Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 248. CNT tool cutter path control

while contouring sculptured surfaces – utilis-

ing nurbs. [Courtesy of Sandvik Coromant]

Until about a decade ago, there existed only one

practical way to represent free-owing curves in a

cutter path. is was despite the fact that CAD/CAM

systems could mathematically dene virtually any

geometric shape with smooth curves. ese CAD/

CAM systems generated pristine forms which would

Machining and Monitoring Strategies 503

have to be converted into a recognisable program-

ming structure that the machine tool’s servo-drives

could understand and apply. is ‘translation’ took

the form of representing complex curves as a series of

straight lines, or linear segments, being joined end-to-

end within a user-dened tolerance band (Fig. 248a).

us, the length of each linear segment was governed

by the curvature of the prole and the tolerance band

previously set. Any tight precision radii on the work-

piece, requires very small tolerance bands, creating a

large number of segments needing considerable pro-

grammed-blocks of toolpath data. is technique is

acceptable in many respects, but its hardly very e-

cient because complex 3-D surfaces need large quan-

tities of data to accurately represent their geometric

proles. is conict between ‘CAD shape-dening

data’ to that of the machine tool’s motional kinemat-

ics necessary to produce the prole, means that trans-

mission rates and corresponding feedrates suer, as

each line segment corresponds to a ‘bottleneck’ in the

part program, this being data point expressed as an

X-Y-Z co-ordinate. To minimise these problems and

more specically, now that HSM capabilities are com-

monplace, CNC builders are incorporating ‘complex

curve interpolation’ capabilities into their controllers,

enabling tool paths to be machined utilising the same

mathematical terms that CAD/CAM systems use to

generate them. In other words, ‘NURBS’ , which in

practice largely means that for the same quantity of

data, the controller can achieve faster, smoother and

more accurate machining.

A ‘NURBS’ is constructed from three discrete pa-

rameters: Poles; Weights; and Knots

. As a result of

‘NURBS’ being dened by non-linear motions, the

tool paths will have continuous transitions, enabling

signicantly higher: acceleration; deceleration; plus

enhanced interpolation speeds; than was previously

47 ‘NURBS’: e rational equation, can be expressed, as follows:

P (t) =

i=

�

Ni,  (t) GiPi

i=

�

Ni,  (t) Gi

e Non-Uniform B-Splines can be expressed, as follows:

Ni, (t) =

�

(Ki � t � Ki + )

 < Ki, Ki+ < t)

Ni, k (t) =

−

Ki) Ni, k −  (t)

Ki + k − 

−

(Ki + k

−t

) Ni +  , k −  (t)

Ki + k

−

Ki + 

Where:

Pi = Control point; Gi = Weight; Ki = Knot vec-

tor.

(Source: Oakham, 1998)

available by CNC controllers without the ‘complex

curve interpolation’ capabilities. As ‘NURBS’ have the

ability to describe any free-form curve, or surface pre-

cisely and eciently, they became immensely popular

with CAD Soware-developers, because it allowed

Design Engineers more freedom to manipulate 3-D

data, than had been available utilising simple ‘line-

segments’ and ‘primitives’. e logical extension for

the application of ‘NURBS’ was followed-up by CAM

developers, as many systems were integrated into one

by the same company that developed the CAD system.

is CAD/CAM integration, enabled these companies

to supply post-processors that supported all the major

digital controller manufacturers oering a ‘NURBS-

capability’.

