Measuring Bored Holes.—The diameters of small holes that are being bored are usually measured with inside calipers or standard gages. If the pulley were being bored to fit over some shaft, the diameter of the shaft would first be measured by using outside calipers, as shown at A, [Fig. 37], the measuring points of the calipers being adjusted until they just made contact with the shaft when passed over it. The inside calipers are then set as at B to correspond with the size of the shaft, and the hole is bored just large enough to admit the inside calipers easily. Very accurate measurements can be made with calipers, but to become expert in their use requires experience. Some mechanics never become proficient in the art of calipering because their hands are “heavy” and they lack the sensitiveness and delicacy of touch that is necessary. For large holes, a gage C is often used, the length l being adjusted to the diameter desired. Small holes are often bored to fit hardened steel plug gages ([Fig. 38]), the cylindrical measuring ends of which are made with great accuracy to standard sizes. This type of gage is particularly useful when a number of holes have to be bored to the same size, all holes being made just large enough to fit the gage without any perceptible play.
Fig. 39. Diagram Illustrating Importance of Setting Work with Reference to Surfaces to be Turned
Setting Work in the Chuck.—When setting a part in a chuck, care should be taken to so locate it that every surface to be turned will be true when machined to the finished size. As a simple illustration, let us assume that the hole through the cast-iron disk, [Fig. 39], has been cored considerably out of center, as shown. If the work is set by the outside surface S, as it would be ordinarily, the hole is so much out of center that it will not be true when bored to the finished size, as indicated by the dotted lines. On the other hand, if the rough hole is set true, the outside cannot be finished all over, without making the diameter too small, when it is finally turned. In such a case, the casting should be shifted, as shown by the arrow, to divide the error between the two surfaces, both of which can then be turned as shown by the dotted lines in the view to the right. This principle of dividing the error when setting work can often be applied in connection with turning and boring. After a casting or other part has been set true by the most important surface, all other surfaces which require machining should be tested to make sure that they all can be finished to the proper size.
Inaccuracy from Pressure of Chuck Jaws.—Work that is held in a chuck is sometimes sprung out of shape by the pressure of the chuck jaws so that when the part is bored or turned, the finished surfaces are untrue after the jaws are released and the work has resumed its normal shape. This applies more particularly to frail parts, such as rings, thin cylindrical parts, etc. Occasionally the distortion can be prevented by so locating the work with relation to the chuck jaws that the latter bear against a rigid part. When the work cannot be held tightly enough for the roughing cuts without springing it, the jaws should be released somewhat before taking the finishing cut, to permit the part to spring back to its natural shape.