The laps alluded to, which are to be mounted on spindles like the circular saw, are composed of wood and metal of determinate forms. First, there is the thin sheet-iron slitting wheel used by lapidaries, and which, when charged with emery, or sand and water, forms the nearest approach to the circular saw. It is, in fact, the circular saw of the stone worker, the ordinary saw used in their trade being a flat blade without teeth, stretched in a wooden frame and similarly fed with sharp sand and water in lieu of being made with teeth, the latter being replaced by the grittiness of the material. The circular plate of iron, brass, or lead alloyed with antimony, mounted on a spindle vertically, is used in a similar manner for grinding flat surfaces with the aid of emery, crocus, oilstone powder, and other substances, and the same is used edgewise for other work, as grinding and polishing tools, the section of the lap being such as will form the article required by reproduction of the revised plan of its own edge. Thus a lap running horizontally with a convex edge will produce the concave form required in beading tools, the latter, however, are more conveniently ground on brass cones mounted like the arbor chucks used for turning washers, rings, &c. The face of the tool or flat side is held towards the small end of the cone, and the latter is armed with flour emery. See [Fig. 196].
Fig. 196.
Before describing the eccentric and other compound chucks, a few examples will be given of the method of turning some of those forms in which the circle does not appear, where, in short, the boundaries of the figures are straight lines. It seems at first sight impossible to produce in the lathe by any simple means such solids as are formed from squares or triangles, of which the cube or die is the most common and most generally understood. This can, however, be accomplished, and a number of mathematical problems may be clearly demonstrated to the eye by such works skilfully made in the lathe. In this kind of work it is absolutely necessary to strive after perfection. In short, as Bergeron rightly says, the work imperfectly done is simply worth nothing at all; but when accomplished with exactness, nothing can be more worthy of a place in a cabinet of curiosities. The usual method of turning a cube is by shaping it out of a perfect sphere, but as the latter can hardly be made without a special slide rest made for the purpose, a method of turning the cube by hand alone will be given, the foundation of it being that which the lathe so readily produces, namely, a cylinder. By far the most proper material for the work in question is sound boxwood, and special care is to be taken to keep all angles as sharp as possible, and therefore to cut clean with sharp tools, and to avoid as far as possible the use of sand or glass paper. [Fig. 197] shows a square described in a circle. The non-mathematical reader may be told to draw through the centre two diameters at right angles to each other, and to complete the square, as in the figure, by joining the extremities. The largest square that is possible to be drawn in the circle is thus described. This square will form one face of the cube, and the diameter of the piece of wood must be regulated according to the proposed size of the finished work. It is evident that this diameter is equal to the line drawn from corner to corner called the diagonal of the square. Now, in turning a ball or sphere, a cylinder would be turned of the exact length of A, C, because every measurement taken on a diameter of the sphere would be of that length. In the present case, however, the cylinder must be of the same length as the line A, B. Turn, therefore, with great care a cylinder of any desired size, gauging it carefully with the callipers and squaring off the ends truly. On one end, in which the centre point just remains visible, draw diameters and construct the figure [197]. Turn out a chuck to fit it exactly, and let the bottom of this chuck be truly square. Take the precise length of the line A, B, with finely pointed compasses that can be fixed with a screw, and measure off the same, and mark it on the side of the cylinder. When the figure is placed on the chuck, a mark must be traced round it at this point, A, B, [Fig. 198], and this must remain to the end. It may be made with a hard pencil or steel scriber, and, though distinct, must be very fine. It is at this line the wood will have to be cut off, but in this operation keep beyond it so as not to erase it. While the piece is in the chuck, rule lines, as E, G, from the points C, D, E, F, 198. Bisect also the length of the cylinder by the line, H, K, also finely drawn. Draw two more diameters, as shown by the dotted lines bisecting the sides of the cylinder, which is now divided round its circumference into eight equal parts. The lines, E, G, &c., can be ruled along the edge of the rest, or, if the latter is untrue, the cylinder may be laid on a plane surface, and a scribing block, or gauge, [Fig. 199], may be drawn across it, or, lastly, a small steel square may be used; one part being carefully placed on the lines at the end of the piece in succession, the other part will lie evenly along the side, and will form a ruler by which to work. A division plate on the lathe pulley will facilitate the above measurements, but they can be readily made without it, and once carefully marked, all the guides required for cutting out the cube will be complete, and the work can be proceeded with, with confidence and decision. Proceed, therefore, to cut off the piece at A, B, with a parting tool, but with the precaution already named of not erasing the line by so doing. Now prepare a chuck which will take the piece lengthwise, [Fig. 1992], and insert it in that position to the depth of the diameter so as to hold it securely, and