It must not be taken for granted, that because these surfaces have been planed or milled, that therefore they are true; because frames of this class cannot, if large, be held without springing them to some extent from the pressure of the bolts or other devices necessary to hold them to be cut.
It is not uncommon to plane the surfaces as true as may be, and put the frames together, bolting them up tight, and then applying the straight-edge trammel and rule to test the truth, correcting any error that may be found by inserting pieces of paper, sheet tin or material of requisite thickness on one side of the surfaces, so as to offset the error in their fit and bring the framing true; but this is not the proper way, because it reduces the area of contact, and furthermore renders a new testing and adjustment necessary whenever the frames are taken apart. It is better therefore to apply a straight-edge to the surfaces and true them to it, testing them vertically as by placing the straight-edge across k l, and longitudinally across s′, at k and the corresponding projection at the other end of the frame, filing them until they appear true.
The holes through the frame may be drilled before filing these surfaces, so as to reduce the area to be filed. Since the end frames t do not in this example carry any journals or mechanism, the position of t is not so particular as it otherwise would be; hence, the holes in its feet may be marked off and drilled independently of the frame, the holes being drilled a little too small to allow for reaming with the holes in the frame. The framing will then be ready to put together (all machine work upon them being supposed to be done). The feet of all such frames should be planed true, so that the frame, when put together, may stand true and steady when placed upon a level floor or foundation, and in this case the distance and parallelism of the feet surfaces will be true with the ways or bearings, affording much assistance in holding the frame while putting it together. The height of the holes may be measured and marked from the feet surface, thus insuring truth as far as height is concerned. Lines may be drawn or marked on each side frame, at the proper distance from and parallel with the jaws of the ways a, b, thus completing on the side frames the marking of the location of the centres of the holes for bolting the end frames on.
If the frames were of a size to be sufficiently easily handled, the end frames might be put in their places, and the whole framework set true, so as to mark the holes in the end frames from those already drilled in the side frame. But if the use of a crane were necessary to lift them, it would be better to mark the holes on the end frames, and drill them before putting the framework together at all, leaving sufficient to ream out of the holes to bring them fair, notwithstanding any slight error in drilling them. In this case, a line denoted by the dotted line x in [Fig. 2564], should be drawn across the frame, and the holes at i and j be made equidistant on each side of it, as well as the proper distance apart. x must be at a right angle to the trued foot surfaces at i j, so as to cause the side frames to stand vertical while their feet are horizontal.
Supposing now the holes to be drilled and the frames are to be bolted together, the whole frame may be held temporarily together by bolts passing through the side frames at each end, or a bolt may be passed through the holes f to steady it. Indeed, if these holes f have been accurately bored, a neatly fitting mandrel passed through them should hold the side frames true. The end frames t having been set to stand at a right angle to the side frames, and with their holes at i j, &c., as near fair as may be with the holes in the side frames, two feet, as i j, may have their holes reamed fair with the holes in the side frames, and tightly fitting bolts be driven in and screwed firmly home. Before reaming the other holes (as k l) of each end frame, the jaws to receive the bearing boxes should be tested for alignment one with the other. Truth, in this respect, being of the utmost consequence for the following reasons:
Suppose the bearing ways on one side frame to stand higher than those on the other, then, the shafts will not stand level in the frame unless (except in the case of the brasses or boxes in b) the lower brasses are made of unequal thickness through the crown, to an amount equal to that of the error. In the case of the brasses in a, c, d, e, the joint faces of all the brasses of one side frame would require to be made thinner beneath the journal than above it on the high frame, and thicker beneath than above on the low frame, This would entail much extra work in planing, marking, and boring the bearing boxes or brasses, and be an inferior job when done.
Again, the bores of all the brasses would not be parallel to the crown or bedding faces, and this error would entail the following extra work: 1. Ascertaining the amount of the error, and allowing for it in marking the brasses; 2. The setting of the ways of the brasses out of true with the ways when clinking them for boring; and, finally, extra fitting or filing the brass bores when fitted with the shafts in place. This extra fitting would be necessary for the following reasons:
When the surfaces of work are to be parallel, they can be measured with calipers. Surfaces to be at a right angle can be tested with a square; those to be in line can be tried with a straight-edge, and in each case the truth or alignment of the surfaces is tested by contact of the testing tool. But in the cases where surfaces at an angle are tested or measured the tools must be set to a line or lines, and the work must be measured or cut to lines, thus: Suppose it were found that the bedding surface of the brass b was a certain amount out of alignment with the corresponding bedding surface on the other side frame, and, by measurement, this amount determined to be 1⁄64 inch, then there is a liability to error in measuring this 1⁄64. The brasses must be marked (for boring the same 1⁄64 out of square, inducing another liability of error in marking that amount); this marking being done by lines, there is a liability to error in setting the work to the lines. From these liabilities to error, it is generally found that work not true in alignment requires, when it comes to be put together, to have each piece fitted to its place and corrected for alignment.
But, suppose the ways are made true and in proper alignment, then the brass bores are simply made of equal thickness at the crown, and on the sides at a right angle to the inside faces of the ways; and truth, in these respects, may be measured by actual contact, with the square or calipers, eliminating the chance of error.
In repairing the machine, or putting in new bearings or brasses; the measurement and transferring of the error in the ways to the brasses has all to be gone through with again, and the parts fitted for alignment; whereas, if the ways are true, the brasses can be made true, and to go together, with but little, if any, adjustment when tried in their places.