Machining and Monitoring Strategies Part 3 doc

Non-Orthogonal Versus Orthogonal Machine Tool Designs The ‘Variax’s’ machine capabilities and benefits dif-fer significantly from those found on conventional slideway-based orthogonal ma

9.3

HSM – with Non-Orthogonal Machine

Tools and Robots

Variax/Hexapod – Design Concept

Non-orthogonal machine tools such as the one

simu-lated, designed and developed for HSM applications

is typically illustrated in various ways in Figs 223 to

225: utilising ‘virtual’ six axes kinematics (i.e namely:

X-, Y-, Z-, A-, B- and C-axes), therefore these axes

operate without having any ‘true’ slideways This

par-ticular kinematic concept has actuators that cross each

other forming X’s instead of meeting at apexes to form

triangles, as they occur in aircraft flight simulators,

which uses conceptually similar mechanisms – known

as Stewart platforms, these configurations being a

form of ‘parallel kinematic link mechanism’ (Fig 223)

To develop this new concept for a machine tool, the

manufacturer utilised computer-aided technology

which played a pivotal role in creating the structural

design (Fig 223) In particular, the application of a

totally three-dimensional design environment was

employed, utilising both finite element analysis (FEA)

in conjunction with kinematic analyses However, the

‘Variax’ design uses a range of uncomplicated, or

stan-dard mechanical components in its design While new

forms of motion actuators were discarded in favour of

conventional and well-proven ballscrew technology,

with its accompanying motor and drive machinery

Even the gimbals that secure the legs at the base and

the spindle carrier, are relatively simple devices

With the design of such a high thrust machine, a

significant problem to overcome was the connection

of the spindle to the six legs (Fig 223a) The answer

to the connection problem was a simple space frame

design, allowing all the forces to be either in tension,

or compression along the structural elements – similar

to a bridge design If one compared this ‘Variax design’

with that of a ‘plate-type design’ to secure the spindle

to the legs, then the former space frame concept

im-proves the mass-to-stiffness ratio by 275% While

an-other key development problem to be overcome was

that of how a spindle supported and driven by six

axes kinematically moves in space, moreover, was it

even mathematically possible to control the motional

members? For example and by way of illustration of

this complex mathematical/control problem: a simple

‘X-axis’ linear kinematic translation requires all six

legs to simultaneously move, but each leg will move at

different speeds, either accelerating, or decelerating at different rates through the whole ‘linear movement’ –

requiring very complex multi-axes mathematical solu-tions to achieve this action By employing a system of novel mathematical transformation runs in real-time

by the CNC’s multi-processor this mathematical trans-lation action was achieved, but from a programmer’s

viewpoint, conventional ‘word-address format’1 of

programming knowledge was all that was needed to successful operate the machine tool

Non-Orthogonal Versus Orthogonal Machine Tool Designs

The ‘Variax’s’ machine capabilities and benefits dif-fer significantly from those found on conventional slideway-based orthogonal machine tools So, on an orthogonal machine the slideways must be perfectly straight, parallel and at 90° to each other On these machines, an axis must have accuracy and precision control along the slideway having linear and rotary degrees of freedom carefully managed by the ground way being scraped to minimise any impending errors/ uncertainties As mentioned earlier, these orthogonal axes have kinematically 21 degrees of freedom, with: linear motion; rotational – i.e yaw, pitch and roll; plus

 ‘Word-address format’ of CNC programming, can be

consid-ered as: A system of coding instructions whereby each word in

a block is addressed by using one, or more alphabetic characters identifying the meaning of the word [Source: Valentino and

Goldenberg, et al 2000]

For example, some typical ‘G- and M-codes’ are:

G00 – rapid movement/ traverse of the tool (modal); G01 – linear interpolation (ie tool moved at a prescribed fee-drate) (modal);

G02 – circular interpolation clockwise – CW (modal); G03 – circular interpolation counter clockwise – CCW (modal);

G04 – programmed dwell (*non-modal);

G40 – Cancel cutter diameter compensation (modal); G41 – tool diameter cutter compensation (i.e radial-offset) on

left-hand side of workpiece (modal);

G42 – tool diameter cutter compensation (i.e radial-offset) on

right-hand side of workpiece (modal);

M00 – Program stop;

M02 – End of program;

M03 – spindle on (CW);

M04 – spindle on (CCW).