In order to more fully comprehend just how

‘NURBS’ works, it is worth a slight digression to briey

discuss the techniques utilised to represent curved

surfaces. By way of illustration, the CAD equivalent

of the Draughtsman’s ‘Flexi-curve’ used to create free-

from curves, is termed a ‘spline’

. e alternative ‘B-

Splines’

dier from that of ‘Splines’ , instead, they

function somewhat like a ‘gravitational pull’ acting on

them, pulling and distorting the curve, but in the con-

trol point’s direction. While, ‘NURBS’ are essentially a

more controllable version of ‘B-Splines’. e resulting

output from ‘NURBS’ is very ecient, as it describes

the curve’s geometry with a fraction of the data output

necessary for linear interpolation. One disadvantage is

that the calculation of ‘NURBS’ are much more com-

plex, necessitating considerable amounts of comput-

ing power to compute them. e ‘Non-Uniform’ term

in ‘NURBS’ , refers to what is called its ‘knot vector’ ,

which indicates the portion of a curve that is aected

by an individual control point, but where it does not

have to be ‘uniform’. By ‘dissecting’ the ‘NURBS’ term

still further, the portion of it aected by the ‘Ratio-

nal’ part of the formula, means that the weight of the

control points’ pull (weighting) – which can be speci-

ed. is ‘weighting’ allows conic sections to be repre-

sented, without having to slice them up to determine

their geometric aspect.

48 ‘Splines’ , can simply be dened as follows: As a series of

equally spaced control points which the computer connects to

create a smooth owing curve’.

49 ‘B-Splines’ , may be dened in a slightly diering manner to

that of ‘Splines’ , such that: Utilising the end and control points

that do not necessarily intersect the curve, thereby they can dis-

tort the curve’. (Source: Oakham, 1998)

504 Chapter 9

When applying ‘NURBS’ to a complex part’s curva-

ture, it is important to recognise that it denes the entire

curve, not just a series of facets, enabling it to express

any curve geometry, utilising less data than for other

‘curve-tting techniques’. Data transmission times are

signicantly improved as a result, this is because one

does not have to transfer all of the curve data, just the:

control points; the order of the polynomial; the knot

vector; and its weighting; as dened by the CAD sys-

tem. Once this has been achieved, the machine tool’s

CNC controller then decodes this information, in or-

der to control its servos. While a single ‘NURBS’ ex-

pression can describe a simple curve, complex curves

(e.g. Fig. 248c) are described by moving ‘weighting’ on

the control points, running the calculation, then mov-

ing the ‘weighting’ again and re-calculating and so on,

in a recursive manner. us, each point moved has

an inuence on the others, but the more the control

points utilised, the less their inuence becomes – in a

similar manner to the so-called: ‘law of diminishing

returns’. ‘NURBS’ is comparable to linear interpolation

in that the greater the accuracy the more the number

of points needed, although it requires less data in to-

tal – with a gure of 60% data-reduction, with an as-

sociated 40% improvement in time, has been claimed.

Although the solution to virtually every curve-tting

eometry can be undertaken by ‘NURBS’ , it cannot

partake in all ‘surface-describing miracles’. If the CAD

system outputs poor data, this will end up with a simi-

arly pitiable ‘curve-tting routine’ , so as the old saying

goes, it’s the equivalent of: ‘Garbage in, garbage out!’ In

time, these ‘NURBS’ will have even more renements

added to enhance the already powerful ‘curve-tting

processes’.

9.8.4 Sculptured Surface Machining –

Cutter Simulation

Once the free-owing curves for the sculptured sur-

faces have been generated and the actual workpiece

is about to be machined, many companies embark on

a ‘cutter simulation routine’ prior to undertaking any

surface machining. Many of the sophisticated surface

machining soware packages, can provide several

variations of complex surface machining routines.

Typical of such routines, is that shown for a particular

leading company’s product for the multi-axis sequen-

tial machining, depicted in Fig. 249a. is specic

‘sequential surface machining’ routine (Fig. 249a), is

an interactive, graphic implementation of ‘drive-part-

check’ surface machining, as dened in the: Automati-

cally Programmed Tool (APT) Standard. is routine

is greatly enhanced when utilised in combination with

two other machining soware packages, namely: ‘Se-

quential machining’; and ‘Drive curve mill’. While an

enhanced function incorporated into the machining

package is termed ‘looping’ ,

which enables the user to

generate multiple passes on a surface, by dening the

inner and outer tool paths, allowing the system to then

generate the intermediate stock-clearance tool path

steps.

