present powers enable artisans to apply it.
To be the useful tool it is, the mouth must not be so narrowed, nor the inserted chisel so withdrawn, that the shaving is thus the thinnest possible. This led to a contrivance now almost universal, that of breaking the fibre so soon as it is separated from the piece of timber. The designer seems to have considered that, as soon as a short length of shaving had been removed, it would be well to destroy the continuity of the fibre, and so prevent an accumulative resistance from this cause. Hence, instead of allowing the cut-off fibres to slide up the inserted chisel, he bent them forward, in fact, cracked them, and so broke the cumulative indrawing force of them. This he accomplished by the use of what is now called the back-iron, and henceforth the boxed-in chisel loses its identity, and must be regarded as part of an independent tool.
The tool thus built up is called a plane, and from its general utility, and capability of adaptation to various forms and conditions, it is well deserving of the high opinions entertained of its powers. Three forms of this tool are in general use in English workshops, called the "jack," the "trying," and the "smoothing" planes. These are on the bench of all workers in smooth, straight-surface wood. Although externally alike except in size, they are yet used for different purposes, and each has a specialty met with in its construction. These specialties may now be considered.
After the wood has passed from the sawyer into the hands of the carpenter, the surface undergoes those operations which render it true and smooth. These three planes do this work. The "jack," usually about fifteen inches long, and the "trying" plane, ranging from eighteen inches to twenty-four inches long, but, in exceptional cases, far exceeding these dimensions, are to external appearances alike; indeed, some regard the different handles as the only distinctions between them, and that these handles show which must be used for rough work and which for smooth (see Fig. 8 as an example of the handle of a "jack-plane," and Fig. 9 as an example of a "trying-plane handle"). This is an error. There are other differences, but the main and leading one is the different form given to the edge of the cutting-iron.
Fig. 6. Fig. 7.
If the iron of the "jack" plane be looked at from the front end of the plane, the form of the edge will be curved, as in Fig. 6; but the iron of the "trying" plane is straight, as in Fig. V. Upon the curvature of the edge depends the efficient action of the "jack."
Sufficient has been said of the tendency of the fibre to draw the tool downward; but it must not be forgotten that the same adhesion of fibre to fibre takes place between the surface-fibres as among those below the surface. If tools excluded from this course of lectures had entered, we should have found that these connecting surface-fibres are separated by the addition of certain supplementary appendages to the tools. The depth to which the plane penetrates has led to the combination in one edge of such supplementary parts.
For the purpose of separating the surface connecting fibres, the jack-iron is convex. Note now its action. The convex sharp edge is pushed along an horizontal plank, penetrating to a depth determined by the projection of each vertical section below the sole of the plane. The ends of this convex edge are actually within the box of the plane; consequently (sideways) all the fibres are separated by cutting, and are therefore smooth and not torn. The effect of this upon the entire surface is to change the surface from the original section to a section irregularly corrugated. The surface after using the "jack" is ploughed, as it were, with a series of valleys and separating hillocks, the valleys being arcs from the convexity of the tool, and the separating hillocks being the intersections of these arcs. All traces of the tearing action of the saw have been removed, and from a roughened but level surface a change has been made to a smooth but in cross-section an undulating one.
The mechanician's next object is to remove these lines of separation between the valleys. For this, the trying-plane is required. The
trying-plane is longer than the jack, because the sole of the plane which is level is, so far as its size goes, the counterpart of that which the surface of the wood is to be; further, the trying-plane should be broader than the jack, because its object is to remove the valleys, and not to interfere with the wood below the bottoms of the valleys. If its action passes below the bottoms of the furrows, then occasion arises for cutting the side-connection of the fibres, and, however a workman may sharpen the edge of his trying-plane for this purpose, he in one respect has destroyed one object of the plane, because, so soon as the iron penetrates below the surface, so soon does the effect of the jack-action begin to reappear, and the cutting edge should pass from the shape shown in Fig. 8 to the shape in Fig. 7. The result of the trying-plane following the jack is to remove all the elevations of wood above the valleys the jack left; and, secondly, to compensate by its great length for any want of lineal truth consequent upon the depth of bite of the jack. Again, the mouth of the trying-plane is much narrower than that of the jack; hence the shavings removed are finer; therefore the slope of the iron, or its inclination to the wood, may be less than is the iron of the "jack"—hence the line of cut is 1 more nearly accordant with that of the fibre, and by so much the surface is left more smooth from the trying-plane than from the jack, as there is more cutting and less tearing action than in the jack. The reasoning hitherto pursued in reference to the purpose of this sequence of a jack and trying plane might and does legitimately produce the conclusion that, after the trying-plane has done its duty, the work is as perfectly finished as it can be. Custom, and perhaps other considerations, have established that after the long trying-plane must follow the short and almost single-handed smoothing-plane. So far as the form of the iron of the smoothing-plane is concerned, there is no difference between it and the one used in the trying-plane; each (as across the plane) is straight, the corners being very slightly curved, but only so much as to insure that they do not project below the line of the cutting edge.
The facet edge and inclination of the cutter to the work, and the position of the back-iron, are now under consideration.
It would seem that, while the trying-plane leveled down all the elevations left by the jack, and brought the surface of the wood as a counterpart of that of the plane, there might be, in the fibre or grain of the wood, twists, curls, and other irregularities, which, while leveled, were yet left rough in consequence of the direction in which the cutting edge came upon them. Indeed, this cutting edge in a long plane, which must advance in the direction of its length, must at times come across a large number of surfaces where the fibre is in opposite directions. The consequence is, that there will be various degrees of smoothness; for good work these must be brought to uniformity. This is effected by passing a short-soled plane over the respective parts of the surface in such directions as observation may suggest. Hence the smoothing-plane is of use chiefly to compensate for such changes in the direction of the fibres of the wood as the longer length of the trying-plane could not conveniently deal with. Hitherto, we have regarded the plane as arranged with a "guide principle" which shall always repeat a straight, level surface. The guide may, however, be the counterpart of any required surface. The plane made of iron, now in my hand, has an elastic steel sole, which, by means of adjusting screws, enables a workman readily to convert a straight-faced sole into one either concave or convex. This is an American production (see Fig. 10).
There is also in this and other planes a mode of fixing the iron which deserves more general adoption than it receives, viz., by a cam-action. It will often be noticed that, where the holding-wedge binds on the box of the plane in our ordinary planes, the wood has split. This arises from a commendable but, in this case, too strict a care for a good fit; hence the wedge is made tight where it should be slack.
- From a lecture delivered before the London Society of Arts.