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Surface features as indicators of tool chipping in single point face milling of aluminium
Surface features as indicators of tool chipping in single point face milling of aluminium
Abstract We describe a set of wear tests in which machined surfaces were produced by single point face milling of an aluminium alloy. The objectives were firstly to identify surface characteristics associated with tool chipping. and secondly to demonstrate that a fibre optic surface profiling pro& capable of in-situ operation on the machine tool had the ability to provide the same wear information as a metrological interference microscope. Spectral energy of the surface profiles decreased significantly with insen damage in a spatial frequency band around the tooth passing frequency of the cutter and also at higher spatial frequencies. 0 I997 EIsevicr Science S.A.
1. Introduction The profile of a machined surface contains information th2t is irzdirative of the stale of wear of the tool producing that surface [ I 1. In consequence a surface descriptor may he a useful input to a system to monitor 1001 wear. Current surface measurement techniques involve either the use of stylus instruments to traverse the surface or of optical instruments such as interference microscopes and optical profilers [ 21. Both these methods require that the workpiece be dismounted from the machine so that measurements may be made in a vibration-free ena, ironment. This involves a loss in production time and precludes the possibility of unmanned operation of machine tools if surface finish parameters are to be us4 as indicators of tool wear. Additionally, a stylus instrument may lead to scoring damage of the surface of soft materials. An instrument that could be used with the workpiece in-situ on the machine would have the clear advantage that surface measurementscould be made inter-operation&y, thus admitting the possibilities of unmanned machining. In this paper, we report systematic measurements on face milIed aluminium alloy made bj a commercial white light
* Corresponding author. ’ Present address: Renishaw pk. Transducer Systems Division. Old Town, Wotron-under-Edge.Glouccstcrshirc.GLI 2 7DH. UK. ’ Presentaddress:Melles-Griot International.Cambridge.CB5 9QR. UK. 0043- f648/97/$17,00 0 1997 Elsevicr ScienceS.A. All rights rcscmd fIfSOO43-1648(97)00149-X
interference microscope. The results obtained are compared with those generated from a novel optical fibre probe mounted in-situ on the machine tool. Aluminium was chosen as an example of a softer material susceptible to scratching by a stylus profiler. for which optical non-contact methoc’s are particularly appropriate. The optical fibre instrument is a differential probe. in which the output is proportional to the height difference of the two sample spots on the surface. We show that the surface spatial frequency corresponding to the tooth passing frequency is suppressed when the insert is damaged. In the context of tool wear assessment,the optical fibre probe gives the same useful data as the interference microscope, but with the advantage that the probe measurements were made on the machine tool.
2. Background In recent years much interest has been generated in the monitoring productivity.
of machining processes with the aim of increasing In particular,
investigations
into the wear state
of the cutting tool by indirect means have been carried out. This requires the measurement of process variables that may be correlated with the evolution of insert wear, such as acoustic emission genera&during cutting [ 31 or the surface finish produced by the cutter.
222
P. Wilkinsm et al. / Wuur 212 (lw7) 221-228
Choudhury and Ramesh 14 3 used an optical displacement sensor to monitor the variation in workpiece dimensions with tool wear during the turning process. The output from the sensor was then used to compensate for these variations by adjusting the depth of cut. Coker and Yung [5 J have investigated the use of an ultrasonic sensor to detect the in-process changes in surface roughES, as &fined by the parameter R,. which result from tool wear in face milling aluminium. The sensor was incorporated inw a control system which could maintain surfaceroughness within 10% of a specified value for tool wear land lengths up to 0.3 mm. Considerable work has beerr carried out on the modelling and simulation of surface generation with the models obtained usually giving an indication of the expected value of I?, in the simulated surface. Zhang et al. [6] produced a model of the intermittent turning process which incorporated tool vibrationand tool geometric motion. Zhang and Kapoor [ 7,8] have simulated the generation of a surface by a single point cutter combining a deterministic component resulting ftom tool geometric motion and a stochastic component arising from random vibrations of the tool holder. Confidence in this model was established using the results from a series of boring machining tests and co.llparing predicted and experimental values of I?,. Moon and Sutherland [9] have studied tbe spatial frequency content of a turned surface identifying a low frequency band associated with the dynamics of the cutting process. It is not the purpose of this paper to present a simulation of the milling process but to extend the work reported in [ IO], on the use of spatial frequency characteristics of a
apparent pixel size was 8.8X8.8 pm, giving a field of view of 2.82 X 2.1 I mm. The data scanning rate was 2.0 pm s- ’ and the data were digitised to 8 bit resolution. The vertical resolution of the instrument was 0. I nm. The optical fibre differential probe is a development of a single-spot surface profile instrument [ I2 ] based on optical interferometry. This first instrument measures the distance between the probe head and a laser light spot (typically I& 20 pm diameter) on the target surface with a vertical resolution of 10 nm. Optical fibre enables the probe head to be separated from the rest of the optical and electronic systems, resuking in a compact device capable of operating in conditions that are more demanding than those necessary fc:,conventional optical metrology. However, thiu Simigk qx3: configuration is still susceptible to target vibration. The differential probe [ 131 is insensitive to out-of-plane target motion, since it measures the height difference between two focused spots on the surface by a Michelson interferometer arrangement. The interferometer output depends on the optical path length difference between the probe head and the two focused spots, so that large-scale out-of-plane surface vibrations are common-mode for both signals. A schematic of the instrument is shown in Fig* 1. The optics necesszuy to generatethe two spots on the surfaceand collect the reflected light were housed in the probe head, linked by a single optical fibre to the rest of the system. The probe head and translation stage were mounted on the machine tool headstock to scan the milled surface of the workpiece. The scanning speed was I mm s-l, and the two spots were 3 mm apart.
