screwthreads mem18003c fitting and machining, campbelltown college1

Fitting and Machining, Campbelltown College

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SCREWTHREADS MEM18003C


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Definition:A screwthread is a helical structure used to

convert between rotational and linear movement or force.

SCREWTHREADS (VEE FORM)


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To fasten parts together, as with bolts and nuts, set screws,

studs or pipe joints.

To adjust the position of parts.

To exert pressure, as with a vice, screw press or screw jack.

Transmit motion. e.g. Lathe cross slide.

Measuring device. e.g. micrometer.

SCREWTHREADS USES


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Pitch is the distance, measured parallel to the axis, between corresponding points on adjacent thread forms.

Pitch

SCREWTHREAD TERMINOLOGY


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Major Diameter of a parallel tried is the diameter of a imaginary cylinder which just touches the crest of the

external thread, and the root of an internal thread. The major diameter is the largest diameter of a screw or nut.

MajorDiameter



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Pitch Diameter of a vee thread is the diameter at which the thickness of the tread is equal to half the pitch.

PitchDiameter



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Minor Diameter of a parallel thread is the diameter of an imaginary cylinder which just touches the crest of an internal thread. The minor diameter is the smallest diameter of the

thread on a screw or nut.

MinorDiameter



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Crest of the thread is the prominent part of the thread, whether the thread is external or internal.

Crest



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Angle of thread or Included Angle is the angle between flanks measured in the axial plane.

Angle



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Root of the thread is the bottom of the groove between adjacent flanks.

Root



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Flank or side of thread is the straight side which connects the crest and the root.

Flank



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Major Diameter – Minor Diameter

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Depth of thread =


Depth of the thread is the Radial Distance between it’s crest and root.

Depth


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Hand of thread: Will either be ‘Right’ or ‘Left’ hand.

Right hand Thread – Nut turns in a clockwise direction.Left hand Thread – Nut turns in a anti clockwise direction.



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To Calculate the Depth of Thread on a Bolt:

.591 x Pitch

METRIC THREAD FORM


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.613 x Pitch

Unified Thread Forms come in two series:

UNC Unified National CourseUNF Unified National FineNPT National Pipe Thread

UNIFIED THREAD FORM


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.64 x Pitch

British Standard Thread Forms:

BSW British Standard WhitworthBSF British Standard FineBSP British Standard Pipe

BRITISH STANDARD THREAD FORM


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To calculate the pitch of imperial threads:

Imperial Threads =

T.P.I = Threads Per Inch

e.g. Pitch of an 8 T.P.I. Thread

=

Pitch = .125

Depth of Thread = .125 x .54

= .0675”

1TPI

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IMPERIAL THREAD FORM


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Screw threads may be formed by casting, rolling or metal removal process.

The metal removal processes in common use are:

Single point threading tool as used in the screw cutting lathe.

Dies and chasers as used in die heads on capstan, turret and automatic lathes.

Dies as used in hand stock and die.

Hand taps and machine taps.

A thread grinding process.

Thread milling process.

METHODS OF PRODUCING SCREW THREADS


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Hand taps are for the cutting of female threads.

Hand taps are made in sets of three and comprise of:TaperIntermediatePlug or ‘Bottoming’

HAND TAPPING PROCESS


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CUTTING AN EXTERNAL THREADWith Hand Stock and Die

Two types of dies are:

Die Nut

Used for repairing or cleaning existing

threads.

Button Die

Used for manufacturing new

threads.


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Set of Tap Wrenches

Set of Taps and Dies

EXAMPLES OF TAP WRENCHES AND STOCKS AND DIES

Tee-tap Wrench

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Different types of fasteners that use Vee Threads


screwthreads mem18003c fitting and machining, campbelltown college1

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