During these automated threading passes, the tool precisely traverses down the bar’s length, is rapidly withdrawn and moved back to its start point, then fed more deeply beginning anothe
Trang 1associated ‘mass effect’ 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-
pensate for die wear, whereas, their split-die nut coun-terparts (Fig. 99b-left), can be manually-adjusted. This
adjustment of the die is achieved by turning a centrally
mounted grub-screw in the stock body, which along
with the fixing screws can be made to open, or close
on the shaft to be threaded. In this manner, achieving
the correct thread tolerance, or ‘play’ for the desired
fitment 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 finish
and prolonging the die’s life – as excessive friction oc-curs during this type of threading process.
The 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 dies0 (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-nal threads. Several types of self-opening die heads
are available, ranging from: radial, tangential, or cir-cular arrangement of the multi-point cutting inserts
and thread chasers. In most cases, it is usual for the
‘Mass effect’ , 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. This is because the
quench media used could not sufficiently 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
often strategically designed in these larger component regions.
Moreover, in many cases the larger component cross-sections
are reduced, so that the ‘mass effect’ does not occur – apart
from the obvious factor of relieving weight, etc.
0 Self-opening dies, are often termed ‘Thread chasing die heads’ ,
whereas in reality a thread is only ‘chased’ once the main
thread form has initially been cut. Thus chasing is employed
to give the required fit and finish to the final thread form.
die-head cutting elements to be preset to take firstly
a roughing cut, followed by finishing cut/chasing of the threads down the bar. At the end of the threaded section, these self-opening dies will automatically open and can then be speedily withdrawn from the threaded portion of the bar. These self-opening dies can be set to give the correct amount of tolerance, con- trolling the ‘play’ on the thread. Moreover, it is pos-sible to fit different thread sizes and forms into the die head, for more universal threading applications. Both the radial and tangential threading elements, create less tool flank 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]
.
Trang 2threading insert. This insert is connected to the lead-screw (i.e normally having a very accurately-hardened
and ground Acme form) which is precisely synchro-nised to that of the headstock’s rotation. On a CNC
turning centre, or similar, this linear motion is
reli-ant on the precision and accuracy of the recirculating
ballscrew coupled to the programmed cutterpath. In
this manner, the threading insert being rigidly held in
either the tool post, or turret, generates a spiral groove
which when at full depth creates a screwthread of the
desired pitch and helix angle. During successive tra-verse feeding passes (i.e. to prescribed depths) along
the workpiece the thread is cut. A typical thread is
routinely produced on CNC turning centres, using its
fixed/canned cycles (i.e. ‘bespoke software’). During
these automated threading passes, the tool precisely
traverses down the bar’s length, is rapidly withdrawn
and moved back to its start point, then fed more deeply beginning another threading pass down the same helical groove, this process being repeated until full thread depth/profile is accomplished. In order to obtain a consistent thread pitch on the workpiece, the feedrate along the threaded portion must exactly co-incide. The thread form is dependent upon the pro-filed geometry of the thread cutting insert. In order to achieve the required final thread profile, the feedrate must be considerably larger than is normally utilised for conventional turning operations.
Any V-form thread point angle geometry, is not
an ideal edge shape for the production of machined threads if the insert is fed in normal to the workpiece’s axis of rotation (i.e. radial/plunge-fed). Chip control here will be compromised, as each flank of the V-form thread gets successively deeper. This narrowing of the
Figure 101 Screwcutting
tech-niques on turning centres and
suggest-ed methods for improvsuggest-ed chip control [Courtesy of Sandvik Coromant]
Trang 3V-shaped threading insert (i.e. see Fig. 103ai), creates
high localised forces and stresses, which will tend to
tear, rather than cut the final V-form thread profile.
In order to minimise these potential high force/shear
components when radial/plunge-cutting a thread, the
radial infeed passes are progressively reduced with
increasing thread depth. The techniques of V-form
thread production by radial infeed techniques will be
the subject of the next section
5.5.1 Radial Infeed Techniques
Utilising single-point threading inserts (i.e. see Fig.
104, where a typical sequence of threading passes are
depicted – as the V-form thread profile is partially
formed), several different techniques of thread turning
are utilised today, they include:
• Radial infeed (Fig. 101 – top-left) – being the most
common method, where the threading insert is fed
at 90° to the workpiece’s rotational axis. The mate-rial being removed on both sides of the tool’s
V-form flanks – producing a ‘soft’ chip-forming action
giving uniform wear to both flanks of the insert,
NB Here the V-form threading insert geometry
forms both flanks with lighter cuts as the thread
depth progressively increases
• Flank infeed (Fig. 103aii) – is often known as the
‘half-angle screwcutting technique’ , mainly utilised
on a conventional engine-/centre-lathe. Here, the
left-hand flank is formed by the tool’s V-form
ge-
ometry, while the right-hand thread flank is gener-ated by successive passes, as the tool is fed down
the face at half the thread’s included angle. Chip
control is improved with all flank infeed techniques
over ‘plunging’ , enabling the chip to be vectored
away from the previously cut surface (i.e see Fig.
