Machining and Monitoring Strategies Part 8 pptx

9.8.3 Sculptured Surface Machining – with NURBS Prior to a discussion on the application ‘curve-fit-ting’ with ‘Non-Uniform Rational Bezier-Splines’ – ‘NURBS’ for short, it is worth a b

Figure 247 By utilising a ball-nosed cutter geometry for die-sinking sculptured surfaces, this reduces finishing stock needed to

be subsequently removed [Courtesy of Sandvik Coromant]

.

9.8.3 Sculptured Surface Machining –

with NURBS

Prior to a discussion on the application

‘curve-fit-ting’ with ‘Non-Uniform Rational Bezier-Splines’

– ‘NURBS’ for short, it is worth a brief review into

the background as to why there has been a

wide-ac-ceptance of them for machining operations involving

sculptured surfaces The technique of curve fitting is

not new, it was devised in the 1960’s, where indirect

methods were found making it relatively easy to

ma-nipulate these curves – without recourse to

modify-ing the different equation parameters that defined

the sculptured surface In a typical system, a complex

curve geometry would be comprised of several discrete

curves – termed a ‘spline’ , equally, a surface is simply

a curve with an extra dimension Thus, for

‘curve-fit-ting’ the cubic method is particularly suited, although

a modified cubic approach that can accommodate the

uneven spacing of ‘nodes’ – the start and end points –

has particular benefits when digitising surfaces

In France, Bezier who at that time was working for

the automotive company Renault, was intrigued by car

body design and found the ‘point-and-slope technique’

for curve-fitting rather crude and inconvenient for

accurate and precise curve design (i.e see Fig 248a)

Hence, Bezier’s philosophy was to find a way of

manip-ulating the individual parameters contained within the

curve’s basic equation, but in a more easy and in-direct

manner Bezier utilised an ‘open polygon’ (i.e a plane

figure of many angles and straight sides), by which a

curve that approximates to passing through the start

and end points of the open polygon: results in a

de-signer having the ability to change the polygon and as

such, achieving different results By having more

de-fined points in the polygon, this produces additional

flexible control for surface manipulation Further, the

curves generated are formed by equations comprised

of parameters raised to higher powers than that of the

cubic varieties, thereby having longer and more

com-plex mathematical expressions Such a curve, is a

dis-crete segment in a complex curve and these segments

must be joined together

In the Bezier ‘curve-fitting’ technique, the transition

between the curve segments, or ‘patches’ – the surface

equivalent to a line segment, requires close study by the

designer A further refinement, but not one developed

by Bezier although incorporating his mathematical

ex-pressions, was that of the ‘B-Splines’, which ensure

 ‘B-Splines’ , were originally introduced by Cops De Bore.

a smooth transition between segments/patches While yet another and improved refinement to the Bezier equations, was the development of non-uniform B-Splines – which could tolerate an uneven spacing of the nodes Terminology which is not usually perceived, but is associated with the term ‘NURBS’ , includes the

‘rational’ and ‘non-rational’ parametric surfaces So,

a ‘rational’ parametric surface may be represented in many forms, with mathematical precision While the cubic non-rational variety cannot express an 90° arc with mathematical precision, although it has adequate accuracy for machining requirements The amalga-mation of the two ‘curve-fitting’ approaches, namely, that of the ‘rational’ parametric surfaces together with

their ‘non-rational’ counterparts, results in Non-Uni-form Rational B-Splines – ‘NURBS’ Hence, ‘NURBS’

in its simplest form, is a data compression algorithm that reduces the data necessary to define curved sur-faces

In order to successfully utilise ‘NURBS’ impressive

‘curve-fitting’ abilities, the term ‘NURBS-interpolation’

was coined by Siemens Energy and Automation – when they first introduced its capabilities onto the market With its ability to reduce data in defining complex curves, ‘NURBS’ offers significant benefits, such as: ties

up less CNC memory producing shorter programs; al-lows higher feedrates to be exploited; produces shorter cycle-times; reduces tool vibrations – hence enhances tool wear rates; improves machined surface geometric definition and finishes; coupled to increased part pro-file accuracy and precision

Today’s CNC controllers have large memories with very high block processing speeds that can

ap-ply sophisticated ‘look-ahead capabilities’ that can scan

the anticipated programmed cutter path for abrupt changes So, these ‘real-time algorithms’ can not only