A typical modular-package might oer: surface con-

touring; parameter line machining; rough-to-depth;

and zig-zag tool paths; having any design modica-

tions, or changes being automatically handled through

what is termed ‘associativity’ ,

thereby signicantly re-

ducing any attendant costly, but otherwise necessary

prove-outs. By utilising cutter simulation, parameters

such as: feedrate; spindle speed; and part clearance; are

instantly accessible and, being ‘modal’ they remain un-

changed, unless the user modies these values. While

at any time during the development of the simulation,

a user can test a setting by generating a tool path with

its accompanying high-resolution graphic display (Fig.

249a). Surface machining will automatically simulate

the cutter’s tool path, being displayed on a graphics

screen and generate textural output into a ‘cutter lo-

cation source le’ (CLSF). Aer simulation, the user

may either choose to accept the tool path simulation

and then save these parameters, or reject it and modify

whatever parameters are necessary to correct for any

attendant problems encountered. It should be stated

that if a problem had occurred when actually cutting

the complex geometric component’s surface – such as

‘

surface gouging’

, this would have probably scrapped

the otherwise expensive stock of workpiece material,

that has also added signicant value to it, by the time-

consuming process of machining this part’s intrinsic

geometric characteristics.

So the application of cutter simulation is not only

economic and scally important, it oers many other

signicant production benets. erefore, with such

enhanced cutter simulation, a range of important fea-

res can be addressed ‘o-line’ , such as:

•

Supporting typical CAD ‘Surfaces and Solids’ pack-

ages,

•

Providing both 3- and 5-axis contouring motion –

including tool orientations that may be oset from

50 ‘Surface gouging’ , is if a cutter unintentionally removes mater-

ial (gouges-out) a portion of surface.

Machining and Monitoring Strategies 505

Figure 249. By utilising a sophisticated cutter and part simulation technique, any potential and very costly ma-

chining mistakes can be avoided

506 Chapter 9

the surface ‘normals’ (i.e. cutter tilt and lead/lag

angles), or being parallel to the surface (i.e. here,

termed: ‘swarf-cutting’),

•

Gouge-checking routines and step-over control

functions, during the non-cutting motions,

•

Allowing complete control over the quality of the

machined surface texture and the attendant stock

to be removed,

•

Control of intrinsic surface directional parameters,

having input of tool paths that are projected onto a

surface to be machined with its associated arbitrary

urves and points ‘sets’ ,

•

Addressing a ‘full-check’ surface capability, having

specied part clearances – for xtures and clamp-

ing, while stipulating both setting and gauging

points in the simulation routine,

•

Allowing for the machining of arrays of multiple

surfaces, including: trimmed and extended sur-

faces; and for any multiple arbitrary holes,

•

Enabling the user to selective in avoiding particu-

lar workpiece features, such as specic holes and

islands.

By utilising sophisticated cutter simulation packages,

the work is undertaken ‘o-line’ by the user, thus

avoiding: costly prove-outs; potential scrappage of

parts; tool breakage; or under extreme circumstances,

even serious damage to the machine tool. In fact, for

any sculptured machining operations (i.e. typied in

Figs.: 245b and c, 246b, 249b), they need to have some

form of cutter path simulation undertaken and its as-

sociated simulated enhancements, otherwise poten-

tially costly production machining mistakes are the

likely outcome.

9.9 Hard-Part Machining

Introduction

Since the development of ultra-hard cutting tool ma-

terials, such as: cubic boron nitride (CBN); and poly-

crystalline diamond (PCD) and their derivatives; to-

gether with ‘sub-micron’ cemented carbides coupled

with their ultra-hard multi-coatings; or diamond-like

coatings (DLC) to these carbide surfaces; it has enabled

the hard-part machining process to become well-es-

tablished and commonplace. Prior to machining parts

in the hardened state, the time-consuming and expen-

sive processes were: rough-out, or nish-machine cer-

tain features of the part, then heat-treat it (i.e. hard-

ening and tempering – as necessary), grinding critical

surfaces and dimensions. Today, by hardening the

wrought stock material before it is machined, this will

eliminate any potential distortions caused by thermal-

inuences on the part when it was heat-treated, this

enables the part to be hard-part machined: roughed

and nished, normally avoiding the subsequent grind-

ing processes: surface and cylindrical – as necessary.

