Fig. 25. Example of Precision Boring on Milling Machine
A method that is a modification of the one in which plugs are used to test the center distance is as follows: All the holes are first drilled with suitable allowance for boring, the location being obtained directly by the feed-screw dials. A special boring-tool, the end of which is ground true with the shank, is then inserted in the spindle and the first hole, as at A in [Fig. 1] is finished, after which the platen is adjusted for hole B by using the dial as before. A close-fitting plug is then inserted in hole A and the accuracy of the setting is obtained by measuring the distance between this plug and the end of the boring-tool, which is a combination tool and test plug. In a similar manner, the tool is moved from one position to another, and, as all the holes have been previously drilled, all are bored without removing the tool from the spindle.
Another modification of the micrometer-and-plug method is illustrated in Figs. [26] and [27]. It is assumed that the plate to be bored is finished on the edges, and that it is fastened to an angle-plate, which is secured to the table of the milling machine and set square with the spindle. A piece of cold-rolled steel or brass is first fastened in the chuck (which is mounted on the spindle) and turned off to any diameter. This diameter should preferably be an even number of thousandths, to make the calculations which are to follow easier. The turning can be done either by holding the tool in the milling machine vise, or by securing it to the table with clamps. In either case, the tool should be located near the end of the table, so as to be out of the way when not in use.
Fig. 26. Obtaining Vertical Adjustment by Means of
Depth Gage and turned Plug in Chuck
After the piece in the chuck is trued, the table and knee are adjusted until the center of the spindle is in alignment with the center of the first hole to be machined. This setting of the jig-plate is effected by measuring with a micrometer depth gage from the top and sides of the work, to the turned plug, as illustrated in [Fig. 26]. When taking these measurements, the radius of the plug in the chuck is, of course, deducted. When the plate is set the plug is removed from the chuck and the first hole drilled and bored or reamed to its proper size. We shall assume that the holes are to be located as shown by the detail view, [Fig. 26], and that hole A is the first one bored. The plug is then again inserted in the chuck and trued with the tool, after which it is set opposite the place where the second hole B is to be bored; this is done by inserting an accurately fitting plug in hole A and measuring from this plug to the turned piece in the chuck, with an outside micrometer as indicated in [Fig. 27]. Allowance is, of course, again made for the radii of the two plugs. The horizontal measurement can be taken from the side of the work with a depth gage as before. The plug is then removed and the hole drilled and bored to the proper size. The plug is again inserted in the chuck and turned true; the table is then moved vertically to a position midway between A and B, and then horizontally to the proper position for hole C, as indicated by the depth gage from the side of the work. The location can be verified by measuring the center distances x with the micrometer. In a similar manner holes D, E, F and G are accurately located.
Fig. 27. Adjusting for Center-to-center Distance
by use of Plugs and Micrometer
If the proper allowances are made for the variation in the size of the plug, which, of course, is made smaller each time it is trued, and if no mistakes are made in the calculations, this method is very accurate. Care should be taken to have the gibs on all sides fairly tight at the beginning, and these should not be tightened after each consecutive alignment, as this generally throws the work out a few thousandths. If the reductions in the size of the plug, each time it is turned, are confusing, new plugs can be used each time a test is made, or the end of the original plug can be cut off so that it can be turned to the same diameter for every test. If the center distances x are not given, it is, of course, far more convenient to make all the geometric calculations before starting to work.