Machining and Monitoring Strategies Part 4 docx

In HSM turning operations, the grip, or clamping force exerted by a chuck on the workpiece changes when the chuck is rotated at speed.. With only 33% of the static clamping force remaini

Figure 228 High-speed milling toolholders (chucks) utilising tapered sleeves with needle rollers for elastic deflection,

when tightened – to precisely hold cutter’s shank along its whole gripping length [Courtesy of Diashowa Tooling]

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quickly heats up the holder and in so doing, expands

the bore allowing the tool’s shank to be placed in-situ

in the toolholder whereupon it contracts around the

shank – taking about 7 seconds to initially expand the

bore by ≈5 µm This thermal expansion gives enough

clearance to allow the speedy withdrawal of an old tool

and its replacement by the new one The heat is

lo-calised at the tool’s sleeve enabling the user to remove

it wearing protective gloves Having removed the new

tooling assembly from the induction machine the user

places this tooling in a solid aluminium cooling-block,

which efficiently dissipates the heat from the tooling

assembly Thus, just a few seconds after the assembly

has resided in the cooling-block, the bore shrinks

back, but is constrained from reaching its equilibrium

diameter by the slightly larger diameter of the tool’s

shank This thermal/mechanical restriction creates the

high, but uniform gripping pressure around and along

the tool’s shank which firmly secures it in place The

entire shrink-fitting process from start-to-finish, only

takes about 30 seconds to complete

The heating control can be adjusted by the user

to accommodate large, or small diameter

toolhold-ers, requiring differing thermal expansion cycles and

with experience, the optimum settings can be quickly

established So, once the correct power and

tempera-ture cycles have been created these parameters can

be employed to regulate the induction machine, thus

avoiding energy wastage If even tighter control is

de-manded of the heating process, an infra-red sensing

device can detect the amount of heat in the toolholder,

so that once it reaches a preset temperature, the

in-duction heating is automatically shut off Many other

features and protection devices are incorporated into

such induction machines allowing them to be

specifi-cally-tailored to the company’s requirements

Typical HSM shrink-fit tool holding assemblies can

be dynamically balanced to G2.5 at 20,000 rev min–1,

while special-purpose toolholders can be purchased

that are balanced to G1.0 t 40,000 rev min–1 More will

be mentioned concerning this critical factor of

dy-namic tool balancing for HSM applications shortly, in

Section 9.5

Cryogenic methods (i.e not shown) are the direct

opposite of thermal techniques Here, instead of the

toolholder’s sleeve being thermally-expanded, the tool

is now the subject As the tool’s shank is inverted and

held at the desired depth beneath the cryogenic liquid

which is held in a suitable container, it is cooled down

by the required ratio of a mixture of liquid

nitrogen-to-methanol This immersion of the tool’s shank in the cryogenic fluid, shrinks the shank’s diameter Thus, the contracted tool’s diameter is then placed into the tool-holder (i.e held at its ambient temperature), which has

its bore as an interference fit with respect to the tool

shank’s diameter Here, the tool shank finally expands and causes an increased gripping pressure as it is re-stricted by the toolholder’s smaller bore As a safety note, the user should always wear cryogenically-pro-tective gloves, otherwise a serious case of prospective

‘frost-bite’ will normally be the likely outcome!

HSM Chucks – for Turning Operations

Although in this section the principal concern has been with the design concepts and techniques of se-curely and accurately locking the tool in its respective toolholder It is worth deviating somewhat here, to consider the major problems encountered when at-tempting an HSM strategy in turning operations, prior

to discussing how and by what mechanical methods, the rotating toolholders – previously alluded to, are held in the spindle’s taper

In HSM turning operations, the grip, or clamping force exerted by a chuck on the workpiece changes when the chuck is rotated at speed This change

be-ing the direct consequence of centrifugal force that acts

on the chuck’s jaw assembly The magnitude of this clamping force is very significant, as a standard power

chuck loses 66% of its ‘static clamping force’ (i.e the

force exerted by the chuck when it is not rotating) at its maximum rated speed (i.e these maximum oper-ating chuck speeds are defined in European Standard: EN1550)

 ‘Interference fits’*, can be best defined, relating to the ‘Limits

and Fits’ of a mating hole and shaft, as follows: ‘The minimum permitted diameter of the shaft is larger than the maximum allowable diameter of the hole.’ (Source: Galyer and Shotbolt,

