Smith G.T. Cutting Tool Technology: Industrial Handbook

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tion, which is determined by how deep the periph-

eral cutting insert’s edges cut, with the insert’s faces

on the edge of the cutter generating the nished

workpiece surface.

•

Peripheral milling – radial (Fig. 78) – utilises pe-

ripherally-located cutting edges that are situated in

a milling cutter body which is horizontally spindle-

mounted. e cutter rotates around a horizontal

axis, this axis being parallel to the tangential feed-

ing direction. Peripheral milling has a D

in a

radial direction that will determine how deep the

cutter’s diameter will penetrate into the workpiece.

ere are two peripheral milling strategies that can

be used with these horizontally-mounted cutters,

Figure 77. Just a few of the ma-

chined features that can be produced

by a range of face milling cutters.

[Courtesy of Sandvik Coromant]

152 Chapter 4

these are either ‘Up-cut’ (Fig. 78 top-right denoted

‘U’), or ‘Down-cut’ (Fig. 78 top-le denoted ‘D’)

milling operations – more will be said on this topic

shortly.

•

Peripheral milling – tangential (Fig. 79) – allows

the cutter to not only face mill, but has the capabil-

ity to work along a third direction feed – axially (i.e.

downward into the part’s surface – Fig. 79 – top).

Essentially, this milling operation is a form of drill-

ing, being performed by the cutting edges on the

cutter’s face, oen termed ‘slot-drilling’

. is tech-

nique allows the cutter – perhaps ground with radi-

used indexable cutting inserts (i.e. see Fig. 79b), to

machine open and closed pockets, or slots, enabling

the cutter’s peripheral edges to complete a range of

cutter-paths to open up the a rectangular, or irregu-

lar ‘

pocketed features’

in the workpiece (Fig. 79a).

Up-Cut and Down-Cut Milling

Here (Fig. 78), the workpiece is fed into the horizon-

tal-mounted peripheral milling cutter, which has its

rotation either clockwise (i.e. termed ‘down-cut mill-

7 ‘Up-cut’ and ‘Down-cut milling’ , these two peripheral milling

techniques are sometimes referred to as either: ‘Conventional

milling’ , or ‘Climb-milling’ operations, respectively.

8 ‘Slot-drilling’ ,

normally utilises two cutting edges on the

cutter’s face. One cutting edge being longer that the other

– crossing the cutter’s centreline, thus as it rotates, its dissimi-

lar length of cutting edges will sweep across the total area of

cut. is action, allows the cutter to be plunged-down into the

workpiece’s surface and then feed along – to produce a slot

– hence the name: ‘slot-drill’.

NB If the slot-drill has its cutting edges rounded (i.e. radiused),

a ‘Ball-nosed slot-drill’ will result, and such a cutter geometry

can produce a range of ‘blended/curved’ workpiece features,

allowing complex-curved proles (i.e ‘sculpture-milling’) op-

erations to be undertaken. In some cases, the curvature of the

radius is modied (i.e. in the case of an ‘APT tool’ – meaning:

automated-programmed tool) geometry is ground, to mini-

mise step-over/cusp height eects, when (post) nishing-o

operations are more speedily rendered on complex-curved

workpiece features. For example, when completing the high-

quality nishing operations on moulds and dies.

9 ‘Pocketed features’ (Fig. 79a), these operations can be gener-

ated by feeding the cutter to a pre-determined series of suc-

cessive depths these being consecutively opened-up by a range

of tool-path strategies. Typical of the techniques for such

pocketing cutter path control, is to employ either ‘Lace-’ ,

‘Non-lace cuts’ – more will be said on this later.

Figure 78. Peripheral milling (radial), can be undertaken by

either: up- or down-cut milling operations

NB These dierent milling cutter rotational directions, impart

totally dissimilar resultant force vectors whilst machining, play-

ing a signicant role in the attendant: power consumption fac-

tors, resultant machined workpiece residual stresses and sur-

face inegrity present, together with geometric shapes/types

of workpieces that can be succesfully machined. [Courtesy of

Sandvik Coromant]

Milling Cutters and Associated Technologies 153

ing’ – Fig. 78 top-le ‘D’), or anti-clockwise (‘up-cut

milling’ Fig. 78 top-right ‘U’). Hence, the workpiece is

fed either with, or against the milling cutter’s rotation

direction, which determines the nature of the begin-

ning, or completion of the cut.

In ‘down-cut milling’ (Fig. 78 top-le), it can be

seen that the workpiece direction of feed is the same

as that of the cutter’s rotation, in the vicinity of the cut.