101 – middle, where the chips can be steered away
from the flank),
NB This ‘half-angle technique’ producing the
thread’s right flank, is generated by the tool’s right-hand flank – which due to frictional effects, creates
here a more pronounced wear rate on the cutting
edge resulting in a poor surface finish
• Modified flank infeed (Fig. 101top-middle left) – in
this cutting action, the tool is fed to depth in suc- cessive passes at a slightly reduced angle (i.e. nor-mally ranging from 1° to 5°). This screwcutting technique provides an improved flank surface fin- ish – compared to the two previous methods, par-ticularly on the either less hard, or for more ductile
workpiece materials. Modified flank infeed methods
are recommended rather than radial infeeding for
larger threads, due to contact on this long flank
length which would otherwise result in vibrational effects being superimposed (i.e chatter) onto the final thread form,
NB If the workpiece material’s characteristics
include potential machining work-hardening problems, then flank infeed techniques should be avoided,
• Incremental feeding (Fig. 101 – top-middle right) –
if the thread form is very large, then the incremen-tal thread feeding strategy is normally utilised. These same radial infeed thread production
tech-niques are used for the manufacture of internal threads
(Fig. 102a), by either ‘Pull-threading’ – depicted in
‘A’ , where the thread form originates from the inter-nal undercut, as opposed to ‘Push threading’ – shown
in ‘B’ – being toward say, an undercut. In both cases
of thread production, the modified flank infeed tech-niques are employed
NB Threads manufactured by method ‘A’ allow for
excellent evacuation of the chips – being an ideal
technique for ‘blind holes’. Conversely, in case ‘B’ , the
swarf would otherwise simply ‘bird’s nest’ in such a hole, unless a through hole is present, as is depicted
in ‘B’.
If thread forms are based upon square threads, or their modified trapezoidal forms: Buttress, or Acme (i.e. see Fig. 95i – for examples of these thread profiles), it
Incremental feeding, is sometimes termed the ‘Alternating flank’ technique, it has the advantage of imparting a uniform
wear to both of the cutting insert’s V-form flanks, thereby sig-nificantly increasing the tool life.
‘Bird
nesting’ , is a term that refers to the rotational entangle-ment and build-up of work-hardened swarf at the bottom of
a ‘blind hole’ , which can create some problems in internal thread production.
Trang 4Figure 102 External and internal threading operations and the effect that the helix has as the diameter changes – for a given
pitch [Courtesy of Sandvik Coromant]
.
Trang 5is advisable to pre-machine the thread with a groov-ing tool – with the tool’s width beis advisable to pre-machine the thread with a groov-ing the equivalent
of the thread’s root spacing dimension. Not only does
this pre-machining strategy of employing a grooving
tool reduce the number of threading passes to just
flank finishing, the tool can have a chip-breaker pres-
ent during the rough machining stage to effectively re-move the bulk stock and its associated swarf.
5.5.2 Thread Helix Angles,
for Single-/Multi-Start Threads
The fundamental basis underpinning any thread form
is the helix angle, which in this case is denoted by the
Greek symbol ‘ϕ’ – as schematically illustrated in Fig.
102b. One way of describing how the helix angle’s
geometry is created, is to imagine that a right-angle
triangle is formed by a thin wire which has been
unwound from a parallel cylindrical shaft, whose
diameter equates to its ‘effective diameter’. Then, this
unwound wire length (i.e. πD) would be its circumfer-ence, acting as a base for the triangle (Fig. 102b). The
perpendicular height of right-angled triangle is equal
to the pitch ‘p’ , or the lead
– in the case of a single-start thread. The angle that the hypotenuse makes with
the base is its helix angle
‘ϕ’. From the schematic dia-gram in Fig. 101c, if the pitch ‘p’ remains constant and
the diameter ‘D ’ is decreased (i.e. ‘D ’ → ‘D ’), then the
helix angle proportionally increases (i.e. ‘ϕ ’ → ‘ϕ ’).
In the case of multi-start threads, the pitch and the
lead differ, as shown in Fig. 106c. In this illustration for
the cutting the triple-start thread, the usual approach
to its manufacture is for the three successive starts to
be individually completed to form the ‘triple-start’ ,
with each start being angularly displaced 120° with
respect to each other. Alternatively, if one start is be-gun with the first threading pass, then the second start
is similarly machined and so on – for the number of
starts required, then the threading insert is advanced
‘Pitch’ – can be defined as the distance between correspond-ing points on adjacent threads, normally expressed in metric
units as ‘mm’ , or in Imperial units as threads per inch.