1990)* In practice, interference fits are those for which, prior

to assembly, the inside (male) component being larger than the outside (female) component, requires either some form of: deformation (i.e pressure); thermal contraction – for a shaft;

or otherwise, thermal expansion – for the hole; to obtain a

‘permanent and secure fitment’ Thus, either plastic deform-ation – for press-fits, or elastic pressure – by thermal effects, have been exerted between these mechanically-mating sur-faces, in order to achieve the desired component assembly (Sources: BS 4500A and B; Childs, 2004; Griffiths et al., 2003)

The capacity for the workpiece, or even a chuck

component to cause damage, or personal injury is

di-rectly related to its kinetic energy (KE) Thus, the

kin-etic energy of a body is proportional to the square of

its speed As a consequence, the capacity of a fast

ro-tating body (i.e chuck assembly) such as a 1 kg

work-piece rotating at 2,000 rev min–1 would have a KE of

≈ 7 Joules , whereas at 6,000 rev min–1 the value would

be 62 Joules – approximately the same as a small

cali-bre (0.25) handgun bullet ballistically hitting a target

at 100 m range! Even though the KE of the workpiece

represents a serious safety hazard, the energy stored in

the clamping jaws is far greater For example, a chuck

assembly fitted with its jaws (i.e shown in partial

sec-tion in Fig 229b) of φ200 mm rotating at 6,000 rev

min–1 would produce a KE of 1460 Joules (Fig 229c),

which is about the same as a 0.44 Magnum handgun

bullet as it ballistically-issues from the muzzle! With

only 33% of the static clamping force remaining at

the maximum chuck rotational speed, there is

con-siderable energy being stored in the rotating chuck

assembly It is important that operators understand

how to determine safe operational speeds when using

non-standard jaws, coupled to its residual grip, while

having an appreciation of the energy stored in these

rotating parts

As one might surmise from the application of HSM

turning chucks, friction plays an important role in

terms of the chuck’s actual performance in-service Of

note, is that a freshly-assembled and greased power

chuck will often exceed its maximum clamping force

by almost 20%, despite this fact, after a few weeks of

use, or indeed non-usage, the maximum-rated

clamp-ing force may be reduced to as little as 30% This

considerable drop-off in clamping performance is

at-tributable to the absence of effective lubricant and the

presence of particulates on the chuck’s sliding surfaces

Substantial losses can result even when the chuck has

been frequently lubricated, resulting from the wrong

grease, or deposits that are not periodically removed

In essence, grease comprises of oils and solids that are

bound together with soap Thus, the oils lubricate the

sliding surfaces and the purpose of the solids are

two-fold: firstly, they impede the escape of oils when the

pressure between the sliding surfaces would usually

squeeze them away; secondly, they directly lubricate by

a shearing mechanism at higher pressures So, greases

having a higher solid content would normally produce

optimum clamping performance However, when

em-ploying the chuck in the regions HSM, the centrifugal

effects act to separate out the solid and oil constitu-ents of the grease, causing the oils to be thrown out

of the chuck and leaving solid matter remnants Over

an extended time period, these solids from the grease collect in various: gaps; recesses; and cavities; combin-ing with small amounts of debris (e.g fines and parti-cles) generated during previous machining operations, which impair the chuck’s performance While the ap-plication of coolant exacerbates this situation still fur-ther, as it tends to leach-away the grease and accelerate its break-down, possibly causing corrosive damage to

a very expensive chuck In order to combat these un-desirable effects, additional compounds are necessary, such as polymers that can improve the lubricant’s: co-hesion; adco-hesion; and water-resistance

Special care must be utilised to ensure that a chuck’s reliable performance occurs when HSM operations are employed, as they are especially susceptible to cen-trifugal separation and leaching of grease – by coolant application Under such circumstances, it is probably advisable to monitor the chuck performance over a period of time and apply grease, or service the chuck once a certain wear pattern emerges allowing one to create a specific maintenance schedule Measurement

of chucking performance is vital and simply applying grease at regular intervals is no guarantee of its per-formance

When a chuck becomes congested with solid mat-ter – separated grease constituents, it prevents fresh grease from reaching the critical surfaces, so in effect provides little, or no improvement Clamping force measurements should be taken both before and after the application of grease, so ensuring that it is evident

as to whether, or not, the chuck needs to be serviced/ cleaned Measurements of the static and dynamic

clamping forces can be simply determined using a ‘Ra-dio Frequency Gripmeter’ (RFG) The RFG essentially

comprises of just a load cell and handset The load cell

is clamped in the chuck’s jaws and the handset displays the measured clamping forces (i.e the ‘grip‘ being accurate to 1kN per jaw), thus avoiding any lengthy and time-consuming calculations These RFG’s are available from reputable chuck manufacturers, with a typical handset being able to store up to 120 separate readings, having a PC-link to Windows© compatible software, allowing graphical trends and further ana-lyses to be undertaken – as necessary

