Modern Mechanical Engineering Vol-I

THE TOOLS

195

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with shoulders and held similarly. Roughing-cutters in fig. 29 are each
adjusted with a grub-screw and locked with a nut. Cutters are set spirally,
and tightened with tapered pins in splits. Wedge bushings and screws are

FJg. 29.—" Wrigley " High-speed Cutters for Aluminium, adjusted with Screw and locked with Nut

used for tightening,
feet in diameter.

Many of the heads with inserted cutters are several

GROUP VI

Grinding Wheels.—The old term " emery " wheels applied to this
group has long been abandoned, since emery is employed to a limited
and ever-lessening extent, having been replaced by more effective grinding
materials.

Emery and Corundum.—The difference between these is one of
purity. Alumina is the chief constituent of each. Corundum contains a
higher proportion of alumina than emery, and its grains split, leaving sharp
edges; while emery wears smoothly, with a glazed surface. Both materials
are impregnated with oxide of iron, which, when present in large quantities,
reduces the cutting capacity. On the other hand, emery wheels produce a
high finish.

Carbide of Silicon Abrasives.—These are prepared in electric

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