Machining of High Strength Steels With Emphasis on Surface Integrity by air force machinability data center_4 pot

7.3.1 Chatter and Chip Formation – Significant Factors Influencing its Generation The stability of the cutting process and the onset of re-generative chatter is influenced by a range o

Figure 156 Vibration and chatter in machining operations, with their machine tool damping characteristics

.

is either ‘pearled’ , or ‘fish-scaled’) superimposed

over the normal cutting insert’s feed marks,

• Visible surface undulations – these effects are

re-produced in the direction of feed, being the

prod-uct of either serrated, or wavy chip formations, of

variable thicknesses

7.3.1 Chatter and Chip Formation –

Significant Factors Influencing

its Generation

The stability of the cutting process and the onset of

re-generative chatter is influenced by a range of factors,

such as the: cutting stiffness (Ks) of the workpiece

material – related to its machinability; parameters

of the machining process (e.g speed, feed, DOC, chip

width – total); insert cutting geometry (e.g rake and

clearance angles, edge preparation, insert shape and

size); cutting process dynamic characteristics (e.g

machine-tooling-workpiece/fixturing) Hence, during

machining operations on the workpiece, the chip is

formed by shearing over the chip area, producing the

cutting, or tangential force (FT) The magnitude of this

tangential force is heavily influenced by the product

of the workpiece material’s stiffness (Ks) and the chip

area, as follows:

FT = Ks × t × w

Where:

FT = tangential force (N),

Ks  = workpiece material’s stiffness (N mm–),

t = chip thickness (mm),

w = chip width (mm).

The direction of the tangential force (FT) is

predomi-nantly affected by the cutting insert’s rake and

clear-ance angles, together with the edge preparation on the

insert In many single-/multi-point machining

opera-tions used to generate for example a milled surface,

there is a requirement to overlap the adjacent cutting

paths (Fig 84c) For most single-point machining

op- ‘Cutting stiffness’ (Ks), is closely associated with that of ‘flow

stress’*, but is more simple to calculate and can be thought

of as a workpiece material property, being dependent on its

hardness.

*‘Flow stress’ , can be defined as: ‘The stress required to sustain

plastic deformation at a particular strain’ ( Kalpakjian, 1997).

erations, this former over-lapping of tool paths does not take place in the same manner, but will only occur after one complete revolution of either the workpiece,

or tool In operations by either milling (Fig 85), or drilling (Fig 50), an overlap takes place in a fraction

of a revolution, this being dependent upon how many cutting edges are present on the tool

In the Degarmo, et al (2003) machining model shown in (Fig 157a), the cutting or tangential force

(F c)0 generation may cause a relative displacement ‘X’

between the cutting insert and the workpiece, affecting

the uncut chip thickness (t), this results in changing

the cutting force This coupled relationship between

displacement in the ‘Y’ direction – modulation

direc-tion – and the resultant cutting force, creates a closed-loop response system Here, the modulation direction

is normally at 90° to the machined surface, so defines the chip thickness As a consequence of these

inter-related factors, there is a phase-shift (ε) between the

subsequent overlapping machined surfaces, resulting

in a variable chip thickness and modulation of the displacement, causing chatter vibration to take place Accordingly, this phase-shift between overlapping cut-ting paths is accountable for the production of chatter (Fig 157b) Moreover, there is a favoured speed cor-responding to a phase-locked condition (e.g when

‘ε=0’), resulting in a constant chip thickness (t) By

obtaining a constant chip thickness, this results in a

‘steady-state’ cutting force generation with it and, the eradication of the feed-back mechanism for regenera-tive chatter In essence, this is the goal for all machin-ing operators, as they attempt to achieve this effect by vary the cutting speeds for a given set of conditions for

a particular machining operation

7.3.2 Chatter – Important Factors

Affecting its Generation

In the previous sections, a brief discussion was made concerning just some of the causes of regenerative chatter mechanisms It is worth looking in greater de-tail at the reasons why this superfluous chatter occurs, explaining how and why it is generated in the hope of

0 In the Degarmo, et al (2003) model diagrammatically shown

in Fig 157a, they use the term and nomenclature of: ‘cutting

force’ and ‘Fc’ , whereas previously in the text, this has been

referred to as the ‘tangential force’ , denoted by ‘F’.

