Fig. 891.
In the promiscuous practice of the general workshop, where it may and often does happen that two pieces of work are rarely of the same shape and size, lines whereby to set the work are an absolute necessity, not only to set the work by in chucking it, but also to denote the quantity of metal requiring to be taken off one face in order to bring its distance correct with relation to other faces. An example of this kind is given in [Fig. 891], which represents a lever to be bored and faced at the two ends, the radial faces standing at different distances from the centre of the lever stem as denoted by the lines (defined by centre punch dots) e, f, g, h, i, j, k, l. It will be noted that at h, i, f, and e there is but little metal to be taken off, while there is ample at l. Suppose then that the face l were the first one turned, and it was only just trued up, then when f or h were turned there would be no metal to turn, for they may be too near the plane of l already.
The necessity for these lines now being shown, we may proceed to show how they should be located and their services in setting the work. The line a is called the centre line, it passing through the centre of the thickness of the link body on both edges of the link. From it all the other lines, as j, f, l, g, e, k, and h, i, are marked.
The first question that arises in the chucking is, which of the holes b, c, or d, shall be bored first. Now the faces k and l are those that project farthest from the centre line a, hence if the hole at that end be bored and the faces k, l, be turned first, we may bolt those faces against the chuck plate, and thus insure that all three holes shall stand axially true one with the other. If the holes b or c were bored first, l projecting beyond j and f (which are the faces of holes b, c) would prevent the radial face first turned from serving as a guide in the subsequent chuckings, unless a parallel piece were placed between the face and the chuck. In this case, however, there is not only the extra trouble of using the parallel piece, but there would obviously be more liability of error, as from the parallel piece not being dead true and the amount of the error multiplying in the length of the lever, and so on.
The hole d is the one, therefore, to be bored first, the chucking proceeding as follows:—Two parallel pieces of sufficient thickness to keep l clear of the chuck plate should be placed one on each side of the hub e, and bolts and plates placed directly over them. The work must be set so that the line a on each side of the link stands exactly parallel with the face of the chuck, the parallelism being tried at each end of the line, because any error that may be made in setting the work by the full length of the line will have a less effect upon the work than the same amount of error in a shorter length of line. For this reason the centre line should always be marked as long as possible and used to set by, unless there is a longer line running parallel to it and marked on both sides of the link, as would be the case if the dotted line at j and that at l were equidistant from a, in which event they may preferably be used.
The work is set true to the lines by a scribing block, or surface gauge, but as that instrument is more used in setting work with chuck dogs its application will be shown in connection with chucking by dogs; hence to proceed: To set the work true to the line a it may be necessary to place a thickness of paper, a piece of sheet tin, or the equivalent, beneath one of the parallel pieces to bring a parallel with the chuck plate surface. This being done, however, and the circle d being set to run true, the hole may be bored and the radial face l turned off so as to just split the dotted line at l, and this radial face may be used instead of the line a for all subsequent chuckings, so as to avoid the errors that might occur in referring to the line, and from the alterations that might occur in the form of the work from removing the surface metal.
Fig. 892.