A simple method of spacing holes that are to be drilled in a straight line is illustrated in [Fig. 19]. Two disks are made, each having a diameter equal to the center-to-center distance required between the holes. These disks must also have holes which are exactly central with the outside to act as a guide for the drill or reamer. The first two holes are drilled in the work while the disks are clamped so that they are in contact with each other and also with the straightedge as shown. One disk is then placed on the opposite side of the other, as indicated by the dotted line, and a third hole is drilled; this process of setting one disk against the opposite side of the other is continued until all the holes are drilled. When it is necessary to drill a parallel row of “staggered” holes, the second row can be located by placing disks of the proper size in contact with the first row of disks.
Fig. 19. Locating Equi-distant Holes in a Straight Line
by Means of Disks and Straightedge
A method of using disks, which is preferable for very accurate work, is shown in [Fig. 20]. The disks are clamped against each other and along straightedge A by the screws shown, and if the outside diameters are correct and the guide holes concentric with the outside, very accurate work can be done. With this device there may be as many disks as there are holes to be drilled, if the number of holes is comparatively small, but if it is necessary to drill a long row of holes, the disks and frame are shifted along an auxiliary straightedge B, the hole in one of the end disks being aligned with the last hole drilled by inserting a close-fitting plug through the disk and hole.
Adjustable Jig for Accurate Hole Spacing
An adjustable jig for accurately spacing small holes is shown in [Fig. 21].
Fig. 20. Special Disk-jig for Precision Drilling
This form is especially adapted for locating a number of equally spaced holes between two previously drilled or bored holes, and the accuracy of the method lies in the fact that a slight error in the original spacing of the guide bushing is multiplied, and, therefore, easily detected. There are two of these guide bushings A and B which are carried by independent slides. These slides can be shifted along a dovetail groove after loosening the screws of clamp-gib C. To illustrate the method of using this jig, suppose five equally-spaced holes are to be located between two holes that are 12 inches apart. As the center-to-center distance between adjacent holes is 2 inches, slides A and B would be set so that the dimension x equals 2 inches plus the radii of the bushings. A straightedge is then clamped to the work in such position that a close-fitting plug can be inserted through the end holes which were previously drilled or bored. Then with a plug inserted through, say, bushing B and one of the end holes, the first hole is drilled and reamed through bushing A; the jig is then shifted to the left until the plug in B enters the hole just made. The second hole is then drilled and reamed through bushing A and this drilling and shifting of the jig is continued until the last hole is finished. The distance between the last hole and the original end hole at the left is next tested by attempting to insert close-fitting plugs through both bushings. Evidently, if there were any inaccuracy in the spacing of the bushings, this would be multiplied as many times as the jig was shifted, the error being accumulative. To illustrate how the error accumulates, suppose that the bushings were 0.001 inch too far apart; then the distance to the first hole would be 2.001 inch, to the second hole, 4.002 inch, and finally the distance from the first to the sixth hole would be 10.005 inches; consequently, the distance between the sixth and seventh holes would equal 12-10.005 = 1.995 inch, or 0.005 inch less than the required spacing, assuming, for the sake of illustration, that the first and last holes were exactly 12 inches apart. In case of an error of 0.005 inch, the bushings would be set closer together an amount equal to one-fifth of this error, as near as could be determined with a micrometer, and all of the holes would then be re-reamed.
