ROBERT GIBSON GRISWOLD

There are many special forms of tools for lathe use, the designs of which had their origin in some particular difficulty experienced while using the common or straight tool. This is especially true of the special spring thread tool about to be described. Many of the readers have doubtless experienced difficulty in cutting smooth threads on the lathe, especially when the material was very tough or hard, or possessed both characteristics. It is almost impossible to get a perfectly smooth thread, no matter how keen the tool is kept or how carefully the tool is fed to the work. This difficulty arises from several causes, which may be : a slight looseness in the tool carriage or post, slight play in the head spindle, or the "springy" nature of the piece upon which the thread is being cut. The latter cause is, perhaps, most frequent, and results in the point of the tool digging into the work and then slipping, leaving the surface either torn and ragged or full of chatter or ripple marks.

The spring tool illustrated in Fig. 1 was primarily designed to overcome the tendency of the tool to dig in, and just how well it performs this function may be realized when the reader is told of a four-inch, double-threaded tap made very recently of a high grade tool steel in which the last or finishing cut left the surface of the thread so true and smooth that perfect reflections could be seen in it of objects held close to it. Repeated attempts had been made to finish it with the ordinary straight tool for roughing out the thread, but to no avail, for no matter how slowly the work revolved or how much lubricant was added, the chattering would persist. The writer has seen some remark-able finishing cuts taken with just such tools, especially when the edge is ground so that it stands at an angle to the horizontal plane through the center of the piece. This will take a shearing cut and leaves the surface remarkably smooth. There may be those who will differ with the writer in this, but he is speaking from an extended experience with some very high class work in which this tool only could be used to secure the desired result.

The shank a is forged from a piece of tool steel to the general outline shown, but without the hole b or slot c. The rough forging is first finished all over ; then the hole b is drilled with a | in. drill and the §lot c cut with a milling cutter if possible, or with any other means that the amateur may find at hand. A 1/4 in. hole is then drilled in the end d to take the tools e. This hole should be reamed so that the tools will be a snug fit therein. A small set screw f, bearing on a flat in the top of the tool e serves to bind it firmly in place. After all work is done on the tool it is hardened and the piece drawn to a spring temper. The tampering of the points e needs no special mention.

When in use, unless on very light work, a small block of wood, such as a piece of cherry, is fitted in between the sides of the slot, as at g, which prevents too great vibration, and yet is yielding enough to allow the tool to recede slightly from the work when a hard spot is run into. Then, when the same spot is reached again, the tool will not back off as much and the piece will be finished very smoothly. Other points besides the thread cutting tool may be used with equal facility.

Probably many of the readers have experienced some difficulty in boring out cylinders of any great length on a small lathe, owing to the spring of the tool when cutting so far away from the point of support. The cut must be very light and the feed very slow if even a fair job is expected. To overcome this difficulty a boring bar having great rigidity is used.

The bar, Fig. 2 a, may be made of a piece of 3/4 in. machinery steel for nine and eleven inch lathes, and perhaps a 1 in. diameter for a thirteen in. lathe. The greater stiffness possessed by this bar the better work will it perform. This bar should be provided with a 1/4 in. or | in. hole in each end as shown, for the reception of the various points to be used with it. A set screw is also provided for clamping the point firmly, and these points may be cut from a piece of Stubb's drill rod, ground to shape and hardened, no forging being necessary. After the bar is finished as smoothly as possible, and of even diameter throughout its length, it is case-hardened. This process is accomplished by heating the piece in a length of iron pipe about two inches in diameter and five inches longer than the bar. The bar is placed in the center and packed all around with a mixture of charcoal and bone-dust, or in place of the latter scraps of old leather mixed with a little ferro-cyanide of potassium. The ends are stopped with clay, and the whole put into a fire and heated to a cherry red for four of five hours ; the bar is then removed and plunged into water, care being exercised to lower the water in a vertical position, quickly, and afterwards rapidly agitating the water by moving the piece to and fro. This should prevent the bar from warping, as it most certainly will if dropped in sideways.

The hole f is put in at an angle in order that a point may be used up into a corner. Thread points may also be used for cutting internal threads. Of course ;he size of the bar limits the diameter of the hole to be bored to slightly more than its own diameter, plus the length of point protruding from the side. For smaller holes than this it is well to have a half-inch bar with a large shank or a split bushing to fit into the clamp d.

The clamp d is made of cast iron and fits the tool post block. The same bolt e that secures it to this block also serves to clamp the bar firmly. The block d should have a tongue fitting into the tool-post slot which will prevent it from turning when the tool begins to take a cut. As the feed screw forces the tool into the work there is a great sidewise pressure which tends to twist the block d and thus relieve the pressure. It should always be remembered in boring that the tool support should be as near the cutting point as possible, as every bit of overhanging bar that is unnecessary gives the bar just that much greater tendency to spring.