Figure 157 A chatter model, with potential chatter conditions and the application of the ‘stability lobe

diagram’ [Source: Degarmo, Black & Kosher, 2003]

.

either entirely eliminating it, or at the very least,

min-imising its affect on the overall machining process

Chatter during machining can result from a range of

multifarious and often linked-factors, they include:

• Depth of cut (DOC) – can be considered as the

prin-cipal cause and, for the prospective control of

chat-ter The DOC delineates the chip width, acting as

the feed-back gain within the closed-loop cutting

process,

NB The machining processes ‘stability limit’ –

be-ing the threshold between stable cuttbe-ing and

chat-ter – can be dechat-termined from trial-and-error by

simply incrementally increasing the DOC until the

commencement of chatter, then‘backing-off’ at this

level The prediction of chatter’s onset can be found

analytically, this value being based upon thorough

knowledge of material stiffness and cutting system

dynamics

• Rotational speed – is probably the simplest

param-eter to modify, thereby altering chatter and its

as-sociated amplitude,

NB The peripheral speed of either the rotating

tool, or workpiece, affects the phase-shift between

overlapping surfaces and its associated vibration

regeneration

• Feed – for milling operations the feed per tooth

de-fines the average uncut chip thickness (t),

influenc-ing the magnitude of the cuttinfluenc-ing process Chatter

is not unduly affected by the feedrate selected, but

feed does have an effect on the predictable severity

of vibration during machining,

NB As no cutting force exists if the vibration

oc-curs in the ‘Y’ direction – resulting in loss of

con-tact between the tool and workpiece – the

maxi-mum amplitude of chatter vibration will be limited

by its feed

 ‘Gain’ , can be practically defined in the following way: ‘The

ratio of the magnitude of the output of a system with respect

to that of the input – the conditions of operation and

measure-ments must be specified’ (Smith, 1993, et al.).

• Cutting stiffness (Ks) – is a material property con-nected to: shear flow stress; hardness, as well as work-hardening characteristics of the workpiece, this factor often being referred to in a metaphorical sense of its material’s machinability characteristics,

NB Materials that might offer poorer comparative

machinability, for example titanium, require con-siderably higher cutting forces leading to a greater

displacement in the ‘Y’ direction and as such, offer

a less stable cutting action

• Width of chip (total) – is equivalent to the product

of the DOC multiplied by the number of cutting edges engaged in the cut Furthermore, the total cut width will influence the stability of the cutting process,

NB At a preset D OC corresponding to that of the

‘stability limit’ , increasing the number of engaged cutting edges, will result in chatter, or vice-versa

• Cutting tool geometry – influences both the

direc-tion and the magnitude of the cutting force, in particular the quantity of the force component in

the modulation direction ‘Y’ So, an increased force occurring in the ‘Y’ direction, causes amplified

dis-placement and vibration at 90° to the surface, creat-ing ideal conditions for chatter Other cuttcreat-ing insert geometrical factors that can influence the cutting stability include the following:

– Back rake angle (α) – as it is inclined to a more

positive angle, the length of the commencement

of the shearing zone decreases, this in turn,

re-duces the magnitude of the cutting force (F c) As the back rake inclination becomes larger, then this directs the cutting force in a more tangential manner, thereby reducing the force component

in the ‘Y’ direction – creating improved stability

at higher speeds,

NB An insufficient feedrate in comparison to the

insert edge radius produces a less efficient cutting action, with more tool deflection and reduced ma-chining stability

– Clearance angle – reduction (γ) – has the effect

of increasing the frictional contact at the inter-face between the tool and workpiece, possibly having a process damping effect This potential stabilising effect could be the result of energy

dissipation – heat transformation, which could

result in decreased tool life, with the

superflu-ous effect of thermal distortion of the machined

part, or an increase in the workpiece’s

heat-af-fected zone (HAZ),

NB On a newly-fitted cutting insert, if initial wear

occurs, this can sometimes have a stabilising effect

for the onset of chatter

– Nose radius – size, insert shape – diamond

tri-angular, square, round, plan approach angle

– positive, neutral, negative – all influence the

area of the chip shape and its corresponding ‘Y’

