Fig. 3. Testing Location of Buttons
[Fig. 7] shows a method of locating buttons from the finished sides of a plate, and this same plate with the five buttons attached is shown in [Fig. 6]. As the dimensions in [Fig. 7] indicate, the holes must not only be accurate with relation to each other, but also with reference to the edges of the templet; therefore, it is necessary to work from the sides as well as the center. The width of the plate was first measured carefully and found to be 5 inches. As the center-to-center distance between buttons B and C and also buttons D and E, is 2½ inches, the distance from the center of each outside button to the edge of the plate is 1¼ inch. A ¼-inch parallel was clamped against the side, as shown in the illustration, and then the distance from the outside of each button to the outside of the parallel (1 ¹³/₁₆ inch) was measured in conjunction with the distance L from the central button. The distance L was obtained by first determining the center-to-center distance M which represents the hypotenuse of a right-angled triangle.
M² = 1.25² + 1.625²
or M = √1.25² + 1.625² = √4.024 = 2.050 inches.
Therefore, L = 2.050 + 0.625 = 2.675 inches.
In this case, the center button was first located correctly from the sides and end and then the other buttons were set. When doing precision work of this kind, the degree of accuracy obtained will depend upon the instruments used, the judgment and skill of the workman, and the care exercised. A good general rule to follow when locating bushings or buttons is to use the method which is the most direct and which requires the least number of measurements, in order to prevent an accumulation of errors.
Locating Work by the Disk Method
Comparatively small precision work is sometimes located by the disk method, which is the same in principle as the button method, the chief difference being that disks are used instead of buttons. These disks are made to such diameters that when their peripheries are in contact, each disk center will coincide with the position of the hole to be bored; the centers are then used for locating the work. To illustrate this method, suppose that the master-plate shown at the left in [Fig. 8] is to have three holes a, b, and c bored into it, to the center distances given.
Fig. 4. Testing Concentricity of Button Preparatory to Boring Hole in Lathe