Surface Integrity Cutting Fluids Machining and Monitoring Strategies_2 doc

Attainable cutting parameters – with differing milling spindles, plus HSM is affected by the feedrate and distance to be traversed, prior to the desired velocity being achieved – for con

Figure 219 Machine tool spindle error plots, illustrating spindle condition [Courtesy of Lion

Precision]

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Figure 220 A typical UHSM spindle cardridge listing some of factors affecting such a spindle’s design and its operation

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Figure 221 Attainable cutting parameters – with differing milling spindles, plus HSM is affected by the feedrate and distance

to be traversed, prior to the desired velocity being achieved – for conventional slideway motions

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NB Pneumatic spindles can be rotated at

excep-tional speeds, by virtue of the ‘ideal condition’ of

minimal metal-to-metal contact, although one

serious disadvantage being they suffer from a low

power output, or to be more specific – torque

• Hybrid spindles (Fig 214-top) – have been

de-veloped to answer the major drawback to utilising

pneumatic spindles The hybrid spindle as its name

suggests, is a combination of conventional

ball-bearing and pneumatic spindles Here, the spindle

design incorporates an aerodynamic thrust bearing

with transversal spiral grooves (Fig 214-top right)

thereby creating an intense pressure wave profile,

which can withstand up to 300% greater static loads

to that of a conventional aerostatic bearing A

typi-cal hybrid aerodynamic spindle bearing allows the

assembly to achieve rotational speeds ranging from:

20,000 to 40,000 rev min–1, with >15.5 kW power at

peak speed

NB Hybrid spindle cartridges are significantly less

expensive than the magnetic ‘active’ spindles, but

more expensive than pneumatic spindle cartridges

In both of these latter versions, they have a

rela-tively long in-service life, as wear-rates are

mini-mised, but do not have the stock-removal capability

of the former cartridge

In Figs 220 and 221a, are shown some of the principal

factors that affect UHSM spindle performance In Fig

220, these factors are represented in an Ishikawa (i.e

‘Cause-and-effect’) diagram Here, the many of the

in-ter-related effects can be seen, although other factors

can also be added, depending upon the local

condi-tions of usage: cutting data; workpiece material; wet,

dry, or near-dry cutting; together with the machine

tool’s overall condition

9.2 HSM Dynamics –

Acceleration

and Deceleration

If the HSM spindle cartridges – mentioned above –

are fitted to conventional CNC machine tools, or the

more likely low-cost scenario would be to simply fit a

mechanically-driven speed-increaser Then the result

of this HSM spindle fitment, will enable high rotational speeds to be produced, but it leaves the CNC-proces-sors somewhat compromised in its ability to produce the desired acceleration and deceleration capabilities

As a practical example of the problems likely to be encountered, the graphs produced in Fig 221b and c, were drawn from an industrial HSM machining expe-rience at a precision metrology company’s premises, using several of the latest vertical machining centres with the spindle of each machine, being fitted with

a mechanically-driven speed-increaser (i.e see Fig 243a)

By utilising the inboard CNC clock – having a reso-lution of 0.0001 seconds, the elapsed times for slide-way motion over varying distances was established In Fig 221b, an exponential relationship is depicted for the X-axis, this being a typical situation for the other axes on the machine tool By a determination of the required motional distance to attain specific veloci-ties, it was possible to illustrate the restrictive nature of both acceleration and deceleration for small slideway motions In Fig 221c, this illustrates the effect of the required distances to be executed at various velocities From Fig 221c – by way of an example, if a feedrate of 8,000 mm min–1 was utilised, then it would be neces-sary for a minimum movement of the slideway to be

16 mm to momentarily achieve the desired feedrate, which is typical for a machining centre having an ac-celeration of 1.08 m sec This physical problem in ac-tual positioning to the required component’s dimen-sional feature is not too great a problem for long linear feeding distances – as the slideway velocity could be reached, but acceleration and deceleration becomes exacerbated by the smaller more intricate prismatic features normally found on the more minute, or smaller parts often produced by HSM milling opera-tions, leading to potential scrappage problems From the results of inspection procedures con-ducted on the HSM over a range of standardised

testpieces, it was concluded that virtually all of the

detrimental dimensional effects introduced by the HSM milling operations, could be attributed to severe

