Example of Taper Turning.—As a practical example of taper turning let us assume that the piece A, [Fig. 4], which has been centered and rough-turned as shown, is to be made into a taper plug, as indicated at B, to fit a ring gage as at C. If the required taper is 11/2 inch per foot and the total length is 8 inches, the tailstock center would be offset 1/2 inch.
To adjust the tailstock, the nuts N ([Fig. 3]) are first loosened and then the upper part A is shifted sidewise by turning screw S. Scales are provided on some tailstocks for measuring the amount of this adjustment; if there is no scale, draw a line across the movable and stationary parts A and B, when the tailstock is set for straight turning. The movement of the upper line in relation to the lower will then show the offset, which can be measured with a scale.
When the adjustment has been made, nuts N are tightened and the part to be turned, with a dog attached, is placed between the centers the same as for straight turning. The taper end is then reduced by turning, but before it is near the finished size, the work is removed and the taper tested by inserting it in the gage. If it is much out, this can be felt, as the end that is too small can be shaken in the hole. Suppose the plug did not taper enough and only the small end came into contact with the gage, as shown somewhat exaggerated at D; in that case the center would be shifted a little more towards the front, whereas if the taper were too steep, the adjustment would, of course, be in the opposite direction. A light cut would then be taken, to be followed by another test. If the plug should fit the gage so well that there was no perceptible shake, it could be tested more closely as follows: Draw three or four chalk lines along the tapering surface, place the work in the gage and turn it a few times. The chalk marks will then show whether the taper of the plug corresponds to that of the gage; for example, if the taper is too great, the marks will be rubbed out on the large end, but if the taper is correct, the lines throughout their length will be partially erased.
Another and more accurate method of testing tapers is to apply a thin coat of Prussian-blue to one-half of the tapering surface, in a lengthwise direction. The work is then inserted in the hole or gage and turned to mark the bearing. If the taper is correct, the bearing marks will be evenly distributed, whereas if the taper is incorrect, they will appear at one end. Tapering pieces that have to be driven tightly into a hole, such as a piston-rod, can be tested by the location of the bearing marks produced by actual contact.
After the taper is found to be correct, the plug is reduced in size until it just enters the gage as at C. The final cut should leave it slightly above the required size, so that a smooth surface can be obtained by filing. It should be mentioned that on work of this kind, especially if great accuracy is required, the final finish is often obtained by grinding in a regular grinding machine, instead of by filing. When this method is employed, a lathe is used merely to rough-turn the part close to size.
Fig. 5. Setting Work for Taper Turning by use of Caliper Gage
When the amount that the tailstock center should be offset is determined by calculating, as in the foregoing example, it is usually necessary to make slight changes afterward, and the work should be tested before it is too near the finished size so that in case one or more trial cuts are necessary, there will be material enough to permit this. When there are a number of tapered pieces to be turned to the same taper, the adjustment of the tailstock center will have to be changed unless the total length of each piece and the depth of the center holes are the same in each case.