In such circumstances of peripheral milling concern-

ing the milled chip’s area, the chip thickness begins

to decrease from the initiation of the cut, eectively

reaching zero on completion of the insert’s peripheral

rotation, prior to the adjacent cutting insert continu-

ing this trend in chip development. Furthermore, as

each milling cutter insert enters the cut with a large

chip thickness, this avoids any potential rubbing and

Figure 79. Axial feed milling using

either: solid carbide end mills, slot drills,

or a ball-nosed milling cutter

NB These type of milling cutters can be

widely utilised across a vast range op po-

tential workpiece geometrically-shaped

features. [Courtesy of Sandvik Coromant]

154 Chapter 4

the likelihood of workpiece surface burnishing, mini-

mising both the probability of temperature increases

and work-hardening tendencies. As a cautionary note,

although this is a very eective and ecient way of

peripheral cutting, if any ‘back-lash’

is present, then

the cutter will attempt to ‘snatch’ ,

or at worst, ‘ride-

over’ the workpiece’s surface, as it is pulled-into the

cut. is possible ‘snatching-eect’ ,

being created by

the resultant cutting force tending toward a back-ward

direction (Fig. 78 middle-le). is adverse action, re-

sulting from presence of ‘back-lash’ could even cause

cutter, or spindle damage, together with part scrap-

page, if it is not minimised/eliminated.

In ‘Up-cut milling’ (Fig. 78 top-right), the work-

piece feed direction opposes that of the milling cutter

rotation, within the cutting vicinity. erefore, the

chip thickness commences at zero and increases as it ap-

proaches the exit point, at the end of the cut. Due to

the fact that at the initiation of the cutting sequence

the cutting edge has no eective forces acting on it,

so it must be forced into the cut and during this time,

some eects of rubbing/burnishing create excessive lo-

calised friction, which in turn, results in an increased

temperature. Here, contact with the workpiece mate-

rial from the previous insert can work-harden the sur-

face. Yet another disadvantage of utilising ‘Up-cut mill-

ing’ ,

is that the resultant force (Fig. 78 middle-right)

can attempt to li the workpiece on the machine tool’s

table, therefore it must be securely clamped/xtured

into place.

e machine tool’s spindle power will depend on

the either the feed force (V

), or the resultant force

10 ‘Back-lash’ , is a problem concerning slideway ‘oat’ (i.e.

slight, but unwanted lateral uncontrolled back-and-forward

motion) in the machine tool’s leadscrew, or ballscrew. On

conventional milling machines (i.e. with no CNC-controlled

axes), these are normally equipped with an Acme thread (i.e.

having a truncated Vee-form thread of 29° included angle),

they require a ‘Back-lash eliminator’ to be tted, which when

rotated/tightened on the split-nut assembly surrounding the

Acme thread which is mechanically-connected to the work-

piece’s table, reduces any back-lash present – thereby allowing

down-cut milling to be successfully performed.

NB Ballscrews, are usually tted to CNC-controlled machine

tools, as they can be pre-loaded by the machine tool builder,

thus minimising any potential back-lash problems. It is nor-

mally desirable to use down-cut milling techniques in CNC

machining, as it is more ecient cutting technique, in com-

parison to that of up-cut milling.

component, the relationship of these ‘Up- and Down-

cut milling’ factors being schematically illustrated in

Fig. 78 – bottom diagrams. is resultant force is a

combination of the tangential and radial cutting forces.

e resultant cutting force will dier signicantly in

its vectored angle, depending upon the cutter position

relative to that of the workpiece’s. ese vectored angles

(Fig. 78 – bottom diagrams), will become larger with

increased D

– for ‘Up-cut milling’ , the net eect be-

ing that the milling process needs more spindle power.

In ‘Down-cut milling’ ,

due to the fact that the resultant

force is in the same direction to that of the feed, it will

signicantly reduce the feed power requirement. Yet

another advantage of using a ‘Down-cut milling’ strat-

egy, is that there will be no reverse change of direction,

so any workpiece clamping is simplied.

