Thread Technologies by JS.Noho_1 doc

Usually, threads have just one start, where the pitch and the lead are identical – more will be mentioned on multi-start threads later in this ‘Root radius’ , is usually a stronger thre

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Threading Technologies

‘But I grow old always learning many things.’

SOLON (640 – 558 BC)  [Plutarch: Solon, xxxi]

5.1 Threads

An Introduction

The  originator  of  the  first  thread  was  Archimedes 

(287–212 BC), although the first modern-day thread 

can be credited to the Engineer and inventor Joseph 

Whitworth  in  1841,  where  he  developed  the 

Stan-dards  for  today’s  screw  thread  systems.  Whitworth’s 

55°  included  angled  V-form  thread,  became  widely 

established  enabling  thread-locking  and  unlocking 

precision  parts  and  of  sub-assemblies  –  paving  the 

way to the build-up of precise and accurate modern-day  equipment  and  instruments.  Standardisation  of 

Imperial thread forms in the USA, Canada, UK, and 

elsewhere,  allowed  for  the  interchangeablity  of  parts 

to become a reality. Around this time, both in France 

and Germany metric threads were in use, but it took 

until 1957 before both the common 60° included an-

gled ISO M-thread and Unified thread profiles to be-come widely accepted and established (Fig. 95). Along 

with 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 

angled truncated Acme form which is a hybrid of a V-

form and Square thread. Tapered: gas, pipe and petro-

leum-type threads, were developed to give a mechani-cal  sealing  of  the  fluid,  or  gas  medium,  with  many 

other types, including multi-start threads that are now 

in use throughout the world. 

V-form  screw  threads  are  based  upon  a  triangle 

(Fig. 95 – top diagram), which has a truncated crest and 

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 

dissimilar lead angles. Usually, threads have just one 

start, where the pitch and the lead are identical – more 

will be mentioned on multi-start threads later in this 

  ‘Root radius’ , is usually a stronger thread form, as it is less 

prone to any form of shear-type failure mode in-service.

 

Pitch, refers to the spacing, or distance between any two cor-responding points on adjacent threads, normally taken at the 

thread’s effective pitch diameter.

NB  The reciprocal of this pitch, is the threads per inch (i.e. 

for Imperial units).

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-der’ ,  its  nominal  size  being  referred  to  as  the  major 

diameter (d). The effective pitch diameter (d ) is the  diameter of a theoretical co-axial cylinder whose outer  surface would pass through a plane where the width of 

the groove, is half the pitch. Therefore, the pitch (p) is 

normally associated with this ‘effective’ diameter (i.e. 

see  Fig.  95 – middle  right).  The  minor  diameter  (d ), 

is the diameter of another co-axial cylinder the outer  surface  of  which  would  touch  the  smallest  diameter.  Thread clearance is normally achieved via truncating  the thread at its crest, or root – depending upon where  the truncation is applied. 

These are the main screw thread factors that con- tribute to a V-form thread, which has similar geom-etry and terminology for its mating nut – for a thread  having single-start. 

5.2 Hand and Machine Taps

Hand Taps

Most ‘solid’ taps come in a variety of shapes and sizes  (Fig.  94),  with  hand  taps  normally  found  in  sets  of  three:  taper,  plug  and  bottoming  (Fig.  96).  The  pro- cess of tapping a hole firstly requires that a specific-sized diameter hole is drilled in the workpiece, this is 

termed its ‘tapping size’. The taper tap along with its  wrench are employed in producing the tapped thread. 

  ‘Solid taps’ , are as their name implies, but it is possible to use 

‘collapsible

taps’. These ‘collapsing taps’ have their cutting ele-ments  automatically  inwardly  collapsing  when  the  thread  is  completed – allowing withdrawal of the tap – without having 

to unscrew it, moreover, these ‘collapsible taps’ can be self-set- ting ready for the next hole to be tapped. They are ‘sized-re-stricted’ by their major diameter.

  ‘Tapping size’ , refers to the diameter of hole to be drilled that 

will produce sufficient thread depth for the threaded section 

to be inserted and screwed down, for a particular engineering  application. For example, the alpha-numeric notation: M6x1,  refers to a metric V-form screw thread of φ6 mm with a pitch 

of 1mm. It is not necessary to state whether the thread is left-, 

or right-handed, as the convention is it will be a right-handed  single-start thread. In this case, for an M6x1 thread, the tap-ping size can be obtained from the tables, as having a drill size 

of φ5 mm. 

