In the form of diamond point or front tool, shown in [Fig. 916], there is an unnecessary amount of surface to grind at b, hence the form shown in [Fig. 919] is also employed on light work, while it is in its main features also employed on large work, hence it will be here employed in preference to that shown in [Fig. 916], the cutting action of the two being precisely alike so long as the angles of the faces are equal in the two tools.
The strength of the cutting edge is determined by the angles of the rake and clearance, but in this combination the clearance has the greater strength value. On the other hand the keenness of the tool though dependent in some degree upon the amount of clearance, is much more dependent upon the angle of the top face.
It follows therefore that for copper, tin, lead, and other metals that may be comparatively easily severed, a tool may be given a maximum of top rake, and it is found in practice that top rake can be employed to advantage upon steel, wrought iron, and cast iron, but the amount must be decreased in proportion as the nature of either of those metals is hard.
For the combinations of copper and tin which are generally termed brass or composition, either no top rake or negative top rake is employed according to the conditions.
Fig. 920.
It may be pointed out, however, that in a given tool the cutting qualification is governed to a great extent by the position in which the tool is presented to the work, thus in [Fig. 920], let c represent a piece of work and b, b, b, b, four tools having their top and bottom faces ground at the same angle to each other. In position 1, the top face of the tool is at an acute angle below the radial line a, hence the tool possesses top rake, the amount being about suitable for hard steel or hard cast iron.
In position 2 the top face is at an acute angle above the radial line a, hence the tool has negative top rake, the amount being about suitable for brass work under some conditions.
In position 3 the top face has no rake of any kind, and the tool is suitable (in this respect) for ordinary brass work.