In any HSM applications for turning, ‘centrifugally-balanced chucks’ can be utilised as they incorporate

a counter-balance mass that equalises the

centrifu-Figure 229 Thermal expansion tooling its operation and high-speed turning chuck details

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gal loss of the jaws – when rotating at typically high

turning speeds (i.e see Fig 229b for a diagrammatic

cutaway assembly of an HSM quick-change chuck)

These quick-change chucks incorporate a traditional

wedge-style and lever mechanism, that instead of

dir-ectly acting on the jaws, the radial force acts through

the actuator and lever mechanism, prior to

transfer-ring the effort to the jaws So, when the chuck rotates

at high-speed, the actuators are thrown outward by

centrifugal force, but are restrained from moving by

the lever, which pivots about the central connected

sphere At the opposite end of the lever, the jaws are

also thrown outward and act to move the pivot in the

opposing direction (i.e to that of the actuators) –

ef-fectively balancing each other The performance of the

counter-balanced chuck depends upon the accuracy of

the balance achieved, as the actuator mass is constant

and the top jaw mass being variable depending upon

the particular top jaws in use, thus the state of the

balance will also vary As a consequence, the

clamp-ing force may fall with rotational speed, or actually

increase with heavy and light jaws, respectively With

standard hardtop jaws, the clamping force remains

al-most constant across the range of the operating speed,

making it unnecessary to calculate the clamping force

losses An additional feature is that the static clamping

force can be much lower, since there is no centrifugal

loss This lower static clamping force application, has

the benefit that when turning either thin-walled, or

more delicate workpieces that may otherwise distort

with higher clamping forces, such chucks are unlikely

to affect these components, when an HSM turning

strategy is utilised

Much more could be said concerning HSM

turn-ing operations, particularly relatturn-ing to the calculations

and working practices, but it was not the intention

here, to give a comprehensive account of such

tech-nical aspects, simply a concise account of the

antici-pated problems and possible solutions when turning

at high rotational speeds In the following section, a

discussion concerning toolholder coupling to the

ma-chine tool’s spindle will be briefly reviewed

9.4.2 Toolholder Design

and Spindle Taper

Introduction

In the past, the taper cone and its associated driving

dogs and pull-stud, provided adequate location and

torque for the cutter assembly when mounted into the machine tool’s spindle The tool’s cone taper an-gle was adequately manufactured so that it perfectly

‘wedged’ into its mating spindle taper and the prob-lem of the single-contact mechanical interface was not really exposed as deficient, until very high rotational speeds were being utilised, coupled to much greater feedrates that the newly-developed tooling geometries and tool materials could now exploit In recent years, both dual- and triple-contact tooling systems have been introduced, these designs will now be briefly re-viewed

Dual-Contact Tool/Spindle Design

One of the most significant developments in maintain-ing a complete mechanical interface between the tool-holder and the machine’s spindle was the dual-contact 7/24 taper system The CAT Standard incorporates this 7/24 taper, but also allows simultaneous contact

on both the toolholder’s flange and taper, when HSM machining is the requirement By achieving this dual-contact, the CAT-shank toolholders minimise any form inherent imbalance at say, 2,000 rev min–1 How-ever, if the cutter assembly is to be rotated at 10,000 rev min–1, the toolholder must cope with a × 25 increase

in centrifugal force, which may compound any unbal-ance present in the tooling assembly Further, if the ro-tational speed is increased still further, into the HSM range, then here, the centrifugal force is × 100 greater and the onset of considerable imbalance may create chattering conditions At such high rotational speeds,

if coolant is utilised in the machining process, the HSM conditions could develop a vortex around the cutting tool, that conventional flood coolant pressures cannot penetrate In these circumstances, possibly the only realistic option is to utilise a through-the-spindle coolant delivery application at pressures of >690 kPa (i.e 1,000 psi), coupled to perhaps, micro-filtration of the coolant with special pipes and couplings The CAT system of dual-contact offers reasonable rotational control of the tooling assembly at moderate-to-high rotational speeds, as the mechanical interface system

of face-and-cone provides a certain security against

 ‘Dual-contact 7/24 taper system’ , refers to the taper being to

the 7 inches of taper per 24 inches of length This 7/24 system incorporates several Standards: CAT and BT 40- and 50-taper tooling.