‘servo-droop’ – more will be mentioned on this subject

shortly Prior to manufacturing the HSM testpieces, they were designed on a CAD system and their respec-tive tool paths were post-processed by the integrated CAM software Hence, with regard to machining cycle times, they were either calculated by the CAD/CAM,

or were the actual in-cut times – see Table 15 With

re-gard to these cycle-times for testpiece manufacture, a

significant improvement accrues when utilising HSM

milling techniques Although the increase of actual

cy-cle-time from that of the theoretical high-speed CAD/

CAM estimation, can be due to a number of factors

as previously noted by Smith and Hanson (1993) Not

least of which was found in this case, where the CAM

system tended to under-estimate the actual time to

machine a component feature This time-difference is

marginally compounded by the ‘servo-droop’ effects

If the machine tool’s G61 (‘Exact-stop’) mode was

employed, the actual cutting time in general showed

only a marginal increase, over the normal HSM

cut-ting time, although the dynamics of motion tended to

be somewhat jerky in action as the command ensured

it reached its targeted positions At normal HSM

mill-ing performance, another exacerbatmill-ing reason for the

increase in cutting time over theoretical, was

attribut-able to the axis acceleration/deceleration parameters

(i.e see Figs 221b and c) Thus, these machine tools

basically failed to reach the required slideway

accel-eration/deceleration then maintain these velocities for

about 20% of the total in-cut times, this was for a

com-ponent of somewhat moderate dimensional size and

pocketing intricacy (i.e the overall testpiece

dimen-sions were approximately: 150 mm in squareness, by

50mm deep)

From the testpiece results, various remarks can

be made concerning the advantages of employing an

HSM strategy over conventional milling practices,

these are:

• Despite a reduced cut depth, the HSM cycle times are a 66% improvement over conventional milling production techniques,

• Using HSM it will significantly reduce burr forma-tion – although not entirely eliminating it, when compared to that of conventional practice,

• Distortion of the thin wall features was minimised

by HSM,

• When employing a speed-increaser (Fig 243a), its bearing’s stiffness is critically important in order to obtain an acceptable milled surface texture Finally, by utilising even the most elementary form of HSM approach – using a speed-increaser, highlights the production advantages to be gained from adopting this strategy, albeit for limited periods of continuous cutting time, which normally dictates such ‘increaser’s’ practical usage

9.2.1 HSM Dynamics – Servo-Lag

Most of today’s CNC machine tools use ‘proportional servo-systems’ , where the axis velocity is proportional

to the difference between the actual position and the command position (Fig 222a) This ‘error signal’ is utilised by the system to determine any acceleration/ deceleration necessary as well as the steady-state ve-locities As one can visualise from Fig 222a, the dis-tance between the actual and commanded positions is

commonly termed ‘servo-lag’ This explanation can be

taken a stage further in Fig 222b, where the illustra-tion depicts how a ‘proporillustra-tional servo-system’ is used

to mill a sloping line In this example, DX and DY are the total programmed changes in position on both the X- and Y-axes, respectively, to go from point ‘A’ to point ‘B’ Conversely, DXL and DYL are the amount of lag on each axis at point ‘C’ along the tool’s path from

‘A’ to ‘B’ Furthermore, in such a system the lag on the X-axis must be proportional to a similar lag in the Y-axis, in order to accurately follow the slope of the line This affect can be mathematically-represented by the following relationship:

DX L

DY L = DX DY = Slope of the line

In Fig 222c, we can gain an appreciation of just what

happens when the servo-lag on both axes is not

pro-portional As the machine tool’s axes travels from point

‘A’ to point ‘B’ , the lag on the X-axis is proportionally

Table 15 A comparison of the theoretical and actual

ma-chining times for the manufacture of testpieces, by various

pro-duction routes

Machining

Method: Theoretical Cad/ Cam Time: Actual In-cut time:

NB All times in minutes

* G61 is the ‘Exact-stop’ mode of machine command and, when

ac-tivated, the machine tool will not initialise another movement

un-til the previous axis command has been completed (i.e the

target-point), thus ensuring an accurate and final slideway positioning.

