Smith G.T. Cutting Tool Technology: Industrial Handbook

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threading insert. is insert is connected to the lead-

screw (i.e normally having a very accurately-hardened

and ground Acme form) which is precisely synchro-

ised to that of the headstock’s rotation. On a CNC

turning centre, or similar, this linear motion is reli-

nt 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

erse 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

xed/canned cycles (i.e. ‘bespoke soware’). 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traverses down the bar’s length, is rapidly withdrawn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/prole is accomplished. In order to

obtain a consistent thread pitch on the workpiece, the

feedrate along the threaded portion must exactly co

ncide. e thread form is dependent upon the pro-



led geometry of the

hread cutting insert. In order to

achieve the required nal thread prole, 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 ank of the V-form

thread gets successively deeper. is narrowing of the

Figure 101. Screwcutting tech-

niques on turning centres and suggest-

ed methods for improved chip control.

[Courtesy of Sandvik Coromant]

192 Chapter 5

chips from each formed and angled ank face of the

V-shaped threading insert (i.e. see Fig. 103ai), creates

high localised forces and stresses, which will tend to

tear, rather than cut the nal V-form thread prole.

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. e 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 prole is partially

formed), several dierent techniques of thread turning

are utilised today, they include:

•

Radial infeed (Fig. 101 – top-le) – being the most

common method, where the threading insert is fed

at 90° to the workpiece’s rotational axis. e mate-

ial being removed on both sides of the tool’s V-

form anks – producing a ‘so’ chip-forming action

giving uniform wear to both anks of the insert,

NB Here the V-form threading insert geometry

forms b

oth anks with lighter cuts as the thread

depth progressively increases.

•

Flank infeed (Fig. 103aii) – is oen known as the

‘half-angle screwcutting technique’ , m

ainly utilised

on a conventional engine-/centre-lathe. Here, the

le-hand ank is formed b

y the tool’s V-form ge-

ometry, while the right-hand thread ank is gener-

ated b

y successive passes, as the tool is fed down

the face at half the thread’s included angle. Chip

control is improved with all 

ank infeed techniques

over ‘plunging’ , enabling the chip to be vectored

way from the previously cut surface (i.e see Fig.

101

–

middle, where the chips can be steered away

from the ank),

NB 

is ‘half-angle technique’ producing the

thread’s right ank, is generated by the tool’s right-

hand ank – which due to frictional eects, creates

here a more pronounced wear rate on the cutting

edge resulting in a poor surface nish.

•

Modied ank infeed (Fig. 101top-middle le) – in

this cutting action, the tool is fed to depth in suc-

essive passes at a slightly reduced angle (i.e. nor-

ally ranging from 1° to 5°). is screwcutting

technique provides an improved ank surface n-

sh – compared to the two previous methods, par-

icularly on the either less hard, or for more ductile

workpiece materials. Modied ank infeed m

ethods

are recommended rather than radial infeeding for

larger t

hreads, due to contact on this long ank

length which would otherwise result in vibrational

eects being superimposed (i.e chatter) onto the

nal thread form,

NB I

f the workpiece material’s characteristics

include potential machining work-hardening

problems, then ank infeed techniques should be

avoided,

•

Incremental feeding

(Fig. 101 – top-middle right) –

if the thread form is very large, then the incremen-

al thread feeding strategy is normally utilised.

ese same radial infeed thread production tech

iques are used for the manufacture of internal threads

(

Fig. 102a), by either ‘Pull-threading’ – d

epicted in

‘A’ , w

here the thread form originates from the inter-

nal undercut, as opposed to ‘Push threading’ – s

hown

in ‘B’ – b

eing toward say, an undercut. In both cases

of thread production, the modied ank infeed tech-

iques are employed.

