Among the different methods employed by toolmakers for accurately locating work such as jigs, etc., on the faceplate of a lathe, one of the most commonly used is known as the “button method.” This method 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 ½ or ⅝ inch in diameter, are ground and lapped to the same size, and the ends are finished perfectly square. The outside diameter should preferably be such that the radius can easily be determined, and the hole through the center should be about ⅛ inch larger than the retaining screw so that the button can be adjusted laterally.
As a simple example of the practical application of the button method, suppose three holes are to be bored in a jig-plate according to the dimensions given in [Fig. 1]. A common method of procedure would be as follows: First lay out the centers of all holes to be bored, by the usual method. Mark these centers with a prick-punch and then drill holes for the machine screws which are used to clamp the buttons. After the buttons are clamped lightly in place, set them 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. The best method of locating a number of buttons depends, to some extent, upon their relative positions, the instruments available, and the accuracy required. When buttons must be located at given distances from the finished sides of a jig, a surface plate and vernier height gage are often used. The method is to place that side from which the button is to be set, upon an accurate surface plate and then set the button by means of the height gage, allowance being made, of course, for the radius of the button. The center-to-center distance between the different buttons can afterwards be verified by taking direct measurements with a micrometer.
Fig. 1. Simple Example of Work Illustrating Application of Button Method
Figs. [2] and [3] illustrate a method which requires only a micrometer. Two of the buttons are set at the correct distance from one edge of the plate by measuring from a parallel strip. Obviously, the micrometer reading will exceed the distance from the center of a button to the edge of the plate, by the amount equal to the thickness of the parallel strip plus the radius of the button. The center-to-center distance between each pair of buttons is also tested as indicated in [Fig. 3], by measuring the overall distance and deducting the diameter of one button.
After the buttons have been set and the screws are tightened, all measurements should be carefully checked. The work is then mounted on the faceplate of the lathe and one of the buttons is set true by the use of a test indicator as shown in [Fig. 4]. When the dial of the indicator ceases to vibrate, thus showing that the button runs true, the latter should be removed so that the hole can be drilled and bored to the required size. In a similar manner 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 the required distance apart within very close limits.
Fig. 2. Determining Distance from Button to Edge of Plate
Another example of work illustrating the application of the button method is shown in [Fig. 5]. The disk-shaped part illustrated is a flange templet which formed a part of a fixture for drilling holes in flanged plates, the holes being located on a circle 6 inches in diameter. It was necessary to space the six holes equi-distantly so that the holes in the flanges would match in any position, thus making them interchangeable. First a plug was turned so that it fitted snugly in the 1¼-inch central hole of the plate and projected above the top surface about ¾ inch. A center was located in this plug and from it a circle of three inches radius was drawn. This circle was divided into six equal parts and then small circles ⅝ inch in diameter were drawn to indicate the outside circumference of the bushings to be placed in the holes. These circles served as a guide when setting the button and enabled the work to be done much more quickly. The centers of the holes were next carefully prick-punched and small holes were drilled and tapped for No. 10 machine screws. After this the six buttons were attached in approximately the correct positions and the screws tightened enough to hold the buttons firmly, but allow them to be moved by tapping lightly. As the radius of the circle is 3 inches, the radius of the central plug, ⅝ inch, and that of each button, ⁵/₁₆ inch, the distance from the outside of the central plug to the outside of any button, when correctly set, must be 3 ¹⁵/₁₆ inches. Since there are six buttons around the circle, the center-to-center distance is equal to the radius, and the distance between the outside or any two buttons should be 3⅝ inches. Having determined these dimensions, each button is set equi-distant from the central plug and the required distance apart, by using a micrometer. As each button is brought into its correct position, it should be tightened down a little so that it will be located firmly when finally set. The work is then strapped to the faceplate of a lathe and each button is indicated for boring the different holes by means of an indicator, as previously described. When the buttons are removed it will be found that in nearly all cases the small screw holes will not run exactly true; therefore, it is advisable to form a true starting point for the drill by using a lathe tool.
