Milling Cutters and Associated Technologies Part 1 docx

In scheduling particular components for milling operations, the se-lection of which CNC type of machine tool configu-ration should be of prime importance, for example a milling machine,

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Milling Cutters and Associated Technologies

‘Mit der Dummheit kämpfen Götter

selbst vergebens.’

TRANSLATION:

‘Against stupidity the Gods themselves struggle in vain ’

FRIEDRICH von SCHILLER

(1759 – 1805) [Die Jungfrau von Orleans, III.vi]

4.1 Milling – an Introduction

At its most basic level, a milling operation involves a

co-ordinated linear, or multiple-axis feeding motion

of the multi-edged cutter as it rotates across and into

the workpiece Milling cutters are usually fitted into a

driven rotating machine spindle for a range of machine

tools The type of machines available is quite diverse

and include: milling machines, machining centres,

mill/turn centres, plano-mills, etc In special cases,

 ‘Fly-cut milling’ , is the exception here, as it is considered a

milling operation that will normally utilise only one cutting

edge, it is similar in design and operation to that of a

two-cut-ting edged trepanning tool A ‘fly-cutter’ is usually employed

for the machining of large diameter circular features, where

either the hole is the required component feature, or the

cir-cular blank is the necessary item from for example, a large

wrought plate/sheet.

 ‘Machining centres’ , are milling machines equipped with an

automatic tool changer – for fast and efficient ‘tool changing’

(i.e to reduce down-time to a minimum), having either a

vertical, or horizontal spindle configuration, although

multi-spindle machines can also be used Predominantly, machining

centres have what is known as: ‘orthogonal axes’ – having each

axis at 90° in relation to each other A ‘basic’ machining centre

has three orthogonal axes (i.e X, Y and Z), but can have

rota-tional axis (i.e A, B and C) incorporated onto them, or built

into the structure of the machine tool Some of these machine

tools, may have 6-axes (i.e 3 linear and 3 rotational), or more,

necessary for any complex free-form sculptured machining

work.

 ‘Mill/turn centres’ , as their name implies, can turn parts and,

with controlled rotational axes, coupled with driven/live

spin-dles, they can accurately and efficiently generate: prismatic

features, faces, splines, keyways, cams, etc., onto the

work-piece at one setting – termed ‘One-hit machining’ In order to

increase the versatility of such machine tools, these mill/turn

centres can also be fitted with ‘co-axial spindles’ , where two

spindles are coincident with each other (i.e their respective

spindle centrelines are in-line, but opposing each other),

al-lowing for example, twin tool turrets to work simultaneously

on two parts These machined components may have quite

distinctly different part geometries/dimensions and, this level

of complex and sophisticated costly plant, requires quite

sig-nificant linear and rotational CNC axes programming

capa-bilities This type of configuration of mill/turn centres with

co-axial spindles, offer two machines in one, but on a quite

small shop-floor ‘footprint’ – an important and perhaps vital

benefit when floor-space is at a premium

 ‘Plano-mills’ , have the ability to generate large flat surfaces

with their traditional range of single-point planning tools, but

with an additional milling capability of having one, or more

milling machine spindles fitted, for large-part surface milling

operations to be conducted on the large part

milling operations can be utilised for multi-axis free-form ‘sculpturing’ of complex-curved workpieces, us-ing equipment such as 5-, or 6-axis machinus-ing centres,

or even robots with ‘slaved’ (i.e fitted) spindles, all these machines being equipped with specially-ground milling cutters – necessary for ‘double-curvature’ ma-chining operations A milling cutter’s design and its respective cutting edges come in a vast range of shapes and sizes (i.e see Fig 76) Each of the individual ‘mills’ cutting edges – as the cutter rotates and is fed into the workpiece, will mill a certain amount of material from the part Milling operations are an efficient way of re-moving either excess stock from previously fabricated parts, or by machining from wrought material The production of a milled surface, offers a machined sur-face texture that is consistently good, having accurate and repeatable dimensions, offering great flexibility

in terms of the geometric types and shapes for these milled components

A milling operation is an ‘intermittent cutting

ac-tion’ , where each individual cutting insert

continu-ously enters and exit’s the cut, unlike turning, which is

basically a continuous machining operation, once the

cut has been engaged It follows that with each cutting

tooth impacting onto the work’s surface (‘intermittent

cutting’), its operation will be affected by: the cutter’s

inherent robustness, the machine tool’s condition and the spindle power availability These factors will have a great influence on the cutter’s ability to efficiently ma-chine the desired component features Milling opera-tions can vary considerably and can be performed by

a wide variety of machine tool configurations, with a diverse range of tooling (Fig 76) and across a large ar-ray of workpiece materials, shapes and geometries

At an early point once the engineering drawing(s) have been designed and produced and, prior to ma-chining the part, an in-depth study of the type of cuts

to be made to produce the desired component features, either out of: wrought, cast, forged, or extruded

mate-rials should be instigated Often at this initial stage in machining assessment, the ‘study’ should address new

approaches to machine the desired part geometries,

by attempting to manufacture the part in the shortest possible cycle-time Moreover, when considering the