e question that could be asked concerning such a

machining application is: ‘What then denes hard-part

a machining process?’ Before answering this question

it is worth metaphorically ‘stepping-back’ somewhat,

to discuss what was considered as extremely ‘hard’

around seventy, or so years ago. At that time, high-

speed steel (HSS) tooling was the norm, as cemented

carbide had not yet been fully-established throughout

the manufacturing industries of the day. Here, HSS

was the favoured tooling material, due to its superior

retained edge hardness at elevated temperatures (i.e

its ‘red hardness’) in combination with its improved

toughness – over other tool materials at that time.

Typically an M2-HSS

had a bulk hardness of between

58-64 HR

. Returning to the question posed above,

concerning what denes a hard-part, according to one

major cutting tool company it is those materials with

a hardness of >42 to 68 HR

– being the equivalent

of machining components from M2-HSS. Previously,

only through grinding operations, could the hardened

omponent be produced to ‘toleranced-size’ , while

51 ‘Hardness’ , can be dened as: e measure of a material’s re-

sistance to deformation by surface indentation, or abrasion’.

(Source: Callister, Jr, et al., 2003)us, a ‘hard material’ can be

considered, when large forces are necessary to cause a perma-

nent indentation [machining] marks. (Source: Schaer, et al.,

1999)

52 ‘High-speed steel’ (M2 – HSS), will have a typical 0.2% yield

tress @ room temperature of ≈3,000 N mm

–1

, while @ 600°C

it is ≈1,800 N mm

–1

, showing good high temperature prop-

erties. Some other relevant mechanical properties, include:

transverse rupture stress @ 4.8 GPa; Fracture toughness (K

)

@ 17 MN m

–3/2

; Izod impact strength (un-notched) @ 33.4 J.

(Source: Trent, 1984)

53 ‘Hard-part machining’ ,

today is utilised widely as both a

roughing and nishing cutting process, which of late, has

seen parts machined from a range of hard workpiece materi-

als, having a bulk hardness of up to 68 HR

. (Source: Huddle,

2002)

Machining and Monitoring Strategies 507

having the necessary surface texture. In the case of

hard-part turning – to be discussed in the next sec-

tion, hard-turned surface texture of better than 2.5 Ra,

is achievable, across a range of workpiece materials.

According to Hanson (2005), hard materials can be

classied into two distinct groups:

1. S

ingle-component materials – might typically in-

clude hardened tool steels, glasses such as Pyrex™

and borosilicate, ceramics including silicon carbide

(SiC) and aluminium oxide (Al

2. C

omposite materials – could be either metal/ce-

ramics such as metal-matrix composites (MMC’s)

and tungsten-carbide /cobalt, glass/ceramics such

as Zerodur™ and Cervit™, as well as ceramic/ce-

ramic composites such as silicon-carbide/silicon.

NB 

ese groupings only list a few of the hard

workpiece materials currently available today, many

more exist, but they can still be classied within

these groupings at present.

e selection of the cutting tool composition and its

associated geometry when hard-part machining, is in-

uenced by the severe demands made by these hard-

ened workpieces. e problems encountered can range

from very rapid tool wear rates, cracks and chipping of

the cutting edge(s), to an unacceptable machined sur-

face condition. Some multi-coated cemented carbides

and aluminium oxide ceramics (Fig. 10) can cope with

some operations on hardened workpiece materials,

but it is more usual to utilise ultra-hard cutting tool

materials, or at the very least, specialised-coatings on

cemented carbide tooling.

