Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 233. A graph to determine high-speed cutter unbalance ‘U’ (ANSI S2.19–1989).

Machining and Monitoring Strategies 473

designated for the HSM ranges, is by dynamically bal-

ancing the tools in an appropriate machine (Fig. 234a).

us, during tool balancing, the cutter is clamped in

a xture that rotates in very rigidly supported bear-

ings (Fig. 234a). Any unbalance in the rotating cut-

ter is directly measured as centrifugal force, which is

transmitted along with its actual rotational position

to a specially-congured computer. e computer

calculates the amount and location of the material to

be removed from the tool’s body, in order to properly

adjust the mass distribution. is unwanted tool ma-

terial can then be removed, by either drilling holes,

or milling ats (i.e. see Fig. 234b – illustrating that a

small amount of material has been removed – milled –

from its ange and is termed ‘Hard-balancing’). Usu-

ally, ats are preferred for removing larger amounts of

tool material stock, because at high rotational speeds a

tool with a hole can generate a unacceptable ‘whistling

noise’: see Fig. 235 to gain an appreciation of the eect

of these high peripheral speeds, here, a large face-mill

is shown in an HSM automotive application. Since

the ‘balancing operation’ can compromise the tool’s

strength and performance, it is important to establish

where any superuous tool material can be safely re-

moved from on the cutter’s body.

alance is a ‘zero-quantity’ , so it is customary to

measure balance in the absence – within acceptable

limits – of unbalance. us, the ‘balance tolerance’ is

the maximum residual unbalance (g-mm) allowed for

a particular tool’s weight and rotational speed. For ex-

ample, the ANSI Standard quality-grades for balance

tolerance range from G0.4 to G6.30, with the lower the

‘G-number’ the closer the balance tolerance. It should

be emphasised, that only when a balanced tool and its

balanced toolholder are balanced together as a com-

plete unit, are they truly dynamically-balanced

27 ‘Dynamic-balancing’ postscript: the cutting inserts, screws

wedges that are retained in the cutter’s pockets must be se-

curely locked into position. If these small items become de-

tached when HSM, this may cause disastrous consequences

for any operator in the vicinity. erefore, machine tool guard-

ing of more than adequate protection is vital here, to minimise

potential safety hazards to the personnel when in use.

9.6 HSM – Research

Applications

Introduction

Possibly the foremost reason for conducting applied re-

search programmes at various universities and similar

research-based organisations, is to ‘push the boundar-

ies’ of our theoretical and practical understanding of

current, or novel machining concepts. Rather than at-

tempt to give a perhaps less-than-informed account of

what is transpiring at other ‘learned organisations’. It

was decided just to deal with some recent work that

was undertaken by the author, in association with in-

dustrial and academic colleagues, both in the UK and

abroad. As these particular research projects were ei-

ther undertaken, or instigated by the author, oen in

close union with others, mainly concerned with indus-

trial-sponsored doctoral programmes, it was felt that

here at least, some continuity concerning HSM-related

research themes could be achieved.

9.6.1 Ultra-High Speed: Face-Milling

Design and Development

Introduction

In order to achieve a cutting speed of say, 1,000 m

min

–1

, a φ10 mm milling cutter has to rotate at ap-

proximately 32,000 rev min

–1

. In fact, this is quite fea-

sible for smaller diameter cutters, as there are quite a

few of today’s machine tools having HSM spindles that

can exploit these speeds. However, it is worth a sight

digression here, prior to continuing with the theme

concerned with special-purpose UHSM by face-mill-

ing, to ask the pertinent question: ‘What do we mean

by HSM?’ Is it:

•

High rotational speed machining?

•

High cutting speed machining?

•

High feed machining?

•

High speed and feed machining?

•

High productivity machining?

