ROBERT GIBSON GRISWOLD

The screw-cutting lathe is a machine peculiarly adapted to the boring of cylinders, field castings for motors and dynamos and, in fact, almost any long, cylindrical holes in any piece that can be fastened to the carriage of the lathe. To do this work properly, the piece must be firmly supported and secured to the carriage. Most lathes have a carriage mounted on very long V bearings which give great stability, and these bearings are provided with T slots into which bolts may be fitted for strapping work to the blocking placed on the carriage. Many of the small amateur lathes are not so provided, but with a little thought it is possible to rig up the carriage so that a very respectable job can be done.

Let us consider the case of boring a three inch cylinder for a small gas or steam engine. A block of hard pine or oak is cut to such a shape as will support the cylinder in question at both ends, and for this purpose the V shaped bearing is by far the best, as it will prevent slipping better than a circular seat. In Fig. 1 is shown how these blocks may be secured to the carriage. The bolts aa are of such a length as to pass completely through the two blocks, and one end is threaded for the greater portion of the body length. This enables the nut to be screwed down to the lower block, and another nut may be brought to bear against the upper block, using only one bolt in each end.

It is a good scheme in doing this class of work to lay out the carriage full size on a piece of paper, then mark the exact height of the lathe center above the carriage, draw in the section of the cylinder, and then the determination of the best sized block is readily determined. The V should not be too shallow, neither should it be so deep as to cut nearly through the piece. While requiring a little more time, it is a very good thing to tack a piece of sheet copper or brass in the V upon which the piece may rest. This has a two-fold object. It not only prevents the cylinder being driven into the softer wood by the pressure of the clamp above, but it adds greatly to the holding power of the clamping arrangements. The metal, being soft, takes a firm grip on the casting, and it will require a greater force to twist it under the pressure of the tool. This also admits of a deeper cut being taken, although in the case of amateur work it is always advisable to work a little more slowly, take smaller cuts and secure a finely finished job, which is not always possible when the work is hurried.

Having determined about the size of our V blocks, the holes are drilled in the ends slightly larger than the bolt diameter so that they may be shifted to bring the cylinder exactly into line. They are then placed in position and clamped in place. If your lathe is not provided with slots for the reception of the bolt-head," it will pay you to drill four holes for this purpose, tapping them to receive the threaded end of the bolt, which then becomes a stud.

The cylinder is now placed in position and lined up by placing small strips of sheet metal between the bearing points and the block so as to raise the center, if necessary, until it coincides with the center-line of the lathe. To determine the exact position of this center is not very difficult, but it should be done with care, else when the cylinder is bored it may be found that the bore is not perpendicular with the flange or parallel with the sides of the casting. Perhaps the easiest method is to stretch a fine iron wire through the spindle of the lathe, securing it at the farthest end by twisting about a small nail which may be placed across the open end of the spindle. The wire is then passed through the center of the casting and secured to the tail spindle, which is not so easy. A wooden plug may be fitted to the tail-center seat and a small hole drilled in its end through which the wire may pass to be fastened at the small end of the taper. The moving of the spindle outwardly will then draw the wire taut. It is now a simple matter to test the center of the casting with a pair of inside calipers, measuring on all four sides until the wire passes exactly along the center-line of the casting, coinciding therewith. This method is used in a great many cases where it is Impossible to get a plane of reference from which to work with a surface gauge. This can, however, be done on a lathe, because the ways are perfectly parallel with the center line. The open ends of the cylinder are provided with sticks driven in so that they pass over the center, and a small piece of tin is fastened to the flat side as shown in Fig. 2, by bending up the four corners of a square and driving into the wood. Then with a pair of hermaphrodite callipers the center is scribed on the tin and located by a very light prick-punch mark.