Some technical diculties can be encountered

when hard-part machining, these might range from:

•

Elevated temperatures in the cutting zone,

•

Greater and more variable cutting force magni-

tudes,

•

Intense pressure on a relatively small cross-section

of the chip – near the edge,

•

Rapid cutting edge wear, or catastrophic break-

down,

•

Workpiece stresses being released during the ma-

chining operation,

•

Poor homogeneity in part material – creating vibra-

tional eects on tool’s edge,

•

Insucient stability and rigidity – created in the

‘machine-tool-workpiece’ loop stiness.

e extreme thermal and mechanical conditions, will

dictate the manufacturing circumstances, concerning:

tool material and its geometry; machining methods

utilised; together with the cutting data selected. So, the

properties demanded from a cutting tool when one is

about to embark on hard-part machining exercise, are

that it has:

•

Superior abrasive wear resistance,

•

Chemical stability at the high temperatures en-

countered,

•

Ability to retain its cutting edge at high tempera-

tures – ‘hot-hardness’ ,

•

High compression and bending strength,

•

Good cutting edge strength and toughness,

•

Cutting edge inertness and resistance to diusion

wear.

e hardness of a part, should not be confused with

ts ‘modulus’ , or the material’s toughness. As it is the

combination of elastic and plastic properties that de-

termine a material’s resistance to yield, namely, its

permanent deformation

. In the following sections,

particular approaches to that of hard-part machining

by various production processes will be concisely re-

viewed.

9.9.1 Hard-Part Turning

For some years now the application of hard-part turn-

ing has increased in popularity, as the time needed to

nish these hardened components – with their hard-

nesses ranging between 45 to ≈68 HR

– has signi-

cantly reduced. e major time-saving is from the vir-

tual elimination of nish-grinding operations, when

‘ne-turned’ surface texture of <2.5 Ra is now possible,

with matching dimensional tolerances. Complex con-

touring of a component’s prole using hard-part turn-

ng operations is achievable (i.e. see Fig. 154 – top),

which overcomes the previously expensive and time-

consuming proling operation of cylindrical grinding

with custom-formed grinding wheels.

However, successful hard-part turning is more than

simply ‘chucking’ a hardened component (Fig. 250a),

54 ‘Yield, or permanent deformation’ , diers here, from that of

either: elastic deformation; or dislocation, slip, etc.; that may

result during machining. (Source: Hanson, 2005)

508 Chapter 9

then utilising for example, either a CBN, or ceramic

cutting insert to machine them. A variety of important

factors require consideration if the turning operation

is to be successful, including the fact that higher cut-

ting forces involved and their aect on the machine

tool’s structural rigidity and stiness (Fig. 250d). ese

hard-part cutting forces can be ≈200% higher than the

forces generated when similar operations are under-

taken on ‘soer parts’.

Due to the fact that either the CBN, or ceramic tool

materials tend to be relatively brittle, cutting tool com-

panies will apply chamfers to the cutting edges – to

strengthen them. A problem with a ‘supporting cham-

fer’ on the insert’s edges is that it reduces its shearing

capability, this causes the cutting forces to increase –

due to the so-called ‘ploughing-eect’. e application

of higher cutting forces here, makes both the insert’s

edge and the part move relative to one another – re-

sulting in chatter. e onset of chatter causes surface

texture degradation – at best, or destroys the cutting

edge/tool and even the part itself – at worst. Moreover,

the size and the shape of the workpiece when hard-

part turning is important, as if too long a length-to-

diameter ratio – without support, this will prove to

be exceedingly dicult to successfully machine. With

some hard-part machining companies stating that L/D

ratios of >3:1 can be a problem – with respect to chat-

ter generation, resulting from the higher imposed cut-

ting forces now in operation.