milled surface as an indicatorof tool wear state, by applying the technique to a face milled aluminium alloy. It will also be demonstrated that it is possible to measure the spatial
4. Experin~ntol
frequency attributes by means of a fibre optic probe mounted on the machine tool and thus provide an indication of the tool wear state.
Absolute profile measurements were made using a ZYGO Corporation New View 100 3D Imaging Surface Structure Analyser. This instrument is a scanning white light interferometer [ I 1 ] which uses a precision translation stage and an interferometric objective to produce an interferogram of the test surface, which is imaged by a Charge coupled device (CCD) camera. Software then translates the interferogram into a quantitative three-dimensional image of the surface. The data resulting from this translation are then available either for display or for further off-line analysis by the user. The microscope in this system has several objective lenses of vacying magnifying powers available. In this work a Michelson objective with a power of X2.5 was used. The CCD camera array density was 320 X 240 pixels with a pixel size of 1 I X 11 pm, and with the magnification employed the
pr~edure
In order to generate specimen surfaces from which surface finish parameters could be extracted, a series of wear tests was performed on blocks of aluminium alloy. The tests were limited to finishing cuts during a face milling process. Work on other materials using single and multi-point cutting has been reported elsewhere [ IO]. Single point cutting was employed in this work to avoid the complications associated with multi-point cutting observed in surface profiles. This was achieved by mounting a single insert in the tool holder. Coolant was not used. Even using single point cutting and cutting conditions at the high end of the manufacturer’s recommended range for the insert, the wear rate observed was exceptionally low, the length of the wear land ( V,) being less th2.n 0.05 mm after 100 tool passes. In consequence it was not practical to use naturally worn inserts for the cutting process wear was induced face miliing quenched tempered En24 prior to aluminium alloy test proper. cutting conditions employed produce the wear a feed tooth of mn= and cutting speed 45 m ‘* The cutting speed employed to that the of edge degradation obtained of that would be produced by prolonged milling of the aluminium alloy. In
P. Wilkinson
toffr0m
launch optics
el al. /
Wear 212 ( 19971221-228
]mWrL probe housing
Fig.
I. Schematic
of differential two-spot pmbe:
223
t a) layoutof optics
2
and data acquisition electronics: (h) detail of fibre optic Michelson interferometer in
probe housing.
this paper a comparison of the surface parameters obtained with a new insert and those obtained with a damaged insert will be presented. The machine tool employed in this work was a WADKIN V5 CNC machining centre. Its headstock natural frequencies are known to be in the range 20 to 120 Hz in the feed direction [ 141 with dynamic flexibiIities of 2 X IO’ to 4 X IO” pm N - ’ . The spindle speeds used in these tests contained forcing frequencies within the dynamic range of the headstock so that deflection of the headstock could in principle be reflected in the measured surface profile as changes in low spatial frequency (waviness) characteristics. Any increase in waviness amplitude is likely to be associated with increased cutting forces with wear. However, the feed deflection generated in cutting the soft aluminium alloy has been estimated to be of the order of 0.25 pm [ IO] and changes in cutting force are likely to be small, resulting in only minor changes in waviness. The tool holder was a SEC0 220. I3 with a cutting rake of + 12” and a lead angle of 45”. Its diameter was 100 mm and it was capable of carrying eight inserts although in this case it was used to fly-cut (single point cutting) the specimen material. SEC0 type SEtCR-AFN grade FIX inserts were
used. They possess a 49 edge with a 20” clearance angle and the cutting edge has a local positive cutting rake angle of 30”. Although the cutting conditions employed were toward the high end of the manufacturer’s recommendations they remained within the range used in common practice. The depth of cut was 0.5 mm and the feed per tooth was 0. I5 mm. The cutting speed was 1600 m min - I. The cutter was located symmetrically with respect to the width of the test blocks. The test material was a heat-treatable aluminium alloy type 6082 in the T6 condition. This material was chosen as an example of a machinable material that would complement work carried out by the authors on more difficult to machine materials suchas annealed En24 st?A and stainless steel type 394 [ IO]. The test specimens were in the form of blocks of width 60 mm and length 250 mm. Insert wear was assessed for each of the pre-worn inserts using an optical microscope. The inserts were grouped in order of a damage parameter which ordered our estimates of the volume of cutting edge removed in the cutting process. The results presented in this paper compare the surface generated by a new insert (no damage) with that generated by the most damaged insert (extensive chipping extending from the cutting edge onto the wiping flat of the insert!. The un-
224
P. Wilkinson et al. / Wear 2 I2 (I 997) 221-228
I’
0.0
*
0.5
*
‘.
