Page 327 - Mechanical Engineer's Data Handbook

GLQSSARY OF TERMS 315

creep Slow plastic deformation of metals under single-point cutting tool for precision machining of
stress, particularly at high temperatures. non-ferrous metals and plastics.
creep resistance Resistance of metals to creep. diamond wheel A grinding wheel with crushed dia-
critical speed A rotational speed corresponding to a monds embedded in resin or metal.
natural frequency of transverse vibrations of the die A tool used to impart shape in many processes,
member. Also called ‘whirling speed’. e.g. blanking, cutting, drawing, forging, punching, etc.
CroBipffOw heat exchanger A heat exchanger in which die casting A casting made in a die. A process where
the two fluids flow at right angles to one another. molten metal is forced by high pressure into a metal
cutting fluid A fluid used in metal cutting to improve mould.
finish, tool life, and accuracy. It acts as a chip remover d#erential pregsure gauge A gauge which measures
and a coolant. the difference between two pressures, e.g. across an
cutting speed The linear or peripheral speed of orifice in fluid flow.
relative motion between a cutting tool and workpiece diode Thermionic or semiconductor device with uni-
in the principal direction of cutting. directional properties used as a rectifier.
cyaniding The introduction of carbon and nitrogen direet current (d.c.) An electric current which flows in
into a solid ferrous alloy by holding it at a suitable high one direction only.
temperature in contact with molten cyanide. dmt current machines Generators or motors oper-
cyeloichl gears Gears with teeth whose flank profile ating on d.c.
consists of a cycloidal curve. discharge Coewcient The rate of actual to theoretical
cylindrical grinding Grinding the outer cylindrical flow of a fluid through an orifice, nozzle, Venturi
surfaces of a rotating part. meter, etc.
disk stresses Radial and hoop stresses in a rotating
disk.
damped vibmtioe Vibrations reduced in amplitude dowel A pin located in mating holes in two or more
due to energy dissipation. parts used to locate them relative to one another.
damping The reduction in amplitude of vibrations draft tube Discharge pipe at a water turbine outlet
due to mechanical friction in a mechanical system or which reduces the water velocity and improves effi-
by electrical resistance in an electrical one. ciency.
deceleration Negative acceleration. The rate of dim- drag The resistance to motion of a body moving
inution of velocity with time. The unit is metres per through a fluid.
second per second (ms-*). drag coefficient A non-dimensional quantity relating
dedemhn The radial distance between pitch circle drag to projected area, velocity and fluid density.
and the bottom of a gear tooth. drawing Forming recessed parts by the plastic flow of
ddection The amount of bending, compression, metal in dies. Reducing the diameter or wire by pulling
tension, or twisting of a part subject to load. through dies of decreasing diameter.
density The mass of a unit volume of a substance. drill A rotating end cutting tool with one or more
The unit is kilograms per metre cubed (kgm-3). cutting lips used for the production of holes.
depth of cut The thickness of material removed from drop forging A forging made using a ‘drop hammer’.
a workpiece in a machine tool during one pass. dry flue gas Gaseous products of combustion exclud-
dm1 gauge A sensitive mechanical instrument in ing water vapour.
which a small displacement, e.g. 0.01 mm, is indicated dryness fraction The proportion by mass of dry
on a dial. steam in a mixture of steam and water, i.e. in ‘wet
diametral clearance The difference in diameter be- steam’.
tween a shaft and the hole into which it fits or runs, e.g. ductility The ability of a material to deform plasti-
in plain journal bearings. cally without fracture.
diamond dust The hardest substance used for abras- Dunkerley’s metbod A method for determining the
ive wheels. natural frequency of transverse vibrations of a shaft or
dnd pyramid hardness An indentation hardness its whirling speed when carrying several masses.
test for materials using a 136“ diamond pyramidal dynamic balancing The technique of eliminating the
indenter and various loads. centrifugal forces in a rotor in order to eliminate
diad tool A diamond shaped to the contour of a vibration.