Workholding plays a crucial role in successful

hard-part turning operations, with a rm all-round

grip – from say a collet positioned and rigidly held

well inside machine’s headstock, is much preferred to

that of a three-jaw chuck – as this latter device may not

necessarily provide either the ‘c

lamping forces’

, nor

inherent rigidity. e location of the turning insert’s

55 ‘ree-jaw chucks – clamping forces’ (Fig. 250 – bottom right),

due to the fact that the ‘conventional’ chuck’s self-centring

mechanism is a face-scroll – this being provided by a spiral

groove cut onto the face of a at disk, then having the equi-

spaced jaws oset by the scroll’s pitch and numbered for re-

placement as either internal, or external jaws. When the chuck

key turns the ‘scroll-pins’ which then rotates the scroll plate

and thus, they simultaneously radially move the jaws – in-

ward/outward. So, depending upon the scroll plate’s rotation,

the torque supplied by the chuck key creates only one third of

the total force supplied at each individual chuck jaw. erefore,

the force may not be adequate to provide sucient force to

grip and rigidly hold the part as it is hard-turned.

positional location, relative to the machine’s spindle

bearings is signicant. e further the distance away

from the front spindle bearing’s location relative to

that of the cutting action, the greater any potential for

the part to ex and chatter – acting like a ‘lever mecha-

nism’ (i.e. its force times distance). So, machine tools

that are designed so that their collets are well-seated

into the headstock and close to the front bearings and,

when hard-part turning, the minimum of workpiece

overhangs should be utilised.

Turning centres that are designed from the out-

set to cope with the demands of hard-part turning,

are usually of greater size and weight, plus being ex-

tremely rigid structures. Even here, concerning a ma-

chine’s rigidity, there are practical limits to the amount

of ‘static stiness’ (i.e. the ratio of applied force to that

of an associated displacement) that can be built into

the turning centre. e design objective is to increase

the machine tool’s ‘

dynamic stiness’

, which involves

dampening the frequency of vibrations via specic ap-

plied technologies, such as ‘

composite-lling’

the ma-

chine bases. While a dierent approach to dampening

vibration, involves the use of hydro-static linear ways,

which ride on non-contact pressurised uid bearing

surfaces. In a similar manner to that of ‘conventional’

linear-ball guide ways, they exhibit low friction and

resist omni-directional loadings (i.e see Fig. 250d – for

a performance comparison of these two linear bear-

ing types). Moreover, hydro-static guide ways provide

dampening as-and-when vibrations occur during

hard-part machining process. Furthermore, hydro-

NB In Fig. 250e on the other hand, the force exerted by this

type of self-centring mechanism, should prove to be able to

cope with the demands of hard-part turning , particularly as

the ‘so-machinable jaws’ have been bored-out, to provide

more circumferential location and support for the part.

56 ‘Dynamic stiness’ ,

can be dened as: e ratio of the applied

force to the displacement, occurring at the frequency of the ex-

citing force’. (Source: Hardinge Inc., USA)

57 ‘Composite-lling’ – machine tool bases. is dampening te-

chinque is traditionally achieved by employing such materials

such as Granitan™ (i.e. typically, being a crushed granite and

epoxy resin), or Harcrete™ – this latter product is a polymer

composite, having an 800% better damping capacity to that of

the equivalent grey cast iron structure. Although these com-

posite-lled structures can be expensive, in order to alleviate

some of this cost, ‘traditional’ castings have strategically-rein-

forced composite-lled cavities. (Source: Kennedy, 2004)

Machining and Monitoring Strategies 509

Figure 250. Hard-part turning operations are replacing some grinding operations: assuming dimensional size and

machined surface texture are acceptable. [Courtesy of Sandvik Coromant]

510 Chapter 9

static bearings

oer the indirect benets of: improved

hard-part machined surface texture; greater compo-

nent accuracy and precision; coupled with increased

tool life. While, due to the fact that hydraulic fuild is

virtually incompressible, if an interrupted cut occurs

for any reason, the tool can later pick-up exactly where

the interruption occurred thereby eliminating the os-

tensibly termed ‘witness marks’ on the machined sur-

face.