NB Many more codes/auxiliary functions exist, utilised in

‘word-address format’ programs.*Non–modal commands are

only active in that actual block [Source, Smith et al., 1993]

Figure 223 Basic kinematics of the non-orthogonal machine tool, with a simulated rendition of the

es-sential elements of the machine’s structure [Courtesy of Giddings & Lewis]

.

squareness errors to be considered, if the machine tool

is to perform within its intended specification In

ef-fect, with an orthogonal machine tool three axes are

controlled (i.e the X-, Y- and Z-axes), and the

rota-tional and squareness parameters must be ‘perfect’

Thus, these errors/uncertainties can be established by

laser-calibration, then compensated for up to a certain extent to create and ‘stable machine’ If for example, one considers the worst case scenario for a machine tool configuration, namely that of a horizontal ma-chining centre Here, if we just deal with one axis, then the spindle travels up and down the column’s

face – as it moves to various points on the workpiece’s

geometry The tooling is held here and in effect, is a

cantilevered beam (i.e the tool is normally only

sup-ported at one end only) In this situation the tooling

assembly can be equated to that of boring tooling, then

its inherent rigidity will decrease by the cube of the

stand-off distance from the ‘gauge line’1 When a

ma-chine tool builder calibrates this orthogonal-designed

machine tool, they will measure along the base of the

column – either optically (i.e by for example,

autocol-limation1 instrumentation), or with laser

interferom-etry, thus feeding-back any potential error sources for

CNC’s dynamic compensation, during execution of

the part program Nonetheless, the column can either

minutely: twist; bend; or even move; if its temperature

varies by only just a few °C In fact, it has been widely

reported that up to 70% of the total errors present on a

machine tool are thermally-related

On conventional machine tools – with slideways,

essentially one does not really know what is

occur-ring in all of the degrees of freedom at any

particu-lar time Although even if we did know the dynamic

status, unless some form of ‘adaptive compensation

system’ 0 was fitted to the machine, having some

so-phisticated multiple sensors positioned on and within

critical positions on the machine’s structure, then

oth-erwise, it would be almost impossible to compensate

 ‘Gauge line’ of the tool, is the distance from where its effective

length is taken, specifically when the tool length

compensa-tion distance is needed for the CNC programming details

Normally, this ‘gauge line’ is datumed from a known point on

the male spindle taper, located in, and standing slightly proud

of its female counterpart This measurement is either

prefer-ably set in a tool presetting machine, or it can be obtained on

the machine tool by some form of table-mounted

touch-trig-ger cube Alternatively, the tool’s length can also be accurately

measured by some form of laser-activated instrumentation,

strategically-positioned at a suitable part of the machine tool’s

structure – where it will not interfere with any subsequent

machining operations

 ‘Autocollimation’ was the ‘traditional’ technique that was

em-ployed for any form of machine tool calibration

Autocollima-tion instruments and equipment, utilise highly-evolved

ac-curate and precision optical apparatus to measure alignments

and squarenesses of the machine tool’s axes, which can then

be utilised to compensate for any error sources detected in the

machine tool’s kinematics [Sources: Taylor Hobson, 1984/

Spectrum Metrology, Leicester (UK)]

0 ‘Adaptive error compensation’ , basically utilises sophisticated

geometric algorithms that allow for compensation of the

geo-metrical elements, via a range of strategically-positioned

sen-sors on orthogonal machine tools (Source: Ford, 1993)

for such error sources – as they occur Clearly then, it

is no coincidence that both the ‘Variax’ operates with six legs and, it also has six degrees of freedom These leg orientations have three pairs of two legs that cross each other and which are secured, but free to swivel

at either end (i.e see the simulated renditions in Fig

223 and partial assembly of an actual ‘Variax’ machine

tool in Fig 224) With these six legs, all of the degrees

of freedom are controlled, which in turn, eradicates the usual sources of errors exhibited by conventional multiple and ‘stacked’ slideway axes Some of the ma-jor benefits obtained from the ‘Variax’s’ design configu-ration are:

• Extremely rigid machine tool – coupled to small

mass making it 500% more rigid than a conven-tional machine, due to the fact that all forces whether they are in compression, or tension are fed through the six legs and its associated space frame,