‘see’ the expected turns coming, but will slow down the feedrate to keep the cutter on its confirmed path and

avoid potentially inconvenient moments of ‘data-star-vation’ Moreover, even these enhanced CNC features

will struggle when a dense cluster of data points gen-erated by linear interpolation possibly causing block processing problems, having the affect of significantly reducing the feedrate as it ‘corners’ from each line seg-ment to the next Consequently, ‘NURBS’ tool paths will undoubtedly alleviate data starvation and feedrate troubles by being more efficient, but like point-to-point toolpaths (Fig 248b), they are not exact representa-tions of the surface The ‘NURBS’ toolpath must be calculated which involves some approximation –

simi-lar to the ‘chordal deviation parameter’ used in many

CAM systems (Fig 248c)

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-flowing curves in a

cutter path This was despite the fact that CAD/CAM

systems could mathematically define virtually any geometric shape with smooth curves These CAD/ CAM systems generated pristine forms which would

have to be converted into a recognisable

program-ming structure that the machine tool’s servo-drives

could understand and apply This ‘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-defined tolerance band (Fig 248a)

Thus, the length of each linear segment was governed

by the curvature of the profile 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 This technique is

acceptable in many respects, but its hardly very

effi-cient because complex 3-D surfaces need large

quan-tities of data to accurately represent their geometric

profiles This conflict between ‘CAD shape-defining

data’ to that of the machine tool’s motional

kinemat-ics necessary to produce the profile, means that

trans-mission rates and corresponding feedrates suffer, 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 specifically, 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 defined by non-linear motions, the

tool paths will have continuous transitions, enabling

significantly higher: acceleration; deceleration; plus

enhanced interpolation speeds; than was previously

 ‘NURBS’: The rational equation, can be expressed, as follows:

P (t) =

i=

�

n Ni,  (t) GiPi

i=

�n Ni,  (t) Gi

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

Ni,  (t) =����

����

�

(Ki � t � Ki + )

< Ki, Ki+ < t)

Ni, k (t) = ( t−Ki ) Ni, k − (t)

Ki + k − −Xi + (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 efficiently, they became immensely popular with CAD Software-developers, because it allowed Design Engineers more freedom to manipulate 3-D data, than had been available utilising simple ‘line-segments’ and ‘primitives’ The 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 This CAD/CAM integration, enabled these companies

to supply post-processors that supported all the major digital controller manufacturers offering a ‘NURBS-capability’

In order to more fully comprehend just how

‘NURBS’ works, it is worth a slight digression to briefly 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’  The alternative ‘B-Splines’  differ 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’ The resulting output from ‘NURBS’ is very efficient, 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 The ‘Non-Uniform’ term

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

which indicates the portion of a curve that is affected

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 affected by the ‘Ratio-nal’ part of the formula, means that the weight of the control points’ pull (weighting) – which can be speci-fied This ‘weighting’ allows conic sections to be repre-sented, without having to slice them up to determine their geometric aspect

 ‘Splines’ , can simply be defined as follows: As a series of

equally spaced control points which the computer connects to create a smooth flowing curve’.

 ‘B-Splines’ , may be defined in a slightly differing 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) 

When applying ‘NURBS’ to a complex part’s

curva-ture, it is important to recognise that it defines the entire

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

any curve geometry, utilising less data than for other

‘curve-fitting techniques’ Data transmission times are

significantly 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 defined 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 ‘weightmov-ing’ again and re-calculatmov-ing and so on,

in a recursive manner Thus, each point moved has

an influence on the others, but the more the control

points utilised, the less their influence 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 figure of 60% data-reduction, with an

as-sociated 40% improvement in time, has been claimed

Although the solution to virtually every curve-fitting

geometry 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-larly pitiable ‘curve-fitting routine’ , so as the old saying

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

time, these ‘NURBS’ will have even more refinements

added to enhance the already powerful ‘curve-fitting

processes’

9.8.4 Sculptured Surface Machining –

Cutter Simulation

Once the free-flowing 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 software 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 This specific

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

an interactive, graphic implementation of

‘drive-part-check’ surface machining, as defined in the:

Automati-cally Programmed Tool (APT) Standard This routine

is greatly enhanced when utilised in combination with

two other machining software 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 defining the inner and outer tool paths, allowing the system to then generate the intermediate stock-clearance tool path steps

A typical modular-package might offer: surface con-touring; parameter line machining; rough-to-depth; and zig-zag tool paths; having any design modifica-tions, or changes being automatically handled through

what is termed ‘associativity’ , thereby significantly

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 modifies 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 file’ (CLSF) After 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’ 0, this would have probably scrapped

the otherwise expensive stock of workpiece material, that has also added significant 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 fiscally important, it offers many other significant production benefits Therefore, with such enhanced cutter simulation, a range of important fea-tures can be addressed ‘off-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 offset from

0 ‘Surface gouging’ , is if a cutter unintentionally removes

mater-ial (gouges-out) a portion of surface

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

ma-chining mistakes can be avoided

.