182 Chapter 5

Figure 94 A range of hand and machine taps and a die for the production of precision threads [Courtesy of Guhring]

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Figure 95 Basic V-form thread nomenclature [Courtesy of Sandvik Coromant]

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184 Chapter 5

name, to lead the tap with progressively deeper cuts as 

it is rotated into the workpiece. As the taper tap enters 

the previously tapping sized drilled hole, care should 

be taken to ensure it remains normal to the work sur-

face, otherwise and angled hole will result. As the ta-

per tap is rotated, after each ¾ turn, it is counter-ro-tated by about a ¼ turn to break the chips, otherwise 

‘galling’, or tap-breakage problems in-situ could arise.  Once the taper tap has been through a ‘running hole’ , 

it is often only just necessary to ‘size’ the hole with the  bottoming tap. However, if a ‘blind’/non-through hole, 

  ‘Galling’ ,  is  when  the  tap,  or  indeed  any  cutter  becomes 

clogged with the remnants of workpiece material, which will 

impair its efficiency, or at worse, cause it to break in the par-tially tapped hole. 

Figure 96 Hand taps and tapping nomenclature [Courtesy of TRW-Greenfield Tap and Die]

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utilise all three taps in the set, as each successive tap 

once rotated to depth, it will have less lead (i.e. taper) 

on the tapped hole, creating a stronger thread – up to 

the thread’s maximum shear strength. 

Very  large  diameter  hand  taps,  require  a  certain 

level  of  skill  in  ensuring  that  not  only  the  tapped 

hole is normal to the surface, but a considerable level 

of  physical  strength  is  necessary  to  tap  such  a  hole! 

Curved surfaces are more difficult to tap, particularly 

concave ones, as it is often difficult to keep the hand 

tap normal to the surface With concave surfaces any 

rotational motion of the tap wrench may be somewhat 

restricted, without a suitable extension chuck/bar – as-suming workpiece access conditions allow. 

For manual tapping operations, it is often useful to 

utilise ‘Tapping chucks’. These chucks have a rotational 

drive, coupled to a sprung-loaded Z-axis. The tapping 

chuck is positioned over the pre-drilled hole and man-ually-fed down into the hole. Once the tap has engaged 

with the hole, it is pulled and simultaneously ‘floated 

down’ the hole being tapped – giving excellent tapped 

hole  accuracy.  At  ‘bottoming-out’  the  tapping  chuck 

automatically  reverses  its  direction  and  ‘drives’  itself 

out of the hole – while the machine’s spindle continues 

to rotate in the tapping direction

Machine Taps

Machine  taps  (Fig.  97)  are  utilised  across  a  diverse 

range  of  machine  tools  and  special-purpose  tapping 

equipment.  They  can  have  a  variety  of  flute  helices, 

ranging  from  quick-to-straight  flutes  (Fig.  97a), 

de-

pending upon the composition of the workpiece ma-terial  to  be  tapped.  When  tapping,  all  machining  is 

undertaken  by  the  cutting  teeth  and  the  chamfer.  In 

general, the form and length of this chamfer will de-pend upon what type of hole is to be tapped. Tapping 

‘through-holes’  is  not  too  difficult,  but  ‘blind-holes’ 

can  present  a  problem,  associated  with  the 

evacu-ation  of  swarf  in  the  reverse  direction  to  that  of  the 

feed.  Tap  flute  spirals  that  are  left-handed  and  those 

with spiral points (Fig. 97bi), remove chips in the cut-  ‘Tapping

depth’ , is an often misleading term, as in many situ-ations holes are tapped too deeply, as its is only necessary to 

have a full thread form for 1.5D*, as this is where the maxi-mum thread shear strength occurs, which in turn, is related 

to the shear strength of the workpiece material.*D = thread’s 

major diameter.

ting  direction,  or  feed  direction  and  are  particularly  useful for tapping through-holes. Whereas, taps with  straight flutes (Fig. 97bii) in conjunction with a long  chamfer lead, can also give good tapping results. For  blind-holes, right-handed flutes, or straight fluted taps  having  shorter  chamfer  lead  lengths  give  acceptable  tapping  results.  These  right-hand  fluted  taps,  allow  chip-flow  in  the  backward  direction  –  up  the  flutes.  The chamfer lead length is such, that it allows return  movement of chips, but they will not jam and are reli-ably sheared off. 

When tapping aluminium, grey cast iron, or certain  brass alloys, the tap should have a short lead length –  regardless of whether the hole is ‘blind’ , or ‘through-running’. If, when tapping these workpiece materials, 

a long chamfer lead length was utilised, the tap would 

behave  like  a  ‘Core-drill’ with  chip-breaker  grooves. 