1
’
.o
*
1.5
”
I
‘.
2.0
2.5
3.0
Dhtuncc scanned (mm )
Fig. 2. SEM photqp&
oftmdmqcd
insert.
2 (I;,
-
;
1 :
.
:
-1
i
-
;
; :
.
’
.
i
i
- ._~____._......_.~...............~.......~,.*...~~.~...........~~.~.......~..*~............. i
L’
.
0.0
: ’
.
’
1 .o
0.5
*
’
1.5
.
’
2.0
.
2.5
1 3.0
Distoncs scanned ( mm ) Fig. 4. Absolute profiles produced by fly-cutting aluminium using (u) insertand (b) damaged insert.
Fig. 3. SEM photopph of damaged insert showing loss of cuttingedge and damageon wiping flat.
damaged insert is shown in Fig. 2 and the damaged inseti is shown in Fig. 3. The surface profile information was obtained from both the interference microscope and the fibre optic differentiai profiIometer. In order to obtain data from the interference microscope, the workpiece was removed from the machine tool. The interference microscope scanned a section of each surface from which profile information was obtained. The
resultant data files consisted of 76 800 points made up of a matrix of 320 X 240 points and, of these, line profiles conof 320 points were used in further processing. Fig. 3(a) and Fig. 4(b) show the absolute profiles as measured by the interference microscope.
sisting
sidering the relationship
new
between integrated energy content
in particular frequency bands and insert chipping. Three spatial frequency regions have been identified
as containing information reIevant to tool wear monitoring [ IO]: a low frequency or ‘waviness’ band containing frequencies below tooth passing frequency, a kinematic frequency band occurring around the tooth passing frequency and a high frequency band containing spatial frequencies in excess
of the fundamental tooth
passing frequency. The
effects of tool wear on the waviness band in machining harder materials have been established [ 101, and since, as indicated above, relatively minor changes in waviness are to be expected in cutting softer aluminium alloys, this work will examine only those changes that occurred in the kinematic
probe
frequency band and the high frequency band. In the present
whilst the workpiece was mounted on the machine tool by traversing the instrument over the surface using a precision stage, A total of 10000 points per surface scanned were
experiments, the feed per tooth was 0.15 mm. Thus we define the kinematic band by the spatial frequency range 2.5-17.0 mm-’ and the high frequency band by spatial frequencies greater than 17 mm - ’ .
Measurements
were obtained from the differential
obtained in this process. The measured differential profiles are shown in Fig. S(a) and Fig. S(b).
surface
It is not easy to derive such common surface profile parameters as R, from the differential profile as measured by the
fibre optic instrument but the characteristics of these profiles are readily available in the spatial frequency domain. In consequence this paper will examine the prolIes obtained from both
instruments in the frequency domain, specifically con-
The spatial frequency response of the differentiai optical probe is not flat. For spatial frequencies corresponding
exactly to the spot separation, 3 mm in the present case, there is a null response [ I3 1. However, for the higher spatial fre-
quencies of interest in the present context, the response is sufficiently unifomr when averaged over bands very much greater than l/3 mm - ‘,
P. Wilkhsnn tv al. / Wp~lr 212
(1W/I 22 I-228
225
Examination of Fig. 4(a) and Fig. 4(b) which show the absolute profiles allows the qualitative conclusion to be drawn that as insert damage occurred. the character of the profile changed markedly. Specifically, the high frequency
components have been attenuated whilst the waviness component in the profile has, as was expected, only changed marginally with tool damage. This is confirmedfrom conwiderationof Fig. 5 ( a) and Fig. S(b) which showthedifferential profiles obtained from the fibrc optic probe and from Figs. 6 and 7.
Distance
0.0
0.1
0.2
0.3
0.4
sconncd (mm )
0.5
0.6
0.7
0.8
0.9
1.0
Fig. 5. Differential profiles of fly-cu~ aluminium produced by and
(b)
damaged
fig. 7. SEM photognph of surface producedby damaged insert showing attenuation of high frequency
Distance scanned ( mm )
( a) new insert
waviness
structure
in presence
of marginallychanged
component.
insert.
