An example of a boring tool which is also used for facing out a pocket on a turret lathe having a flat turret is shown in the lower part of fig. 3. This tool is of the "shovel nose" type and its cutting action is rather hard on account of the bluntness of the nose and the amount of stock which is removes. The work F is a machine steel forging and the shoulder is not recessed at all in the blank. The tool G is of rectangular section and it is forged and ground on the cutting end to the shape shown. The tool-holder H is supported by the steel bracket J which is fastened down on the turret face by screws L. The slots K are T-shaped and permit various settings and combinations of tools to be made.

Fig. 4 shows a very simple type of single-point adjustable boring-bar for machining the bushing A (see upper part of the illustration). Although this bar is simple in its construction, there are several important points in design which should be carefully noted. The bar E is of a low grade of tool steel and is hardened and ground to fit the turret hole. The reason for making the bar of tool steel is simply to obtain all the rigidity possible and thereby obviate chatter. The tool B is of round section and is put through the bar at a slight angle, being held in position by the two screws C. A backing-up screw D permits careful adjustment to be made. The bar is cut away where the tool comes through to provide chip clearance, but it is cylindrical on the end except in this one place. By making it this way, it is found an easy matter to use micrometer calipers across the bar and tool as indicated at F, so that accurate settings may be readily made without resorting to "cut-and-try" methods. It is very bad practice to bevel the end of the bar at G and put the holding screws through at this point, because a calipering point is sacrificed by so doing. A simple formula is here given for setting tools of this type for turning a given diameter:

Fig. 4. (Upper View) Boring-bar with Adjustable Cutter; (Lower View) Boring-bar with Double-ended Cutter.

Let F = required calipering distance for a given size hole;

X = diameter of the bar at the end where the tool is;

Y = diameter of the hole to be bored.

Then .

X + Y

F =--------- 2

This formula will be found useful for setting tools very close to the desired diameter, although the spring of the bar will cause slight variations and the amount of stock which is to be removed also makes some difference.

The lower part of Fig. 4 shows a boring tool of an entirely different type. The cutter is double-ended, and a bar of this sort is often used for removing stock rapidly. Although it is a faster cutting tool than a bar having only a single tool or cutting point, it cannot be depended upon to produce a hole which is absolutely concentric with other surfaces machined at the same setting. The work A is the same as in the upper part of the illustration, and the bar B fits the turret hole. It is flattened slightly on two sides at points D, and a rectangular slot contains the cutter C and the wedge E. It will be noted that the cutter is shouldered so that it is a close fit at the points D. Tools of this type cannot be ground radially without changing their diameters, but this is seldom necessary as the cutting edge is at the forward end. A land of about 1/8 inch is usually left just back of the bevel, and the cutter can be ground back to this point without changing the diameter. Beyond this, however, there is a slight back taper for the sake of clearance, so that the life of the cutter does not extend beyond it.