Page:Practical Treatise on Milling and Milling Machines.djvu/54

48 casting in which is mounted a spindle that is connected to an index crank through a worm and wheel. Fig. 4 shows the construction of this part. The head casting has dove-tailed bearings at each side that fit the contour of a base plate, which can be clamped to the surface of the table. The alignment of the head with the table longitudinally is provided by means of a tongue on the under side of the base plate that fits a T slot in the table.

The spiral head spindle passes through the head, and is held in place by means of a nut at the small end. The front end is threaded and has a taper hole corresponding to that of the machine spindle.

Fig. 4

It is rotated by means of the worm wheel B, which is driven by the hardened worm A that is located on the shaft to which the index crank is fastened. In order to insure accuracy the worm threads are ground after hardening. Through gearing, the index plate and worm A can be driven together from the table feed screw when the index pin is in position in any hole of the pla. When worm A is turned by means of the index crank, indexing may be accomplished, and when it is geared to the table feed screw, spiral milling, in addition to indexing, is made possible. The cutting of the spiral is due to the turning of the table feed screw, which through the interposition of change gears between this screw and the gears that drive the shaft carrying worm A, causes the spindle of the spiral head to rotate as