Page:Practical Treatise on Milling and Milling Machines.djvu/56

50 consists of two arms which may be spread apart when the screw A, is loosened slightly. The correct number of holes may be counted and the sector arms set to include them; or better, the graduations on the dial may be used in connection with the tables given on pages 210 to 218. To set the sector arms by this last method, follow down the column headed "Graduation" in the tables referred to, until opposite the number of divisions that is desired. Take the number that is found here and set the arms by bringing the left one against the index pin, which should be inserted in any convenient hole in the required circle, and moving the right one until the graduation corresponding to the number obtained from the table coincides with the zero on the left arm. The correct number of holes will then be contained between the two arms, and counting is unnecessary.

When setting the arms by counting the holes, the left arm should be brought against the index pin as directed above, and then the required number of holes for each division should be counted from the hole that the pin is in, considering this hole as zero.

Adjustable Index Crank. The index crank of the spiral head is adjustable circumferentially. This is shown in Fig. 6. Many times it is desired to make a delicate ajustment of the work, or to bring the index pin to the nearest hole without disturbing the setting of the work. To adjust the index crank after the work has been placed in position, turn thumb screws A-A, Fig. 6, until the pin enters the nearest hole in the index plate. To rotate the work relative to the index plate, both the stop pin at the back of the plate and the index crank pin should be engaged, the adjustment being made by means of the thumb screws as before.

Throwing Worm Out of Mesh. When it is desired to turn the spindle by hand and index work by means of the plate on the front end of the spindle, it is necessary to disengage the driving worm A, Fig. 4.  To do this, turn the knob E, by means of a pin wrench furnished, about one-quarter of a revolution in the reverse direction to that indicated by an arrow stamped on the knob. This will loosen nut G, that clamps eccentric bushing H; then with the fingers turn both knobs E and F, at the same time, and the bushing H, will revolve,