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.
Fig. 18. Jig-plate with Buttons attached, ready for Boring
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.
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 A1, 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 A1 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 A1 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.
Fig. 19. Setting a Button True Preparatory to Boring, by use of Test Indicator
After the buttons have been tightened, all measurements should be carefully checked; the work is then mounted on the faceplate of the lathe, and one of the buttons, say b, is set true by the use of a test indicator as shown in [Fig. 19]. When the end of this indicator (which is one of a number of types on the market) is brought into contact with the revolving button, the vibration of the pointer I shows how much the button runs out of true. When the pointer remains practically stationary, thus showing that the button runs true, the latter should be removed. The hole is then drilled nearly to the required size, after which it is bored to the finish diameter. In a similar manner the other buttons are indicated and the holes bored, one at a time. It is evident that if each button is correctly located and set perfectly true in the lathe, the various holes will be located at the required center-to-center dimensions within very close limits.