placed in the center, and if that runs out of truth it will greatly magnify the amount of running out which will be indicated by the degree of movement of the indicator point.

The internal construction of the usual pattern of three-jaw universal chuck is clearly outlined at B, C and D, Fig. 76. The view at A shows the chuck assembled. That at B shows the bottom half, which is attached to the head stock spindle. At C the bevel rack that is employed to cause the jaw-regulating screws to move in unison is shown. This bevel rack engages the bevel pinion shown on the adjusting screws at D. A movement of any one of the screws will therefore produce a corresponding and equal movement of the other two. At E the internal mechanism of a fourjaw chuck is shown. At F, a face plate fitted with chuck jaws is depicted. When work is supported on a mandrel it is necessary to provide some means of turning it, because the frictional contact on the live center (that carried by the head stock) is not sufficient to turn the arbor against the resistance of the cutting tool. Lathe dogs are used to turn the work, these being simple clamp members having projecting tails designed to engage one of the slots of the face plate. For round work the form shown at G 1, Fig. 76, is commonly used. The shaft or arbor to be driven is placed in the hole A, and firmly secured by tightening the clamp screw. For work other than round, the lathe dog shown at G 2 is very satisfactory. The lathe dogs at G 3 and 4 are special forms that can be used with either round or irregular work. The simplest form of arbor and that commonly used is shown at Fig. 76, H. This is a piece of steel ground to some standard size, but having a slight taper with the ends flattened to permit of secure holding by the lathe dog clamp screws. While the simple mandrel is a popular form, it has the disadvantage that the constant driving on and off of the work will produce depreciation and the mandrel will become reduced in size. Solid arbors are usually obtained in sets ground to standard diameters varying by thirty-seconds or sixteenths of an inch. It is conceivable that there would be many pieces to be handled that would not fit any standard solid arbor. In order to handle these odd size pieces an expanding support, either of the form shown at

Fig. 75, E, or 76, I, can be used. A mandrel of this kind is arranged so that the pieces designed to grip the work can be forced tightly against the piece to be turned by locating the expander farther on the taper, which is greater than that of the solid form.

No lathe is complete without a well-selected outfit of cutting

tools, which may be obtained in a great variety of forms. of the most popular types for all around work is the "Champion" tool holder, which takes various cutting points, which can be readily changed in the master holder. This is shown at Fig. 77, A, with a simple round nose-turning tool in place. As will be ap

parent, the tool which does the cutting is ground from a piece of square tool steel of proper size to fit the square hole in the body of the tool holder, which is usually a steel forging. Another form of "Champion" tool holder used for thread cutting is shown at B, Fig. 77. The cutter may be adjusted as it wears or is reduced in size by grinding by loosening the clamp screw and raising the cutting tool which is provided with a series of ratchet teeth, and then once again tightening the clamp screws which brings the ratchet teeth on the cutter and on the tool holder into positive engagement. The proper angles for a cutting-down tool are shown at C, and a side tool at D. The angle on the front edge of the tool is known as a clearance angle, while that from the cutting point back is known as the rake on a straight cutting-down tool. On the side tools there is another angle to be considered, known as the top angle. This is clearly indicated. The form of the cutting point used depends largely upon the characteristics of the material to be cut. The first consideration relates to the softness of the material, the other to structure, whether it is crystalline like cast iron or fibrous like wrought iron. The clearance is added to the tool to prevent it from rubbing on the work, while the degree of rake determines the cutting ability or sharpness of the cutting edge, and gives freedom for the chip to leave the work. A lathe tool should always be set so the cutting edge will be very nearly at a point that would correspond to a horizontal line drawn through the center of the work. If a tool is set too low, it will tend to dig into the work and force it from the centers, whereas if it is set too high, the angle of clearance will be reduced and the work will rub against the bottom of the tool. Many machinists favor setting the cutting edge just a little above the center or at a point corresponding to about five degrees above the horizontal line drawn through the axis.

A complete set of cutting tools for use with the Champion tool holder, shown at Fig. 77, A, is clearly outlined at Fig. 77, E. The tools are made of various grades of tool steel, which is a high carbon alloy capable of being hardened by raising its temperature to about 1500 degrees Fahr. and then quenching in water, oil or brine, according to the degree of hardness desired. Various alloy

steels containing tungsten, molybdenum, cobalt, and other substances are also obtainable, these possessing desirable qualities such as ability to keep their edge at greater heat than tool steel of the straight high carbon form, or of having greater resistance for cutting tough metals such as the chrome-nickel or chrome-vanadium steels so widely used in automobile construction.

The Armstrong tool holders are made in a variety of patterns,

Fig. 78.—Armstrong Tool Holders for Many Uses.

the most common of which are shown at Fig. 78. As each is named in the illustration, further description is unnecessary. The general construction is the same for all tools, the main portion or body being a steel forging not subject to deterioration, while the cutting point or tool is readily removable for grinding or replacing. While the tool holder has many advantages, it is often necessary to make special lathe tools such as when turning filets, boring and other operations where the conventional form

of tool holder could not be used to advantage. Before the advent of the tool holder, lathe tools were forged of tool steel by the blacksmith according to the individual preferences of the machinist having the work done. While the forged tools are entirely satisfactory, it is not practical to use special cutting steel on ac

Fig. 79.-Set of Ordinary Hand Forged Cutting Tools for Use in Lathes.

count of the great cost of this material. As practically the only point where the high grade steel is desired is around the cutting edge, it will be apparent that it would be very wasteful to use that material for the body of the tool, which could be just as well made of a cheaper grade of steel. This, of course, is the great advantage of the Armstrong tool holder and similar devices.