The Position of Turning Tools.—The production of accurate lathe work depends partly on the condition of the lathe used and also on the care and judgment exercised by the man operating it. Even though a lathe is properly adjusted and in good condition otherwise, errors are often made which are due to other causes which should be carefully avoided. If the turning tool is clamped so that the cutting end extends too far from the supporting block, the downward spring of the tool, owing to the thrust of the cut, sometimes results in spoiled work, especially when an attempt is made to turn close to the finished size by taking a heavy roughing cut. Suppose the end of a cylindrical part is first reduced for a short distance by taking several trial cuts until the diameter d, [Fig. 8], is slightly above the finished size and the power feed is then engaged. When the tool begins to take the full depth e of the cut, the point, which ordinarily would be set a little above the center, tends to spring downward into the work, and if there were considerable springing action, the part would probably be turned below the finished size, the increased reduction beginning at the point where the full cut started.
This springing action, as far as the tool is concerned, can be practically eliminated by locating the tool so that the distance A between the tool-block and cutting end, or the “overhang,” is as short as possible. Even though the tool has little overhang it may tilt downward because the toolslide is loose on its ways, and for this reason the slide should have a snug adjustment that will permit an easy movement without unnecessary play. The toolslides of all lathes are provided with gibs which can be adjusted by screws to compensate for wear, or to secure a more rigid bearing.
Fig. 9. (A) The Way in which Tool is sometimes displaced by Thrust of Cut, when set at an Angle.
(B) Tool Set for Finishing both Cylindrical and Radial Surfaces
When roughing cuts are to be taken, the tool should be located so that any change in its position which might be caused by the pressure of the cut will not spoil the work. This point is illustrated at A in [Fig. 9]. Suppose the end of a rod has been reduced by taking a number of trial cuts, until it is 1/32 inch above the finished size. If the power feed is then engaged with the tool clamped in an oblique position, as shown, when the full cut is encountered at c, the tool, unless very tightly clamped, may be shifted backward by the lateral thrust of the cut, as indicated by the dotted lines. The point will then begin turning smaller than the finished size and the work will be spoiled. To prevent any change of position, it is good practice, especially when roughing, to clamp the tool square with the surface being turned, or in other words, at right angles to its direction of movement. Occasionally, however, there is a decided advantage in having the tool set at an angle. For example, if it is held about as shown at B, when turning the flange casting C, the surfaces s and s1 can be finished without changing the tool's position. Cylindrical and radial surfaces are often turned in this way in order to avoid shifting the tool, especially when machining parts in quantity.
Tool Grinding.—In the grinding of lathe tools there are three things of importance to be considered: First, the cutting edge of the tool (as viewed from the top) needs to be given a certain shape; second, there must be a sufficient amount of clearance for the cutting edge; and third, tools, with certain exceptions, are ground with a backward slope or a side slope, or with a combination of these two slopes on that part against which the chip bears when the tool is in use.