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

curves and points ‘sets’ ,

• Addressing a ‘full-check’ surface capability, having

specified part clearances – for fixtures 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 specific holes and

islands

By utilising sophisticated cutter simulation packages,

the work is undertaken ‘off-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 typified 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 finish-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-influences on the part when it was heat-treated, this enables the part to be hard-part machined: roughed and finished, normally avoiding the subsequent grind-ing processes: surface and cylindrical – as necessary The question that could be asked concerning such a

machining application is: ‘What then defines hard-part

a machining process?’ Before answering this question

it is worth metaphorically ‘stepping-back’ somewhat,

to discuss what was considered as extremely ‘hard’1

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 HRC Returning to the question posed above, concerning what defines a hard-part, according to one major cutting tool company it is those materials with

a hardness of >42 to 68 HRC   – being the equivalent

of machining components from M2-HSS Previously, only through grinding operations, could the hardened component be produced to ‘toleranced-size’ , while

 ‘Hardness’ , can be defined as: The measure of a material’s

re-sistance to deformation by surface indentation, or abrasion’

(Source: Callister, Jr, et al., 2003)Thus, a ‘hard material’ can be considered, when large forces are necessary to cause a

perma-nent indentation [machining] marks (Source: Schaffer, et al.,

1999)

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

stress @ 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 (KIC)

@ 17 MN m–/; Izod impact strength (un-notched) @ 33.4 J (Source: Trent, 1984)

 ‘Hard-part machining’ , today is utilised widely as both a

roughing and finishing 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 HRC (Source: Huddle, 2002)

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

classified into two distinct groups:

1 Single-component materials – might typically

in-clude hardened tool steels, glasses such as Pyrex™

and borosilicate, ceramics including silicon carbide

(SiC) and aluminium oxide (AlO),

2 Composite 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 These groupings only list a few of the hard

workpiece materials currently available today, many

more exist, but they can still be classified within

these groupings at present

The selection of the cutting tool composition and its

associated geometry when hard-part machining, is

in-fluenced by the severe demands made by these

hard-ened workpieces The 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 difficulties 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 effects on tool’s edge,

• Insufficient stability and rigidity – created in the

‘machine-tool-workpiece’ loop stiffness

The 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 diffusion wear

The hardness of a part, should not be confused with its ‘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 finish these hardened components – with their hard-nesses ranging between 45 to ≈68 HRC – has signifi-cantly reduced The major time-saving is from the vir-tual elimination of finish-grinding operations, when

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

with matching dimensional tolerances Complex con-touring of a component’s profile using hard-part turn-ing operations is achievable (i.e see Fig 154 – top), which overcomes the previously expensive and time-consuming profiling operation of cylindrical grinding with custom-formed grinding wheels

However, successful hard-part turning is more than simply ‘chucking’ a hardened component (Fig 250a),

 ‘Yield, or permanent deformation’ , differs here, from that of

either: elastic deformation; or dislocation, slip, etc.; that may result during machining (Source: Hanson, 2005)

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 affect on the machine

tool’s structural rigidity and stiffness (Fig 250d) These

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

forces generated when similar operations are

under-taken on ‘softer 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-effect’ The application

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

edge and the part move relative to one another –

re-sulting in chatter The 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 difficult 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 firm 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 ‘clamping forces’ , nor

inherent rigidity The location of the turning insert’s

 ‘Three-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 flat disk, then having the

equi-spaced jaws offset 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 Therefore,

the force may not be adequate to provide sufficient force to

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

positional location, relative to the machine’s spindle bearings is significant The 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 flex 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 stiffness’ (i.e the ratio of applied force to that

of an associated displacement) that can be built into the turning centre The design objective is to increase

the machine tool’s ‘dynamic stiffness’ , which involves dampening the frequency of vibrations via specific

ap-plied technologies, such as ‘composite-filling’  the

ma-chine bases While a different approach to dampening vibration, involves the use of hydro-static linear ways, which ride on non-contact pressurised fluid 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 ‘soft-machinable jaws’ have been bored-out, to provide more circumferential location and support for the part.

 ‘Dynamic stiffness’ , can be defined as: The ratio of the applied

force to the displacement, occurring at the frequency of the ex-citing force’ (Source: Hardinge Inc., USA)

 ‘Composifilling’ – machine tool bases This 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-filled structures can be expensive, in order to alleviate some of this cost, ‘traditional’ castings have strategically-rein-forced composite-filled cavities (Source: Kennedy, 2004)

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

machined surface texture are acceptable [Courtesy of Sandvik Coromant]

.