Great demands for increased spindle power are

not necessary, as typical D

’s are ≈0.75 mm, coupled

to small feedrates. With for example, the cutting data

for a φ1

2 mm, hard-turned part, might be by utilising

a spindle speed of 4,500 rev min

–1

, which equates to a

surface speed of ≈170 m min

–1

. is is well within the

capabilities of some turning centres today, that have

pindle speeds of 10,000 rev min

–1

. As one can gather

from this discussion, hard-part turning and thread-

ing (Fig. 251a) is principally concerned with saving

money, by removing additional and now, superuous

operations from the overall product’s cycle time. It has

been reported by industrial-users, that by utilising a

hard-part turning strategy, then the total time to com-

plete the component has been reduced by up to 75%,

when compared to the traditional approach, of: rough-

turning; then heat-treating; and nally cylindrically

grinding. is latter process of cylindrical grinding,

can still be valid where extremely tight tolerances are to

be held, coupled to when high-quality surface texture

demands are to be met. Oen, both for large-batches,

or if continuous production runs are necessary, for ei-

ther hard-part nished turning, or when cylindrical

grinding, they require critical dimensional tolerances

to be held across certain diameters, or indeed, for their

lengths. In this metrological situation, a ‘receiver gaug-

58 ‘Hydro-static bearing performance’ , is characterised by three

factors, these are its: load-carrying capacity; oil ow-rate; and

pumping power. e magnitudes of the hydro-static coe-

cients depend very much on the pad design, such that:

W = a

Q = q

W/A

× h

/ µ

P = p

Q = H

(W/A

)

× h

/ µ

Where: W = Load on the bearing (N); Q = Volume ow-rate of

oil (m

–1

); P = Pumping power (N × m s

–1

≡ W);a

= Pad load

coecient (dimensionless); q

= Pad ow coecient (dimen-

sionless); H

= Pad power coecient (dimensionless, because

= q

); A

= Pad area (m

);p

= Oil pressure in the recess of

the pad (Pa); h = Film thickness (m); µ = Dynamic viscosity of

the oil (Pa.s). (Source: Mott, 1985)

ing’ unit specially-congured to measure such part

dimensional features can be built-up from modular

units and its associated instrumentation (Fig. 252).

Not only can this custom-built receiver gauge assem-

bly (Fig. 252b), simultaneously measure many compo-

nent features quickly, but through the instrumentation

unit (Fig. 252a), ‘Statistical Process Control’ (SPC) can

be employed to up-date the whole process through

‘closed-lop’ feedback to the machine’s controller. is

up-dating will automatically modify the tool osets,

thereby minutely adjusting the process and as such, re-

ducing variability in the process to a minimum. Parts

can be loaded into the adjacent ‘receiver gauge unit’ ,

via

a gantry-robot (Fig. 250e), or alternatively by a ‘dedi-

cated robotic device’. Once held in the robot’s gripper,

the nished hard-turned/-ground part, is manipulated

into the desired orientation by its axes, then steadily

lowers the completed workpiece onto the component

support plates within the receiver unit. In the case of

Fig. 252b, the centres will automatically engage with

the centre-drilled holes, slightly liing the compo-

nent to its measuring height, prior to the measuring

heads progressively moving forward to contact each

machined component’s surface feature – for automatic

measurement and control.

9.9.2 Hard-Part Milling

Introduction

Until relatively recently, the application of HSM by

milling of hardened die and mould steels was consid-

ered something of a ‘black-art’. is is not the case, as

by adhering to some basic machining principles and

guidelines the whole process becomes a somewhat:

straightforward; predictable; and a protable activity.

In eect, there are three primary machining meth-

ods utilised to produce hardened dies and moulds,

although it should be stated that the die/mould con-

guration, along with its respective hardness will de-

termine which technique, or combination of processes

produces the optimum manufacturing route. However,

notwithstanding these circumstances, the primary

methods can be classied as follows:

1. ‘

So machining’ – milling – this is where the part is

‘roughed-out’ prior to its hardening heat treatment.

e technique of ‘so-machining’ is normally con-

idered when milling large workpieces, or com-

ponents requiring deep-cuts, or wide features – such

Machining and Monitoring Strategies 511

Figure 251. Hard-part machining operations undertaken on many components by through-hardening,

hard-facing or by surface-hardening heat-treatment; are acceptable. [Courtesy of Sandvik Coromant]

512 Chapter 9