Threading Technologies

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Threading Technologies 5 ThreadingTechnologies ‘But I grow old always learning many things.’ SOLON (640 – 558 BC)  [Plutarch: Solon, xxxi] 182 Chapter 55 .1 Threads chapter.  Referring  to  Fig.  95,  the  angle  enclosed  by  the thread flanks is termed the included thread angle  (β – as illustrated in Fig. 95 – middle right). This thread  form  is  uniformly  spaced  along  an  ‘imaginary  cylin-An Introduction der’ ,  its  nominal  size  being  referred  to  as  the  major  diameter  (d).  The  effective  pitch  diameter  (d)  is  the The  originator  of  the  first  thread  was  Archimedes (287–212  BC),  although  the  first  modern-day  thread  diameter of a theoretical co-axial cylinder whose outer can  be  credited  to  the  Engineer  and  inventor  Joseph  surface would pass through a plane where the width of  the groove, is half the pitch. Therefore, the pitch (p) is Whitworth  in  1841,  where  he  developed  the  Stan-dards  for  today’s  screw  thread  systems.  Whitworth’s  normally  associated  with  this  ‘effective’  diameter  (i.e.  see  Fig.  95 – middle  right).  The  minor  diameter  (d), 55°  included  angled  V-form  thread,  became  widely established  enabling  thread-locking  and  unlocking  is the diameter of another co-axial cylinder the outer precision  parts  and  of  sub-assemblies  –  paving  the  surface  of  which  would  touch  the  smallest  diameter. way to the build-up of precise and accurate modern- Thread clearance is normally achieved via truncating day  equipment  and  instruments.  Standardisation  of  the thread at its crest, or root – depending upon where Imperial  thread  forms  in  the  USA,  Canada,  UK,  and  the truncation is applied.  These  are  the  main  screw  thread  factors  that  con-elsewhere,  allowed  for  the  interchangeablity  of  parts  tribute  to  a  V-form  thread,  which  has  similar  geom-to become a reality. Around this time, both in France  etry and terminology for its mating nut – for a thread and Germany metric threads were in use, but it took until 1957 before both the common 60° included an- having single-start. gled ISO M-thread and Unified thread profiles to be-come widely accepted and established (Fig. 95). Along  5.2 HandandMachineTapswith these and other various V-form threads that have been  developed  (Fig.  95i),  they  include  quick-release threads such as the Buttress thread: this being a modi-fied form of square thread, along with the 29° included  Hand Tapsangled truncated Acme form which is a hybrid of a V-form and Square thread. Tapered: gas, pipe and petro- Most ‘solid’ taps come in a variety of shapes and sizes leum-type threads, were developed to give a mechani- (Fig.  94),  with  hand  taps  normally  found  in  sets  of cal  sealing  of  the  fluid,  or  gas  medium,  with  many  three:  taper,  plug  and  bottoming  (Fig.  96).  The  pro-other types, including multi-start threads that are now  cess  of  tapping  a  hole  firstly  requires  that  a  specific-in use throughout the world.  sized diameter hole is drilled in the workpiece, this is  termed its  ‘tapping size’. The taper tap along with its  V-form  screw  threads  are  based  upon  a  triangle (Fig. 95 – top diagram), which has a truncated crest and  wrench are employed in producing the tapped thread. root, with the root either having a flat (as depicted), or a more likely, a radius – depending upon the specifi-cation.  If  screw  threads  have  an  identical  pitch,  but different  diameters,  it  follows  that  they  would  have  ‘Solid taps’ , are as their name implies, but it is possible to use   dissimilar  lead  angles.  Usually,  threads  have  just  one  ‘collapsible taps’. These ‘collapsing taps’ have their cutting ele-start, where the pitch and the lead are identical – more  ments  automatically  inwardly  collapsing  when  the  thread  is  completed – allowing withdrawal of the tap – without having will be mentioned on multi-start threads later in thi ...