0.6
5. Resultsand discussion
0.5
:i_.____. ..... Ii a 1....1..,..,:..........._..
0.4
Even in the absence of wear, the generation of milled surfaces includes a combination of dynamic and kinematic effects. The formerareassociated with the natural frequencies and dynamic Bexibilities of the machine and the latter are assaciated with the geometry of the cutting edge and, for milling, the feed per tooth. Long spatial wavelength variations due to machine vibrations are normally distinguished from the shorter wavelength variations associated with the tool profile and its movement relative to the cut surface.
0.5
c-i ... ....
1..
,
.
,;.
.
.
.
.
.
.
.
.
1 .o
1.5
Distance
scanned
.
.
.
.
.
.
Fig. 8. Filtered absolute surface
damagedinsert.
2.5
3.0
~,_.,_,.____._.i............,.~.,..,..........~...........~..~
Distance
Fig. 6. SEM photographof surfaceproducedby undamagedinsertshowing high frequencystructurein presenceof waviness.
2.0 (mm)
scanned
(mm)
profilesproducedby (a) new inscrland (h)
P. Wilkinson
226
ef al. / Wear
212
(I 997)
22 1-228
t ,o 0.a 0.6 0.1 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1 .a
0.0
0.1
0.2
0.3 0.4 0.5 0.6 0.7 Ohtanca scanned (mm)
0.8
0.9
0
1.a
20 spotid
0 Distctnce sconfwd
10
(mm)
20
10
insert. Fig. 9(a) and Fig. 9( b j show the corresponding filtereddifferentialdata. The filteredabsolute profile recordsproducedby the interference microscope contained 318 points and spatial frequency spectra of these profiles were obtained by processing all 3 18 points. Initially profiles from six locations on both of the specimen surfaces wea obtained so that rqeatability of the surface profiles could be investigated. Typical spectra are shown
in Fig. IO(a) for the surface produced
by the new
insert and Fig. 10(b) for that produced by the chipped insert. Subsequently profiles from a further sixteen locations (Fig. 11) on each specimen surface were measured and processed in a like manner. The sixteen profiles were made up of eight centre line measurements, four on the insert entry edge of the specimen and four on the exit edge. In order to demonstrate that a 1 mm scan length contains sufficient information to allow assessment of cutting edgecondition, alength of 1.Omm C1 I3 points) was extracted from the profile data and processed to produce spectral information.
-
1
40
*
1
50
60
(cycles/mm)
40
30
frequency
50
60
(cycl~s/mm)
Fig. 10. Spatial frequency spectra corresponding to the filtered absolute profiles produced by {a) new insert and (b) damaged insert.
In order to remove the effects of the low frequency components of the surface profiles from the spatial frequency spectra, the data from both tbe interference microscope and the differential probe were processed by two first order highpass digital filters both with a cut-off spatial frequency of 4 cycles mm - ’ . Fig. 8(a) and Fig. 8(b) show the ftlteredabsolute profiles produced by the new insert and the damaged
fi 30
frequency
Spatid
Fig. 9. Fittered differential surface profiles produced by [a) new insert and (b) damaged insert.
*
nun
Nonbock
Nocbsefthon15mm 10 edge
DD
cutzw\
4 qipoctd
xons
OII line 6 mm
fromMl side Fig. II. Diagram &owing approximate locations of scans of specimen surfaces.
Each filtered differential profile record consisted of 9998 points. To reduce the computational burden every second point was selected from these records resulting in data sets of 5000 points from which the middle batch of 4096 points was processed to produce a spatial frequency spectrum of the differential profile, The spatial frequency spectra thus obtained are shown in Fig. 12(a) and Fig. 12(b) . The spectra were integrated over the selected frequency bands shown in Figs. 10 and I2 and the resulting band energies defined a value of *integrated spectral content’ for each surface. Table 1 shows the integrated spectral content of the filtered data obtained from the interference microscope in the kinematic band and the high frequency band for the surface generated by a new insert and that generated by a damaged insert. The means and standard deviations obtained from the six
P. Wilkinson er ~4. /Wear 212 I lUY7) 221-22X
227
Table 3 One millinletre sari lengthintegratedspectralcontentof milled aluminium surfaces ( arbirrary units) from the interference microscope over 16 additionat sites Damaged insert
New inset-l 0.06
Kinematic band High frequency
L...
*
’
*
10
’
.
’
.
’
*
’
I 60
*
50
20 30 40 Spatial frequency (cycles/mm)
band
Meall
S.D.
Mean
S.D.
0.23
0.05
0.10
0.09
0.48
0.09
0.18
0.13
Table 4 Probabilities [hat data sets produced by different insert cutting edge condi-
tions arise from the same population at the 5% level Scan length
Kinematic band High frequency band
2.83 mm
l.Omm
3.2 x IO-” 4x lo-‘”
2.9 X 1O-5 1.2x Io-w
Table 5 Integrated spectr-I content of milled aluminium surfaces (arbitrary units) 0.02 0.00
from the fibre optic probe I.
