Disk-and-Button Method of
Locating Holes
The accuracy of work done by the button method previously described is limited only by the skill and painstaking care of the workman, but setting the buttons requires a great deal of time. By a little modification, using what is sometimes called the “disk-and-button method,” a large part of this time can be saved without any sacrifice of accuracy. The disk-and-button method is extensively used in many shops. Buttons are used, but they are located in the centers of disks of whatever diameters are necessary to give the required locations. As three disks are used in each step of the process, it is sometimes called the “three-disk method.”
To illustrate the practical application of this method, suppose six equally-spaced holes are to be located in the circumference of a circle six inches in diameter. To locate these, one needs, besides the buttons, three disks three inches in diameter, each having a central hole exactly fitting the buttons. It is best to have, also, a bushing of the same diameter as the buttons, which has a center-punch fitted to slide in it. First the center button is screwed to the templet, and one of the disks A, [Fig. 9], is slipped over it; then a second disk B carrying a bushing and center-punch is placed in contact with disk A and a light blow on the punch marks the place to drill and tap for No. 2 button, which is kept in its proper place while tightening the screw by holding the two disks A and B in contact. Next the third disk C is placed in contact with disks A and B and locates No. 3 button, and so on until the seven buttons are secured in position. The templet is then ready to be strapped to the lathe faceplate for boring.
Fig. 8. An Example of Precision Work, and Method
of Locating Holes by Use of Disks in Contact
Of course, it is not possible to use disks of “standard” sizes for many operations, but making a special disk is easy, and its cost is insignificant as compared with the time saved by its use. One who employs this method, especially if he also uses disks to lay out angles, soon accumulates a stock of various sizes. While it is desirable to have disks of tool steel, hardened and ground, or, in the larger sizes, of machine steel, case-hardened and ground, a disk for occasional use will be entirely satisfactory if left soft.
Another example of work is shown in [Fig. 10]. This is a jig templet similar to the one illustrated in Figs. [6] and [7]. Sketch A gives its dimensions and sketch B shows the disk-and-button way of locating the holes. A steel square is clamped with its stock against the right-hand edge of the templet and its blade extending across the top. The lower edge of the blade should be located 0.250 inch from the upper edge of the templet by the use of size blocks. A 2½-inch disk, touching both blade and stock, locates hole C. Another 2½-inch disk, touching the first disk and the square blade, locates hole B. Next a disk 1.600 inch diameter is placed in contact with the two upper disks and locates the center hole A; and, finally, the disks for holes B and C are used to locate holes D and E.
Fig. 9. Locating Holes on a Circle and Equi-distant
by using Disks and Buttons in Combination
Two other jobs that illustrate this method may be of interest. The first one, shown in [Fig. 11], required the locating of nine equally-spaced holes on a circumference of 7⅜ inches diameter. In any such case, the size of the smaller disks is found by multiplying the diameter of the circle upon which the centers of the disks are located by the sine of half the angle between two adjacent disks. The angle between the centers of adjacent disks equals 360 ÷ number of disks. 360 ÷ 9 = 40; hence, in this case, the diameter of the smaller disks equals 7⅜ multiplied by the sine of 20 degrees, or 7⅜ × 0.34202 = 2.5224 inches. 7⅜-2.5224 = 4.8526 inches, which is the diameter of the central disk.