4.1.2 Milling Cutter Geometry – Insert

Axial and Radial Rake Angles

With any machining operation, the combination of

the rake and clearance angles determines the cutting

edge’s wedge angle, this greatly inuences the insert’s

strength. Oen, cemented carbide cutting inserts have

a small negative primary land present, this helps to

avoid fracture during the intermittent cutting action

associated with a milling operation. A helix angle is

present on many milling cutters

, be it a: side-and-

11 Helix angles, can vary considerably, the helices being selected

for the workpiece material to be cut. For example, when mill-

ing aluminium, a quick helix is necessary to take advantage of

the low shear characteristics of the material, conversely, when

milling grey cast iron a slow helix is preferable, as this mate-

rial is somewhat brittle in nature. In many instances when ma-

chining the so-called ‘sticky’ materials such most aluminium

grades, etc., then the milling cutter is designed to have a larger

‘chip-gusset’/‘chip-pocket’ (i.e. clearance space for the chip).

is larger ‘chip-pocket’ may take the form of alternating the

bias of the helices for the cutting edges on say, a peripheral

milling cutter, having a positive helix on one tooth, with the

adjacent tooth being of negative helix – typically a milling

cutter of this design is the so-called: ‘staggered-toothed side-

and-face cutter’.

Milling Cutters and Associated Technologies 155

face cutter, face-mill, end mill

, slot-drill, etc. e

helix angle brings the cutting edge progressively into

ut, resulting in ‘quieter running’ , but instead of an

orthogonal cutting action (i.e. with two component

forces – tangential and radial – acting on the cutting

edge), an oblique cutting action occurs (i.e having an

additional axial force component present). is axial

force component resulting from the geometry of the

helix, has a tendency to either ‘pull’ the cutter out of

the spindle, or push it towards it, depending upon

whether it is of a le- or right-hand helix.

For all of the designs for Endmills and Slotdrills

currently available, there are basically three types of

axial and rake angled cutting edge geometries, theses

are:

•

Double-positive cutting edges (i.e shown in Fig.

80C) – normally employs a single-sided insert,

as this geometry allows a relatively ‘free’ cutting/

shearing action. Here, the positive axial and rake

geometries, produce low cutting forces, owing to

the reduced chip thickness and a shorter length of

contact at the chip/tool interface. As a result, less

spindle power is necessary, enabling a lower insert

strength requirement enabling high-shear cutting

availability, when compared to either of the fol-

lowing insert geometries. e manner in which

the chip formation occurs is benecial, in that spi-

ral chips are formed, which can easily be broken

and exhausted from their respective chip pockets.

When milling ductile materials such as grades of:

aluminium, steel, as well as some stainless and heat-

resistant steels, where there is a tendency to form

BUE, the double-positive geometry is the only suc-

cessful solution to machining such workpiece ma-

terials. If the workpiece has a tendency to be some-

what unstable, perhaps due to either its fragility, or

12 ‘Endmills’ , can not only have the cutting edges designed with

a helix angle, but for the ‘solid’ end-milling varieties, they may

have either their cutting edge land lengths interrupted by a

groove this feature being a long-lead spiral groove – to act as

a form of chip-breaker (i.e. they are oen called ‘Rippa-cut-

ters’ – utilised for high stock removal rates), for long-chipping

materials. For some cutter designs, such as: ‘Roller-, or Slab-

mills’ , they can have cutting inserts staggered along the tool’s

periphery covering the length of the body, to disrupt the long-

chipping swarf (i.e. see Fig. 76). For Endmills versions of this

inserted-toothed design, they are oen termed ‘Porcupine

cutters’ (i.e. an example of a ball-nosed version, is depicted in

Fig. 79b).

clamping method, then the double-positive insert

geometry is once again, the most suitable milling

geometry to use.

•

Double-negative cutting edges (Fig. 80B – shown

here with either a round insert – bottom-le, or

square insert – top-right) – in this case, the radial

and axial angles are both negative. When a double-

negative insert is used, the required clearance is ob-

tained by tilting the insert. is ‘tilt’ of the insert,

has the added economical benet of allowing both

sides of the insert to be utilised, enabling the avail-

ability of more cutting edges coupled to stronger

edges, when compared to the former insert geom-

etry. is double-negative milling cutter insert ge-

ometry, is most suitable for machining conditions

involving heavy impact stresses, associated with

workpieces produced from: hard steels, certain

cast iron matrices and on some of their ‘chilled-sur-

faces’

, or ‘induction-hardened surfaces’

– nor-

mally using the ultra-hard PCBN-grades of inserts.

With these ‘Double-negative’ insert cutting geom-

etries, the demands on spindle power requirement

and its stability are considerable, owing to the large

cutting forces and chip thickness factors associated

with this type of geometry.