NB 

reads manufactured by method ‘A’ allow for

xcellent evacuation of the chips – being an ideal

technique for ‘blind holes’. Conversely, in case ‘B’ , t

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

modied trapezoidal forms: Buttress, or Acme (i.e.

see Fig. 95i – for examples of these thread proles), it

11 Incremental feeding, is sometimes termed the ‘Alternating

ank’ technique, it has the advantage of imparting a uniform

wear to both of the cutting insert’s V-form anks, thereby sig-

nicantly increasing the tool life.

12 ‘Bird nesting’ , i

s 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.

Threading Technologies 193

Figure 102. External and internal threading operations and the eect that the helix has as the diameter changes – for a given

pitch. [Courtesy of Sandvik Coromant]

194 Chapter 5

is advisable to pre-machine the thread with a groov-

ing tool – with the tool’s width being 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

ank nishing, the tool can have a chip-breaker pres

nt during the rough machining stage to eectively re-

ove the bulk stock and its associated swarf.

5.5.2 Thread Helix Angles,

for Single-/Multi-Start Threads

e fundamental basis underpinning any thread form

is the helix angle, w

hich in this case is denoted by the

Greek symbol ‘ϕ’ – a

s schematically illustrated in Fig.

102b. One way of describing

ow 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 sha, whose

diameter equates to its ‘eective diameter’. en, this

unwound wire length (i.e. πD) would be its circumfer

nce, acting as a base for the triangle (Fig. 102b). e

perpendicular height of right-angled triangle is equal

to the

pitch

‘p’ , or the lead

– in the case of a single-

start thread. e angle t

hat the hypotenuse makes with

the base i

s its helix angle ‘ϕ’. From the schematic dia-

gram in Fig. 101c,

f 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, t

he pitch and the

lead dier, a

s 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

un with the rst threading pass,

hen the second start

is similarly machined and so on – for the number of

starts required, then the threading insert is advanced

13 ‘Pitch’ – can be dened 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.

14 ‘Lead’ – b

eing dened as the axial distance through which

a point on the thread advances during one revolution of the

thread ×. is 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-

iously 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:

= number of starts,

= pitch (mm).

For example, in the case of the triple-start thread illus-

rated in Fig. 106c, for say, a V-form metric thread of

6 mm pitch, then the lead w

ill be: 3 × 6 mm = 18 mm.

NB 

is 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

ix 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

e threading insert is carefully ground by the tool-

ing manufacturer to provide the correct thread prole.

is 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). e lead angle of the

ank surface varies at dierent 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 eect, the actual cutting rake

varies a

long the

insert’s cutting edge, becoming more positive o

n the

leading edge a

nd more negative on the trailing edge – the

loser 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

15 reading shimming – the tool holder is delivered tted with

a shim that gives an eective 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 tting a dier-

ent shim angle. Likewise, internal threading tool holder incli-

nations can be changed, by tting such shims.

Threading Technologies 195

line indicating the mean lead angle ‘λ’

– measured at

the pitch diameter (Fig. 103b). is insert inclination

16 reading insert top face geometry – instead of a at/straight

top face to the insert, today, it is oen angled (i.e. shown in

Fig. 103bi – bottom le), which enables improved control of

the developing chip.

produces a symmetrical side clearance (i.e. depicted

in Fig. 103bi – bottom le diagram) and is important

in ensuring a uniform edge wear on both anks, re-

ulting in increased insert useful life. e fact that this

small threading insert inclination, causes one ank to

cut slightly below, while the other cutting marginally

above the centre-line of the workpiece – for a at top

faced insert, is of no practical signicance at

ormal

lead angles for either the function of cutting, or the

Figure 103. Thread for-

mation by radial and ank

infeeds, to-gether with

threading insert inclina-

tion angles. [Courtesy of

Sandvik Coromant]

196 Chapter 5

thread’s prole. Further, a small deviation from the

exact symmetry required in the insert’s inclination is

also acceptable, without too obvious a disadvantage.

us, 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. is 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]

Threading Technologies 197

threads (e.g. Buttress), it should be borne in mind that

the ‘straight-anked’ ones – those with angles between

0° to 7° – in particular, oer side clearances which may

be adequate. In Fig. 103c a graph depicting the thread

ng insert inclination angles is given for diering helix

angles, with the helix angle calculation derived as fol-

lows:

an λ = P / D × π

Where:

= Helix angle (°),

= Pitch (mm, or threads per inch),

= Eective pitch diameter (mm, or inches).

