Chapter XIX.—DRILLS AND CUTTERS FOR DRILLING MACHINES.
Drilling Jigs, Guides, or Fixtures.—When a large number of pieces are to be drilled alike, as in the case when work is done to special gauges, special chucking devices called jigs, or fixtures, are employed to guide the drill, and insure that the holes shall be pierced accurately in the required location, and test pieces or gauges are provided to test the work from time to time to insure that errors have not arisen by reason of the wear of these drill-guiding devices.
Fig. 1699.
Fig. 1700.
Suppose, for example, that we have a link, such as in [Fig. 1699], and that we require to have the holes throughout a large number of them of equal diameter at each end and the same distance apart, and if we could prevent the wear of the tools, and so continue to produce any number of links all exactly alike, we could provide a simple test gauge, such as shown in the figure, making it pass the proper distance apart, and of a diameter to fit the holes; but as we cannot prevent wear to the tools we must fix a limit to which such wear may be permitted to occur, and having reached that point they must be restored and corrected. We must at the same time possess means of testing in what direction the wear has induced error. Let it be assumed that the bore at a should be 1⁄2 inch and that at b 3⁄8 inch in diameter, that their distance from centre is to be, say, six inches, and that either bore may vary in diameter to the amount of 1⁄1000 inch, while the distance from centre to centre of the bores may also vary 1⁄1000 inch. Now let it be noted that if one piece be made 1⁄2000 inch too short, and another 1⁄2000 inch too long we have reached the extent of the limit, there being 1⁄1000 inch difference between them, although neither piece varies more than 1⁄2000 inch from the standard. Similarly in the bore diameters, if the bore, say at a, is 1⁄2000 inch too large in one piece and 1⁄2000 too small in another, there is a difference of 1⁄1000 between them, although each varies only the 1⁄2000 inch from the standard. In making test gauges for the holes, therefore, we must consider in what direction the tool will wear; thus, suppose that the finishing reamer for the holes is made when new to the standard diameter, and it can only wear smaller, hence a plug gauge of the standard diameter and 1⁄1000 inch smaller would serve thus, as so long as the smaller one will go in the limit of wear is not reached; when it will not go in sufficiently easily the reamer must be restored to fit the standard gauge. On the other hand, the reamer when new may be made 1⁄1000 inch above the standard size and restored when it has worn down to the standard size. In this case the bore diameter is still within the limit as long as the small gauge will enter; but when it fits too tight the reamer must be restored to the large plug gauge, the forms of these gauges being shown in [Fig. 1700].