Now when the casting is ready for setting, a surface gauge is placed on the top of the lathe bed, and the point adjusted so that it exactly coincides with the points of the lathe centers at either end. If the lathe centers are out of line in this respect they should be fixed at once, as accurate work cannot be be done on them. If found correct as to height, the casting is placed on the blocks and set up with "shims" (the small strips of sheet metal spoken of above) until the center marked on the tin coincides exactly with the point of the surface gauge at either end. Yon will find when adjusting this casting that the movement of one end to secure exact coincidence will probably throw the other end out, especially if it has just been accurately set. But this can be nothing more than a " cut and try " method, and the differences must be halved at either end by repeated trials until it is just right.

Then the gauge is set (it is better to have another gauge for this work) so that it may be used from the side of the ways for lining the casting in that direction, as shown in Fig. 1. The upper blocks must be screwed down on the castings very firmly after the setting is about right, and the alignment tested again, as the pressure of the upper block will very likely throw it out somewhat.

Another very simple method, but not always applicable, is to use the points of newly sharpened centers, bringing them into coincidence with the centers marked on the tin. Sometimes these centers cannot be readily seen, but if they can, the method may be used, but it will be found somewhat more difficult to work in the cramped space thus provided between the ends of the casting and head and tail-stock.

Presuming that the casting has been properly set and secured, a boring bar is passed through the middle of the casting and swung on the points of the head and tail centers. (If this bar is perfectly true and straight, it may be used instead of stretching a wire from center, measuring from its periphery to the inside of the bore exactly the same as was directed for the wire.) The cutter is sharpened and placed in position so that it extends exactly the same amount on either side.

For those not familiar with the boring bar, it may be well to give a description of how one may be made. No matter what the work is upon which it is to be used, make it stiff - of as large a diameter as practicable. The bar is supported on the lathe centers, and the pressure of the cut will easily spring it out of line, if not quite stiff. For the 3 in. cylinder above mentioned, a bar not less than 1 1/4 in. in diameter should be used, and about 18 in. in length. One or more holes may be drilled into it to accommodate the cutters, which may be made of 3/8-in. Stubb's drill rod, or self-hardening steel. A set screw is let into the side to firmly clamp the cutter in position, as shown in Fig. 3.

It may be possible that the novice will experience some difficulty in setting a cutter that cuts on both ends so that it will cut even, and it would, in that case be better to Use a shorter cutter, cutting on one side only. The bar will spring some, especially when cutting through the scale. But any irregularity may be corrected by taking smaller cuts afterwards. Never crowd the work nor expect the cutter to do too much. You may move your casting slightly on the blocks, which would cost more time in setting than would be required in running half a dozen small cuts.

Start the first cut so that not more than a good thirty-second of an inch is taken off, and with a feed of not over 1-100 in. to the revolution. The feed, of course, is secured by moving the carriage, and the change gears at the end of the lathe may be so set as to secure almost any feed desired. After the hard scale is cut through and cleaned out then the feed may be increased to a 64th, and the depth of the cut increased slightly, but if foot power is being used, this will be found quite as much as one wishes to push.

For the finishing cut grind the tool to round point shown in Fig. 4, but do not have it take too broad a cut as it will cause chattering. Measure carefully both ends of the cylinder to make sure that the bore is perfectly cylindrical, as lathes will sometimes bore slightly taper, owing to the centers not being exactly in line. If there are any ports in the cylinder, which is usually the case with a two-cycle engine, it may be necessary to take exceedingly fine cuts while passing these openings, as the pressure of the tool is then relieved and the bar will spring back to its original shape again, causing the tool to cut slightly out of true when it takes hold again on the opposite side of the hole.

While the cylinder may and should be faced while thus set to insure that it will be exactly perpendicular with the bore, it requires great patience and care. The cutter will have to be sharpened on the side so as to make a side tool of it, as shown in Fig. 5, and very light cuts taken while the carriage is fed along by hand, A very good job may be done in this way, however, and it will be truly perpendicular with the cylinder bore. After the cylinder has once been moved or removed from its setting, it will be next to impossible to get it exactly into line again, so one must use great care throughout the work to insure that it does not slip.