• Exceedingly fast in operation – as all the ballscrews

must move together, they only have a light mass to contend with, making it up to 5 times faster than an orthogonal machine tool,

• Very high continuous thrust – due to concurrent

and synchronised use of the six ballscrews,

• High accuracy and precision – this is due to the

machine’s inherent rigidity, coupled to the fact that all six degrees of freedom are controlled, with laser feedback through the centre of the ballscrews1,

• No supporting structure – the ‘Variax’ is

self-con-tained, so it does not need foundations The ma-chine has a three-point location and the weight

of the spindle head is neutralised, as far as the

ballscews are concerned – by the three large

gas-spring supports (i.e see Figs 224 and 225a)

 ‘Laser-controlled feedback’ , through the centre of the

ballscrews, negates the Abbé offeset error, with any errors in the legs (i.e axes) being averaged, rather than ‘stacked-up’ as

is the situation in conventional machine tools Hence, volu-metric accuracies are in line with that of a Co-ordinate Meas-uring Machine CMM – see details in Fig 223a.

 ‘Gas-spring supports’ , are fitted outside the actuator platform

(i.e see Figs 224 and 225a), they are strategically positioned

so that they carry actuator platform’s gravity-induced load Thus, these gas-spring support placements and their operation, means that the actuators have to only overcome inertia and momentum to move the machining head This has the kinematic benefit of enabling the machine spindle to move through space faster than any single actuator changes its length, this combination allows the ‘Variax’ to move and accelerate up to five times faster than a conventional machine tool.

NB The machine tool also has large rubberised

compliance mountings, so when it is in operation

it seems to ‘visually-bounce around’ , but this is

something of an optical illusion, as the overall rigid

integrity of the ‘Variax’ is maintained

Variax/Hexapod – Specification

and Machining Performance

The ‘Variax’ is equipped with a 630 mm square pallet

and can both move and cut at 66 m min–1 (i.e see Fig

225b), accelerating at >1g while providing >3 tonnes

of continuous thrust in the Z-axis The machine tool is

compliant with Standard: ASME B5.54, obtaining

ac-curacies of <10 µm, while its rigidity is >175 Nm µm–1

Pallet change time is 10 seconds, with a tool change

time of 6 seconds (chip-to-chip), having a tool

stor-age capacity of 50 tools (i.e expandable) The standard tool spindle cartridge is rated at 22 kW , with spindle speed ranging from 100 to 16,000 rev min–1, having an angle of tool spindle inclination of up to 25°

In demonstrations and in-house trials at the man-ufacturer’s premises, the initial prototype ‘Variax’ ma-chine was operated and machining components for three years On one large aerospace machining appli-cation of a critical component, the original cycle-time was 19.3 hours on a conventional machining centre, but when this same part was cut on the ‘Variax’ , it took just 3 hours to complete, with the additional benefit of being both more accurate and precise Equally, when

a smaller aerospace component – a landing bracket – was originally machined it took 1.65 hours on the con-ventional machine tool, but when the same part was placed on the ‘Variax’ , it took only 0.55 hours cycle-time to complete

Figure 224 The Variax/

Hexapod Machining Cen-tre here seen during final assembly at Nottingham University, equipped with

an up-rated high-fre-quency spindle: 40,000 rev min–1 @ 40 kW power-rat-ing, with a ski taper.

NB Laser transducers –

giving an accurate and precise control, can clearly

be seen located on each

of the actuator legs [Courtesy of Giddings & Lewis/The University of Nottingham (UK)]

Figure 225 Variax/Hexapod offers a unique and rigid design solution, with many production benefits

[Cour-tesy of Giddings & Lewis]

.

