Smith G.T. Cutting Tool Technology: Industrial Handbook
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5.1 Threads
An Introduction
e originator of the rst thread was Archimedes
(287–212 BC), although the rst modern-day thread
can be credited to the Engineer and inventor Joseph
Whitworth in 1841, where he developed the Stan
-
d
ards 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
-
g
led ISO M-thread and Unied thread proles to be-
c
ome 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
-
ed 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
-
l
eum-type threads, were developed to give a mechani-
c
al sealing of the uid, or gas
m
edium, 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 at (as depicted), or
a more likely, a radius
1
– depending upon the speci-
cation. If screw threads have an identical pitch
2
, but
dierent 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
1 ‘Root radius’ , is usually a stronger thread form, as it is less
prone to any form of shear-type failure mode in-service.
2 Pitch, r
efers to the spacing, or distance between any two cor-
responding points on adjacent threads, normally taken at the
thread’s eective pitch diameter.
NB e 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 anks is termed the included thread angle
(β – a
s illustrated in Fig. 95 – middle right). is thread
form is uniformly spaced along an ‘imaginary cylin-
d
er’ , its nominal size being referred to
a
s the major
diameter (d)
. e eective pitch diameter (d
2
) 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. erefore, the pitch (
p) i
s
normally associated with this ‘eective’ diameter (i.e.
see Fig. 95 – middle right). e minor diameter (d
1
),
is the diameter of another co-axial cylinder the outer
surface of which would touch the smallest diameter.
read clearance is normally achieved via truncating
the thread at its crest, or root – depending upon where
the truncation is applied.
ese are the main screw thread factors that con
-
t
ribute to a V-form thread, which has similar geom-
e
try and terminology for its mating nut – for a thread
having single-start.
5.2 Hand and Machine Taps
Hand Taps
Most ‘solid’
3
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). e pro-
c
ess of tapping a hole rstly requires that a specic-
sized diameter hole is drilled in the workpiece, this is
termed its
‘tapping size’
4
. e taper tap along with its
wrench are employed in producing the tapped thread.
3 ‘Solid taps’ , are as their name implies, but it is possible to use
‘collapsible taps’. ese ‘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. ey are ‘sized-re-
stricted’ by their major diameter.
4 ‘Tapping size’ , r
efers to the diameter of hole to be drilled that
will produce sucient 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 le-,
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].
Threading Technologies 183
Figure 95. Basic V-form thread nomenclature. [Courtesy of Sandvik Coromant].
184 Chapter 5
is taper tap has a large length of taper – hence its
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
-
f
ace, otherwise and angled hole will result. As the ta-
p
er tap is rotated, aer each ¾ turn, it is counter-ro-
t
ated by about a ¼ turn to break the chips, otherwise
‘galling’
5
, or tap-breakage problems in-situ could arise.
Once the taper tap has been through a ‘running hole’ ,
it is oen only just necessary to ‘size’ the hole with the
bottoming tap. However, if a ‘blind’/non-through hole,
5 ‘Galling’ , is when the tap, or indeed any cutter becomes
clogged with the remnants of workpiece material, which will
impair its eciency, or at worse, cause it to break in the par-
tially tapped hole.
Figure 96. Hand taps and tapping nomenclature. [Courtesy of TRW-Greeneld Tap and Die].
Threading Technologies 185
is to be tapped to depth
6
, then it may be necessary to
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 dicult to tap, particularly
concave ones, as it is oen dicult 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 oen useful to
utilise
‘Tapping chucks’. ese chucks have a rotational
drive, coupled to a sprung-loaded Z-axis. e 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 ‘oated
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. ey can have a variety of ute helices,
ranging from quick-to-straight utes (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 dicult, but ‘blind-holes’
can present a problem, associated with the evacu
-
ation of swarf in the reverse direction to that of the
feed. Tap ute spirals that are le-handed and those
with spiral points (Fig. 97bi), remove chips in the cut
-
6 ‘Tapping depth’ , is an oen 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 utes (Fig. 97bii) in conjunction with a long
chamfer lead, can also give good tapping results. For
blind-holes, right-handed utes, or straight uted taps
having shorter chamfer lead lengths give acceptable
tapping results. ese right-hand uted taps, allow
chip-ow in the backward direction – up the utes.
e chamfer lead length is such, that it allows return
movement of chips, but they will not jam and are reli
-
ably sheared o.
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.
is eect 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 soware
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
– modied for metric units), using the following equa
-
tion:
T
m
= L
n
�N =
πDL
n
�
V
+ A
L
+ A
R
Where:
T
m
= Cutting time (min.),
L = Depth of tapped hole, or Length of cut (mm),
n = Feedrate (mm min
–1
),
N = Spindle (rpm),
V = Cutting speed (m min
–1
),
A
L
= Allowance to start the tap (min),
A
R
= 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 utes. [Courtesy of Guhring].
Threading Technologies 187
Figure 98. Fluteles tapping and tool geometry. [Courtesy of Guhring].