[Source: Smith and Maxted, 1995]

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Figure 222 The CNC control problem of servo-lag and its affect on the associated HSM motional kinematics

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less than the lag on the Y-axis This error might be the

result of the ‘servo-gains’1 between the X- and Y-axes

not being properly synchronised Normally,

‘servo-gain’ can be expressed in units of: mm min–1 [i.e

velo-city / mm (i.e distance in 0.001)] of lag Thus, lag can

be determined using the following relationship:

Lag L (mm) = Feedrate F Gain G = , �. =. mm

For example:

If a machine tool’s moving axis is travelling along

its slideway at 2,500 mm min–1 and the servo has a gain

of 2, the lag will be 1.25 mm, as indicated in the

fol-lowing calculation:

L (mm) = F G = , �. =.mm

9.2.2 Effect of Servo-lag

and Gain on Corner Milling

If two axes with correctly matched servo-lags can

move in a straight line from point ‘A’ to point ‘B’ , then

to comprehend the effect of gain, let us consider what

occurs when milling a right-angled corner at a

con-stant feedrate without stopping (Fig 222d)

Whilst milling the corner from ‘A’ to ‘B’ and the

onward to ‘D’ , the servo develops a steady lag (DXL),

until sufficient command signals have been generated

to reach point ‘B’ It is at this position that the

con-trol begins to generate commands toward point ‘D’ ,

although the actual slideway has not yet reached point

‘B’ , owing to the servo-lag (DXL) At this point the

X-axis will begin to decelerate and, simultaneously, the

Y-axis begins to accelerate, that is the velocity is

pro-portional to the distance between the command signal

and the actual position Acceleration factors affect the

slideway motions producing the result that the

dis-tance from ‘B’ to ‘C’ is always greater than DXL

Fur-thermore, this curved path is not a circular arc, but an

exponential curve, with the amount of variance from

the sharp right-angled corner being dependent on the

 ‘Gain’ or to be more specific: ‘servo-gain’ , in this instance, is

a measure of the servo’s responsiveness Thus, the higher the

gain, the lower the lag.

magnitude of servo-lag1, which itself depends upon the affect of feedrate and gain – according to the previ-ous formula

9.2.3 Effect of Servo-Lag and Gain

Whilst Generating Circular Paths

For one to fully understand just what happens when milling complex contours, it will be helpful to consider the simple case of a milled path where two straight lines are joined by a semi-circle (Fig 222e) In this situation, the milling operation occurs at a constant feedrate moving from point ‘A’ in a straight line until the command dimension reaches point ‘B’ However,

at this point, because of the effect of servo-lag, the axis motion will have only reached point ‘BL’ Therefore, as the control command is moving forward at a constant rate, it begins to generate commands toward point ‘C’ This action results in the axis motion beginning to move away from the desired path at point ‘BL’ The dot-ted line depicdot-ted in Fig 222e, shows the actual path taken by the cutter and as one can visually observe, from points ‘BL’ to ‘CL’ , the deviation from the desired path is shown as ‘e’

In this example, the magnitude of ‘e’ is determined

as a function of the: feedrate; gain; plus the desired radius When the radius error approaches the pro-grammed radius, the resulting machined profile ap-pears distorted and is hence, impracticable Specifi-cally, if one needed to mill a 25 mm radius at a feedrate

of 2,500 mm min–1 with a machine tool gain being:

25 mm/min/0.001, then the error generated would

be approximately 0.125 mm, equally, if the gain was increased to 100 mm/min/0.001, the maximum er-ror ‘e’ will be considerably reduced to approximately 0.008 mm

A machined curve is an approximation on CNC machine tools, in that the profile is constructed from

a series of short connected segments, or chords The controlling factor on the length of such segments is the deviation between the centrepoint of any chord

 ‘Servo-lag’ , is sometimes referred to in the literature as:

‘Servo-droop’ – due to its ‘rounding-effect’ at the corners, this

being particularly prevalent when fast feedrates are selected, creating fast tool path velocities, particularly when normally undertaking high-speed milling operations.