direction The orientation of the modulation

direction ‘Y’ toward a dynamically more-rigid

direction angle, allows a decrease in vibrational

response, giving greater overall process stability

– having notably less chattering tendencies

As machining process stability is a direct result of

characteristics of dynamic force displacement between

both the workpiece and the cutting insert, all of the

various factors of a machining system: machine tool;

spindle; tooling; workpiece; workholding – in varying

degrees, can influence chatter To increase process

sta-bility of the machining system, it is necessary to

maxi-mise the dynamics, this being the overall product of its

static stiffness and damping capacity Further,

machin-ing stability can be increased by utilismachin-ing toolmachin-ing with

the greatest possible diameter with the minimum of

tool overhang By way of a caution concerning chatter

frequency, this normally occurs near the most flexible

vibrational mode of the machining system

7.3.3 Stability Lobe Diagrams

In Fig 157c, a ‘Stability lobe diagram’ (SLD) is

de-picted, which relates to the: total cut width that can be

machined, to the tooling’s rotational speed, for a

speci-fied number of cutting inserts For example referring

to the: Degarmo, et al (2003) diagram, suppose the

total width of cut was maintained below a minimum

level, then the process stability would exhibit ‘speed

 If the total cut width was maintained below a minimum level,

in practical terms this would be of limited value for many

ma-chining systems.

independence’ , or an ‘unconditional stability’ Hence,

at relatively slow speeds an increased stability can

be achieved within the process damping region – as

shown The ‘conditional stability’ lobe regions of the

diagram, permit an increased total cut width (i.e the

DOC x number of cutting edges, these being engaged

in the cut) at dynamically preferred speeds, at which

the phase-shift ‘ε’ between overlapping, or consecutive

cutting paths approaches zero In Fig 157c, stability

lobe number ‘N’ refers to the complete vibration cycles

existing between overlapping surfaces Moreover, the higher speeds correspond to lower lobe numbers, pro-viding the utmost potential increase in the total cut width and material removal rate – this being due to the greater lobe height and width If the total cut width exceeds the stability threshold – even assuming that the cutting process is operating at the desired speed, chatter will occur So, the larger the total cut width above the ‘stability limit’ , the more unstable and ag-gressive the chatter vibration becomes

Referring to the diagrammatic representation of the SLD on the graph in Fig 157c, if a chatter

con-dition arises, such as that found at point ‘a’ , the

ro-tational speed is attuned to the initial recommended

speed (i.e when ‘N=’), resulting in stable machining

at point ‘b’ on this diagram The D OC can be incremen-tally increased until the onset of chatter again – as the

threshold stability is crossed at point ‘c’ By utilising a

hand-held ‘speed analyser’ whilst the chatter

contin-ues – under the previously-selected operating

condi-tions, this will result in the ‘analyser’ giving a modified

speed recommendation that corresponds to point ‘d’

Now, if required, the DOC can be progressively

incre- ‘Speed analysers’ , are normally hand-held devices that

pro-duce dynamically-favoured speed recommendations and are

commercially available Such ‘speed analyser’* equipment

when utilised for a cutting process, can show the relative mo-tion between the tooling and the workpiece and recommends the appropriate speed to avoid chatter-effects

*‘Speed analysers’ can be successfully used for many industrial

applications, such as those involving: High-speed; Thin-chip, hardened-die machining; multi-point cutting operations – milling, etc.; Turning and boring operations These ‘speed anal-ysers’ can also be employed for workpiece compositions rang-ing from ductile metals (i.e aluminium and steel grades) and brittle materials (i.e cast irons and brasses, etc.), together with some non-metallics (plastics, etc.) and composite materials (carbon fibre, etc.)

