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.
Size Block and Gage Method
Another method which can at times be employed for accurately locating a jig-plate in different positions on an angle-plate, is shown in [Fig. 29]. The angle-plate is, of course, set at right angles to the spindle and depth gages and size blocks are used for measuring directly the amount of adjustment. Both the angle-plate and work should have finished surfaces on two sides at right angles to each other, from which measurements can be taken. After the first hole has been bored, the plate is adjusted the required distance both horizontally and vertically, by using micrometer depth gages, which should preferably be clamped to the angle-plate. If the capacity of the gages is exceeded, measurements may be taken by using standard size blocks in conjunction with the depth gages.
Fig. 29. Locating Work from Edges of Angle-Plate
by means of Depth Gages and Size Blocks
It is frequently necessary to bore holes in cast jig-plates or machine parts, which either have irregularly shaped or unfinished edges. A good method of locating such work is illustrated in [Fig. 30]. The part to be bored is attached to an auxiliary plate A which should have parallel sides and at least two edges which are straight and at right angles to each other. This auxiliary plate with the work, is clamped against an accurate angle-plate B, which should be set square with the axis of the machine spindle. A parallel strip is bolted to the angle-plate and the inner edge is set square with the machine table. After the first hole is bored, the work is located for boring the other to the edge of the auxiliary plate, and horizontal measurements y between the parallel and the plate. These measurements, if quite large, might be taken with micrometer gages, whereas, for comparatively small adjustments, size blocks might be more convenient.