and a point at right angles on the programmed curve

The linear distance between these two points is usually

termed the ‘maximum allowable chordal deviation’ and

is a function of the CNC controller’s executive software

So, the resultant machined curve is a combination of

the chordal deviation and the servo-lag for a

particu-lar machine tool To illustrate this condition, Fig 220f

shows the culmination of servo-lag when following

a contour, with the curve ‘C1’ being the desired

con-tour, ‘C2’ a linear approximation (i.e the programmed

path), ‘C3’ is the actual generated path resulting from

servo-lag utilising a high gain servo and finally, ‘C4’

being the path generated by a low gain servo Through

servo-lag, a smoothing of any contour occurs owing to

the lagged cutter path, this causes severe contour

prob-lems with respect to part accuracy and precision for

the simple arc geometry depicted in Fig 222e Clearly

then, servo-lag and gain promote a variety of effects

on complex shapes, depending upon their geometry

and tolerance, with these errors becoming still more

complicated when one considers three-dimensional

milling contouring In many circumstances the cutting

of three-dimensional profiles may necessitate utilising

four, or more axes with either one, or two of them

be-ing rotary axes bebe-ing necessary to create the required

tool paths to produce the component The servo-lag

and gain on all axes must be considered when

manu-facturing complex part geometries Regardless of the

workpiece’s geometry, or the number of axes utilised,

there is one factor that should be emphasised

concern-ing potential errors created by servo-lag If servo-lag

is extremely large, then this ‘lag’ can easily exceed the

positioning errors in the machine tool’s basic

specifi-cations

9.2.4 CNC Processing Speed

Possibly the main factor limiting contouring speed is

the CNC’s inherent processing speed, with each

pro-grammed-block1 generated for every axis having to

 ‘Programmed blocks’ , these are basically the ‘G-’ and ‘M-’

and ‘Auxiliary-codes’ which make up each individual block’s

line, with successive blocks in a logical sequence containing

the whole CNC program Generally speaking, the smaller the

number of blocks – for the successful production machining

of the part, the more efficient and refined has been the

pro-gramming (Smith et al., 1993)

be read, interpreted, the activated to obtain dynamic slideway motions This CNC exercise is usually

re-ferred to as the ‘block processing time’ The maximum

allocated time for block processing of information is dependent on the length of slideway stroke (i.e chord length) and its associated feedrate It is possible to cal-culate the maximum block processing time (Tb), as follows:

Tb= Maximum stroke length

Feedrate For example: if we require a profile’s chord length (i.e stroke length) of 0.50 mm, in order to maintain con-touring accuracy whist milling at 3,000 mm min–1, or

50 mm s–1, with a maximum block processing time, then this ‘time’ should be less than:

Tb= ., � =. = .s, or  ms Many CNC’s have block processing times typically within the range of 30 to 60 ms, as can be seen from the above example, the CNC program would suffer from ‘data starvation’ , whilst the controller attempts to catch up on its data processing Such ‘starvation’ would cause hesitation in the slide motions, slowing down

the cutting time and leaving ‘dwell marks’1 on the

ma-chined workpiece’s surface Since this ‘data starvation’ effect is unacceptable, a lower feedrate must now be programmed to overcome the problem and as a re-sult, the cycle-time increases In the above example,

if an older CNC was fitted to the machine tool with the controller’s block processing time being 60 ms, the

cut would have taken six times longer to generate the

profile, than a more modern CNC controller having a processor capable of 10 ms

So that we can fully-comprehend the CNC process-ing speed problem, let us now consider two widely dif-fering machining applications:

1 Complex three-dimensional milling of a hob – to

manufacture a die utilised in the production of in-tricate and expensive military metal buttons Such

 ‘Dwell marks’ , here are the result of an ‘untimed delay’ in the

program’s execution, created in this instance, by data starva-tion (i.e block processing speed was simply not fast enough) These untimely delays in the activation of programming blocks cause the rotating cutting tool to rest and press against machined surface and thus, generate minute ‘gouging-effects’

in the surface (Source: Seames, 1990; Smith, 1993)

