Fig. 1603.
Thus, if there be four gibs as at a, b, and e, f, set up by their respective set-screws, the very means provided to take up the wear affords a means of setting the bed out of line, so that the slots in the table (and, therefore, the chucks fitting to these slots) will not be in the line of motion of the table, and the work depending upon these chucks will not be true. This may be avoided by taking up the wear on two edges only, as in [Fig. 1603] at a, b, but in this case the bearing at e and f would eventually cease by reason of the wear.
Suppose, for example, that the pressure of the tool cut tends to throw the table in the direction of arrow j, and the surfaces at a and f resist the thrust and both will wear. But when the strain on the table is in the direction of arrow k, the surfaces b, e, will both wear; hence while the width apart of the table slides becomes greater, the width apart of the bed slideways wears less, and the fit cannot be maintained on the inner edges of the guideways. It is furthermore to be noted that with flat guideways the table will move sideways very easily, since there is nothing but the friction of the slides to prevent it, but in the case of V-guides the table must lift before it can move sideways; hence, it lies very firmly in its seat, its weight resisting any side motion.
It is found in practice that the wear of the guides and guideways in planer tables and beds is greatest at the ends, and the reason of this is as follows:—
Fig. 1604.
In [Fig. 1604] is a top view of a planer table, the cutting tool being assumed to be at t, and as the driving gear is at g forcing the table in the direction of the arrow a, and the resistance is at t, the tendency is to throw the table around in the direction of arrows b and c. When the tool is on the other side of the middle of the table width as at f, the tendency is to throw the table in the opposite direction as denoted by the arrows d and e, which obviously causes the most wear to be at the ends of the slides.
As the feed motions are placed on the right-hand side of the machines the operator stands on that side of the machine at x, and starts the cut from that side of the table; hence unless the work is placed in the middle of the table width, the wear will be most in the direction of arrows b and c.