 Cycle-times for the manufacture of parts, should include the

productive operations (i.e all machining times) and non-productive elements (i.e including: tooling and workpiece set-ups, tool-changing operations and any necessary form of workpiece measurement), in producing the overall completed component

individual part feature to be machined, judge whether

just one cut, or several passes are the best machining

strategy for its subsequent production In scheduling

particular components for milling operations, the

se-lection of which CNC type of machine tool

configu-ration should be of prime importance, for example a

milling machine, or machining centre equipped with

either a: vertical, or horizontal spindle, or a

univer-sal mill, or even a large gantry mill Once the CNC

machine tool has been selected, other secondary, but

perhaps nonetheless important factors should be

ad-dressed, such as the potential accuracy and

repeatabil- Scheduling of parts, is based upon a range of crucial

produc-tion decisions Typically the final decision may be due to a

number of interrelated factors: when the parts are needed,

the quantity of parts in the batch, their geometry and size,

the availability of the correct machine tool, any potential

cycle-time reductions when utilising a specific machine tool

Other important machining economic factors may need to be

addressed such as: workholding methods, cutting tool

moni-toring, automated part loading/unloading, etc In many

large-scale volume production environments, ‘line-balance’

deci-sions that may have to be made could arise Particularly when

a diverse sequence of operations on component features that

must be machined, these being part of a series of operations

across several machine tools

ity of the production process including the attainable milled surface texture, together with rigidity/instabil-ity of the overall process for the selected machine This latter factor of milling stability, via the rigidity of the machine-tool-workpiece loop, will dictate not only the anticipated milling cutter’s tool life, but affects the to-tal performance of the overall production process with large-scale ramifications for potential component part economics

4.1.1 Basic Milling Operations

Regardless of the type of milling cutter selected, a ma-chining operation utilises one, or more of the follow-ing production millfollow-ing techniques, with any variations

in methods being related to feed directions in relation

to the tool’s rotational axis The three basic milling op-erations are:

• Face milling (Fig 77a, b and c – depicts some

typi-cal machined features produced by facemilling)

A facemilling operation is a combined cutting ac-tion by the inserts, in the main on the tool’s periph-ery and, to a lesser extent by insert edges on the cutter’s face In facemilling, the cutter rotates at 90°

to that of the direction of radial feed against the workpiece Facemilling has a DOC in an axial

direc-Figure 76 Just a small selection of

the vast range of milling cutters avail-able for both machining and mill/turn centres [Courtesy of Seco Tools]

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 finished

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 The cutter rotates around a horizontal axis, this axis being parallel to the tangential feed-ing direction Peripheral millfeed-ing has a DOC in a radial direction that will determine how deep the cutter’s diameter will penetrate into the workpiece

There 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]

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

‘U’), or ‘Down-cut’ (Fig 78 top-left 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, often termed ‘slot-drilling’ This

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

techniques are sometimes referred to as either: ‘Conventional

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

 ‘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 This 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 profiles (i.e ‘sculpture-milling’)

op-erations to be undertaken In some cases, the curvature of the

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

automated-programmed tool) geometry is ground, to

mini-mise step-over/cusp height effects, when (post) finishing-off

operations are more speedily rendered on complex-curved

workpiece features For example, when completing the

high-quality finishing operations on moulds and dies.

 ‘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-’ , or

‘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 different milling cutter rotational directions, impart

totally dissimilar resultant force vectors whilst machining, play-ing a significant 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]

.

ing’ – Fig 78 top-left ‘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-left), 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, effectively 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]

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 effective and efficient way of

peripheral cutting, if any ‘back-lash’0 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 This possible ‘snatching-effect’ , being created by

the resultant cutting force tending toward a back-ward

direction (Fig 78 middle-left) This 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 Therefore, 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 effective forces acting on it,

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

some effects 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 lift the workpiece on the machine tool’s

table, therefore it must be securely clamped/fixtured

into place

The machine tool’s spindle power will depend on

the either the feed force (Vf), or the resultant force

0 ‘Back-lash’ , is a problem concerning slideway ‘float’ (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 fitted, 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 fitted 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 efficient 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 This resultant force is a

combination of the tangential and radial cutting forces

The resultant cutting force will differ significantly in its vectored angle, depending upon the cutter position

relative to that of the workpiece’s These vectored angles (Fig 78 – bottom diagrams), will become larger with

increased DOC – for ‘Up-cut milling’ , the net effect be-ing that the millbe-ing 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

significantly 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 simplified

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 influences the insert’s strength Often, 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- 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)

This 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’

face cutter, face-mill, end mill, slot-drill, etc The

helix angle brings the cutting edge progressively into

cut, 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) This 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 left- 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 The manner in which

the chip formation occurs is beneficial, 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

 ‘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 often 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 often 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-left, 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 This ‘tilt’ of the insert, has the added economical benefit 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 This 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-left, or an inclined and chamfered square insert – middle-left) For ex-ample, in the case of Fig.79a (top-left), the insert

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

hard-wearing surface, this being necessary for example on the Vee-and-flat 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 This

‘metallic chill’ acts as a ‘heat-sink’ to quickly allow solidifica-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.

 ‘Induction-hardened surfaces’ , are usually obtained by a

trav-elling electric resistance induction heating/cooling unit This 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.*This induction-hardsurface-hard-ened surface now consists (i.e metallurgically) of a very hard ‘white-iron’ , appearance – after suitable metallographical preparation – with a white surface layer, once it has been suitably acid-etched

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 The 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 DOC’s can be

achieved, because the negative radial rake provides

high insert strength, whilst the positive axial rake

offers 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

 ‘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]

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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)

The 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 – left), as their name implies, they are normally utilised when machining up to a shoulder, or perhaps

a stepped-feature on the workpiece There 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]

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