The capital outlay for a ‘Variax/Hexapod’ machine

tool, costs about the same as a similar specification –

component size capacity, to that of a conventional

five-axis machining centre, but with the above additional

performance benefits

Robotic Machining

Robotic machining applications have been utilised for

some years, currently up to a thousand such

installa-tions are to be found world-wide Probably the biggest

user of robotic machining is the aerospace industries,

although the automotive industry is catching up fast

Most of the current research work into robot

machin-ing is undertaken with an anthropometric type of

robot configuration, usually having either five, or six

axes (i.e see Fig 226)

If a six axis robot is employed (Fig 226), it gives

several benefits over say, a five-axis machining centre,

with probably the greatest production advantage being

access to the workpiece’s surface features (Fig 226b)

This extra degree of freedom, allows the alternative

wrist positions to achieve identical tool positional

orientation – regardless of the workpiece contour,

enabling the robot axis the ability to rotate the wrist

about the tool’s axis Moreover, where speed and

ac-celeration are important, robots normally out-perform

the more traditional machine tool structures, mainly

due to both their low weight and minimal inertial

ef-fects Within the robot’s programming language,

pa-rameters exist that allow a balance to be made between

robotic arm speed and accuracy So, when

roughing-out higher speeds can be employed – at the expense

of accuracy, then the parameters can be changed to a

slower speed, but with greater finished machined part

accuracies

Limitations also occur when using robots for

ma-chining, with perhaps the most obvious one being

their intrinsic lack of rigidity, when compared to that

of virtually any machine tool If a robot is employed

at conventional spindle speeds the tool’s cutting forces

are simply too great, creating both vibrations and

de-flections in the robotic arm, which badly impacts on

the component’s machined surface texture and

di-mensional accuracy In order to mitigate against any

cutting force and accuracy deficiencies, it is essential

to utilise HSM spindles, in combination with taking

small DOC’s, to minimise these effects

The robot’s kinematic structural complexity also

causes positional problems, due to innate

manufactur-ing tolerance effects to the location of the joints in the

robotic arm This dimensional and geometric toler-ancing build-up, means that the mathematical model used to control individual joint positions in space, will minutely differ from the actual reality to that of the joint placements This positional difference being most apparent and exaggerated around the perimeter

of the robot’s working envelope Often, it is impossible

to calibrate the mathematical model for these com-plex inter-related errors, as the overall comcom-plexity of robot’s control algorithms then become such that they cannot execute the kinematic motions fast enough to dynamically control the robot at the required axis tra-jectories

In calibration trials on a typical six-axis anthropo-metric robot undertaken by Young (1999) while work-ing in cartesian co-ordinates for the robot’s positional accuracy – when assessed with laser interferometer instrumentation, the results produced linear errors of

± 0.8 mm in each cartesian co-ordinate (i.e X-, Y- and Z-axes) – across the complete working range The er-ror curves produced were in fact, symptomatic of the kinematic structure of these robots Characteristics included a decrease in backlash to almost zero – to-ward the perimeter of the working envelope (i.e due

to gravitational effects), while a combination of linear and sinusoidal effects combined to produce the total error Even though these robot errors are large with respect to those found on a machine tool, they should not deter robotic usage for suitable HSM applications The repeatability shown by most robots is usually far superior to that of its accuracy

In practice, any multiple axis robotic arm utilised for sculptured machining applications, needs to have

their axes biased and offset in order to eliminate the

‘paradox’ that might overwhelm them when all axes are attempting to keep the end-effector (i.e tool) nor-mal to the contoured surface (Fig 226b) For example,

on a three-roll wrist (Fig 226), the combination of the kinematic linear and rotational build-up, may result

in instead of one of these axes angularly moving just 1°, it causes it to actually move 359° instead – thereby scrapping the workpiece in the process! To alleviate this problem, if the workstation/stand, is offset to one side and angled, this offset and compound angle will minimise the axis predicament that can afflict the ro-bot’s subsequent programmed motions The worksta-tion posiworksta-tion in its ‘known space’ with respect to the robot axes datums must be known and accurately cali-brated, thus ensuring that the angled grid-plate for re-sulting workpiece fixturing is both ‘fixed and qualified’

By slowing down the robotic axis trajectories in the

fi-Figure 226 Robotic high-speed machining with a multi-axis anthropometric robot, equipped with HSM

milling head [Courtesy of Southampton Solent University/Westwind Air Bearing Ltd./Smith, G.T.]