0
I.
I.
I.
9
20
30
40
50
I.
10
Spatial frequency (cycles/mm)
I
Integrated spectmtcontentof milled aluminium surfaces(arbitrary units) from the interference microscope New inset7
Damaged insert
Mean
SD.
Mean
S.D.
Kinematic band
0.15
0.02
0.07
0.02
High frequencyband
0.26
0.02
0.13
0.03
Tabie 2 Full scan length integrated spectral content of milled aluminium surfaces
(arbitrary units) from the interference microscope over 16 additional sites New insert
Kinematic band High frequency band
Damaged insert
Kinematic band
0.86
0.59
High frequency band
I *OS
0.68
60
Fig. 12. Spatial frequency spectra corresponding to Lhe filtered differential profiles produced by ( a 1newinsert and (b 1damaged insert. Table
New insert
*
Darllaged inset-i
Mean
S.D.
Mean
S.D.
0.14 0.29
0.02 0.03
0.06 0.12
0.03 0.05
locations on the specimen surfaces are given in Table I ; it is clear from the small standard deviations that the data are representative of the entire milled surface. Table 2 shows the means and standard deviations in the spectral content obtained from the further sixteen sites on each test specimen for the full scan length of the interference microscope with results consistent with those of Table 1,
Table 3 shows the same information acquired from the 1 mm scan !ength. The use of a shorterscan length resuh carries the penalty of a diminished frequency discrimination in the associated spectra, This may be seen in the larger standard deviations of Table 3 as compared with those of Table 2. In order to establish that the data produced by the new insert and those produced by the damaged insert came from statistically different populations when viewed at the two scan lengths, a one-way analysis of variance was performed on the data sets, The re5ulLs of this analysis for both scan lengths and for both frequency bands are contained in Table 4. The values given are the probabilities that the data sets for the surface,produced by the new insert and those produced by the damaged insert come from the same population at the 5% level. We may conclude from these resufts that the data do represent changes observed in the specimen surfaces as a resuit of insert damage and further that a I mm scan length contains sufficient information to confirm such changes. In the kinematic band most of the energy is associated with the feed per tooth. This energy is observed to reduce sign& candy with insert damage, a trend that is in agreement with that observed in machining En24 steel following a chipping event [ IO]. Similarly the energy associated with the high frequency band is observed to decrease with damage. This is also in agreement with observations made in machining En24 [ 101. Table 5 shows the integratedspectral content in both
228
P. Wilkinson
etal. / Weur 212 (1997) 221-228 metric probe. Further, we have shown that, in the spatial frequency domain, the differential profiles measured in-situ contain similar tool wear information to the absolute prcfiles measured conventionally after removing the workpiece from the machine tool. Hence a differential surface profile obtained
by a non-contact optical method can be used as the basis for monitoring the condition of the culling tool insert by means of the spatial frequency spectrum. Acknowle&ements
This work was supported in part by the UK Engineering and Physical Sciences Research Council under its ACME programme, which the authors gratefully acknowledge, J. MuhI, Department of Mechanical Engineering, University of Edinburgh, is thanked for provision of the interference microscope facility.
References [ I 1D.J. Whitehvuse. Handbook of Surface Metrology, Institute of Physics,
Fig*14sEMphoro%rophofstufacepmducedby smearing of work-hardened material0-m surface.
insert hewing
the kinematic frequency band and the high irequency band obtained fromtheoptical fibreprobeover a I mm scan length. The trend of reducing spectralcontent is readily seen in this table, thus confirming the systematic results obtained from the interference microscope. The reduction is less marked than that observed in the absolute profile (Table 3), which is to be expected from the presence of the nulls in the response spectrum of the differential probe. The reason for the reduced spectral content with insert damage most probably lies in the non-ideal chip formation characteristicsassociated with the damaged insert. This gives rise to a cut surfacewhere work hardenedmaterialis smeared over the surface during passage of the cutting edge and wiping flat. Such un effect can be seen by comparing magnified images of the cut surfaces as shown in Figs. 13 and 14.