Metric threading inserts are characterised by their

thread prole and the associated pitch, being ex

ressed in millimetres. e shape and size of the in-

ert will determine the

ompleted thread form. One

threading insert can be utilised to cut all t

hreads of

this prole and size, irrespective of their thread diam-

ter, or whether they are: right- or, le-hand, single-

or, multi-start (i.e. see Figs. 105b and 106a, for internal

and external le- and right-hand threading congura-

ions, 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. Oen, 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’ , i

s 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

e prole of a thread can be cut by several dierent

techniques and diering types of inserts – depending

whether ‘topping’ the is required. For example, if a V-

form proled threading insert is utilised (Fig. 106bi),

no actual machining is undertaken of the thread’s

op. 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]

198 Chapter 5

Figure 106. External threading operations and insert forms. [Courtesy of Sandvik Coromant].

Threading Technologies 199

pre-machined workpiece – when producing external

threads – has the exact size for the major diameter re-

ired, 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 oen necessary to

‘chase the thread’

aerward.

In the case of proled t

hreading inserts, the com-

plete thread prole is cut from a slightly oversized

lank. Usually, three distinct proling inserts could be

used in thread production, these are:

•

V-form (Fig. 106bi) – has the ability to machine a

range of thread proles, with the nose radius pre-

isely and accurately ground

or the smallest pitch

to be cut. As a result of this tightly ground nose

radius, the insert’s life is shorter than with other

proling insert versions, as its size has not been

optimised for individual thread geometries. From

an economic viewpoint, due to the V-form prol

ng 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 prole

the thread’s crest and is therefore manufactured to

exactly the specication of the required thread pro-



le. Such full-prole inserts simplify thread pro-

uction, as no prole is deeper than its

pecica-

ion, allowing them to be a stronger insert thereby

resulting in improved tool life,

•

Multi-point form (Fig. 106biii) – with this multiple-

pointed proling insert, the rst tooth roughs-out

the thread and is therefore slightly set back in com-

arison to the second tooth on the insert, which acts

almost like a ‘chaser’ which fully-forms and blends

the various thread proling elements upon the nal

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

er of threading passes can be reduced by almost

50% – as it cuts deeper than its counterparts, when

compared to the single-proling insert forms.

17 ‘Chasing a thread’ , refers to using a chasing tool with the ex-

act thread prole which is utilised to follow the thread along,

thereby deburring and forming the desired prole 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-

rally quite dicult to vary such factors as the: cutting

speed, feed and, to a lesser extent the D

, indepen-

dently of one another, without certain consideration of

some limiting factors. e 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

he cutting 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 soen, causing potential cutting edge

plastic deformation. e remedy here seems quite

easy, simply reduce the cutting speed, but this may in

rease the risk of BUE. is BUE may cause the chips

to become welded onto the cutting edge from which

they are

hortly fragmented and continuously carried

away – taking a minute portions of the insert’s edge

along with them. e 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

–1

, when machining with any

cemented carbides.

Feed and D

e feed in millimetres per revolution, must coincide

with the desired pitch, or lead – when cutting multi-

start threads. Hence, if the cutting speed is modied,

the feedrate will also have to be increased, or de

reased, so that the

eed per revolution is constantly

maintained. So, the critical factor here is to achieve

some f

orm of control over the D

when threading.