‘Lead’ – being defined as the axial distance through which
a point on the thread advances during one revolution of the
thread ×. This helix angle ‘ϕ’ is also known as its ‘lead angle’
NB Both the pitch and the lead are identical for single-start
threads.
to a deeper thread depth and the process is repeated until the full thread form has been completed. As pre-viously mentioned, the pitch is not the same as the lead for multi-start threads and the lead can be easily calculated as follows:
Lead = np
Where:
n = number of starts,
p = pitch (mm).
For example, in the case of the triple-start thread illus-trated in Fig. 106c, for say, a V-form metric thread of
6 mm pitch, then the lead will be: 3 × 6 mm = 18 mm.
NB This means that if a mating nut was rotated down
this triple-start thread, it would be linearly displaced
by 18 mm in one revolution – allowing the nut to be rotated in, or out quickly (i.e. because of its larger he-lix angle), but to the detriment of an increased axial loading. Although this load is distributed across the contact between all the multi-start threads
5.5.3 Threading Insert Inclination
The threading insert is carefully ground by the tool-ing manufacturer to provide the correct thread profile. This insert must operate with a radial cutting rake of 0°, if the correct thread form is to actually imparted
to the formed thread (Fig. 103). The lead angle of the flank surface varies at different points between the crest and the root of the thread, increasing toward the root – the opposite is true on an internal thread. Due
to this effect, the actual cutting rake varies along the insert’s cutting edge, becoming more positive on the leading edge and more negative on the trailing edge – the
closer it gets to the thread’s root. In order to minimise such threading insert rake angle variation the insert
is inclined, so that its top face is perpendicular to a
Threading shimming – the tool holder is delivered fitted with
a shim that gives an effective side inclination angle of 1° – be-ing the most common type. Although shims can be changed
in degree increments from: –2° to 4°, by simply fitting a differ- ent shim angle. Likewise, internal threading tool holder incli-nations can be changed, by fitting such shims.
Trang 6line indicating the mean lead angle ‘λ’ – measured at
the pitch diameter (Fig. 103b). This insert inclination
Threading insert top face geometry – instead of a flat/straight
top face to the insert, today, it is often angled (i.e. shown in
Fig. 103bi – bottom left), which enables improved control of
the developing chip.
produces a symmetrical side clearance (i.e. depicted
in Fig. 103bi – bottom left diagram) and is important
in ensuring a uniform edge wear on both flanks, re-sulting in increased insert useful life. The fact that this small threading insert inclination, causes one flank to cut slightly below, while the other cutting marginally above the centre-line of the workpiece – for a flat top faced insert, is of no practical significance at normal lead angles for either the function of cutting, or the
Figure 103 Thread
for-mation by radial and flank infeeds, to-gether with threading insert inclina-tion angles [Courtesy of Sandvik Coromant]
Trang 7thread’s profile. Further, a small deviation from the
exact symmetry required in the insert’s inclination is
also acceptable, without too obvious a disadvantage.
Thus, the inclined insert can be utilised to cut threads
of between 0° and 2° with an inclination of 1° and, still produce a satisfactory thread. This thread production technique is only true for the normal, symmetrical threads (i.e. ‘V-forms’); in the case of ‘saw-toothed’
Figure 104 External threading with an indexable insert – chip formation in a partially-formed/generated thread for a single pass
along the bar [Courtesy of Sandvik Coromant]
.
Trang 8the ‘straight-flanked’ ones – those with angles between
0° to 7° – in particular, offer side clearances which may
be adequate. In Fig. 103c a graph depicting the thread-ing insert inclination angles is given for differing helix
angles, with the helix angle calculation derived as fol-lows:
tan λ = P / D × π
Where:
λ = Helix angle (°),
P = Pitch (mm, or threads per inch),
D = Effective pitch diameter (mm, or inches).
Metric threading inserts are characterised by their
thread profile and the associated pitch, being
ex-
pressed in millimetres. The shape and size of the in-sert will determine the completed thread form. One
threading insert can be utilised to cut all threads of
this profile and size, irrespective of their thread diam-eter, or whether they are: right- or, left-hand, single-
or, multi-start (i.e. see Figs. 105b and 106a, for internal
and external left- and right-hand threading configura-tions, respectively).
In the case of internal thread inclination angles, the
tool must be ‘canted-over’ at the angle ‘λ’ (i.e. see Fig.
