The Button-and-plug Method
The use of the button method as applied to the milling machine, is illustrated in [Fig. 28], where a plain jig-plate is shown set up for boring. The jig, with buttons B accurately located in positions corresponding to the holes to be bored, is clamped to the angle-plate A that is set at right angles to the spindle. Inserted in the spindle there is a plug P, the end of which is ground to the exact size of the indicating buttons. A sliding sleeve S is accurately fitted to this plug and when the work is to be set for boring a hole, the table and knee of the machine are adjusted until the sleeve S will pass over the button representing the location of the hole, which brings the button and spindle into alignment. When setting the button in alignment, all lost motion or backlash should be taken up in the feed-screws. For instance, if the button on the jig should be a little higher than the plug in the spindle, do not lower the knee until the bushing slips over the button, but lower the knee more than is required and then raise it until the bushing will pass over the button. This same rule should be followed for longitudinal adjustments.
Fig. 28. Accurate Method of Aligning Spindle with Button on Jig-Plate
After the button is set by this method, it is removed and the plug in the spindle is replaced by a drill and then by a boring-tool or reamer for finishing the hole to size. In a similar manner the work is set for the remaining holes. The plug P for the spindle must be accurately made so that the outer end is concentric with the shank, and the latter should always be inserted in the spindle in the same relative position. With a reasonable degree of care, work can be set with considerable precision by this method, providing, of course, the buttons are properly set.
Some toolmakers use, instead of the plug and sleeve referred to, a test indicator for setting the buttons concentric with the machine spindle. This indicator is attached to and revolves with the spindle, while the point is brought into contact with the button to be set. The difficulty of seeing the pointer as it turns is a disadvantage, but with care accurate results can be obtained.