Machining and Monitoring Strategies Part 6 ppt

In order to achieve the desired 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 m

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

Wyatt & Berry, 2003]

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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

desired 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 Yousefi

and Ichida (2000), they utilised rotational speeds up

to 15,000 m min–1 by this technique, showing that the

turned surface texture parameter ‘Ra’ values (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

to 15,000 m min–1 This 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 significant 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

effects which would otherwise impede both the cutting

process and affect 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

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

machine tool error diagnostics The Ballbar as its name

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

measur-ing element, is positioned between the fixed and telescopmeasur-ing

balls) This 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 software

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

position (i.e radially offset the required distance for the

orien-set at the radial turning distance in the plane of the cut (i.e see the Ballbar configuration 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:

Sn1 → Sn = 2000 + {n1/2 [2a1 + (n1 – 1) d1 ]} – 1000

+ {n/2 [2a + (n – 1) d ]} – 500 + {n/2 [2a +

(n – 1) d]} – 250 + {n/2 [2a + (n – 1) d]} – 125 + …

Snn

Where:

a1 = 2,000, a = 4,000, a = 5,000, a = 5,500;

d1 = 1,000, d = 500, d = 250, d = 125;

n1 = 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 significant 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 rotaconven-tional turning process at the beginning of the turning process

A special-purpose workholding device – being dual-plane balanced to G2.5 @ 10,000 rev min–1, was fitted into the spindle of a vertical machining centre (Fig 237) This BT40 tapered workpiece holder was constructed from one-piece of EN24T steel hardened

by nitriding to >50 HRC, 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, finally rotating another 180° – to slow down This complete cycle is then repeated CCW Then polar plots are generated with a diagnostic printout, which ‘ranks’ these errors, so that they can then be eliminated,

or significantly reduced, accordingly This is a speedy, efficient diagnostic ‘health-check’ of the machine tool errors in the two measured planes, providing significant 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.

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Figure 238 A ‘variable quasi-pilgrim stepped arithmetic progression’ – being utilised for UHSM (turning)

[Source: Smith, Littlefair, Wyatt & Berry, 2003]

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situ These pre-shaped testpiece disks were: φ300 mm

by 6 mm thick, made from aluminium 2017F The tool

with various cemented carbide tooling insert grades,

was held on a platform dynamometer (Kistler model:

9257B) – having complemntary charge amplifiers

coupled to suitable data analysis software A range of

DOC’s were utilised: 0.5, 1.0 and 1.5 mm, with a

con-stant and rapid feedrate of 30 m min–1 Apart from

cut-ting force analysis, turned surface texture, harmonic

roundness and micro-hardness results were obtained,

together with metallographical inspection of the

sub-surface regions Hence, from this testpiece setup and

utilising the ‘progression’ for peripheral workpiece

speed strategy described above, the speeds ranged

from the conventional, through to UHSM

UHSM: Turning Trends

Unlike the previous findings of Yousefi and Ichida

(2000), where they suggested that the cutting forces

remained relatively constant across a broad spectrum

of UHSM – for turning operations This UHSM

turn-ing work indicated that there was a decrease in mean

cutting forces between 2,000 to 6,000 m min–1, with a

corresponding improvement in turned surface texture

(i.e ‘Ra’) across this range The harmonic

departures-from-roundness were influenced by the sinusoidal

ef-fect of the fluctuating tangential force as it progressed

around and along the turned surface’s periphery This

harmonic behaviour was evident in the cutting force

data, where the analysis software showed both a rising

and falling relationship, as the turning insert passed

over the rotating workpiece’s surface at great peripheral

speed Such cutting force traces occur in high-speed

interpolation by the milling process, where there is a

general undulating increase/decrease in force

genera-tion, this being related to the axis transition cross-over

during cutter interpolation around the workpiece (i.e

see Fig 159) This cutting force undulation is the result

of the machine tool’s servo-motors reversing

direc-tion at these transidirec-tions, albeit, at significantly slower

speeds than utilised for this UHSM turning work

At such high turning speeds, chip-streaming was

apparent at peripheral speeds >4,000 m min–1

Chip-streaming in UHSM by turning is the preferred

chip-form, as it exhausts the work-hardened swarf away

from the cutting vicinity, thereby minimising

entan-glement around the newly-formed turned workpiece

surface At such high turning peripheral velocities,

the chip-streamed swarf is directed radially-away

from the work surface Conversely, at lower rotational workpiece-to-insert velocities, there was a marked tendency for ‘chip-curling’ As a result of the influence