the onset of imbalance Typical applications for these

HSM dual-contact systems include: aerospace part

production; precision die and mould making;

automo-tive component production; as well as medical

compo-nent manufacturing

It is worth digressing somewhat, to explain the

situ-ation of why the single-cone mechanical interface is

simply not effective for HSM production applications

When rotational speeds begin to approach 20,000 rev

min–1, it is not an unusual occurrence for the

single-contact conventional, or standard CAT V-flange

tool-ing assembly to be effectively sucked into the spindle

(i.e as there is no mechanical contact at the flange),

this being the result of a combination of the pull-stud

pressure and the machine’s spindle ‘taper swelling’ – due

to the very high centrifugal force acting at such high

rotational speeds In fact, this minute amount of ‘taper

swelling’ can cause the tool holder to separate from

the spindle’s surface and as a result cause considerable

damage to both the cone’s male and female surfaces

In order to alleviate this HSM problem and run the

tooling assemblies at even faster rotational speeds, the

HSK dual-contact toolholders were developed, which

will now be briefly mentioned

Hsk Dual-Contact Tooling

There are a number of toolholder designs that are

al-ternatives to the conventional steep-taper spindle

con-nection Probably the most popular version for HSM

is the HSK-designed tooling connection (i.e see

perti-nent HSK tooling details in Fig 126c) HSK toolholder

connections offer simultaneous fitment on both the

taper and face, at the front of the spindle The reason

for their acknowledged popularity amongst the HSM

machining companies, is because the increased

rigid-ity of the joint, coupled with their inherent reduction

in dimensions, compared to the equivalent

conven-tional steep-taper connection In Fig 126c, the HSK

8° (included angle) short taper with its gauge face

con-tact and simultaneous taper interference can be seen,

which was designed in Germany to Standard: DIN

69893, being introduced in 1993 HSK is a German

acronym that translates into English as: ‘Hollow short

taper’ Thus, the HSK connection provides:

• both high static and dynamic stiffness,

• offering great axial and radial repeatable accuracy,

• with low mass and stroke,

• having inner clamping

Therefore, with all these proven design advantages over conventional spindle connections, it allows the HSK tooling assemblies to utilise the increased rota-tional speeds necessary for an HSM strategy

Triple-Contact Tool/Spindle Design

The triple-contact connection is being offered by a few toolholder manufacturers (i.e shown in Fig 230) The triple-contact design relies on an inner expand-ing sleeve which maintains uniform contact between the machine tool spindle and the: toolholder’s top ta-per; bottom tata-per; and flange; this being regardless of the spindle speed employed Of particular note is the inner expanding sleeve which functions particularly well at high spindle speeds So, as the centrifugal forces increase – with higher rotational speeds, it causes the spindle to grow (i.e ‘swell’), the toolholder’s spring mechanism forces the split-cone sleeve to proportion-ally-expand with the spindle Further, the expanding sleeve also acts as a vibration-dampening device The expanding sleeve extends the tool’s life on average by between 300 to 500%, by virtually eliminating vibra-tion As a result of this ‘vibration-free interface’ be-tween the tool and workpiece, it provides smoother machining of: tool steels; aluminium alloys; plus other metallic alloys This triple-contact connection system, also performs efficiently with extra-long tools (i.e see Fig 231), notably when utilised on horizontal machin-ing centres The main reason for the enhanced triple-contact tool’s cutting performance with extended tooling assemblies, is the result of the ‘floating’ inner sleeve (Fig 230) which acts to minimise any potential Z-axis deflection, thus maintaining its rotational con-centricity

Such triple-contact tooling is not inexpensive to purchase, but these toolholders really do amortise their cost, by significantly extending cutter life, while improving part production rates Further, it is claimed

by the tooling manufacturer that the toolholder is

‘maintenance-free’ , while its spring-mechanism in

‘life-testing’ has achieved upward of one million tool changes With the advent of either the double- and triple-contact systems, enabling contact between the machine tool’s spindle and the toolholder’s mechanical interface: top-taper; bottom-taper; plus flange; while

‘eliminating vibration’; this has been achieved under the unique conditions that arise with today’s HSM and high-accuracy and precision manufacturing needs

9.5 Dynamic Balance of

Toolholding Assemblies

Introduction

Balancing tools that are intended for HSM

applica-tions is vitally important and there are quite a few

In-dustrial/Manufacturing engineers and users who do not really understand the concept of how to achieve balanced tooling, or why it is really necessary Either very long extended tooling required for say, for deep-pocketing (Fig 231), or tooling that is out-of-balance, will more than likely produce: chattering effects; goug-ing of a step, or face; loss of workpiece accuracy and precision; not to mention uneven and premature cut-ter wear Whenever a new tooling assembly is destined