a hob will more than likely have very fine detailed

work on its surface, perhaps with radii as small as

0.25 mm, requiring a tool tip radius of 0.025 mm

In order to machine the button’s elaborate features

with such a small milling cutter, the spindle speeds

might need to reach 40,000 rev min–1, utilising a

feed per revolution of 0.008 mm, giving a feedrate of

320 mm min–1 Many production engineers would

not consider this as an example of high-speed

mill-ing, but let us look more closely at this particular

machining problem If the controller has a

servo-gain of 4, with a feedrate of 320 mm min–1, this

means that the servo-lag would be 0.75 mm min–1,

which is consistent with milling radii of 0.25 mm

However, if the servo-gain was 1, this would cause

a servo-lag of 0.320 mm min–1 and in this case, it

obviously could not machine that button’s

intri-cate detailing In such circumstances, it would be

necessary to appreciably reduce the feedrate to

say, 75 mm min–1 to generate the button’s contours,

leading to the cycle-time increasing by 400% Let

us also now consider the impact of block

process-ing time under these conditions To mill a radius as

small as 0.25 mm, we would need to produce linear

stroke lengths of just 0.075 mm – to reproduce

ac-ceptable button detailing This intricate contouring

work requires a block processing time of 15 ms If

the CNC controller has a block processing time

of just 60 ms, then the feedrate must be limited to

75 mm run–1 which again, increases milling time by

a factor of four

2 ECM pattern electrode for a Turbine fan (i.e large

aluminium casting) – here, the electrode’s

geom-etry has very gentle three-dimensional curves In

this situation the chosen CNC machine tool’s

mill-ing spindle has a 250,000 mm min–1 capability,

cou-pled to adequate power to cut at a feed of 0.25 mm

rev–1 This production requirement produces a

feed-rate of 62,500 mm min–1 (i.e being the product of:

250,000 x 0.25) would be possible For accuracy

and precision, a chordal deviation (Cd) of 0.005 mm

would indicate a stroke length of 0.75 mm – if the

minimum radius of curvature for the Turbine fan’s

geometry was 25 mm Assuming that the

servo-gain of 1 was available, then we would obtain errors

as large as 0.125 mm and with such errors, the

ma-chine tool would not produce an acceptable part

Further, at 62,500 mm min–1, if the block

process-ing time (Tb) was 60 ms, this ‘timing’ would require

stroke lengths of 2.5 mm instead of the 0.75 mm we

needed for the required accuracy and precision

Therefore, in order to eliminate the effects of low

gain, or slow processing time, it is necessary to de-press the feedrate, resulting in the cutting time be-ing increased up to 400%

When considering these two practical examples from

a metaphorical sense, the former method can be com-pared to that of racing a go-kart on a small tight track, while the latter method is similar to a highly tuned sports car racing on a longer and smoother track The go-kart may only reach speed a of 30 km h–1, whereas the sports car may hit speeds of >200 km h–1 The corner forces and reaction times are similar for both methods, even though the speeds are vastly different Looked at from yet another viewpoint, we can say that the frequency of response of both the drive and car, that is their servo-gain and processing time, are very similar in both examples even though the speeds (feed-rates) are radically different

In the day-to-day production environment, the du-plication of specific and precise contours is the end re-sult of a combination of many inter-related factors As the number of machine tool axes required to produce sculptured part surfaces increases, the difficulty of ob-taining the desired profile also becomes proportion-ally problematic So, machine tools that would nor-mally produce excellent general-purpose machining work, may not be either accurate, nor efficient enough

to manufacture complex part contouring geometries That is, unless their CNC processors can achieve block processing speeds of <10 ms, with servo-system gains

of up to 4, having sufficient ‘look-ahead’1 capabilities

(i.e see Fig 222g) that are required, for any realistic and practical HSM applications

 ‘Look-ahead’ facilities, are when the controller has the

abil-ity to look-ahead through the following sequenced program-ming blocks* to determine successive motions and actions – an important feature for any HSM applications Many of

today’s sophisticated CNC controllers can look-ahead through

a considerable number of these blocks, thereby prompting the controller’s response, prior to undertaking any command exe-cutions *A ‘Block’ can be defined as: A set of words, characters, digits or other elements handled as a unit – hence, the term

‘block’ – which creates a sequence of lines of a computer pro-gramming language, that can then be activated upon by the machine tool‘s CNC controller, producing the necessary pro-grammed-actions (Source: Smith et al., 1993)

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]