Fig. 1499.
To take up the wear of the sliding bar a, various forms of guideways and guides are employed, a common form being shown in [Fig. 1499]. There are two gibs, one on each side of the bar, and these gibs are set up by screws to adjust the fit. In some cases only one gib is used, and in that event the wear causes the slide to move to one side, but as the wear proceeds exceedingly slowly in consequence of the long bearing surface of the bar in its guides, this is of but little practical moment. On the other hand, when two gibs are used great care must be taken to so adjust the screws that the slide bar is maintained in a line at a right angle to the jaws of the work-holding vice, so that the tool will cut the vertical surfaces or side faces of the work at a right angle to the work surface that is gripped by the vice.
To enable the length of stroke of slide a, [Fig. 1496], to be varied to suit the length of the work, and thus not lose time by uselessly traversing that slide, e is provided with a T-slot as before stated, and the distance of the wrist pin (in this slot) from the centre of wheel e determines the amount of motion imparted to the connecting rod, and therefore to slide a. The wrist pin is set so as to give to a a rather longer stroke than the work requires, so that this tool may pass clear of the work on the forward stroke, and an inch or so past the work on the return stroke, the latter giving time to feed the tool down before it meets the work.
The length of the stroke being set, the crank piece e (for its slot and wrist pin correspond to a crank) is, by pulling round the pulley p, brought to the end of a stroke, the connecting rod being in line with slide a. The nut d is then loosened and slide a may then be moved by hand in its slideway until the tool clears the work at the end corresponding to the connecting rod position when nut d is tightened and the stroke is set.
Fig. 1500.
Now suppose it is required to shape or surface the faces f and f′, the round curve s and the hollow curve c of the piece of work shown held in a vice chuck in [Fig. 1500], and during the cutting stroke the slide a will travel in the direction of n in the figure, while during its return stroke it will traverse back in the direction of i. The sliding table w in [Fig. 1496] would continuously but gradually be fed or moved (so much per tool traverse, and by the feeding mechanism described with reference to [Fig. 1501]) carrying with it the vice chuck, and therefore the work. When this feeding brought the surface of curve s, [Fig. 1500], into contact with the tool, the feed screw handle in figure would be operated by hand so much per feed traverse, thus raising the slider, and therefore the tool, in the direction of l, and motion of the work to the right and the left of the tool (by means of the feed handle) would (if the amount of tool lift per tool stroke is properly proportioned to the amount of work feed to the right) cause the tool to cut the work to the required curvature. When the work had traversed until the tool had arrived at the top of curve s, the direction of motion of the feed-screw handle z in [Fig. 1496] must be reversed, the tool being fed down so much per tool traverse (in the direction of m) so as to cut out the curves from the top of s to the bottom of c, the face f′ being shaped by the automatic feed motion only.