Even these ve potential contenders for what amounts

to HSM, are by no means an exhaustive list, one could

also add more obscure factors such as: the power de-

mand on the spindle; tool assembly balance speeds;

474 Chapter 9

Figure 234. Dynamic dual-plane (i.e. radial and axial) cutter balancing. [Courtesy of Ingersoll].

Machining and Monitoring Strategies 475

Figure 235. High-speed large diameter face (nish) milling on a grey cast iron engine block. [Courtesy of Sumitomo

Electric Hardmetal Ltd.]

476 Chapter 9

the taper size in relation to its rotational speed; thin-

walled machining capability; etc.; the list will grow,

depending upon what we consider to constitute as go-

ing either very fast, or what speed allows us to machine

certain types of part features! Although even here, the

major benets of an ultra-high speed machining strat-

egy are somewhat lost if a ‘working denition’ is not

clearly stated. In this current discussion on the sub-

ject, one measure of HSM could be if the peripheral

speed of the cutter, or workpiece is >1,500 m min

–1

Hence, this could represent a base-line for the tran-

sition from conventional to HSM, so for comparison,

as in the case for previously mentioned small diam-

eter milling cutter of φ10 mm, it would need to rotate

>47,750 rev min

–1

, or conversely, a larger face-mill of

say, φ300 mm, would have to rotate at approximately

1,600 rev min

–1

– in order to sustain a peripheral cutter

speed of at least 1,500 m min

–1

. is latter rotational

speed although considerably slower, is a much greater

problem that that presented for the former small diam-

eter cutter. e reasons for this are three-fold: rstly,

has the machine tool got enough spindle power to

achieve the necessary stock removal rates required, or

will it be likely to stall? Secondly, is the spindle taper

tment robust enough to cope with the torque eects

and bending moments imparted during machining?

irdly, will the cutting inserts still be retained at the

high centrifugal forces generated in association with

and exacerbated by the imparted cutting forces? ese

and other lesser important questions and decisions

need to be addressed, if the large diameter face-mill

is to successfully mill across a wide workpiece surface

with any degree of eciency. is former point of the

manner in how workpiece surface stock is removed is

important, for example, two markedly diering ma-

chining strategies could be adopted, such as:

I. Shallow depths of cut combined with rapid tra-

verse rates and small step-overs, utilising smaller

diameter cutters at high peripheral speeds,

II. Deep and wide cuts with a large diameter face-

mill with slower traverse rates, having much

lower rotational speeds.

NB Both machining strategies will remove simi-

lar amounts of part stock!

UHSM: Face-Milling Cutter Design

When designing large diameter face-milling cutter

assemblies for production applications in the UHSM

range, a number of critical features need consideration,

such as: cutter-body material; its weight and rigidity;

taper tment; dual-plane balancing; as well as its aero-

dynamic behaviour – at fast peripheral velocities.

If one attempts to design and develop a large face-

milling cutter with an insert cutting circumference

designed to rotate at 3,000 m min

–1

, which at rst

glance, may not seem that fast. However, if we equate

this cutting speed to that of the same φ10 mm mill-

ing cutter previously mentioned, then this smaller

tool would have to rotate at ≈95,500 rev min

–1

, but

for the larger diameter cutter, it would also require

to be dual-plane balanced

. Without dynamic bal-

ancing, the large cutter may be prone to a disastrous

series of vibrational problems, which may ultimately

lead to premature cutter failure – with all its attendant

safety hazards. e cutter design in Fig. 236 for this

applied research programme, was dual-plane balanced

to Standard dened in ISO:1940/1, being to a specic

‘G-number’. is Standard was initially conceived

for the rotational balancing of impellers and similar

high-speed equipment, across a large speed range. e

large face-mills had been dual-plane balanced to G2.5

@ 10,000 rev min

–1

. As mentioned in Section 9.5.1, this

‘G-number’ refers to the maximum tolerable imbal-

ance for the complete tooling assembly, being based

upon the previously described formula (i.e given be-

low again for clarication), resulting in the following

calculations:

Unbalance: U =

 � M � G

(g-mm)

[or] Force: F = U �



�



(N)

Where:

U = allowable unbalance (g-mm);

9549 = a constant;

M = mass, or weight of the total cutter assembly

(kg);

G = pre-selected balance tolerance number from

ISO: 1940/1;

28 ‘Dual-plane balanced cutters’ , are those cutters that have

been dynamically balanced in two planes: having both radial

and axial balance.