.

nal machining passes over a sculptured workpiece

sur-face, a certain degree of accuracy can be achieved, but

even here, it does not approach that of any moderately

accurate and precise machine tool Tool centre-point

programming is the preferred option, as when utilising

cartesian co-ordinates, the programmed points

repre-sent the cutter’s end – once the required offset has been

established This tool centre-point program allows the

programmer to effectively disregard the tool’s length

in any subsequent machining applications However,

care must be taken with any flexible coupling

connec-tions, such as pneumatics, water, or electrical wiring

to the HSM spindle – allowing sufficient slack in the

piping (i.e see Fig 226), thus ensuring that the robot

will not inadvertently de-couple these services, as it

at-tempts to manufacture the complex part

9.4 HSM – Toolholders/

Chucks

Introduction

Rotational speeds for tooling assemblies subjected to

HSM applications must of necessity be very high, this

can create several problems for any tooling utilised in

such machining activities Notably, due to the

centrif-ugal force the toolholder could swell and slacken its

gripping force on the tool’s shank In an extreme

situa-tion due to the applicasitua-tion of cutting forces this might

cause the tool to speedily exit from its holder, in so

do-ing either scrappdo-ing the workpiece, or become a severe

safety hazard to any operator in attendance at the

ma-chine tool Therefore, both a good fit and connection

is an essential requirement of any HSM machining

ap-plication Thus, the mechanical interface between the

toolholder and the machine’s spindle, together with

that of the tool’s shank and its respective toolholder

are the prerequisites for a successful HSM application

9.4.1 Toolshank Design

and Gripping Pressures

Tool and Sleeve – Mechanical Interface

The latter point briefly mentioned in the previous

section concerning the tool shank’s fitment in the

holder, is an important criterion in any HSM

applica-tion Nonetheless, of greater interest and note, is the actual gripping pressure exerted at the toolholder’s mechanical interface with its mating tool shank Some interesting toolholder designs have been attempted to increase the gripping pressure here, with the level ex-erted at the free-end of the holder not being too great

a problem, due to elastic compliance of the sleeve in this region The notable difficulty arises as the shank

is being gripped toward the flange end of this sleeve, where with most conventional toolholder sleeve de-signs, the gripping pressure drops-off significantly at this locality At high rotational speed and under the application of the cutting forces, the cutter will tend

to become unstable and present a distinct ‘wobbling motion’ due to the lack of gripping pressure and sup-port here, being exacerbated by the higher imparted centrifugal forces In order to try to alleviate this lack

of grip problem, significant design effort has been expended over the years to attempt to increase elas-tic behaviour and hence amplify gripping pressure at this sleeve’s region In the graph shown in Fig 227b, the elastic behaviour and its associated contraction are plotted from the tool sleeve’s free-end in linear steps along the sleeve and toward the flange In most prior designs the sleeve contraction on the tool’s shank near the flange was approximately 50 µm (i.e shown by plot

‘B’) By introducing a radially-plunged undercut to the sleeve’s outside diameter at the juncture of the flange and sleeve (i.e see Fig 227a), this creates significantly increased elastic behaviour and hence improved grip-ping pressure at this region of the sleeve, as illustrated

by the plotted relationship of the test results – produc-ing a sleeve contraction of 145 µm at the flange, as in-dicated by ‘A’ , in Fig 227b This vastly improved grip-ping pressure with the ‘undercut flange’ is of the order

≈150%, when compared to other design techniques, which restricts any attempt at the tool’s ‘wobbling be-haviour’ as a result of the improved elastic contraction

on the shank

In order to obtain a good overall elastic gripping pressure along the length of the sleeve, multiple hard-ened needle rollers are positioned – at a slight angular inclination, both around and along the male shallow-angled tapered sleeve’s periphery (i.e items 2 and 4 in Fig 227a) As the female tapered sleeve (item 3) is ro-tated, the caged inclined needle rollers rotate and be-gin to steadily move up the male tapered sleeve toward the flange (item 1) and, in so doing, elastically com-press the sleeve (item 2), which in turn will tighten

on a tool’s shank As the tapered female sleeve (item 3) is fully tightened with its ‘C-spanner’ , it will

com-Figure 227 High-speed milling toolholders (chucks), holding cutter’s shank along its whole gripping length [Courtesy of

Di-ashowa Tooling]

.

press its face against the rubberised contact seal and

the flange (item 1), thereby acting as both a vibration

damper and sealing against particle/debris ingress into

the mechanism The front face already being sealed

against such potential particle ingress (i.e see Fig 228

sectional detail diagram) In order to increase the

elas-tic behaviour of the male tapered sleeve (item 2 – Fig

227) and improve its gripping pressure here, the end of

the sleeve is partially slit along its length and around

it at eight equally-spaced positions (i.e see Fig 228)

