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The Tap & Die Co
445 West Green Road
London N15 3PL - UK
Tel:+44 (0)20 88881865
Fax:+44 (0)20 88884613

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Established since 1978

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Taps+Dies Terminology

Taps + Dies Terminology or Screw Threading Terminology for taps & dies:
BSW,BSF,UNC,UNF,Metric Standard,Metric Fine,BA,BSB,ME 32tpi ME 40 threads,BSP,BSPT,NPT,Whitform Special taps and dies

 Angle of Thread: Angle included between the side of the thread measured in an axial plane
Basic Size: Theoretical or nominal standard size from which all variations are made
    For taps:-  The taper at the thread end of a tap or the throat of the die, made by cutting away the crests of the first few threads is to distribute the work of cutting, over several threads and act as guide in starting the tap or die.  The chamfer is relieved to facilitate cutting and the TAP is classed Taper(T), Second(S) or Bottoming(B), according to the length of chamfer, approximates are:
   Taper tap is 4o per side (6 to 8 threads)
   Second tap 8o per side (3.1/2 to 6 threads)
   Bottoming tap 22o per side (1.1/2 to 2.1/2 threads)
 Note: The name ‘PLUG’ is commonly used to indicate a Bottoming tap. In America it is used to indicate a Second tap. To avoid confusion the terminology that should be used as stated by British Standard 949,1979 is ‘Bottoming’ tap
    For dies:   stamped-size side is 45o   and   back side is 60o
Clearance Hole: Holes in the die that provide the cutting rake and chip clearance
Cutting Edge:  Leading edge of the land in the direction of rotation for cutting and which does the actual cutting.
Crest:  The top surface joining the two flanks of a thread
Depth of Thread:  The depth of thread, in profile is the distance between the crest and the root of the thread measured normal to the axis
Effective Diameter:  On a parallel screw thread the diameter of an imaginary cylinder which would pass through the threads at such points as to make width of the thread at these points equal
Flute:   The grooves in the tap that provide the cutting rake and chip clearance
Helix Angle:   The angle made by the helix of a thread at the pitch diameter with a plane perpendicular to the axis.  The helix angle increases continuously from the crest to the root of the thread
Land:   That portion of the thread not cut away by the flutes or clearance holes.
Land Width: The chordal width of the land between the cutting edge and the heel measured normal to the cutting edge
Lead / Pitch:   Distance a screw thread advances axially in one complete turn. On a single thread the lead and pitch are identical
Major Diameter:   Largest diameter on a parallel screw thread or tap.  The term ‘Major Diameter’replaces the term ‘Full Diameter’as applied to the thread of a nut
Minor Diameter:   Smallest diameter on a parallel screw thread or a tap. The term ‘Minor Diameter’ replaces the term ‘Core Diameter’ and ‘Root Diameter’as applied to the thread of a screw and also the term ‘Inside Diameter’ as applied to the thread of a nut
Number of Threads / Tpi:   The number of threads in a length of one inch (threads per inch)
Tapping Drill Sizes:   We will show you here how to calculate tapping-drill sizes: (generally)
For Metric (mm) threads a tapping-drill size is calculated by subtracting ‘pitch’ from diameter of the thread eg. for 4x.7mm thread: tapping-drill sizes is 4mm dia - .7 pitch = 3.3mm
For Imperial (inch) threads a tapping-drill size is calculated by subtracting ‘pitch’ from diameter of the thread eg. for 3/4 x 10 UNC thread is  3/4= .75 dia  & 10 pitch=1/10 = 0.1  
Tapping-drill sizes is .75 dia - 0.1 pitch = 0.65   x 25.4 = 16.5mm    (Note: 25.4mm=1 inch)
Tolerance:   Taps: General Dimensions to BSS 949 & American series NC NF NS to ASA standard -
                        Dies circular: Basic dimensions to BSS 1127
 - On our website there is a lot of other very useful ‘Technical’ Data & Information for you to view

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 The Tap & Die Co, London