Fig. 231.
Figs. 232, 233, 234.
X
Combinations like [231], in which the ellipses intersect, cannot be so obtained. Hence the oval chuck is provided with a wheel, either racked to work by a tangent screw or fitted with a spring catch, by which it becomes a dividing plate. This wheel revolves on a central pin[18] fixed to the middle of the sliding plate, and carries a screw of the same pitch as that upon the mandrel to which other chucks can be attached. By this means the axial lines of the ellipse can be varied in direction. This addition is shown in [Fig. 232], which is a section, and [233], which is a front view. In the former, A is the wheel, which, as previously explained, should be so arranged as to contain a number of cogs divisible by as many figures as possible; 96 is such a number, being divisible by 2, 3, 4, 6, 8, 12; 72 is also a good number, as it will divide by 2, 3, 4, 6, 8, 9. If the edge is racked and moved by a tangent-screw with divided head a greater range can be taken and finer work done. In this case the face of the wheel can be marked with divisions, and a fine steel pointer, as shown at F, added to count by. The pin B, which is firmly attached to the centre of the slider plate, must be strong, and the lower part at least should be conical. It is drilled and tapped at the smallest end, and when the circular plate with its screw is slipped upon it, a screw, E, the head of which is countersunk into the face of the large screw, retains it in place. The slide, C, has a recess turned to fit the wheel plate, and the latter is cut as shown at [X], which ensures a more accurate bearing than if it was left flat on the lower surface. In making this chuck certain precautions are necessary. In the first place, the guide ring fixed to the poppet must be exactly concentric with the mandrel when in its central position; and when it is drawn by the adjusting screws to the right or left the central line must remain parallel with the surface of the lathe bed. To ensure this centrality it is necessary to turn its outer surface when it is in position on the lathe head. So at least says Bergeron; and it is perhaps the best method whereby to ensure the accuracy that is required.[19] For this purpose Bergeron directs the use of a cutter similar to [Fig. 234] attached to the mandrel as a chuck, the edge which is on the inside of the bent part at a acting on the exterior of the ring as the mandrel revolves. The screws allow the tool to be advanced closer to the ring as the work proceeds, while they secure it at any desired point. Such a contrivance as this, used merely as a finishing tool to correct any slight error, is no doubt sufficiently satisfactory. The various parts of this and other compound chucks should be first turned separately to near the required size, and accurately finished when in their respective places upon the chuck. Any parts which present a difficulty from the impossibility of retaining them in place while operated upon, may be soldered with tinman's solder, and thus turned, after which the application of moderate heat will detach them, and the fluid solder can at the same time be wiped off with a pledget of tow or cotton waste. As many of our readers may wish to make such apparatus as the above, it may be desirable to add a few directions for the preparation of chamfered edges such as those of the slide and guide bars, the latter of which should be of iron or steel. Let the slide, for instance, be cast as a rectangular plate and the two flat surfaces be roughly levelled with a file. One of these must now be made perfectly true, either by mounting it with solder upon the face plate of the lathe, and levelling it with the aid of the slide rest, which is perhaps the safest plan, or by careful working with the file, using a straight-edge in all directions, and finishing by careful grinding upon a flat stone slab with water, or on a wooden grinder charged with emery and oil. After one side is finished, the opposite face may be similarly treated; but for this the plate may be secured to the finished surface of the lower plate of the chuck itself, and turned with a tool fixed in the slide rest. The edges must now be filed truly at right angles to the sides, care being taken to keep the long sides of the plate parallel. (The short sides or ends will be rounded by being turned true with the edge of the chuck.) The work must now be tested with the straight-edge and small steel square, and any error carefully corrected. Of course, if the reader is the happy possessor of a planing machine, all these operations will be facilitated and accuracy more likely to be ensured. It may here be mentioned that, to supply the want of such planing machine (a want often felt by amateurs who have not mastered the use of file and scraper), Monro, of Gibson-street, has cleverly added a planing apparatus to the ordinary foot lathe, rendering the latter tool complete for all purposes of amateur engineering.
[18] This pin should have been shown of a conical not cylindrical form, and much stronger in proportion.