Figure 230 Triple-contact tool connection

system is ideal for any potential HSM operations [Courtesy of Heartech Precision Inc (HPI)]

Figure 231 Tool runout (≥10 µm) should be of prime importance when machining deep pockets [Courtesy of

Sandvik Coromant]

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for HSM applications on a workpiece, a balancing

operation needs to be undertaken, this statement is

also true for many sub-HSM applications, particularly

when extended tooling is used for whatever reason

(Fig 231) In fact, every rotating object (i.e chuck, or

tooling assembly, etc.), will generate vibration

As has been explained in the previous section, this

vibration results from a number of sources, but

princi-pally here, from centrifugal forces produced by the

ro-tation of an unbalanced mass There are several types

of unbalance that could arise, but here, we are mainly

concerned with what is termed dynamic unbalance,

which increases by the square of the rotational

vel-ocity For example, any vibration produced by a

tool-ing assembly at 3,000 rev min–1, is × 100 greater than

an identical tooling configuration that is rotating at

300 rev min–1 Moreover, what is often either

misun-derstood, or indeed overlooked, is that any change to

the tooling assembly – no matter how small it might

seem, requires re-balancing! These tooling

modifica-tions include any occasion when a cutting tool is

ad-justed, or changed, or similarly if the toolholder is also

either adjusted, or changed Such changes to the

‘sta-tus quo’ of the tooling, will directly affect its ensuing

balance, even minutely when just a ‘few microns’! So

that, these miniscule changes to the tooling’s dynamic

condition, causes a degree of tooling oscillation, hence

an out-of-balance condition – with the likely problems

that this creates

With the wide variety of tooling that is held in: tool

storage carousels; magazines; turrets; etc.; they must

all be ‘balanceable’ by some means A range of

balanc-ing techniques can be employed here for either sbalanc-ingle-,

or dual-plane balancing – more will be said concerning

these effects will be made in the following section The

techniques utilised in achieving tool balance could

in-clude:

• ‘Hard-balancing’ (i.e see Fig 234b) – when the

complete assembly either has to have material

re-moved, or added at a certain part of its assembly

NB The major problem associated with

‘hard-bal-ancing’ is that if the tooling setup changes, so will

the likely rotating mass change, which will mean

modifying the amount of material to be either

added, or subtracted from this newly-distributed

mass,

• ‘Adjustable balancing rings’ (i.e see Fig 232) – by

rotating the twin lower and higher balance rings

either clockwise, or anti-clockwise they minutely modify the balance-condition, allowing single-plane balance to be achieved

NB These matched pair of balance rings are in a

symmetrical state of unbalance (i.e they are both

‘unbalanced’ to the same degree) Letting the user adjust the pair to counter any unbalance in the cut-ting tool/toolholder assembly and locking them into place – usually achieved on commercially-available balancing machines (i.e see Fig 234a) The state of unbalance is not merely a subject to the

‘caprice’ of the machine tool operator, a tool assembly’s balance is given by various quality Standards, such

as ISO 1940/1, or ANSI S2.19 – being basically exact reflections of each other In the following related sec-tions, they deal with how and in what manner rotat-ing cutter assembly balance is achieved, utilisrotat-ing such HSM balance calculations and associated graphical details as necessary, from these Standards

9.5.1 HSM – Problem of Tool Balance

Unbalance of a rotating body (i.e here we are con-cerned with a complete tooling assembly), can be

defined as: ‘The condition existing when the principal mass – axis of inertia – does not coincide with its ro-tational axis’ (i.e shown schematically in Fig 232)

For example, such an undesirable state of affairs can

be comprehended by considering the following situ-ation: if a φ50 mm face mill assembly is rotated at 15,000 rev min–1, it will produce a peripheral speed

>240 km hr–1, which may prove to be disastrous if it is unbalanced!

Basically there exists, three types of unbalance con-ditions for rotating assemblies – such as tooling, these are:

1 ‘Static unbalance’ – single-plane This type of

un-balance occurs when the mass does not coincide with the rotational axis, but is parallel to it and the force created by such unbalancing, is equal to the magnitude at both ends of the rotating body Thus,

if some relief – metal removal (i.e see Fig 234b) –

on the toolholder body equal to the out-of-balance mass that occurs, then a nominal static unbalance is

achieved,

2 ‘Couple unbalance’ – Under these circumstances,

the cutter assembly – mass axis – does not coincide

Figure 232 The taper fitment against runout/eccentricity for a milling cutter and its associated balanced

tool-holder

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