We have demonstrated that energies in two spatial frequency bands (kinematic frequencies and high frequencies)
of the machined surfaces decrease significantly with insert damage for single point face milling of aluminium alloy. We have also demonstratedthat differential surface profile measurements can be made in-situ using a fibre optic jnterfero-
199J, pp. 732-738. [2] J.M. Bennett, Recent developnffnts in surface roughness characterization. Meas. Sci. Technal. 3 ( 1992) I t 19-I 127. I3 1 EN Diei. D.A. Domfeld.Acoustic emission sensing of tool wem in facemilling. Trans. ASMF 1. Eng. ind. 109 ( 1987) 234L240. [4] SX. Chwdhury, S. Ramesh, On-line tool wear sensing and cotnpensatioc in turning. J. Mater. Process. Technol. 49 ( 1995 ) 247254. [ 5 1 S.A. Coker, C.S. Yung. In-process control of surface roughness due to tool wear using a new ultrasonic system. Int. J. Mach. Tools Mzuluf. 36 t 19%) 411422. 161 GM. zhang, S. Ycmmareddy,S.M. Lee. SC-Y. la, Simuhuion of s~rfacctopography fbrmed during the intermittent turning process. Trans. ASME 3. Dyne Sys. 113 I 1991) 27>239. [ 7 1 GM. Bang. S.G. K.av+ Dynamic generation of machined surfaces, part 1: description of 9 random ercitation system, Trans. ASME J. Eng. Ittd, 113 ( 1991) 137-144. I8 1 G.M. Zhang. S.G. Kapow, Dynamic generation of machined surfaces. part 2: construction of surface topography, Trans. ASME J. Eng. Ind, I13 ( 1991)145-153. 191 K.S. Moon, J.W. Sutherland. The origin and interpretation of spatial frccpenciesin a turned surface profile, Trans. ASME 3. Eng. Ind. 116 ( 1994) 340-347. 1101P. Wilkinm. R.L. Reuben,J.D.C. Jones, J.S. Barton, D.P. Hand,T.h. Carolan. S.R. Kidd. :: *ace finish parameters as diagnostics of tool wear in face milling, Wear ( 19961, in press. I 111 T. Connolly, Scanning interferometer characterizes surfaces, Laser Focus World 3 1 ( 19%) 85-87. 1121 D.P. Hand. T.C. Carolan. J.S. Barton. J.D.C. Jones, Profile measuremen of optically rough surface by fibre-optic interferometry, Opt. htt. IS (1993) 1361-1363. II3 1S.R. Kiti. D-P. Hand. T.A. Carolan. J-S. Barton. j.D.C. Jones, Measurement of aspects of surface form using an optica; differential height measurement technique, Meas. Sci. Technol. 7 ( 1996) 15791582. I 141 S.J. Wilcox, W.K.D. Botthwick, R.L. Reuben, An investigation of the influence of machine/fixture stiffness in AE and workpiece vibration generated during face milling on bright mild steel. 4th Int. Conf. on Condition Monitoring and Diagnostic Engtneering Management COMADEM ‘92. Sanlis. France. July 1992.
Abstract We describe a set of wear tests in which machined surfaces were produced by single point face milling of an aluminium alloy. The objectives were firstly to identify surface characteristics associated with tool chipping. and secondly to demonstrate that a fibre optic surface profiling pro& capable of in-situ operation on the machine tool had the ability to provide the same wear information as a metrological interference microscope. Spectral energy of the surface profiles decreased significantly with insen damage in a spatial frequency band around the tooth passing frequency of the cutter and also at higher spatial frequencies. 0 I997 EIsevicr Science S.A.
1. Introduction The profile of a machined surface contains information th2t is irzdirative of the stale of wear of the tool producing that surface [ I 1. In consequence a surface descriptor may he a useful input to a system to monitor 1001 wear. Current surface measurement techniques involve either the use of stylus instruments to traverse the surface or of optical instruments such as interference microscopes and optical profilers [ 21. Both these methods require that the workpiece be dismounted from the machine so that measurements may be made in a vibration-free ena, ironment. This involves a loss in production time and precludes the possibility of unmanned operation of machine tools if surface finish parameters are to be us4 as indicators of tool wear. Additionally, a stylus instrument may lead to scoring damage of the surface of soft materials. An instrument that could be used with the workpiece in-situ on the machine would have the clear advantage that surface measurementscould be made inter-operation&y, thus admitting the possibilities of unmanned machining. In this paper, we report systematic measurements on face milIed aluminium alloy made bj a commercial white light
* Corresponding author. ’ Present address: Renishaw pk. Transducer Systems Division. Old Town, Wotron-under-Edge.Glouccstcrshirc.GLI 2 7DH. UK. ’ Presentaddress:Melles-Griot International.Cambridge.CB5 9QR. UK. 0043- f648/97/$17,00 0 1997 Elsevicr ScienceS.A. All rights rcscmd fIfSOO43-1648(97)00149-X
interference microscope. The results obtained are compared with those generated from a novel optical fibre probe mounted in-situ on the machine tool. Aluminium was chosen as an example of a softer material susceptible to scratching by a stylus profiler. for which optical non-contact methoc’s are particularly appropriate. The optical fibre instrument is a differential probe. in which the output is proportional to the height difference of the two sample spots on the surface. We show that the surface spatial frequency corresponding to the tooth passing frequency is suppressed when the insert is damaged. In the context of tool wear assessment,the optical fibre probe gives the same useful data as the interference microscope, but with the advantage that the probe measurements were made on the machine tool.
