Now, if plug A is removed from bushing b the point of the centering-rod K may be made to describe a circle. At some point within this circle is located the center-punch mark on the work to be bored. The holes in the rim of the faceplate all being exactly the same distance from an axial line through both the arbor I and centering-rod K, it follows that the center mark on the work must be so located by horizontal and vertical movements of the work table that pin A may be freely entered in all the four holes in the rim of disk D. When that occurs, the center coincides with the axis of the spindle.
The point of the center-punch A ([Fig. 37]) should have an angle slightly greater than the angle on the centering-rod K, as it is impossible to locate the work in the preliminary trials so that the center of the work will be coincident with the axis of the spindle, and unless the precaution mentioned is taken, the true center on the work is liable to be drawn from its proper location when trying to bring the work into such a position that the plug will enter all the holes in the disk. As the work being operated on is brought nearer to the proper location by the movements of the milling machine table, spring G will be compressed, the center rod sliding back into barrel L. This spring is made so that it will hold the center against the work firmly, but without interfering with the free rotation of the sector C around disk D. When the work is located so that the plug enters the holes, the gibs of the machine should be tightened up and the plug tried once more, to make sure that the knee of the machine has not moved sufficiently to cause the work on the table to be out of line. The work table is now clamped to prevent accidental horizontal shifting, and the work is drilled and bored.
Fig. 39. Sectional View of Indicator shown in Fig. 38
In using this indicator the milling machine spindle is not rotated together with arbor I, only the sector being turned around the disk. The tool is set, however, in the beginning, so that the axes of two of the bushings b are at right angles to the horizontal plane of the machine table, while the axes of the other holes in the disk are parallel with the top of the work table. The centering-rods are made interchangeable and of various lengths, to reach more or less accessible centers. [Fig. 38] shows the indicator with one of the long center-rods in the foreground.
The only part of the milling machine on which dependence must be placed for accuracy is the hole in the spindle, and this is less liable to get out of truth, from wear such as would impair the accuracy, than are the knee, table, or micrometer screws. The only moving part is the sector, and this, being light, is very sensitive.
A series of 24 holes was laid out and bored in one and one-half day by the method described in the foregoing. Measurements across accurately lapped plugs in the holes, showed the greatest deviation from truth to be 0.0002 inch, and running from that to accuracy so great that no error was measurable. This same work with buttons would have required considerably more time.
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