Machining and Monitoring Strategies 477

N = maximum rotational speed of tooling assembly

(rev min

–1

);

F =

force (N).

When the above equation for cutter unbalance is

utilised, for these tooling assemblies, utilising the fol-

owing values: G = 2.5; M = 4.294; N = 6,000; which

then gave an allowable imbalance U = 17.9 g-mm. is

level of imbalance means that the cutter’s mass can-

not rotationally shi by more than 17.9 g-mm, if it is

to maintain dual-plane balance at a peripheral speed

of at least 3

,000 m min

–1

. In fact, two identical cutter

assemblies were designed and manufactured, hav-

ing a BT40 taper tment – for the vertical machining

centre (i.e. Cincinnati Milacron Sabre 500). In order

to maintain both structural rigidity and integrity,

the complete cutter bodies and their associated ta-

pers were each produced from single stock of EN24T

steel. Aer precision turning and milling the complete

bodies and insert pockets, these cutters were nitride-

hardened

to HR

52, prior to a ‘very light-grinding’

process and then balancing. e four cutting insert

pockets were equally-spaced (pitched) and the but-

ton-style cemented carbide inserts were: φ1

2 mm by

4mm thick, single-sided and TiN-coated (Stellram:

RPET 1204 DFZ). e insert pocket geometry had a

11° toe angle with a neutral geometry. Button inserts

were selected as they give the strongest shape cutting

geometry available (see Fig. 23), producing an innite

approach angle to the workpiece (see Fig. 83b), thus

minimising impact load at entry to the cut while of-

fering multiple cutting edges – when subsequently

turned in their seatings. As one might expect, insert

security is vitally important, due to the great centrifu-

gal eects and applied cutting forces. Due to the pre-

vious nitride-hardening process, hardened insert seats

were unnecessary, once the retaining screws had been

‘torqued-up’ locking and then sealing them – for se-

29 ‘Nitride-hardening’ , produces a very hard surface with a

soer and tougher matrix. e UHSM cutters were held in a

pressure-tight furnace and heated to between 500-550°C for

some hours in an ammonia gas*, allowing the nitrogen atoms

to diuse into the surface and to form ne stable nitride pre-

cipitates with aluminium constituents, allowing the nitrided-

steel surface to be precipitation- hardened. No subsequent

heat-treatment is necessary.

*Approximately 30% of the ammonia disassociates ( NH

→

←

3H+N) and part of the nascent nitrogen is absorbed by the

surface layers of the steel. (Source: Cotrell et al., 1979)

curity. e actual seatings for the inserts had consid-

erable body-support around their periphery, just hav-

ing a working-clearance at the insert’s cutting region.

ese UHSM face-mills were extremely compact, with

the minimum stand-o height from the cutter’s gauge

line (i.e. see Fig. 236), which reduced the eects of the

previously mentioned ‘rigidity rule’.

Due to the relatively large diameter and weight of

these face-mills and the fact that the machining centre

had limited spindle power, these cutters could, if used

ppropriately, exploit the ‘mass’ , or ‘ywheel-eect’ of

their weight in conjunction with rotational speed to

‘store inertia’. So, when the spindle power is restricted,

cutters with high mass must be taken up to their de-

sired rotational speed in a progressive manner, other-

wise they are likely to ‘trip’ a ‘spindle over-load’ in the

CNC controller. is steady and progressive increase in

he cutter’s rotational speed occurred at 500 rev min

–1

increments – dwelling for several seconds to minimise

inertial power overload, between increases to the de-

sired peripheral speed. Due to the machine tool hav-

ng a maximum spindle speed of 6,000 m min

–1

, this

equated to a peripheral cutting speed of 3,000 m min

–1

with the face-mill having 0.5 m cutting circumference.

