Arc of approach. That part in the revolution of a pair of gear wheels in which the teeth in contact approach the line of centres of the two wheels.
Arc of recess. That part in the revolution of a pair of gear wheels in which the teeth in contact recede from the line of centres of the two wheels.
Arc-pitch. The pitch of gear wheel teeth when measured around the pitch circle.
Attachment. A work-holding device that may be attached to a machine.
Auger. A wood-boring tool having two spiral plates and a pointed screw to feed it, the cutting edge being at the end of the tool.
Axle-box. The bearing in which an axle revolves.
B
Back-gear. The toothed wheels on the spindle of a lathe and at the back of the lathe-head, by means of which the speed of the lathe is reduced.
Back-geared lathe. A lathe having a back gear to reduce its motion.
Back-knife gauge-lathe. A lathe in which the work is finished and cut to size and shape by a knife at the back of the lathe.