Each threading pass along the workpiece causes an in-

reasingly larger portion of the insert’s cutting edge to

become in contact in the threading operation, accord-

ngly, tool forces will proportionally increase. If the

is kept constant during several passes, the chip-

200 Chapter 5

removal rate may increase by up to 300% at each suc-

cessive infeed. erefore, in order to minimise t

hese

induced stresses o

n the cutting edge and keep them as

uniform a

s possible, the D

must be reduced with each

pass along the workpiece.

Thread Finishing and Close Tolerancing

In order to obtain a good surface nish/texture, or

a close tolerance on the nished thread anks, the

CNC machine tool can be programmed to make ei-

her one, or two additional nishing passes. ese ad-

itional passes are termed ‘spring-cuts’

and improve

both accuracy and precision in the nal thread form.

‘Spring-cuts’ cannot be utilised on workpiece materi-

ls that have a tendency to work-harden,

or example

when thread turning stainless steels and so on, as they

cause high tool wear. With work-hardening materials,

cuts of <0.03 mm should be avoided, as these materials

elastically-deform instead of being cut. e severity of

the problem of work-hardening is even greater when

thread turning austenitic steels and their equivalents

and here, it is recommended that an infeed pass should

always be >0.08 mm. ese comments are conned to

steels and their alloys and even here, an appropriate

number of infeeds by trial-and-error may be neces

ary. When a threading insert breaks, it is normally the

result of induced high stresses, so the remedy is usually

to increase the number of infeed threading passes. is

solution is also true for machining threads in most cast

iron grades, the exception here being for austempered

ductile irons (ADF). It seems somewhat obvious that

the greater the number of passes the threading opera

ion is divided into, the smaller the D

and the stresses

on the threading insert’s tip. Moreover, if this philoso-

hy is pursued too far and very many infeed passes are

programmed, the tool will simply not be able to cut

at all – owing to insucient D

and this in turn, will

result in simply elastic deformation of the workpiece

material. Such a ‘timid approach’ to thread cutting will

lead to a higher wear-rate, so it becomes necessary to

further reduce the number of passes – thereby becom

ng a self-defeating machining objective!

18 ‘Spring-cuts’ , are cuts that create a very light tool pressure on

the threaded workpiece, to minimise the elastic deections in

the ‘loop’ produced by the tool-machine-workpiece system,

thereby improving machined quality.

Cutting Forces

If a comparison is made between the cutting forces

for a threading operation with that of external turn-

ng, then the power requirements are higher for thread

formation, especially when the chip thicknesses are

small. If however, the chip thickness is increased, then

the values for plain turning are approached. Hence,

one should

always a

ttempt to utilise higher chip thick-

nesses, as the benets are two-fold: a decrease in the

power consumption, combined with an increase in the

subsequent production rate.

General Comments on Cutting Inserts

read cutting demands an insert with a: sharp cut-

ting edge, good wear resistance and the ability to with-

tand temperature uctuations. A sharp insert cutting

edge combined with a favourable geometry are neces-

ary, so that a good nal thread surface nish occurs,

while simultaneously reducing vibrational tendencies.

High wear resistance is crucial, as otherwise the sur

ace nish would be impaired and thread tolerance de-

iations would occur. Temperature uctuations must

be withstood, as the very operation of threading in-

roduces uctuations in the insert’s edge temperature:

the fast machining pass along the thread causes heat-

ng, then the tool is rapidly withdrawn and returned

to the start-point – during which time the edge cools.

is cyclical process is continuously repeated up to

20 times in quick succession, which could potentially

promote fatigue cracks in the insert – termed ‘comb

cracks’ (i.e oen previously present aer milling oper

tions – as they had nite ne lines at regular intervals

on the tool’s edge, giving them the physical appearance

of a ‘comb eect’). is was a real problem some years

ago – which has now been overcome with suitable

coating technology – and could in the

ast may poten

ially shorten the insert’s useful life.

NB S

ee Appendix 8 for a Trouble-shooting Guide to

conventional thread turning problems and remedies.

Threading Technologies 201