Once the cutter has reached its top speed, it can then

be rapidly progressed (i.e. fed) across the workpiece at

rate of 20 m min

–1

. In this case the workpiece ma-

terials were a range of stainless steel alloy testpieces

(Fig. 236). Aer rapidly face-milling these ‘stainless

estpieces’ , the cutter’s rotation was decremented in

500 rev min

–1

intervals until stationary. e cutter once

stationary, could have its edge wear (inserts) assused

and workpiece milled surface texture and surface in-

tegrity could be inspected and investigated.

One factor to bear in mind concerning UHSM with

large face-milling cutters being utilised for their ‘in-

rtial eect’ , is to design them without driving dogs.

If these ‘dogs’ were tted, not only can they introduce

out-of-balance eects, but tend to signicantly disrupt

the air-ow and introduce alarming and high noise

factors. is aspect of cutter design is important, if

the cutter cannot ‘cleave through the air’ with aerody-

namic eciency, turbulent air ow will result and op-

erational noise becomes excessive. One problem that

these particular cutters did not suer from (i.e despite

the conventional taper-cone angle) – unlike many of

their higher rotational speed counterparts, was ‘spin-

le nose swelling’ , which can cause a lack of regis-

ter if the taper tment connection is not of either the

double-, or triple-contact face-and-cone types. One

unexpected aspect of employing such large face-mills

478 Chapter 9

Figure 236. A specially-designed dual-plane (radial and axial) face mill, for ultra-high-speed milling. [Source: Smith,

Wyatt & Hope, 1998]

Machining and Monitoring Strategies 479

in UHSM, was found to be due to the very high peri-

pheral speed. A ‘suction-eect’ resulted from the un-

derside clearance of the cutter’s body, created a ‘low-

pressure region’. is ‘virtual vacuum’ here, meant that

conventional-pressure ood coolant application was

not possible, as it simply vaporised to a mist!

UHSM: Cutting Trials

e neutral geometry enabled the cutting inserts to

present a strong cutting edge to these stainless steel

testpieces, enabling an undistorted cut path and ma-

chined cusp to be generated. is insert geometry

feature, allowed the milled surface topography to be

unaected by insert inclination angles. A stringent test

for any cutter is to machine stainless steel by UHSM

(

ie. being least × 10 faster than any work previously

undertaken) and here, tests were conducted on vari-

ous grades (Fig. 236). e subsequent milled surface

analysis showed little in the way of sub-surface plas-

tic deformation – aer UHSM. e surface layers

exhibiting only marginal increases in the vicinity of

the surface when tapered sections were micro-Knoop

‘

foot-printed’ , over these stainless steel’s substrate (i.e.

see Footnotes: 15, Chapter 1; 85, Chapter 7; and Fig.

187c – concerning Knoop indentors usage). Milled

surface topography can be viewed visually and surface

parameters taken by either: 3-Dimensional contact; or

non-contact instruments; but in this case, by utilising

an SEM

with its unique ‘Stereo-imaging and topog-

30 ‘Scanning Electron Microscopes’ (SEM’s), operational princi-

ple is relatively simple. In that, at the top of the SEM’s column

an electron gun resides having a tungsten lament held in a

strong electrical eld. is results in the electron gun emitting

electrons (i.e. negatively-charged atomic particles), which ac-

celerate to very high speeds. ese high speed electrons – held

in a vacuum – travel down the column, being inuenced by

lenses lower in the column, which squeeze them together to

form an electron beam of very small diameter. is minute

diameter electron beam is then focussed prior to colliding

with the test specimen in the microscope’s specimen chamber,

now as a diminutive spot. is minuscule spot will then scan

both to the le and to right as well as up and down over the

test surface, the information from which is then brought to a

screen as an image. Prior to this, as the electron beam strikes

the test sample’s surface, many dierent processes occur, such

as: secondary electrons; backscattered electrons; Auger elec-

trons; X-rays; Cathodo-luminescence; etc.; these being emit-

ted, collected and counted, then utilised for further analyses.

raphy soware’ – for 3-D visual assessment coupled to

its height-to-depth proling application.