103bii), so that the cutting edge is situated normal to the centreline. Often, the toolholder shank has to be ground-away to avoid fouling on the internal hole’s diameter as shown in Fig. 105a. Here (Fig. 105a), the distance from the tool tip to the rear of the toolholder
shank – denoted by the dimension ‘D’ , is relieved to
‘D mod’ to avoid fouling on the curvature of the hole, as the tool is fed-out of the thread depth at the end of its successive ‘threading passes’.
5.5.4 Thread Profile Generation
The profile of a thread can be cut by several different techniques and differing types of inserts – depending whether ‘topping’ the is required. For example, if a V-form profiled threading insert is utilised (Fig. 106bi),
no actual machining is undertaken of the thread’s top. In this situation, it is necessary to ensure that the
Figure 105 Internal
threading operations for right- and left-hand threads [Courtesy of Sandvik Coromant]
Trang 9Figure 106 External threading operations and insert forms [Courtesy of Sandvik Coromant]
.
Trang 10pre-machined workpiece – when producing external
threads – has the exact size for the major diameter re-quired, conversely, for internal threads this must be
the minor diameter that is pre-machined. Due to the
sharpness of the thread produced by this technique, it
is often necessary to ‘chase the thread’ afterward.
In the case of profiled threading inserts, the com-plete thread profile is cut from a slightly oversized
blank. Usually, three distinct profiling inserts could be
used in thread production, these are:
• V-form (Fig. 106bi) – has the ability to machine a
range of thread profiles, with the nose radius pre-cisely and accurately ground for the smallest pitch
to be cut. As a result of this tightly ground nose
radius, the insert’s life is shorter than with other
profiling insert versions, as its size has not been
optimised for individual thread geometries. From
an economic viewpoint, due to the V-form profil-ing insert’s ability to cut a wide variety of thread
pitches, less inserts need to be stocked,
• Full-form (Fig. 106bii) – has the ability to profile
the thread’s crest and is therefore manufactured to
exactly the specification of the required thread file. Such full-profile inserts simplify thread
pro-
duction, as no profile is deeper than its specifica-tion, allowing them to be a stronger insert thereby
resulting in improved tool life,
• Multi-point form
(Fig. 106biii) – with this multiple-pointed profiling insert, the first tooth roughs-out
the thread and is therefore slightly set back in com-parison to the second tooth on the insert, which acts
almost like a ‘chaser’ which fully-forms and blends
the various thread profiling elements upon the final
threading pass. Cutting conditions need to be rigid
and stable for this type of insert to operate correctly
– due to its longer cutting edge length. It is essential
to ensure that the recommended in-feed values are
used, to ensure that cutting forces are balanced for
both of the cutting teeth. One advantage of utilising
these multi-point threading inserts is that the num-ber of threading passes can be reduced by almost
50% – as it cuts deeper than its counterparts, when
compared to the single-profiling insert forms
‘Chasing a thread’ , refers to using a chasing tool
with the ex-act thread profile which is utilised to follow the thread along,
thereby deburring and forming the desired profile simultane-ously.
5.5.5 Threading Turning –
Cutting Data and Other Important Factors
Whatever type of thread to be cut, whether it is a: V-form, Multi-start, Trapezoidal, or Tapered, it is gen-erally quite difficult to vary such factors as the: cutting speed, feed and, to a lesser extent the DOC , indepen-dently of one another, without certain consideration of some limiting factors. The typical limitations imposed when cutting a thread, will now be discussed.
Cutting Speed
Typical limitations imposed by the action of cut-ting a thread include, reducing the cutcut-ting speed by 25% – compared to ordinary turning, as the insert’s shape limits heat dissipation. If a high a chip load occurs due too great a cutting speed selected, then the cutting temperature can approach that of say, a cemented carbide’s original sintering temperature.
As a result of this elevated temperature, the binder phase may soften, causing potential cutting edge plastic deformation. The remedy here seems quite easy, simply reduce the cutting speed, but this may in-crease the risk of BUE. This BUE may cause the chips
to become welded onto the cutting edge from which they are shortly fragmented and continuously carried away – taking a minute portions of the insert’s edge along with them. The problem can be minimised by specifying a tougher grade of carbide for the threading insert, or choosing a multi-coated insert. Normally, the cutting speeds for any threading operation should not be less than 40 m min–, when machining with any cemented carbides.
Feed and DOC
The feed in millimetres per revolution, must coincide with the desired pitch, or lead – when cutting multi-start threads. Hence, if the cutting speed is modified, the feedrate will also have to be increased, or de-creased, so that the feed per revolution is constantly maintained. So, the critical factor here is to achieve
some form of control over the DOC when threading. Each threading pass along the workpiece causes an in-creasingly larger portion of the insert’s cutting edge to become in contact in the threading operation, accord-ingly, tool forces will proportionally increase. If the
DOC