of the insert’s geometry: nose radius and DOC

relation-ship, the so-called ‘theta-effect’ in conjunction with the

feed-per-revolution occurs (i.e see Figs 34c and d) There is a direct and predictable relationship to chip-curl tendency when certain conditions arise at the lower peripheral turning speeds, which is not apparent

at the UHSM turning range

This UHSM turning applied research work, has shown that it is feasible to employ ultra-fast turning practices to the relevant components if the correct tooling, workpiece and machine tool relationships can

be met

9.6.3 Ultra-High Speed:

Trepanning Operations

Intoduction

Trepanning has been a well-recognised production process for many years, it is principally utilised to pro-duce large hole diameters, since this technique does not require as much spindle power as solid drilling Moreover, in the ‘conventional’ approach to trepan-ning, it is undertaken in one operation, but instead of all the workpiece material being removed in the form of

a large volume of swarf, a cylindrically-shaped core is left behind at the centre of the hole Thus, this method must be utilised for through-hole applications, assum-ing that the internal feature – hole manufactured is the scrap material Conversely, in the UHSM trepanning work shortly to be discussed, the two cutting edges are externally set against the workpiece’s periphery, mak-ing the ‘slug’ the product of the machinmak-ing operations (Fig 239c)

UHSM – Trepanning Fixture Design

As in the case of vertical turning, UHSM by trepan-ning was undertaken on a vertical machitrepan-ning centre utilising the same workholding arrangement (Fig 239) Here, a special-purpose trepanning fixture -

600 mm in overall length, was designed and manufac-tured with twin-opposing tools (Fig 239a) The tool-ing was conventional TiN-coated cemented carbide turning inserts – having straight toolholders these being positioned on their sides in opposing directions

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Figure 239 An ultra-high-speed trepanning fixture and dual-plane balanced workpiece holder: utilised for an UHSM research

programme of work [Source: Smith, Hills & Littlefair, 2005]

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(Fig 239b) So, by the simple action of turning a hand

wheel at its end, the tools could be simultaneously

opened and closed – for the required trepanned

diam-eter This simultaneous tooling action was achieved,

by the singular rotating action of both the φ20mm by

4 mm pitch left- and right-hand (i.e M 20 × 4)

square-threaded leadscrews (Fig 239c)

One of the major advantages of an UHSM

trepan-ning operation over its equivalent turtrepan-ning counterpart,

is that the cutting forces are virtually ‘cancelled-out’ , in

a similar fashion to a conventional ‘balanced turning’