For the milled testpieces produced from 316-aus-

tenitic stainless steel, subjected to UHSM by this face-

ill at 3,000 m min

–1

, the surface topography showed

the inuence of the wear land at produced by the

four φ1

2 mm TiN-coated inserts, although the remain-

der of periodic surface oered little sign of any surface

modication. e 303 stainless steel grade testpieces,

indicating a slight improvement over the former 316

grade. While, 416 martensitic stainless steel testpieces

under identical cutting data generated no appreciable

surface blemishes, with the additional benets of: an

extended cutting insert life; signicant reductions in

both cutting forces and power requirements, over the

303 and 316 stainless steel grades.

9.6.2 Ultra-High Speed:

Turning Operations

Introduction

As has been shown in the previous sections with ref-

erence to HSM by milling, considerable applied and

fundamental research eort has occurred, conversely,

little endeavour has been made regarding high-speed

turning operations. Possibly the major reason for the

lack of interest here into HSM by turning operations,

is because a dierent approach to the workholding is-

sues needs to be taken. In that, on a CNC turning cen-

tre, or lathe, the ‘bursting-pressures’

resulting from

signicant centrifugal forces with conventional work-

NB e depth of elds from an SEM are considerably deeper

than that produced by conventional microscopes, allowing

some exacting surface topography analysis to be undertaken.

(Source: Smith et al., 2002)

31 ‘Bursting-pressure’ problems, have been well-known in tra-

ditional turning activities for many years. is aspect of

safe-working practice, was particularly relevant for large cast

iron face-plate work, where a rotational speed limitation is

imposed by a machine tool builder. If this restricted speed is

exceeded, then the cast iron – being poor in terms of tensile

strength, will literally fragment (i.e. ‘burst’), due to the exces-

sive centrifugal forces imposed. While, the problem is not as

severe for chuck and collet work, the lack of gripping-pressure

on the part – at high rotations, will aect the workpiece if long

slender parts supported on one end only are turned – possibly

causing a ‘whipping-eect’ and attendant safety hazard.

480 Chapter 9

Figure 237. An ultra-high-speed turning operation undertaken on a vertical machining centre. [Source: Smith, Littlefair,

Wyatt & Berry, 2003]

Machining and Monitoring Strategies 481

holding: chucks and face-plates; in combination with

a reduction in gripping-force at high rotational speed

is a potential safety hazard. In order to achieve the

esired UHSM rotational speeds of >1,500 m min

–1

which can be considered as the ‘threshold’ for such a

machining strategy, then the workpiece must be held

in a machining centre spindle – with the tool station-

ary, in a similar manner to vertical turning (i.e. see Fig.

237). Relatively recently in pioneering work by Youse

and Ichida (2000), they utilised rotational speeds up

o 15,000 m min

–1

by this technique, showing that the

turned surface texture parameter ‘Ra’ v

alues (i.e see

Section 7.5.1 in Chapter 7) steadily reduced with in-

creases in cutting speed. Moreover, this Japanese ma-

chining study found that the cutting forces remained

relatively constant throughout the test range of: 1,200

o 15,000 m min

–1

. is trend of reasonably constant

turning forces with increased speed is contrary to that

normally found for UHSM by milling operations (Ko-

manduri, 1995, et al.), where a signicant reduction

in milling forces results from high cutter rotations,

allowing large length-to-diameter cutter ratios to be

used (Gough et al., 1991).

