=Locating Work by the Button Method.=--Among the different methods employed by machinists and toolmakers for accurately locating work such as jigs, etc., on the faceplate of a lathe, the one most commonly used is known as the button method. This scheme is so named because cylindrical bushings or buttons are attached to the work in positions corresponding to the holes to be bored, after which they are used in locating the work. These buttons, which are ordinarily about 1/2 inch in diameter, are ground and lapped to the same size and the ends squared. The diameter should, preferably, be such that the radius can be determined easily, and the hole through the center should be about 1/8 inch larger than the retaining screw, so that the button can be shifted.
As an illustration of the practical application of the button method, we shall consider, briefly, the way the holes would be accurately machined in the jig-plate in Fig. 18. First the centers of the seven holes should be laid off approximately correct by the usual methods, after which small holes should be drilled and tapped for the clamping screws _S_. After the buttons _B_ are clamped lightly in place, they are all set in correct relation with each other and with the jig-plate. The proper location of the buttons is very important as their positions largely determine the accuracy of the work. A definite method of procedure that would be applicable in all cases cannot, of course, be given, as the nature of the work as well as the tools available make it necessary to employ different methods.
[Illustration: Fig. 19. Setting a Button True Preparatory to Boring, by use of Test Indicator]
In this particular case, the three buttons _a_, _b_ and _c_ should be set first, beginning with the one in the center. As this central hole must be 2.30 and 2.65 inches from the finished sides _A_ and _A_{ 1}_, respectively, the work is first placed on an accurate surface-plate as shown; by resting it first on one of these sides and then on the other, and measuring with a vernier height gage, the central button can be accurately set. The buttons _a_ and _c_ are also set to the correct height from side _A_{ 1}_ by using the height gage, and in proper relation to the central button by using a micrometer or a vernier caliper and measuring the over-all dimension _x_. When measuring in this way, the diameter of one button would be deducted to obtain the correct center-to-center distance. After buttons _a_, _b_ and _c_ are set equidistant from side A_{ 1} and in proper relation to each other, the remaining buttons should be set radially from the central button _b_ and the right distance apart. By having two micrometers or gages, one set for the radial dimension _x_ and the other for the chordal distance _y_, the work may be done in a comparatively short time.