The boring tool is, upon cast iron and brass, exceedingly liable to chatter, but this may always be avoided by making the angles forming the cutting edge less acute: thus, in [Fig. 1110] are three boring tools, a, b, c, operating in a piece of work d. Now the lateral pressure of a cut is exerted upon the tool at a right angle to the length of the cutting edge; hence (in addition to the vertical pressure) the lateral pressure of the tool a will be in the direction of the dotted line and arrow a, that on b in the direction of dotted line and arrow b, and that on c in the direction of dotted line and arrow c; hence the pressure of the cut would tend to force a towards the centre of the hole and off or away from its cut, b back from its cut, and c deeper into its cut. Now as the cut proceeds, the tool edge dulls, hence it would appear that a compromise between c and b would be the most desirable, as giving to the tool enough of the tendency to deepen its cut to compensate for the tendency to spring away from its cut, as the cutting edge dulls (which it does from the moment the cut begins). This is quite practicable in tools to be used on wrought iron, as shown in [Fig. 1111], which represents the most desirable form.
Fig. 1111.
In this form the part of the cutting edge performing duty under a deep cut will be mainly in front of the tool, but in light cuts the cutting edge would be farther back, where it is more nearly parallel to the line of the work bore, and will hence cut smoother.
Fig. 1112.
Where a boring tool is intended for light cuts only on wrought iron it may have all, or nearly all, its rake at the top, as shown in [Fig. 1112], from a to b representing the cut, and c the tool.