Mechanical general terms

ACCURATE – Without error within tolerances allowed, precise, correct.

ACHME 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

machine.

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.

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

line.

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 operat
ed 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

semicircle.

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

size.

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.

OFF CENTER – Not on the true center line or axis, offset, eccentric or

inaccurate.

PEEN – The end of the head of a hammer opposite the face, such as ball,

straight or cross peen, and used for peening or riveting.

PILOT – A guide at the end of the counter bore which fits freely into the

drilled hole and align the body of the counterbore while cutting takes

place.

PILOT SHAFT – A shaft positioned in or through a hole of a component

as a means of aligning the components.

PILOT VALVE – A valve used to control the operation of another valve.

PINION – The smaller of the pair of gears regardless of the size or type.

PIPE THREAD – A 60° thread having flattened crest and roots which are

cut on a taper. Pipe thread is used on piping and tubing.

PITCH – In screw threads, the distance from a point on o

ne thread to a

corresponding point on the next thread measured parallel to the axis. In

the case of spur gears, indicates the size of the gear teeth and is correctly

called diametral pitch.

PITCH DIAMETER – For screw threads, the diameter of an imaginary

cylinder, the surface of which would pass through the threads at such

points that would make the width of the groove and width of the land

equal to one half the pitch.

PLAY – The movement between two components.

PULLEY – A wheel having a plain or V groove rim over which a belt runs

for the transmission of power from one shaft to another.

QUILL – A hollow shaft that revolves on a solid shaft carrying pulleys, gears

or clutches. When the clutch is closed, the quill and shaft revolve

together.

RACK – A straight metal strip having teeth that mesh with those of a gear to

convert rotary into reciprocating motion or just the opposite.

RECESS – A groove cut below the normal surface of a workpiece.

RIGHT HAND THREAD – A screw thread which advances into the mating

part when turned clockwise or to the right.

RIVET – A one piece fastener consisting of a head and a body and used for

fastening two or more pieces together by passing the body through a

hole in each piece and then forming a second head on the body end. It

cannot be removed except by taking off the head.

SCREW – A helix formed or cut on a cylindrical surface which may advance

along the axis to the right or left. The helix may be single or multiple.

SCREW THREAD – A ridge of uniform section or shape in the form of a

helix on the external or internal surface of a cylinder, or in the form of

a conical spiral on the external or internal surface of a cone.

SET SCREW – Usually a hardened steel screw having either no head or a

square head and with various degrees of points or ends to lock or tighten

adjustable machine parts in position on a shaft.

SHOULDER SCREW – A screw having two or more diameters or shoulders

and commonly used for supporting levers and other machine parts that

have to operate freely.

SINGLE THREAD – A screw thread cut around a cylinder having a single

start in which the lead is equal to the pitch.

SOCKET HEAD – Screw head having a hexagonal or other form of recessed

socket in the head so that the screw can be turned with a wrench or key,

as a hexagon key.

SPLINE – Slot or groove cut in a shaft or bore, a splined shaft onto which a

hub, wheel, gear etc. with matching splines in its bore is assembled so

that the two must turn together.

SPOOL VALVE – A hydraulic directional control valve in which the direction

of the fluid is controlled by the means of a grooved cylindrical shaft

(spool).

SPRING – An elastic device which yields under stress or pressure but returns

to its original state or position when the stress or pressure is removed.

SPUR GEAR – A toothed wheel having external radial teeth.

SQUARE THREAD – A form of screw thread in which the cross–section of

the thread forms a square, making the width of the thread equal to the

space between the threads.

STEP BLOCK – A block of steel or cast iron having a series of steps and used

for supporting the ends of machine clamps when clamping work to the

table.

STOPS – Devices attached to the movable parts of a machine tool to limit

the amount of travel.

STUD – A rod having thread on both ends.

STUFFING BOX – A chamber having manual adjustment device for sealing.

TAPER – A shaft or hole that gets gradually smaller toward one end.

TAPER PINS – Steel pins used for locating and holding the machine parts

in position on a shaft.

T-BOLT – A threaded bolt having a square or rectangular end which fits into

the T slot of a machine table for clamping workpieces.

TEMPLATE – A flat pattern or guide plate usually made from sheet metal

and used as a gauge or guide when laying out, drilling, forming in a

machine or filing irregular shapes on metal pieces.

THUMB SCREW – A type of screw having a winged or knurled head for

turning by hand when a quick and light clamping effect is desired.

TOLERANCE – A fractional allowance for variation from the specifications.

T-SLOT

 A recessed or undercut slot made with a special T shaped cutter in

the tables of machine tools to receive the square head of a T bolt for

clamping workpiece.

U-BOLT – An externally threaded fastener bent in the shape of the letter U

and with both ends threaded.

VALVE – Any device or arrangement used to open or close an opening to

permit or restrict the flow of a liquid, gas or vapour.

V-BLOCKS – Square or rectangular shaped blocks of steel that are usually

hardened and accurately ground. These have 90° V groove through the

center and are provided with clamps for holding round workpiece for

laying out, drilling, milling etc.

VISE – A mechanical device of many designs and sizes in which work pieces

are clamped for hand or machine operations.

V-WAYS – The top of the bed of a lathe, planer or other machine tool which

acts as bearing surface for aligning and guiding the moving parts such as

the carriage of a lathe.

WORM – A threaded cylinder which meshes with and drives a worm gear,

the thread being specially designed to mate with the teeth in the worm

gear.

WORM GEARS – Gears with teeth cut at an angle to be driven by a worm.

The teeth are usually cut out with a hob to fit the worm.