Fig. 1130.
If a boring bar is to be used only for work that does not require facing at the ends, the cutter, slot, or keyway should be placed in such position in the length of the bar as will best suit the work (keeping in mind the desirability of having the bar as short as possible), and the bar should be tapering from the middle towards each end, as shown in [Fig. 1130]. This will make the bar stronger in proportion to its weight, and better able to resist the pressure of the cut and the tendency to deflect. The parallel part at a is to receive the driving clamp, but sometimes a lug cast on at that end is used instead of a clamp.
For bores too large to be bored by the bar alone, a tool-carrying head is provided, being sometimes fixed upon the bar by means of a locking key, and at others fed along the bar by a feed screw provided on the bar.
When the head is fixed on the bar the latter must be twice as long as the bore of the work, as the work is on one side of the head at the beginning, and on the other at the end of a cut; hence it follows that the sliding or feeding head is preferable, being the shortest, and therefore the most rigid, unless the bar slides through bearings at each end of the head.
Fig. 1131.
[Fig. 1131] represents a bar with a fixed head in operation in a cylinder, and having three cutting tools, and it will be observed that if tool a meets a low spot and loses its cut, the pressure on tools b and c, both being on the opposite side of the head, would cause the bar to spring over towards a, producing a hole or bore out of round, and it follows that four tools are preferable.