2. Background In recent years much interest has been generated in the monitoring productivity.
of machining processes with the aim of increasing In particular,
investigations
into the wear state
of the cutting tool by indirect means have been carried out. This requires the measurement of process variables that may be correlated with the evolution of insert wear, such as acoustic emission genera&during cutting [ 31 or the surface finish produced by the cutter.
222
P. Wilkinsm et al. / Wuur 212 (lw7) 221-228
Choudhury and Ramesh 14 3 used an optical displacement sensor to monitor the variation in workpiece dimensions with tool wear during the turning process. The output from the sensor was then used to compensate for these variations by adjusting the depth of cut. Coker and Yung [5 J have investigated the use of an ultrasonic sensor to detect the in-process changes in surface roughES, as &fined by the parameter R,. which result from tool wear in face milling aluminium. The sensor was incorporated inw a control system which could maintain surfaceroughness within 10% of a specified value for tool wear land lengths up to 0.3 mm. Considerable work has beerr carried out on the modelling and simulation of surface generation with the models obtained usually giving an indication of the expected value of I?, in the simulated surface. Zhang et al. [6] produced a model of the intermittent turning process which incorporated tool vibrationand tool geometric motion. Zhang and Kapoor [ 7,8] have simulated the generation of a surface by a single point cutter combining a deterministic component resulting ftom tool geometric motion and a stochastic component arising from random vibrations of the tool holder. Confidence in this model was established using the results from a series of boring machining tests and co.llparing predicted and experimental values of I?,. Moon and Sutherland [9] have studied tbe spatial frequency content of a turned surface identifying a low frequency band associated with the dynamics of the cutting process. It is not the purpose of this paper to present a simulation of the milling process but to extend the work reported in [ IO], on the use of spatial frequency characteristics of a
apparent pixel size was 8.8X8.8 pm, giving a field of view of 2.82 X 2.1 I mm. The data scanning rate was 2.0 pm s- ’ and the data were digitised to 8 bit resolution. The vertical resolution of the instrument was 0. I nm. The optical fibre differential probe is a development of a single-spot surface profile instrument [ I2 ] based on optical interferometry. This first instrument measures the distance between the probe head and a laser light spot (typically I& 20 pm diameter) on the target surface with a vertical resolution of 10 nm. Optical fibre enables the probe head to be separated from the rest of the optical and electronic systems, resuking in a compact device capable of operating in conditions that are more demanding than those necessary fc:,conventional optical metrology. However, thiu Simigk qx3: configuration is still susceptible to target vibration. The differential probe [ 131 is insensitive to out-of-plane target motion, since it measures the height difference between two focused spots on the surface by a Michelson interferometer arrangement. The interferometer output depends on the optical path length difference between the probe head and the two focused spots, so that large-scale out-of-plane surface vibrations are common-mode for both signals. A schematic of the instrument is shown in Fig* 1. The optics necesszuy to generatethe two spots on the surfaceand collect the reflected light were housed in the probe head, linked by a single optical fibre to the rest of the system. The probe head and translation stage were mounted on the machine tool headstock to scan the milled surface of the workpiece. The scanning speed was I mm s-l, and the two spots were 3 mm apart.
milled surface as an indicatorof tool wear state, by applying the technique to a face milled aluminium alloy. It will also be demonstrated that it is possible to measure the spatial
4. Experin~ntol
frequency attributes by means of a fibre optic probe mounted on the machine tool and thus provide an indication of the tool wear state.
Absolute profile measurements were made using a ZYGO Corporation New View 100 3D Imaging Surface Structure Analyser. This instrument is a scanning white light interferometer [ I 1 ] which uses a precision translation stage and an interferometric objective to produce an interferogram of the test surface, which is imaged by a Charge coupled device (CCD) camera. Software then translates the interferogram into a quantitative three-dimensional image of the surface. The data resulting from this translation are then available either for display or for further off-line analysis by the user. The microscope in this system has several objective lenses of vacying magnifying powers available. In this work a Michelson objective with a power of X2.5 was used. The CCD camera array density was 320 X 240 pixels with a pixel size of 1 I X 11 pm, and with the magnification employed the
pr~edure
In order to generate specimen surfaces from which surface finish parameters could be extracted, a series of wear tests was performed on blocks of aluminium alloy. The tests were limited to finishing cuts during a face milling process. Work on other materials using single and multi-point cutting has been reported elsewhere [ IO]. Single point cutting was employed in this work to avoid the complications associated with multi-point cutting observed in surface profiles. This was achieved by mounting a single insert in the tool holder. Coolant was not used. Even using single point cutting and cutting conditions at the high end of the manufacturer’s recommended range for the insert, the wear rate observed was exceptionally low, the length of the wear land ( V,) being less th2.n 0.05 mm after 100 tool passes. In consequence it was not practical to use naturally worn inserts for the cutting process wear was induced face miliing quenched tempered En24 prior to aluminium alloy test proper. cutting conditions employed produce the wear a feed tooth of mn= and cutting speed 45 m ‘* The cutting speed employed to that the of edge degradation obtained of that would be produced by prolonged milling of the aluminium alloy. In
P. Wilkinson
toffr0m
launch optics
el al. /
Wear 212 ( 19971221-228
]mWrL probe housing
Fig.