mentally increased to point ‘e’  – this being a

‘safe-limit’ for the optimum machining operation

7.4 Milled Roundness –

Interpolated Diameters

Circular features such as bosses, circular rebates, etc.,

can be CNC milled by utilising a specific

word-ad-dress ‘circular interpolation’  command This CNC

function creates precise and accurate circular control

in two slideways simultaneously, while the milling

cutter mills around the workpiece, as depicted in Fig

158 Here, the milling cutter’s rigidity plays an

impor-tant role in the quality of the final machined feature,

this being based upon the ‘rigidity square rule’  The

deflected milling cutter illustrated in Fig 158-right,

having lack-of-rigidity will produce some unwanted

effects on the final milled part Cutter deflection not

only introduces the potential for chatter vibration,

but if used to mill up to square shoulder, its deflection

distorts the component geometry and introduces

har-monic variation to the circular interpolated feature

So that minimal change takes place in a milled profile,

it is advisable to keep to cutter lengths having short

 Generally-speaking, it is not advisable to attempt to maintain

both the DOC and the total cut width at the stability

thresh-old , because any variation in the: workpiece affecting its

cut-ting stiffness ‘K s’; speed errors; or perhaps small changes in

the overall dynamic characteristics of the machining system,

could result in crossing the stability limit, creating severe

chatter For example, in a milling application, the amplitude

of chatter vibration can be limited by a provisional feed per

tooth reduction , until an established and desired speed has

been achieved offering a stable DOC.

 ‘Circular interpolation’ , is a block of entered information

di-recting the CNC system to cut, either an arc, or a circle, (e.g

G02 – in a clockwise, or G03 anti-clockwise direction).

 ‘Rigidity square rule’ – for milling cutters states: ‘Cutter

rigid-ity decreases by the ‘square’* of the distance from the holder’

(Smith, 1993, et al.).

*For example, if a cutter ‘stood-out’ from its respective

tool-holder by 50 mm to mill a circular feature (Fig.158 – left), then,

if all other machining conditions remained the same and, then

cutter was replaced by one of 100 mm long (Fig 158 – right),

it would now be 4 times less rigid, causing serious tool

deflec-tion.

stand-off distances, conducive with correct and cur-rent operational practices

There are several distinct problems involved in the milling high-quality circular interpolated features and,

a slight digression into basic machine tool induced-er-rors is necessary to clarify the circumstances for the problems exhibited in Fig 159 Most of today’s

ma-chine tools have what is termed ‘orthogonally-orien-tated axes’ and in the case of the popular three-axis

vertical machining centre configurations, if the axes have not been recently calibrated, then considerable

‘error’8 can be introduced into the final milled part

features It has been well-proven that a machine tool

equipped with three orthogonal sideways: ‘X-axis’;

‘Y-axis’ – in the horizontal plane, together with the

‘Z-axis’ – in the vertical plane, can introduce up to 21

kinematic ‘errors’ into the cutting process The kine-matics for any machine tool are quite complex, when it has the ability to provide motion to all its axes simulta-neously, although these errors are often small, they are

 ‘Orthogonally-orientated axes’ , (is briefly mentioned in

Foot-note 2) refers to the fact that each axis is positioned at 90° with respect to each other, often situated on top of another axis For example, on a typical 3-axis vertical machining centre,

the ‘Y-axis’ sits on top of the ‘X-axis’ , but at right-angles to it, conversely, the ‘Z-axis’ is situated at 90° to these axes – hence

the term ‘orthogonal’

NB Non-orthogonal machine tools exist, often having

com-plex ‘kinematics’* between five and six axes Therefore with

these machine tools, in order to machine (i.e mill) a

straight-line all the axes must be in synchronised control to achieve

this linear action.

*Kinematics, comes from the Greek word ‘Kinesis’ , which

means ‘Motion’ It can be defined as: ‘The study of motion

with-out regard for the cause‘ (Lombardi, 2001) In machine tool

terminology, it refers to the translational effects of both lin-ear and angular motions It is principally concerned with the

effects of the ‘degrees of freedom’ for a ‘free-body’ in

three-di-mensional space (also see: Footnote 47, in Chapter 3).

 ‘Error’ is now not considered as an appropriate metrological

term for any form of calibration, the recommended term

to-day, is: ‘uncertainty’*.

*‘Uncertainty’ , has been simply defined as: ‘The doubt that

exists about the result of any measurement’ (Bell/NPL, 1999)

This is why today, uncertainty in measurement is a

combina-tion of many factors, some physical, while others are induced

Hence, another term, along with all of these uncertainty

fac-tors has been coined, which is its ‘Uncertainty budget’ – this

being a simple mathematical calculation, based upon a sum-mary of these uncertainty calculations.

Figure 158 The effect of increased milling cutter length on the resultant circular interpolated profile on the workpiece

.

