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
equal radii.

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, 
sound etc. 

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.

CENTER LINE - A line used on drawings to show the centers of objects 
and holes. The center line consists of alternate long and short dashes. 

CHAMFER – To bevel or remove the sharp edge of a machined part.

CHECK VALVE – A valve which permits flow in one direction only.

CIRCULAR PITCH – The distance from the center of one gear tooth to
the center of the next gear tooth measured on the pitch line.

CIRCUMFERENCE – A curved line forming a circle and the length of this

COIL SPRING – A spring steel wire wound in a spiral pattern.

COMMUTATOR – A number of copper bars connected to the armature
windings but insulated from each other and from the armature.

CONVOLUTION – One full turn of screw.

CORE – The central or innermost part of an object.

COUNTER BORING – The operation of enlarging a portion of a hole for
part of its depth and to a given diameter, as for the head of a fillister
head screw.

COUNTER SINK – To cut or shape a depression in an object so that the
head of a screw may set flush or below the surface.

CREST CLEARANCE – Defined on a screw form as the space between the
top of a thread and the root of its mating thread.

CREST OF SCREW THREAD – The top surface joining the two sides of
flanks of a thread.

CROWNED – A slight curve in a surface e.g., on a roller or race way.

DEDENDUM – The depth of a gear tooth space below the pitch line or
circle. Also, the radial distance between the pitch circle and the root
circle, which also includes the clearance.

DIAPHRAGM – A flexible dividing partition separating two compartments.

DOUBLE FLARE – A flared end of the tubing having two wall thickness.

DOWEL – A pin, usually of circular shape like a cylinder, used to pin or
fasten something in position temporarily or permanently.

DRIFT PIN – A round tapered steel pin used to align rivet holes so that the
rivet will pass through the holes easily.

ECCENTRIC – A circle or cylinder having a different center from another
coinciding circle or cylinder. Also, a device for converting rotary motion

to reciprocating motion.

END PLAY – As applied to a shaft, the amount that the shaft can move
backward and forward.

EYE BOLT – A bolt threaded at one end and bent to a loop at the other end.

FEATHER – A sliding key, sometimes called splint. Used to prevent a pulley,
gear or other part from turning on a shaft but allows it to move
lengthwise. The feather is usually fastened to the sliding piece.

FILLET – A concave surface connecting the two surfaces meeting at an angle.

FLANGE– A metal part which is spread out like a rim, the action of working
a piece or part to spread out.

FLANK (Side of thread) – The straight part of the thread which connects
the crest with the root.

FLARE – To open or spread outwardly.

FULCRUM – The pivot point of a lever.

FLUSH – When the surfaces of different parts are on the same level, they are
said to be flush.

FLUTE – A straight or helical groove of angular or radial form machined in
a cutting tool to provide cutting edges and to permit chips to escape
and the cutting fluid to reach the cutting edges.

GATE VALVE – A common type of manually operated valve in which a
sliding gate is used to obstruct the flow of fluid.

GEAR – A general term applied to types of toothed wheels, valve motion,
pump works, lifting tackle and ropes.

GEARING – A train of gears or an arrangement of gears for transmitting
motion in a machinery.

GIB – An angular or wedge like strip of metal placed between two machine
parts, usually sliding bearings, to ensure a proper fit and provide
adjustment for wear.

GLAND – A device to prevent the leakage of gas or liquid past a joint.

HAND WHEEL – Any of the various wheels found on machine tools for
moving or positioning parts of the machine by hand feed, as the tailstock
handwheel on a lathe.

HALF MOON KEY – A fastening device in a shape somewhat similar to a


HELICAL GEAR – A gear in which the teeth are cut at some angle other
than a right angle across the gear face.

HELICOIL – A thread insert used to repair worn or damaged threads. It is
installed in a retapped hole to bring the screw thread down to original

HELIX – The curve formed by a line drawn or wrapped around a cylinder
which advances uniformly along the axis for each revolution, as the
thread on a screw or the flute on a twist drill. A helix is often called a
spiral in the shop.

HELIX ANGLE OF A THREAD – The angle made by the helix of the
thread at the pitch diameter line with a line at right angle to the axis.

HEXAGONAL NUT – A nut having six sides and shaped like a hexagon.

INVOLUTE GEAR TOOTH – A curved tooth generated by unwinding a
string from a cylinder to form the curve.

JOURNAL – The part of a shaft or axle that has been machined or finished
to fit into a bearing.

KEYS – Metal pieces of various designs that fit into a slot in a shaft and
project above the shaft to fit into a mating slot in the center hole of a
gear or pulley to provide a positive drive between the shaft and the gear
or pulley.

KEYSEAT– The slot or recessed groove either in the shaft or gear, which is
made to receive the key. Also, it is called a KEYWAY.

KNURL – A uniform roughened or checked surface of either a diamond, a
straight or other pattern.

LAND – The top surface of a tooth of cutting tools, such as taps, reamers
and milling cutters.

LEAD ANGLE – The angle of the helix of a screw thread or worm thread. It
is the measure of the inclination of a screw thread from a plane
perpendicular to the axis of the screw.

LEAD HOLE – A small hole drilled in a workpiece to reduce the feed
pressure, aid in obtaining greater accuracy, and guide a large drill.
Sometimes called PILOT HOLE.

LEAD OF THREAD – On a single threaded screw, the distance the screw
or nut advances in one complete revolution.

LEFT HAND SCREW – One that screws into the mating part or advances
when turned to the left or counter clockwise.

LIMITS OF SIZE – The minimum and maximum sizes permissible for
specific dimensions.

LINE – A tube, pipe or hose which is used as a conductor of fluid.

LINKAGE – A movable connection between two units.

LOBE – The projecting part such as rotor lobe or the cam lobe.

LOCK NUT – A type of nut that is prevented from loosening under vibration.
The locking action is accomplished by squeezing, gripping or jamming
against the bolt threads.

LOOSE PULLEY – A pulley which turns freely on a shaft so that a belt can
be shifted from the driving pulley to the loose pulley in order to stop a
machine driven by an overhead belt drive.

MAJOR DIAMETER – On a straight thread, the diameter of the imaginary
cylinder that just touches the crest of an external thread or the root of
an internal thread.

MALE PART – The external part of any workpiece which fits into a hole,
slot or groove of the mating part.

MANUAL VALVE – A valve which is opened or closed or adjusted by hand.

MESH – Engaging one part with another, as the teeth of one gear mesh with
the teeth of a mating gear.

MICRON – One millionth of a meter or 0.039370 inch.

MINOR DIAMETER – On a straight thread, the diameter of the imaginary
cylinder which just touches the root of an external thread or the crest of
an internal thread.

MORSE TAPER – A self holding, standard taper largely used on drilling
tools, drilling machine spindles, and some lathes.

MULTIPLE THREADED SCREW – A screw with two or more threads
cut around the periphery of the workpiece to provide an increased lead
with a specified pitch.

NUT – A metal fastener of square, hexagon or other shape, having an internal

thread which screws onto a bolt, stud or arbor.

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