the central line will show whether it lies evenly (which is, in fact, the use of this line). If even, a point-tool held on this line will form a mere dot upon it; if uneven, it will make a circle as it revolves. Place the rest across the face of the work, and, beginning at the centre, cut carefully towards the outside until you have cut away the wood as far as the line, A, B. You will thus complete one face of the cube, and an inspection of the end of the piece, shown black in the sketch to show the quantity removed, will prove that you thus produce a right line, C, D. Take out the work, and reverse it, and operate similarly on the opposite face; but in every case do not quite obliterate the lines first marked as guides. You have now a piece shaped like [Fig. 200], A and B, and must make a smaller chuck to hold it; you have then to cut away in a similar manner the remaining parts of the cylindrical portion, and the cube will be complete. To finish the sides more neatly, lay upon the table the finest sand paper, and tack it at the corners, and with gentle movements work down precisely to the guide lines. This requires extreme care, for if once the piece is but in the slightest degree tilted up, an angle will lose its sharpness, and the beauty of the work will be marred. Hence we recommend to cut with sharp tools, instead of trusting to the finishing process. To cut, however, exactly to the line is very delicate work, and to the less practised hand the use of the sand paper on the faces in succession may be the necessary expedient. The main secret of sharp edges in works of wood and metal is to finish with hard substances, as emery stick or glass paper glued to a piece of wood, or the same nailed on the bench, and to try always to work on the centre, leaving the edges or angles to take care of themselves. When the reader has made a cube, as above, that will bear delicate measurement, he will be more than usually gratified, and will be qualified for still more difficult work. A chuck figured by Bergeron, [Fig. 201], is very convenient, as it holds the work truly central; the jaws work simultaneously by a right and left-handed screw as in the die chuck. It is, however, perfectly easy to do good work without it, but the chucks should be carefully made, turned very flat at the bottom, with side truly perpendicular. A little extra care bestowed upon chucks will save many disappointments, and conduce to good work. The formation of a four-sided solid, consisting of triangles solely, in the lathe is a work of difficulty, owing to the impossibility of fixing the work satisfactorily in an ordinary chuck. The natural way to form such a solid is to turn a cone A, B, [Fig. 202], and on its base to mark a triangle of the required size. It is then necessary to place the cone in a chuck, so that the ends of one of the lines thus marked and the apex of the cone are precisely level with the surface of the chuck, as shown in [Fig. 203]. But it is evident that adequate support is not thus obtainable. The apex of the cone cannot, in point of fact, be inside the chuck at all, as it is necessary to cut clean to its extremity, and even the base of the cone is imperfectly held at two points. Hence it becomes necessary to make use of "turner's cement," and to imbed the work fairly in it, while both are warm, to such a depth as will hold it securely and still allow the guide lines to be seen. The latter should be carried from the three angles of the triangle marked on the base to the apex. On the whole, this is the easiest method of fixing such work in the lathe; and if the piece is itself warmed before being placed, there will be time to adjust it precisely before the cement is cold. To do this, place the rest parallel to the lathe bed, hold a pointed tool steadily upon it, and note whether, as the work slowly revolves, the three points in question, viz., the two angles of the triangle and the top of the cone, are in one plane. When they are so placed, the rest is turned to face the work, and the material is carefully cut away till the gauge lines are just reached.[14] A pyramidical solid with a square base may be similarly turned. The following is the method of preparing the above turner's cement:—Burgundy pitch, 2 lb.; yellow wax, 2 oz. Melt together in a pipkin, and stir in 2 lb. of Spanish white. When the whole is well mixed, pour it out on a marble slab and roll it into sticks. Fine brickdust, whiteing, or any similar substance finely pulverised, will answer equally well to add to the pitch and beeswax. This cement is very useful, as it will hold the work firm enough to turn carefully, and nevertheless a slight blow will loosen it. To clean it off, warm or dip the work in hot water and wipe quickly with a piece of cloth. The above is from the "Handbook of Turning," the author of which has copied from Bergeron both the recipes given in his work; the one here described is stated to be specially serviceable in cold weather. It is perhaps rather less brittle than the first of Bergeron's, and for this cause is the best for general use. Holtzapffel sells this at one shilling the stick, which is of convenient size and generally of excellent quality. Bergeron gives a method of turning pilasters or balustrades, which is of great ingenuity, and applicable to work of various sections. The rectangular and triangular sections illustrated are not, indeed, strictly rectilineal figures, as the sides of the balustrades thus formed are not flat but rounded surfaces; they are, however, sufficiently curious, and when well turned are interesting specimens of lathe work. Bergeron's description relates to the pole lathe, or to work mounted between two centres with a pulley cut on the work itself to receive the lathe cord. The ordinary method of mounting on a foot-lathe will, of course, be much better