•

Positive/negative cutting edges (Fig. 80A – illus-

trating a square insert – top-le, or an inclined

and chamfered square insert – middle-le). For ex-

ample, in the case of Fig.79a (top-le), the insert

13 ‘Chilled cast iron surfaces’ , are used in order to produce a

hard-wearing surface, this being necessary for example on the

Vee-and-at slideways’ for cast lathe beds. Here, a ‘chill’ (i.e.

normally a metallic interface) is for example strategically-po-

sitioned in the cavity, prior to pouring the liquid melt. is

‘metallic chill’ acts as a ‘heat-sink’ to quickly allow solidica-

tion at the liquid/wall interface, producing a high tempera-

ture/cooling gradient and subsequent crystalline growth of

small grains, that have become both locally hard and wear re-

sistant, but are surrounded by a graphite-based matrix that of-

fers excellent ‘damping’ qualities to the overall cast structure.

14 ‘Induction-hardened surfaces’ ,

are usually obtained by a trav-

elling electric resistance induction heating/cooling unit. is

equipment, once it has locally heated the surface, will be im-

mediately quenched*, as the apparatus slowly moves along the

required surface to be heat-treated, imparting a surface-hard-

ened layer to the cast iron.*is induction-hardened surface

now consists (i.e. metallurgically) of a very hard ‘white-iron’ ,

appearance – aer suitable metallographical preparation

– with a white surface layer, once it has been suitably acid-

etched.

156 Chapter 4

geometry comprises of a combined positive axial

angle and a negative radial angle. Although the ba-

sic form of the insert may have a negative geometry,

the edge on its end face must be positive in order to

give a positive axial rake. e spindle power require-

ments for this combined geometry are a compromise

between the lower demands of the double-positive

insert geometry and the higher ones associated

with that of its double-negative counterparts. With

this positive/negative milling insert geometry, high

feeds per tooth combined with large D

’s can be

achieved, because the negative radial rake provides

high insert strength, whilst the positive axial rake

oers good chip formation, with the added bonus

of directing the chips away from the cutter body

For any general-purpose milling applications, the

cutters having positive/negative cutting insert ge-

ometries are usually ideal.

15 ‘Chip-evacuation/-exhaust’ , are terms that are readily used to

explain how the chips are removed from the milling cutter’s

body. It is a very important consideration, as any chip-jam-

ming tendencies must be avoided at all cost, as the cutter,

workpiece, or both, can be severely-damaged if this potential

and avoidable problem arises.

Figure 80. The rake and clearance angles for various types of face-milling cutter insert geometries. [Courtesy of Sandvik Coro-

mant]

Milling Cutters and Associated Technologies 157

4.1.3 Milling Cutter – Approach Angles

Although cutting insert axial and radial rake angles

are important to correctly select, probably of similar

importance is the milling cutter’s approach, or enter-

ing angle (i.e described and illustrated in Fig. 81).

e insert’s inclination can vary by pre-selecting the

most suitable one for the workpiece to be milled. Of-

ten a compromise has to be made when selecting the

cutting insert’s inclination/approach angle. For ex-

ample, inserts having an approach angle of 90°, are

termed ‘Square-shoulder cutters’ (i.e. depicted in Fig.

83a – le), as their name implies, they are normally

utilised when machining up to a shoulder, or perhaps

a stepped-feature on the workpiece. ere are some

problems associated with 90° approach angled milling

cutters, their limitations are:

•

Chip thickness is at a maximum – for a given

feedrate, resulting in high loads on the cutting in-

serts,

Figure 81. Face milling cutter insert approach angles. [Courtesy of Stellram].

158 Chapter 4

Figure 82. Face-milling cutters, having inclined approach angles, with either high-shear (i.e. tangentially-mounted) inserts, or

cutter insert density variations. [Courtesy of Ingersoll, Courtesy of Sendrik Coromant]

Milling Cutters and Associated Technologies 159

•

Chip ow may be hampered – the vectored angle

for exhausting chips may be compromised,

•

High radial force component

– i.e. in relation to

the axial force, produces unfavourable loads on the

spindle, creating vibration tendencies, hence the

feeds must be restricted,

•

Positive geometry triangular inserts should not be

used – these weaken insert corners, whereas rhom-

boid-shaped insert geometries, or similar, oer

much stronger insert cutting edges.

For general-purpose milling operations, intermedi-

ate approach angles such as inserts having an 75° ap-

proach are common (i.e see Fig. 82a), as they provide

good edge strength in combination with a favourable

relationship between insert size and cutting depth.

Moreover, if these 75° approach-angled inserts are

tangentially-mounted (i.e. as depicted in Fig. 82a), the

edges are even stronger because of the ‘body’ of the in-

sert is more fully-supported, than is generally the case

for most of the radially-mounted variants. Equally, an

insert with an 45° approach angle, will spread the load

over a longer cutting edge (Fig. 83b). is 45° insert

approach geometry, provides good chip-ow for long-

chipping materials, with a low radial force component

in comparison to that of the axial force, what is more a

strong insert edge allows higher feedrates to be utilised

(Fig 83a).

