Locating Work by means of Size Blocks
The size-block method of locating a jig-plate or other part, in different positions on a lathe faceplate, for boring holes accurately at given center-to-center distances, is illustrated in [Fig. 16]. The way the size blocks are used in this particular instance is as follows: A pair of accurate parallels are attached to a faceplate at right angles to each other and they are so located that the center of one of the holes to be bored will coincide with the lathe spindle. The hole which is aligned in this way should be that one on the work which is nearest the outer corner, so that the remaining holes can be set in a central position by adjusting the work away from the parallels. After the first hole is bored, the work is located for boring each additional hole by placing size blocks of the required width between the edges of the work and the parallels. For instance, to set the plate for boring hole D, size blocks (or a combination of blocks or gages) equal in width to dimension A₁ would be inserted at A, and other blocks equal in width to dimension B₁ beneath the work as at B. As will be seen, the dimensions of these blocks equal the horizontal and vertical distances between holes C and D. With the use of other combinations of gage blocks, any additional holes that might be required are located in the central position. While only two holes are shown in this case, it will be understood that the plate could be located accurately for boring almost any number of holes by this method.
Fig. 16. Method of setting Work on Faceplate
with Size Blocks or Gages
Incidentally, such gages as the Johansson combination gages are particularly suited for work of this kind, as any dimension within the minimum and maximum limits of a set can be obtained by simply placing the required sizes together. Sometimes when such gages are not available, disks which have been ground to the required diameter are interposed between the parallels and the work for securing accurate locations. Another method of securing a positive adjustment of the work is to use parallels composed of two tapering sections, which can be adjusted to vary the width and be locked together by means of screws. Each half has the same taper so that outer edges are parallel for any position, and the width is measured by using a micrometer. The size-block method is usually applied to work having accurately machined edges, although a part having edges which are of a rough or irregular shape can be located by this method, if it is mounted on an auxiliary plate having accurately finished square edges. For instance, if holes were to be bored in the casting for a jig templet which had simply been planed on the top and bottom, the casting could be bolted to a finished plate having square edges and the latter be set in the different positions required, by means of size blocks. Comparatively large jig plates are sometimes located for boring in this way and the milling machine is often used instead of a lathe.