Mechanical Engineering Terms
ACCURATE – Without error within tolerances allowed, precise, correct,
confirming exactly to standard.
ACME THREAD – A screw thread having an included angle of 29° and
largely used for feed screws on machine tools.
ACUTE ANGLE – An angle which is less than a right angle, 90°.
ADDENDUM – The portion of the tooth of a gear that extends from the
pitch line to the outside.
ALIGN – To bring two or more components of a unit into correct positions
with respect to one another.
ALLOWANCE – The intentional or desired difference between the maximum
limits of mating parts to provide a certain class of fit.
ANGLE – The amount of opening or divergence between two straight lines
that meet at a vertex or that intersect each other.
ANGLE OF THREAD – The included angle between the sides forming the
groove of the screw thread.
ANNULUS – A figure bounded by concentric circles or cylinders (e.g.,a
washer, ring, sleeve etc.).
ARC – A circular section of the circumference of a circle bounded by two
ASSEMBLY – A unit that contains the parts that make up a mechanism or a
AXIS – The line real or imaginary, which passes through the center of a body
and about which the body would rotate if set revolving.
BACKLASH – The clearance or amount of movement between the tooth
profiles of a pair or train of gears in mesh. Also refers to the looseness or
lost motion between screw threads which have been badly worn.
BAFFLE – A device which slows down or diverts the flow of gases, liquid,
BASIC SIZE – The theoretical or nominal standard size from which all
variations are made.
BASTARD – Not standard, irregular. A bastard cut file is a rough cut file
having coarse teeth than a second cut file.
BELL MOUTHED HOLE – A hole which is rounded or tapered slightly
larger at one end or both ends and is not exactly cylindrical throughout
its entire length.
BEVEL – Any surface not at right angle to the rest of the workpiece. If a
bevel is at a 45° angle, it is frequently called a MITER.
BIMETALLIC STRIP – A strip of metal consisting of one metal (or alloy)
in the top portion bonded to a different metal in the bottom portion.
A straight strip becomes curved when heated.
BLIND HOLE – A hole which is made to a certain depth of a workpiece
but does not pass through it.
BISECTING AN ANGLE – Dividing an angle into two equal parts.
BOND – The holding together of different parts.
BORE – The inside diameter of a cylinder, or a hole for a shaft. Also the
operation of machining a circular hole in a metal workpiece.
BRUSH – Pieces of carbon or copper that make a sliding contact against the
commutator or slip rings.
CAM – A plate or cylinder which transmits variable motion to a part of a
machine by means of a follower.
CAP SCREW – A finished screw 5mm or larger, used for fastening two
pieces together by passing the screw through a clearance hole in one
part and screwing in into a tapped hole in the other.
CENTER – A fixed point about which the radius of a circle or an arc moves.