and the whole operation of more easy performance. Let it be required to turn a moulded pillar or balustrade of the section [Fig. 204], viz., a triangle with slightly curved sides. A piece of wood of the requisite length is planed up accurately to a triangular form, or, as it generally happens that a set of such pilasters are required, a number of such pieces are prepared which must be accurately planed to the same size, for which purpose a gauge or template, [Fig. 205], should be made use of. Six or eight of these, if not too large, may be operated upon at the same time; but as six pieces of two inches across each face require a cylinder of about eight inches diameter, the number must depend on height of centres. The larger the cylinder, however (which, in fact, forms a chuck), the more nearly will the sides of the finished work approach to plane surfaces, as they will form arcs of a larger circle. Let a cylinder, then, be turned of sound wood with a reduced part at one end so as to form a shoulder. Divide the circumference into twice as many equal parts as there are pieces to be turned, the divisions being equal to one of the sides of these pieces as measured in a cross section. These divisions are to mark the positions of the grooves or channels in which the triangular strips are destined to lie. Half the circumference will be so cut out, the alternate divisions being left. Thus, to turn four pieces, each two inches wide, a cylinder of sixteen inches will be required, affording four grooves two inches wide, and four intermediate pieces forming the partitions between the grooves. The latter must, by means of the saw and chisel or other tools, be made to receive the strips exactly, and the ends of the latter being carefully squared off, are to be made to rest against the plate C, [Fig. 206], which is cut out and screwed against the shoulder, after the above-mentioned grooves have been cut. The whole are then secured by two rings, with screws. Probably the stoutest india-rubber rings now made would answer as well as the iron clamps in Bergeron's description. Lay the strips in their places, but as they are flat they will not form part of the cylindrical surface, but will lie lower, as [Fig. 207], or higher, [Fig. 208]; the latter is the best, and the pieces may, for this cause, be cut out a trifle deeper than ultimately required and they may be planed down a little to remove the angles and assimilate them to the cylindrical surface. The whole may then be turned together so as to form a plain cylinder, the clamping rings, [Fig. 209], being shifted when requisite. The whole is now to be formed into a balustrade, but as the proportions of the mouldings of such a large cylinder would not suit the small pieces, the hollows must be less deep, and the raised parts less prominent than they would ordinarily be made. The clamps are now to be loosened, and the pieces reinserted with another face upwards, the flange or plate against which their ends rest forming a gauge or stop to ensure their position, without which precaution the mouldings would not eventually meet at the angles. In cutting a fresh side the utmost care is requisite, for if the work on the original cylinder is cut by the tool, it will be impossible to restore it, and the work will be spoiled. In replacing the strips, let the finished part lie below, so as to come first in contact with the tool, by which the angle will be clean. Extra care is for this purpose required in cutting the last side. It appears to the writer that another precaution should be taken which Bergeron omits, namely, to arrange the mouldings so that certain parts are left of the original size of the wood, in order to retain a certain number of points of contact with the sides of the grooves, so that the strips shall not fall deeper into them than at first. The extremities of the strips should certainly not be left smaller than the central portion, or the pieces will rock on the latter while in process of being turned. The two ends, therefore, should be allowed to retain their original triangular form, forming base and capital of the pillar, or the pattern may be so planned that, after the extremities have been left as supports they may be cut off when the work is complete. Care must be taken, in working as above, to have the cylinders so large above the estimated size that the inner apices of the triangles do not nearly meet at the centre, else the whole chuck would be very weak and split into triangular strips. The lathe called a spoke-turning lathe would accomplish this kind of work in a far more easy and speedy manner. The balustrade would have to be made by hand as a pattern, and cast in metal, and any number could be produced precisely similar. The lathe in question is on the following principle:—The frame carrying the tool (a set of revolving gauges) is made to oscillate backwards and forwards to and from the piece to be operated on. This is accomplished by its having attached to it a roller or rubber working against the cast-iron pattern placed parallel to the work, and below or one side of it. The rubber and frame are kept against the pattern by a strong steel spring. The cutters also travel in a direction parallel to the axis of the piece. Hence any elevated part of the pattern causes the tool to recede from the work in a corresponding degree, and a hollow allows a nearer approach of the tool. Thus, as the tool is carried by a screw slowly from end to end of the work, it is made to advance and recede in exact correspondence with the form of the pattern. An immense deal of work is done in this way, such as balustrades, spokes of wheels, the long handles of the American felling axes, and similar irregular forms.
[14] With the universal cutting frame this kind of work is much more readily accomplished.
Figs. 197, 198.
Figs. 1992, 199.