operation (Figs 41 and 238 – top right) Here in this

instance, one tool is set and positioned slightly ahead

of the other, thereby not only reducing the overall DOC,

but allowing the ‘trailing tool edge’ to effectively act as

a ‘finishing tool’ This tooling positioning strategy

pro-duced an improved trepanned surface texture, while it

significantly reduced the harmonic

departures-from-roundness, as metrologically assessed later on the

roundness testing machine Moreover, by effectively

‘halving’ the DOC, this allowed for an improvement in

the chip-streaming behaviour to be attained

In a later modification to the trepanning fixture (i.e

not shown), a large micrometer drum with its

inte-grated vernier scale was fitted in place of the knurled

adjustable hand-wheel (i.e see Fig 239a), allowing for

some considerable discretion over the linear tooling’s

diametral adjustment With such a large trepanning

fixture – having the opposing tooling widely-spaced, it

is vital that these tools are centralised directly beneath

the machine’s spindle Otherwise, there is a possibility

of both sine and cosine errors being present, creating

‘Abbé-type errors’ , when adjusting and setting these

tools for their diametral in-feed

UHSM – Trepanning Operation

This preliminary work on UHSM by trepanning, has

shown that with a suitably robust tooling fixturing and

allowing a large (indirect) range of tooling diameter

adjustment – via the twin leadscrews, then not only is

the process feasible, but it offers considerably improved

machining performance and an inherent improvement

in trepanned surface and roundness characteristics,

over vertical turning processes Possibly in a later

modification to a heavily-revised tooling adjustment

system, it might be possible to employ twin coaxial

ballscrews, with CNC servo-control, allowing

auto-matic control for machining tapers and profiling to the

workpiece – by utilising the supplementary rotary axis

control in the machine’s CNC controller Moreover,

one limitation to this UHSM trepanning technique is

the length of longitudinal cut that can be taken, prior

to the Z-axis motion causing the rotating part to foul

on the central portion of the trepanning fixture This problem can be mitigated against, by increasing the relative stand-off height of the twin-tooling from the top of the fixture by mounting each toolholder in an extended tool block, so allowing greater Z-axis feeding

to be undertaken Moreover by rearranging the tools

in relation to the workpiece, it would be possible to

‘turn’ shallow, depth internal trepanned features UHSM by trepanning offers significant advantages over ‘conventional’ vertical turning, in that, in this cur-rent work, if was found that the trepanned workpiece surface and roundness were significantly improved from the previously discussed UHSM by vertical turn-ing, described in Section 9.6.2

9.6.4 Artefact Stereometry:

for Dynamic Machine Tool Comparative Assessments

Introduction

The use of machinable artefacts for the assessment of machine tools such as machining centres, has been utilised for some of years (i.e typically: NAS Stan-dard 979: 1969; ISO StanStan-dard: 10791-7: 1997; Knapp, 1997), being developed just for this purpose Both the NAS and ISO Standard testpieces incorporated nota-ble prismatic and rotational characteristics, manu-factured to specific geometric and dimensional toler-ances, such as: at the top, an φ110 mm circular feature;

6 mm below this round shape, an 110 mm diagonal feature is cut; a central φ30 mm though-running hole

is produced; with a series of counter-bored holes at four equi-spaced quadrants are generated these be-ing situated 6 mm below the diagonal shape Taken in cross-section, the geometry of the machinable arte-facts resembles a stepped component, having an over-all height of 50 mm In fact, this type of artefact has long been employed by industry to establish the over-all machining performance capabilities of a particu-lar machine tool under test However, although this prismatic and rotational featured machinable artefact achieves some measure of conformance and indicates the likely operational performance of the machine tool, it does tend to have several significant limita-tions, such as the:

• Overall dimensional size of the artefact is quite small – when compared to that of the volumetric envelope of typical industrial machining centres,

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Figure 240 Artefact stereometry, illustrating its integrated volume geometries, for a:

1 (right) conic frustum,

2 (right) cylinder,

3 rectangular volume of machine tool’s axes.

[Source: Smith, Sims, Hope & Gull, 2001]

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• Circular feature cannot be directly compared to

that of diagnostic instrumentation – such as the

Ballbar, as the diameter of this rotational feature

differs from that of the standard Ballbar sizes,

• Weight of the artefact does not realistically

com-pare to any workspaces normally placed on the

ma-chine tool in its ‘loaded-state’ , meaning that ‘true’

machine tool loading-conditions are not directly

comparable

With these machinable testpiece limitations in mind,

it was thought worthwhile developing a new

calibra-tion strategy for such machine tools, but here, under

more realistic ‘loaded conditions’ , also this new

arte-fact being more directly comparable to diagnostic

instrumentation (i.e such as the Ballbar), but having

considerably larger volumetric size and weight, with

the capacity for reuse of the expensively-produced

precision part of the machinable artefact’s assembly

Stereometric Artefact – Conceptual Design

Stereometry has been a concept that has often been

over-looked, but it deals with the volumetric content of

a range of geometric shapes However, if this

‘volumet-ric concept’ is carefully integrated into a single artefact,

it could be employed for calibration work on machine

tools such as machining centres (i.e see Fig 240) Here,

the cylinder was represented by three machinable

aero-space aluminium disks (grade: 2017F – produced from

6 mm sheet, to nominally slightly >φ300 mm) each one

being set 100 mm apart in height (i.e disks: 1, 2 and 3)