In any HSM operations, it is essential that the ro-

tational mass of either the workpiece, or cutter – de-

pending upon which is the rotating item, is dynami-

cally balanced, in order to minimise out-of-balance

eects which would otherwise impede both the cutting

process and aect machined surface texture. Ideally,

workpieces, or a cutter should be rigidly held in-situ

during machining and at the very least, be single-plane

balanced.

UHSM: Turning Strategy

Prior to undertaking the UHSM turning operations

(Fig. 237), the machining centre was checked for di-

agnostic errors by a ‘

Telescoping Ballbar’

assessment

32 ‘Telescoping Ballbar’ (Fig. 242a), is a powerful instrument for

machine tool error diagnostics. e Ballbar as its name im-

plies, is a ball-ended length transducer (i.e. an LVDT- measur-

ing element, is positioned between the xed and telescoping

balls). is LVDT has a range of ± 0.75 mm with a resolution

of 0.1 µm and accuracy of 1 µm. It is held in kinematic (mag-

netic) seatings between the machine spindle and its base.

Extension bars can increase the radial length up to 300 mm,

covering a large volumetric sweep for the two axes being diag-

nostically monitored. Any kinematic plane can be rotationally

swept by the Ballbar. In operation – from the soware pro-

gram, the machine’s CNC will move the Ballbar to the start

position (i.e. radially oset the required distance for the orien-

set at the radial turning distance in the plane of the

cut (i.e. see the Ballbar conguration in Fig. 242a). A

range of rotational Ballbar speeds were utilised, albeit

at considerably lower peripheral speeds than those

which were employed for the UHSM turning trials.

For this current UHSM work, a machining strategy

was adopted utilising a ‘Variable quasi-pilgrim stepped

arithmetic progression’ (i.e the progression is schemati-

cally depicted in Fig. 238), for the selection of ‘turned

testpiece’ rotational speeds, being based upon the fol-

lowing general progression case criteria:

→ Sn

= 2000 + {n

2 [2a

+ (n

– 1) d

]} – 1000

+ {n

2 [2a

+ (n

– 1) d

]} – 500 + {n

2 [2a

– 1) d

]} – 250 + {n

/2 [2a

+ (n

– 1) d

]} – 125 + …

Where:

= 2,000, a

= 4,000, a

= 5,000, a

= 5,500;

= 1,000, d

= 500, d

= 250, d

= 125;

= 4,000, n

= 2,000, n

= 1,000, n

= 500.

Such an unusual ‘progression’ mathematically de-

scribed above (i.e also being shown schematically in

Fig. 238), enables signicant discrimination of rota-

tional results coupled to data analyses toward the up-

per limit of the UHSM turning process, while giving

‘traceability’ to rotational speeds within the conven-

tional range of the rotational turning process at the

beginning of the turning process.

A special-purpose workholding device – being

ual-plane balanced to G2.5 @ 10,000 rev min

–1

, was

tted into the spindle of a vertical machining centre

(Fig. 237). is BT40 tapered workpiece holder was

constructed from one-piece of EN24T steel hardened

by nitriding to >50 HR

, weighing ≈2.5 kg with the

aerospace-grade aluminium disk-shaped testpiece in-

tated planes), then it slowly rotates CW for 180° – to pick up

uniform rotational velocity, where it rotates through a further

360° – for polar measurement, nally rotating another 180°

– to slow down. is complete cycle is then repeated CCW.

en polar plots are generated with a diagnostic printout,

which ‘ranks’ these errors, so that they can then be eliminated,

or signicantly reduced, accordingly. is is a speedy, ecient

diagnostic ‘health-check’ of the machine tool errors in the two

measured planes, providing signicant information, which

can be utilised to improve the machine tool’s overall perfor-

mance. (Source: Renishaw Ballbar Training Manual)

NB Typical ‘polar plots’ are shown in Appendix 16, together

with a diagnostic print-out of the results.

482 Chapter 9