Heading-block. A block used in upsetting the heads of bolts or pins.
Heart-shake. A split radiating from the centre of a log.
Heating-surface. That part of the surface of a steam-boiler that receives heat on one side and has water on the other.
Heel-tool. A hand turning tool having a projecting heel to cross the tool rest, and usually held in a wooden stock or handle.
Herring-bone tooth. A form of gear wheel tooth in which the tooth, instead of passing direct across the wheel face, curves partly around the circumference and then back again, so that the two ends of the tooth only are opposite to each other.
Hindley’s-screw. A short length of screw used to drive a worm wheel, and sometimes termed an endless screw.
Hob or hub. A tool for cutting the threads on screw cutting tools, such as chaser dies.
Hour-glass screw. A worm or tangent screw which is formed to envelop part of the arc of circumference of a worm wheel, and therefore assumes in outline the form of a sand hour-glass.
Hunting-tooth. An extra tooth put into a pair of gear wheels that would otherwise contain the same number of teeth, the object being to prevent the same teeth from always falling together.
Hypocycloid (hī-po-sī′kloid). A cycloidal curve in which the rolling circle is rolled within the fixed or base circle.