Figure 159 The generated errors produced when circular interpolating at high feedrates when high-speed machining

.

but significant ‘errors’ , which can be said to be

simplis-tically produced as a result of:

• Linear motions (six) – created by the displacement

of the forward-and-backward motion of the X-, Y-

and Z-axes slideway movements, introducing

par-ticular non-linearities into the slideway

position-ing,

• Rotational motions (three) – yaw, pitch and roll for

each axis All of these partial rotational motions can

be practically-described in the following manner:

• Yaw is the side-to-side ‘crabbing-motion’ along the

slideway,

NB ‘Yaw’ is normally the result of too much

clear-ance (i.e ‘slop’) in the adjacent slideway members

• Pitch introduces a backward-and-forward

rock-ing (pitchrock-ing) action normal to the slideway, as the

moving element traverses along the axis,

NB ‘Pitching’ is probably due to the

‘profile/wavi-ness’ (i.e long-frequency effects) in its respective

slideway

• Roll is the clockwise-and-anticlockwise rotational

motion along the slideway

NB ’Roll’ could be introduced by two ‘adjacent

ways’ situated on each slideway, but not being

coin-cident with respect to each other (i.e laying in the

same respective plane), causing a limited pivoting

action – along the ‘line-of-sight’ of the axis as it

tra-verses along its length

• Squareness (three) – these ‘errors’ occur due to the

fact that each axis may not be at 90° (i.e square) to

one another

These types of 21 ‘kinematic machine-induced

er-rors’ can be appreciably reduced by the application of

calibration through laser-based techniques To a lesser

extent, these ‘errors’ can be minimised via ballbar

ar-tifact-based methods, offering a quick ‘health-check’

by either static, or dynamic assessment techniques

The results of either the laser, or ballbar, can be fed

back into the machine’s CNC controller for dynamic

corrections as cutting takes place, offering a

consid-erable improvement in the machine’s subsequent

ac-curacy and precision The above machine tool

calibra-tion techniques are somewhat beyond the scope of the

present discussion, the same could be said for

‘ther-mally-induced errors’ , however, they can also influence

the machined part surface and the machine tool’s

pro-filing abilities Moreover, ‘error-mapping techniques’

and sophisticated in-process control by an associated

‘dynamic error compensation system’ , have been shown

to extensively reduce the effects of the ‘variety of

er-rors’ that can be present on the machine tool, but once again, these topics are mentioned only for further re-search applications – as necessary

The circular interpolated milled profile shown in Fig 159, shows significant departures from roundness

of the milled workpiece, which is a function of most of the previously discussed kinematically- and thermally-induced machine tool ‘errors’ , together with the possi-bility of some ‘load-induced errors’ This diagrammatic representation (i.e Fig 159), indicates that several

‘errors’ on the milled circular interpolated profile are present At relatively slow simultaneous

feeding-mo-tions of the two axes (‘X-’ and ‘Y-axis’), it will generate

a reasonable facsimile of the required circular feature However, then by somewhat increasing this milled in-terpolation speed, the apparent roundness will appre-ciably degrade, the reasons for this degradation, might

be the result of:

• Servo-spikes – these unwanted effects occur at the

‘axis transition points’  at their respective 90° angu-lar intervals, often termed ‘quadrant-points’ ,

• Back-lash – possibly resulting from any form of

axis reversals, originating from the recirculating ballscrews0, creating a slight ‘off-set’ , or ‘mismatch’

at the axis transition points,

• Servo-errors – when both axes are simultaneously

moving, their respective linear speed should be

 ‘Axis transition points’ , are where the ‘servo-spikes’ occur

They result from a reversal of one of the axes at this angular position and, its associated motor power-surge creating this

‘spike’ Normally, the ‘spike’ is associated afterward by a cor-responding, but very small localised slack here, as axis take-up

begins once more at these ‘quadrant-points’ on the

circular-interpolated feature (i.e see the inset and magnified diagram

in Fig 159).

0 ‘Recirculating ballscrews’ , are not supposed to have any

ap-preciable back-lash present, as they are normally pre-stressed

by applying loads by the application of either: tension-, or

compression-shimming However, as the pitch of any the screw has minute errors present, these are usually ‘mapped-out’ by

the original machine tool builder – using the recognised

In-ternational Standard laser-calibration techniques Although,

once the machine tool has been operating for sometime and either local ballscrew-wear occurs, or perhaps the machine

has had the occasional ‘tool-crash’ , this can introduce and

af-fect both its pitching- and back-lash-errors.