When roughing-cuts are necessary, or dicult-to-

machine workpiece materials must be milled requir-

ing strong insert edges, then a round insert might be

the answer. In general, round inserts have a positive

geometry with no sharp edges and as a result, oer

very strong cutting edges and chip-loads are relatively

evenly distributed along the rounded contact region

(Fig. 83b – right). Furthermore, a round insert usu-

ally has a positive insert geometry, which can then be

turned in its seating to simply provide additional cut-

ting edges.

16 ‘Square-shoulder cutters’ , can produce a high radial force

component, which means that feedrates must be limited, as

they may cause ‘edge frittering’ (i.e. see Fig. 81 – bottom le,

illustrating an edge break-out condition), this unacceptable

machining condition is particularly prevalent on brittle-types

of workpiece materials, such as: (most) Brasses, (many) Cast

irons, (some) Powder Metallurgy compacts, together with

(many) non-metallic materials – Plastics, Perspex, Tufnol,

Carbon-bre, etc.

4.1.4 Face-Milling Engagement –

Angles and Insert Density

Face-Milling Engagement

In any milling machining operation involving the po-

sitioning of the cutter in relation to a workpiece, some

thought should be given to not only the cutter’s: diam-

eter; number of teeth, or cutting inserts; width of the

workpiece; but also how the resultant cutting force(s)

might inuence the overall eectiveness of the pro-

duction process. is latter point, not only inuences

method of component clamping, dictating: how, where

and what will be the optimum method of ‘location and

restraint’

of the workpiece, but on exit from cut, the

sudden disengagement and release of cutting forces

will potentially create not only a exit-burr on ductile

materials, or frittered edge on a brittle material. e

cutter’s exit can inuence the type of stress induced

into the workpiece surface – a compressive stress be-

ing preferred (more will be said on this topic later, in

the section dealing with ‘Machined Surface Integrity’).

In most milling operations the term: ‘engagement’ con-

cerns the relationship of cutter-to-workpiece position-

ing and, in virtually all face-milling operations, one

tries to prevent at the exit of the cut, the chip being at

its thickest, as this is an unfavourable machining strat-

egy. e objective when milling, is to always try to get

the thinnest possible chip at exit from the cut. Some

of these engagement positioning relationships are de-

picted in Fig. 84, indicating where the most favourable

cutter/workpiece relationships are present. Also in

Fig. 84, are depicted some unfavourable engagements

that should be avoided, this may be possible by either

changing the milling cutter’s diameter, or its tool path

if possible, to avoid such engagements.

e milled cut length is inuenced by the position

of the cutter with respect to the workpiece, with tool

life being related to each cutting insert’s amount of

time engaged in the actual cut. For example, in Fig.

84g, a cutter has been positioned centrally over the

workpiece, this produces the shortest possible time

17 ‘Location and restraint’ , are important factors when work-

holding. A component needs to be not only accurately located

with respect to either a ‘datum’ , or held on a ‘grid-plate’ in a

known relationship to that of the cutter’s position, but it must

also be properly restrained – to prevent any, compliance of

its ‘degrees of freedom’ while it is clamped during machining.

Hence the term: ‘Location and restraint’.

160 Chapter 4

in-cut, conversely, in Fig. 84h the cutter has been

moved just o-centre, causing a longer arc of cut for

each insert, which is likely to reduce the tool’s ‘cutting

life’ somewhat, but this is only part of the problem of

o-centre cutting. Returning to the cutter positioned

centrally (i.e. Fig. 84g), here the direction of the radial

component cutting forces will uctuate, with respect

to the cutting edges start and nish cutting, which

may create potential vibrational problems, or prema-

ture edge breakdown. However, with o-centre mill-

ing (i.e. Fig. 84h), this machining strategy introduces

a constant force direction, moreover, as the cutter is

positioned not quite centrally over the workpiece, this

central region produces the largest average chip thick-

ness. Just to complicate matters still further, if the cut-

ter is positioned even further o-centre, this will allow

even more inserts to be simultaneously brought into

ut (i.e. shown as ‘α’ , in Fig. 85). ere are oen many

Figure 83. The importance of cutting insert approach angle inclination on the resultant chip shape. [Sources: Fig. a: Tooling

University, 2003; Fig. b: Heuwinkel & Richter, 2005]

Milling Cutters and Associated Technologies 161