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. reads
produced by uteless taps are much stronger than
their equivalent machined taps (Fig. 98b). e 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
-
r
ial movement and to reduce tap wear rates. Like the
conventional machine taps (Fig. 97), uteless taps
have a lead to the tap’s edge – termed a ‘forming lead’
(Fig. 98b – le), as opposed to a conventional machine
tap which has a ‘chamfer lead’ (Fig. 98b-right) which
forms part of the cutting action. erefore, the chi
-
p
less tap in operation (Fig. 98bi), plastically moves
workpiece material from the pre-drilled hole into the
spaces between the tap’s anks 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
-
i
sing uteless taps on engineering components, these
are:
•
Over-sized diameter of pre-drilled hole – if the
hole is too large, then insucient workpiece mate-
r
ial will be available to fully form the rolled thread,
•
Undersized diameter of pre-drilled hole – too
small
a h
ole will be likely to cause the chipless tap
to jam – as it attempts to roll the thread, possibly
leading to tap breakage,
NB
erefore, 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-
c
al 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-
t
en some form of experimentation is necessary in
order to obtain the optimum diameter, as this pre-
drilled
d
iameter will vary according to the work
-
p
iece 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 shas, having either straight and tapered external
threads these can be manually cut, up to a realistic max-
i
mum φ4
0 mm, with threading dies. In essence, these
threading dies can be considered as analogous to hard-
e
ned threaded nuts with multiple cutting edges (Fig.
99a). e cutting edges on the front die face are usually
bevelled, or have a spiral lead to assist in
s
tarting 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-o ’ 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 sha
and die, a few threads on the bar’s end will not be to
full thread depth. Care
m
ust be taken when initially
starting to cut the thread, as if it is not square to the
bar’s axis, then a
‘drunken thread’
7
will result. Previ-
ously, most dies were manufactured from high carbon
steel and, due to their size, their ‘ruling section’
8
and its
7 ‘Drunken threads’ , are the result of variations in the helix
angle and its associated pitch diering 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 sha – hence, its name: ‘drunken thread’.
8 ‘Ruling section’ , t
his term relates to the cross-sectional area
that can normally be hardened, being signicantly inuenced
by the component’s geometry which aects its ‘critical cool-
ing velocity’ (i.e. usually around 1,000°C sec
–1
) when being
quenched. is quenching rate is necessary if the part’s metal-
lurgical structure is to fully transform into a martensitic state,
prior to subsequent tempering.
Threading Technologies 189
Figure 99. Die geometries and their nomenclature. [Courtesy of Guhring].
190 Chapter 5
associated ‘mass eect’
9
meant that the dies could be
through-hardened – which gave them an overall ‘bulk
hardness’ of greater than HSS. Today, basically dies
are either manufactured from micro-grained HSS, or
coated cemented carbide.
Solid dies (Fig. 99a), do not have any means to com
-
p
ensate for die wear, whereas, their split-die nut coun-
t
erparts (Fig. 99b-le), can be manually-adjusted. is
adjustment of the die is achieved by turning a centrally
mounted grub-screw in the stock body, which along
with the xing screws can be made to open, or close
on the sha to be threaded. In this manner, achieving
the correct thread tolerance, or ‘play’ for the desired
tment of its associated mating nut. It is also usual
practice, to use a suitable die lubricant, to facilitate in
the thread’s production while improving surface nish
and prolonging the die’s life – as excessive friction oc
-
c
urs during this type of threading process.
e major disadvantage of using the solid-type
threading dies is that they either have to be unscrewed
from the threaded workpiece, or rewound from the
thread, using up unproductive time elements, this
being particularly important for large batch runs, or
in a continuous production environment.
Self-open-
ing dies
10
(i.e. not depicted) have been utilised for
many years on: capstan and turret lathes, single- and
multi-spindle automatics and so on, for cutting exter-
n
al threads. Several types of self-opening die heads
are available, ranging from: radial, tangential, or cir-
c
ular arrangement of the multi-point cutting inserts
and thread chasers. In most cases, it is usual for the
9 ‘Mass eect’ , is related to the component’s ‘ruling section’. For
example, if the part has a large cross-sectional area, when it is
quenched from the hardening temperature zone (i.e. this can
be found from its associated thermal equilibrium diagram –
for the present), it will not exceed the ‘critical cooling velocity’
and only a partial martensitic state occurs. is is because the
quench media used could not suciently drastically reduce
the part’s temperature with an incomplete atomic transforma-
tion occurring and in so doing, the heat-treated component
will retain some austenite in the matrix. For this reason, large
holes (e.g. designed into in through-hardened Sine-bars) are
oen strategically designed in these larger component regions.
Moreover, in many cases the larger component cross-sections
are reduced, so that the ‘mass eect’ does not occur – apart
from the obvious factor of relieving weight, etc.
10
Self-opening dies, a
re oen termed ‘read chasing die heads’ ,
whereas in reality a thread is only ‘chased’ once the main
thread form has initially been cut. us chasing is employed
to give the required t and nish to the nal thread form.
die-head cutting elements to be preset to take rstly
a roughing cut, followed by nishing cut/chasing of
the threads down the bar. At the end of the threaded
section, these self-opening dies will
a
utomatically
open and can then be speedily withdrawn from the
threaded portion of the bar. ese self-opening dies
can be set to give the correct amount of tolerance, con
-
t
rolling the ‘play’ on the thread. Moreover, it is pos-
s
ible to t dierent thread sizes and forms into the die
head, for more universal threading applications. Both
the radial and tangential threading elements, create
less tool ank contact and frictional rubbing on the
cut thread.
5.5 Thread Turning –
Introduction
On conventional engine-/centre-lathes, a single-point
thread cutting tool (Fig. 100), has a synchronised and
combined linear and rotary kinematic motion for its
Figure 100. External and internal threading tool holders and
in-serts. [Courtesy of Seco Tools]
.
Threading Technologies 191