This effect would create ‘drilling’ a tapping-sized hole 

to the major diameter – instead of actually cutting the  required thread

On some machining and turning centres, it is pos-sible to ‘solid tap’ the workpiece, using CNC software 

developed just for this task. A ‘solid tapping’ operation  requires  that  the  rotation  of  the  spindle  and  the 

Z-axis control are fully synchronised, otherwise tapping 

errors would arise. It is possible to calculate the time  required for a tapping operation (Degamo, et al. 2003  – modified for metric units), using the following equa-tion:

Tm = L n �N = πDL n

�V + AL+ AR

Where:

 Tm = Cutting time (min.),

  L = Depth of tapped hole, or Length of cut (mm),

  n = Feedrate (mm min–),

  N = Spindle (rpm),

  V = Cutting speed (m min–),

 AL = Allowance to start the tap (min),

 AR = Allowance to withdraw the tap (min)

* To convert to inches, substitute 12 for the 1000 con-stant in the equation and modify the metric units to  inches

186 Chapter 5

Figure 97 Machine taps: with and without flutes [Courtesy of Guhring]

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Figure 98 Fluteles tapping and tool geometry [Courtesy of Guhring]

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188 Chapter 5

5.3 Fluteless Taps

Fluteless  taps  (Fig.  98a),  do  not  have  cutting  edges 

(Fig. 98ai) and produce the desired thread geometry 

by a ‘rolling action’ of the workpiece material. Threads 

produced  by  fluteless  taps  are  much  stronger  than 

their  equivalent  machined  taps  (Fig.  98b).  The  bulk 

workpiece material approximately follows the thread’s 

contour,  thereby  imparting  additional  shear  strength 

to  each  thread.  Oil  grooves  are  usually  incorporated 

into the taps periphery, to facilitate workpiece mate-rial movement and to reduce tap wear rates. Like the 

conventional  machine  taps  (Fig.  97),  fluteless  taps 

have a lead to the tap’s edge – termed a ‘forming lead’ 

(Fig. 98b – left), as opposed to a conventional machine 

tap which has a ‘chamfer lead’ (Fig. 98b-right) which 

forms  part  of  the  cutting  action.  Therefore,  the 

chi-pless  tap  in  operation  (Fig.  98bi),  plastically  moves 

workpiece material from the pre-drilled hole into the 

spaces between the tap’s flanks and in so doing, locally 

work-hardening this material to a limited depth in the 

workpiece’s substrate. 

Several factors need to be considered prior to util-ising fluteless taps on engineering components, these 

are:

• Over-sized diameter of pre-drilled hole –  if  the 

hole is too large, then insufficient workpiece mate-rial will be available to fully form the rolled thread,

• Undersized diameter of pre-drilled hole –  too 

small a hole will be likely to cause the chipless tap 

to jam – as it attempts to roll the thread, possibly 

leading to tap breakage,

NB  Therefore, precise control over the diameter of 

the pre-drilled hole is imperative

• Workpiece material’s characteristics –  both  the 

bulk hardness and more importantly, its mechanical 

working ability and as a result of this action its lo-cal hardening, are important factors when ‘rolling’ 

a thread form. 

NB 

A ‘start-point’ for the size of pre-drilling diam-

eter can be obtained from the tooling suppliers. Of-ten some form of experimentation is necessary in 

order to obtain the optimum diameter, as this pre-drilled  diameter  will  vary  according  to  the 

work-piece material’s previous processing route

In Appendix 7, some tapping problems are given, with  possible  causes  and  solutions  that  may  be  of  use  in  identifying  any  potential  remedial  machining  action 

to be taken

5.4 Threading Dies

On shafts, having either straight and tapered external  threads these can be manually cut, up to a realistic max-imum φ40 mm, with threading dies. In essence, these  threading dies can be considered as analogous to hard-ened threaded nuts with multiple cutting edges (Fig.  99a). The cutting edges on the front die face are usually  bevelled, or have a spiral lead to assist in starting the  thread on the workpiece. Likewise, it is normal to add 

a reasonable chamfer to the bar’s end to be threaded, as  this also helps to gently introduce the thread to depth, 

as  the  stock  and  die  are  manually-rotated  down  its  length. As is the case for tapping, it is normal practice 

to ‘back-off’ the ‘stock’s’ rotation about every ¾ of a  turn by approximately ¼ of a turn, to facilitate chip-breaking. As a result of these ‘leads’ on both the shaft  and die, a few threads on the bar’s end will not be to  full thread depth. Care must be taken when initially  starting to cut the thread, as if it is not square to the 

bar’s  axis,  then  a  ‘drunken thread’  will  result.  Previ-ously, most dies were manufactured from high carbon 

steel and, due to their size, their ‘ruling section’ and its 

  ‘Drunken threads’ ,  are  the  result  of  variations  in  the  helix 

angle and its associated pitch differing in uniformity on each  side of the thread’s diameter. Hence, a ‘true’ mating nut, would 

‘wobble’ somewhat as it is rotated down such poorly manufac-tured threaded shaft – hence, its name: ‘drunken thread’.

  ‘Ruling section’ , this term relates to the cross-sectional area 

that can normally be hardened, being significantly influenced 

by  the  component’s  geometry  which  affects  its  ‘critical

cool-ing velocity’ (i.e.  usually  around  1,000°C  sec–)  when  being  quenched. This quenching rate is necessary if the part’s metal-lurgical structure is to fully transform into a martensitic state,  prior to subsequent tempering.

Figure 99 Die geometries and their nomenclature [Courtesy of Guhring]

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190 Chapter 5