and after machining, the disks were exactly φ300 mm

(i.e see Fig 241) The conic frustum included angle

was 22.5°, this being the result of producing 4

equi-spaced holes in each disk Starting on the bottom (disk

1), then stopping the machine and fitting the middle

disk (disk 2) and drilling the 4 holes and likewise

up-ward to the top disk (disk 3), while simultaneously

producing a 3-dimensional Isosceles triangle (Fig

241) Each disk had these individual holes being set

at an angular relationship of 90° equi-spaced apart, so,

when they are taken as a ‘volume’ , a conic frustum is

produced (Fig 242b) These geometric and

volumet- ‘Isosceles triangle’ , has two sides with two angles being equal,

but in this case, with the geometry of a right-angled triangle

NB These side lengths and associated angles can be varied,

so long as they both (i.e lengths, or angles) remain of identical

proportions.

ric relationships were intrinsically set and datumed to

a centrally-machined slot in the base of the precision

mandrel This fact, meant that the exact angular and

volumetric relationships remained in-situ, when the stereometric artefact was then taken off the machine tool for subsequent analyses

Stereometric Artefact – Machining Trials

Prior to the stereometric artefact having its machin-able disks milled, the initial test machine tool (i.e in the initial trials on a Cincinnati Milacron Sabre 500 equipped with a Fanuc OM CNC controller) was fully diagnostically calibrated by: Laser interferometry; long-term dynamic thermal monitioring of its duty-cycles in both a loaded and unloaded condition; to-gether with Ballbar assessment Prior to discussing the actual machining of the disks, it is worth taking a few moments to consider the precision mandrel that accurately and precisely locates each disk in the de-sired orientation, with respect to each other and the machine tool’s axes This mandrel body was produced from a eutectic steel (0.83% carbon), which after through-hardening to 54 HRC, was precision

cylindri- ‘Eutectic steel’ or ‘Silver-steel’ as it is generally known, due to

its almost ‘shiny appearance’ when compared to other grades

of plain carbon steels In brief, this 0.83% carbon content steel

is so-called a eutectic* steel as it relates to the eutectic com-position derived from the iron-carbon thermal equilibrium

diagram Producing an 100% pearlitic structure (i.e hence its

‘metallographic-brilliance’ , or its ‘irridescence’) when viewed under a microscope, exhibiting fine alternate layers of: FeC and Fe To harden eutectic steel, its temperature is raised slightly above the ‘arrest point’ (i.e arrest point here, equals 723°C, so hardening could be undertaken at ≈765°C) into

‘γ-solid solution’ (i.e austenitic region), then rapidly quenched

and agitated in water to prevent carbon atomic diffusion (i.e

undertaken at greater than the ‘critical cooling velocity’), with

the carbon atoms now being effectively ‘fixed’ – though not

intrinsically part – of the atomic lattice structure This carbon

entrapment, creates intense local strains that block dislocation movement Hence, the resulting structure is both hard and

ex-tremely strong, but also very brittle Microscopically, the

hard-ened structure appears as an array of random needles, being completely different from the original pearlitic structure This needle-like structure formed by trapped carbon atoms in an

iron crystal lattice is termed, ‘martensite’ Thus, the degree of

hardness – after quenching, being proportional to its lattice

strain After hardening, the mandrel needed to be tempered

Tempering is a controlled heat-treatment process to allow some of the trapped carbon to escape from the interstitial spaces between the iron atoms distorted lattice structure,

where they eventually form particles of cementite

488 Chapter 9

cally-ground on the three register diameters, with the

top and bottom faces being surface ground Previous to

this heat-treatment and the grinding processes,

dow-elling datums (i.e φ6 mm) were drilled and reamed,

then 3 equi-spaced tapped clamping holes were

pro-duced for each disk, along with a ground tenon groove

in the base – all these features being orientated to the

geometry of the machines axes (Fig 241)

Several unique features are introduced within the

machinable portions of the disks, such as:

• These aerospace-grade aluminium disks were

milled to φ300 mm diameter, which directly

cor-responded to the radial path of the Ballbar (i.e see

Fig 242a) – used previously for diagnostic machine

tool assessment, ensuring that some degree of

cor-relation occurred between them,

• The three Z-plane disk heights of: 70, 170 and

270 mm (i.e modified from the original design Fig

241), coincided with both the X-Y plane table

po-sition and vertical heights utilised for the Ballbar

plots, creating a reasonably large cylindrical

volu-metric envelope (Fig 242b) Moreover, the

stereo-metric artefact was both designed and orientated to

coincide with the start and finish positions of the

Ballbar’s polar traces,

• The 4 circular interpolated holes (φ10 mm) on each

disk (i.e see Fig 241), were geometrically

posi-tioned to form a three-dimensional Isosceles

tri-angle at the three Z-axis heights for each quadrant

of these disks – with the 1st an 3rd holes relating to

the axes transition points in the X-Y planes Thus,

each of the interpolated milled holes in the face of

separate disk’s, produced the geometric

stereom-etry of a conic frustum, having an included angle of

22.5° – when the angular orientation of the middle

disk is ‘software-realigned’ to produce a

straight-line relationship (i.e see Fig 242b),

NB The temperature at which tempering is undertaken is

critical, thus between 200–300°C, atomic diffusion rates are

slow with only a small amount of carbon being released,

thereby the component retains most of the hardness So if

higher ‘soaking-temperatures’ are employed (i.e between

300–500°C), then this creates greater carbon diffusion

form-ing cementite, with a correspondform-ing drop in the component’s

bulk hardness.

* A eutectic structure is a two-phase microstructure resulting

from the solidification of a liquid having the eutectic

compo-sition: the phases exist as fine lamellae that alternate with one

another (Sources: Thelning, 1981, Alexander et al., 1985;

Cal-lister, Jr et al., 2003)

• Overall weight of the mandrel and three disk as-sembly was 38 kg, consequently, this could be considered as a realistic ‘loaded condition’ for the machine tool to operate under, from a practical sense

In order to minimise the milling forces on the ma-chinable disks, HSM was employed using a

spindle-mounted ‘Speed-increaser’ (Fig 243a) equipped with

a φ6 mm slot drill The HSM speed-increaser was oper-ated under the following conditions: 18,000 rev min–1;

at a circular interpolation feed of 750 mm min–1; with the disks having 1 mm of excess stock for each ma-chinable disk – to be milled by circular interpolation

In Fig 243a, the last machinable disk has been located and clamped and the whole mandrel-and-disk assem-bly was nearing completion, having previously had its φ10 mm quadrant-positioned holes for each disk ma-chined by small circular interpolated motions by the slot drill (i.e see the sectional details of the φ10 mm hole geometry in each disk’s quadrant co-ordinates, as illustrated in Fig 241)

Stereometric Artefact – HSM Results

After HSM by milled interpolation on the vertical machining centre, the complete artefact with its ma-chinable disks in-situ, was carefully removed from the machine tool, then automatically-inspected for its quadrant hole positions and disk diameters, on an Eastman bridge-type Co-ordinate Measuring Machine (CMM) This CMM having previously been thermally

error-mapped, then checked with a ‘Machine

Check-ing Gauge’  (MCG) – prior to artefact inspection The CMM utilised a specially-made and calibrated

 ‘Speed-increasers’ , are a means of multiplying the rotational

speed of the machine’s spindle, by utilising a fixed relationship geared head Here, this actual speed-increaser had a 3:1

gear-ing ratio, equatgear-ing to a top speed of 18,000 rev min–1, when it

is operating at the top speed for this particular machine tool (i.e 6,000 rev min–1)

NB Normally, these HSM milling/drilling geared heads are

limited to a certain proportion of running time per hour at

its top speed, as they could over-heat and thereby damage the

bearing/gearing mechanism.

 ‘Machine Checking Gauge’ (MCG), is utilised to check a

CMM’s repeatability and accuracy and to detect for any po-tential ‘lobing-type errors’ from the ‘triggering-positioning’ mechanism of the touch-trigger probe, these being invariably used on such machines

Figure 241 Artefact stereometry was designed for the volumetric and positional uncertainites on machining

centres, by: HSM interpolation of machinable disks [Source: Smith, Sims, Hope & Gull, 2001]

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