Fig. 2563.
An example in the erection or setting of framed work is shown in [Fig. 2563], which represents a side elevation of a frame put together in four parts, two side and two end frames. a and b are journal bearings requiring to stand parallel and true one to the other, b being capable for adjustment in distance from a by means of the adjusting screws g, h. The bearings c, d, e, f, are to be parallel one to the other and to a, b. Their proper relative distances apart, and the axes of all the shafts, are to stand at a right angle to the side frames.
Fig. 2564.
[Fig. 2564] represents an end view of the frame, the ends t being bolted to the side frames s and s′ at i, j, k, and l.
Now it is obvious that the ways for the bearings a, b, c, &c., may be trued out, ready to have the brasses fitted before the framework is put together, and that from their positions they would have to be planed out at separate chuckings; supposing, of course, the frame to be too large to be within the capacity of the machine table. It would be difficult to cut all the surfaces of the bearing ways to stand in the same plane, unless there were some true plane to which all might be made common for parallelism.
Furthermore, unless the surfaces where t is fastened to s and s′ are properly bedded to fit each other, bolting them up would spring and bend the frames out of their normal planes. To meet these requirements, there are given to the side frames a slightly projecting surface where the feet of t meet them, and furthermore, the feet of t themselves project beyond the sides of t as shown. These projecting pieces may therefore be planed to a common plane without planing the sides of the respective frames; and this plane should be as nearly as can be parallel with the body of each frame surface. The surfaces of the bearing ways may then be planed parallel to those of the projections, and the jaw surfaces true to the side surfaces, and all the bearing ways will stand true if the frames be properly set—when put together with the bolts. But unless the bedding surfaces at i, j, k, l, be made to bed and fit properly, the whole truth of the bearing ways and their distances apart across the framework may be altered. Thus, supposing the feet of t at i and j to meet s as denoted by the dotted lines o r, and whether the fault lie with the feet of t or with the projections on s the result will be that the pressure of the bolts holding i j to s will bend s so that its plane will be a curve as denoted by the dotted line p p, and the distances apart of the journal ways b b and d d respectively will be wrong, being too wide on account of the bend outward of s.
But the feet may touch on the opposite corners, the surfaces of s′ or of t being out of true or out of full contact, as denoted by the dotted lines v w on k, l; in this case the frame s′ would be bent to the curve q q, and the journal ways would be too close together.
On the other hand, the want of fit between these surfaces may be in the direction of the length of the frame instead of the direction of its height, as has been supposed; or it may be in one direction on one foot and in another direction on another foot. But in whatever direction it does exist, it will inevitably bend and twist the frame.