THE TOOLS 183 and cranked. They have no top rake, but only front and side clearances. There is also a slight clearance from the front backwards. Tools with Profiled Edges.—These form a very large group, used only for finishing without traverse. They have no top rake, are used in- differently for all materials, largely in the lathes, and only to a very limited extent in reciprocating machines, since the work is done better with pro- filed milling-cutters. A familiar form is the " vee " tool, employed in cutting screw threads. As these have to traverse, 8° is a suitable side clear- ance for them. They are often made right- and left-handed, with a larger clearance on the leading edge. They are also straightforward and bent. GROUP II The Drills.—Few drills are used now except those of the twist design. None of these are strictly standardized, except in the practice of individual manufacturers. The true drills have two cutting lips only—single edges in balance. Those with three or with four lips link the drills with the reamers, and are used for finishing holes. Drill Angles.—Twist-drills are true cutting- tools. The old flat drills were scrapes. -The straight- fluted drills used for brass are scrapes. With these exceptions drills are right-handed cutting-tools, only a few for special purposes being made left-handed. The helix angle—that imparted to the flutes (fig. 5, B), corresponds with the top rake of the common single- edged tools. An average is 25°, but in some designs it is as high as 30°. In the " increase " twist-drill the angle changes as the lips are ground, becoming less acute. The exit of the cuttings is facilitated, and the thickness of the web increases. The in- creased thickness of web provides additional strength to resist torsional stress. A slight disadvantage is the reduction in the cutting angle. The increased twist in drills standardized by different firms varies (fig. 5) from 26° at A to 21° at B, and from 32° at A to 27° at B, at extremes. The angle c of the lips of drills varies only slightly, ranging from 58° to 61°. The usual angle is 59°. The clearance angle or backing-off, D, varies from about 6° to 15°. Fig. 5.—Elements of Twist DHII A usual amount is 12°, but it need not be so large. It should increase from the periphery towards the centre. On this clear- ance depends the angle which the " chisel edge" or drill point makes with the flat portion of the flute, which is properly 135° (fig. 6, B). If this angle is much larger, as at A, the point is too keen for endurance; if obtuse, as at c, the edge will not cut but will rub only.