This mechanical design solution for the tightening of

the male tapered sleeve, gives possibly the optimum

gripping pressure for such a mechanical interface on

the tool’s parallel shank

Tool and Sleeve – Hydraulic Designs

For an alternative tool-gripping pressure design

con-cept, the hydraulic toolholder (i.e see Fig 228 – top

right – for a section through the tool sleeve), offers an

ideal alternative to the mechanical tool interface

previ-ously mentioned above Here, the hydraulic toolholder

manufacturer guarantees a 0.005 mm

tolerance-in-roundness (TIR) at 100 mm from the front face, with

<0.0013 mm repeatability for such hydraulic

toolhold-ers The normal contraction for a φ25 mm toolholder is

<0.13mm, although the contraction is proportionally

higher for larger diameters, conversely, it will be lower

for smaller diameters The benefits of utilising

hydrau-lic toolholders are that they provide both rigidity and

balance, while holding the tool’s shank axially-straight

along its own centreline It is also claimed that

hydrau-lic toolholders produce less vibrations resulting in

im-proved machined surface textures, with a possibility of

less chatter This latter benefit it is claimed to be the

result of the hydraulic fluid in the toolholder acting as

a natural vibration damper and impact cushion

Hydraulic toolholders are balanced to ISO/ANSI

Standards, to G2.5 at 15,000 rev min–1 in practice, but

are claimed to perform successfully in spindles

rotat-ing at 50,000 rev min–1 Moreover, other benefits are

made that tool presetting is much quicker to achieve

than mechanical designed toolholder assemblies, as it

is stated that due to the hydraulics having an automatic

centring action, only the tool’s length needs to be

pre-set Further, as there are fewer moving parts, these

toolholders need little maintenance and the sleeve and

piston should have a fatigue life of >100,000 cycles,

prior to servicing, this ‘in-service life’ representing

many years of practical usage

The disadvantages to hydraulic toolholders are few, but may prove a significant obstacle to their introduc-tion, including the fact that their purchase costs are

higher than their mechanical counterparts, with the

other limitation being a new hydraulic sleeve is

re-quired for different tool shank diameters However, this latter point can be mitigated against, by conduct-ing a programme of: rationalisation; consolidation; and optimisation on the various production require-ments for toolholder varieties and shank diameters (i.e see Chapter 1, Sections 1.1.1 to 1.1.3 for details of such tool survey)

Tool and Sleeve – Thermal and Cryogenic Designs

Shrink-fit systems require specially made tool hold-ers, being designed for a specific tool shank diameter, although they can accommodate any style of shank Once in-situ the tool in its thermal sleeve behaves in

a very similar way to that of a one-piece tool The high gripping pressure coupled to excellent concentric-ity (i.e <5 µm), allows these toolholders to increase speeds and feeds by >20%, when compared to most

of their mechanically-designed counterparts Due to greater rotating concentricity of shrink-fit toolholders, there is a better wear pattern developed on the milling cutter’s teeth, or drill’s lips, etc., which it is claimed, increases tool life by >30% over conventional holders In a similar fashion to that of hydraulic tool-holders, thermal contraction occurs both around the periphery and along the whole mechanical interface, which will automatically centre the tool’s shank within its mating bore This complete toolholder-to-tool fit-ment, minimises centrifugal force when operating in

an HSM mode Thus, the contraction of the toolholder rigidly locks the tool in-situ, this gripping pressure is

at least 500% greater than for conventional toolhold-ers In fact the pressure exerted here, is even greater than that of the pull-stud (i.e retention knob), mean-ing that in the presence of high forces, the whole as-sembly would be pulled out of the spindle before the tool would be released by its mating holder

The significant component of a shrink-fit toolhold-ing system is the induction heattoolhold-ing unit, as schemati-cally depicted in Fig 229a Typischemati-cally, the solid-state and self-contained unit is relatively compact and in operation to change tools, the user positions the tool holder in a receptacle built into a shelf at the front of the unit An induction coil is located beneath the shelf and encircles the toolholder’s sleeve (i.e collar), which