I. Schematic
of differential two-spot pmbe:
223
t a) layoutof optics
2
and data acquisition electronics: (h) detail of fibre optic Michelson interferometer in
probe housing.
this paper a comparison of the surface parameters obtained with a new insert and those obtained with a damaged insert will be presented. The machine tool employed in this work was a WADKIN V5 CNC machining centre. Its headstock natural frequencies are known to be in the range 20 to 120 Hz in the feed direction [ 141 with dynamic flexibiIities of 2 X IO’ to 4 X IO” pm N - ’ . The spindle speeds used in these tests contained forcing frequencies within the dynamic range of the headstock so that deflection of the headstock could in principle be reflected in the measured surface profile as changes in low spatial frequency (waviness) characteristics. Any increase in waviness amplitude is likely to be associated with increased cutting forces with wear. However, the feed deflection generated in cutting the soft aluminium alloy has been estimated to be of the order of 0.25 pm [ IO] and changes in cutting force are likely to be small, resulting in only minor changes in waviness. The tool holder was a SEC0 220. I3 with a cutting rake of + 12” and a lead angle of 45”. Its diameter was 100 mm and it was capable of carrying eight inserts although in this case it was used to fly-cut (single point cutting) the specimen material. SEC0 type SEtCR-AFN grade FIX inserts were
used. They possess a 49 edge with a 20” clearance angle and the cutting edge has a local positive cutting rake angle of 30”. Although the cutting conditions employed were toward the high end of the manufacturer’s recommendations they remained within the range used in common practice. The depth of cut was 0.5 mm and the feed per tooth was 0. I5 mm. The cutting speed was 1600 m min - I. The cutter was located symmetrically with respect to the width of the test blocks. The test material was a heat-treatable aluminium alloy type 6082 in the T6 condition. This material was chosen as an example of a machinable material that would complement work carried out by the authors on more difficult to machine materials suchas annealed En24 st?A and stainless steel type 394 [ IO]. The test specimens were in the form of blocks of width 60 mm and length 250 mm. Insert wear was assessed for each of the pre-worn inserts using an optical microscope. The inserts were grouped in order of a damage parameter which ordered our estimates of the volume of cutting edge removed in the cutting process. The results presented in this paper compare the surface generated by a new insert (no damage) with that generated by the most damaged insert (extensive chipping extending from the cutting edge onto the wiping flat of the insert!. The un-
224
P. Wilkinson et al. / Wear 2 I2 (I 997) 221-228
I’
0.0
*
0.5
*
‘.
1
’
.o
*
1.5
”
I
‘.
2.0
2.5
3.0
Dhtuncc scanned (mm )
Fig. 2. SEM photqp&
oftmdmqcd
insert.
2 (I;,
-
;
1 :
.
:
-1
i
-
;
; :
.
’
.
i
i
- ._~____._......_.~...............~.......~,.*...~~.~...........~~.~.......~..*~............. i
L’
.
0.0
: ’
.
’
1 .o
0.5
*
’
1.5
.
’
2.0
.
2.5
1 3.0
Distoncs scanned ( mm ) Fig. 4. Absolute profiles produced by fly-cutting aluminium using (u) insertand (b) damaged insert.
Fig. 3. SEM photopph of damaged insert showing loss of cuttingedge and damageon wiping flat.
damaged insert is shown in Fig. 2 and the damaged inseti is shown in Fig. 3. The surface profile information was obtained from both the interference microscope and the fibre optic differentiai profiIometer. In order to obtain data from the interference microscope, the workpiece was removed from the machine tool. The interference microscope scanned a section of each surface from which profile information was obtained. The
resultant data files consisted of 76 800 points made up of a matrix of 320 X 240 points and, of these, line profiles conof 320 points were used in further processing. Fig. 3(a) and Fig. 4(b) show the absolute profiles as measured by the interference microscope.
sisting
sidering the relationship
new
between integrated energy content
in particular frequency bands and insert chipping. Three spatial frequency regions have been identified
as containing information reIevant to tool wear monitoring [ IO]: a low frequency or ‘waviness’ band containing frequencies below tooth passing frequency, a kinematic frequency band occurring around the tooth passing frequency and a high frequency band containing spatial frequencies in excess
of the fundamental tooth
passing frequency. The
effects of tool wear on the waviness band in machining harder materials have been established [ 101, and since, as indicated above, relatively minor changes in waviness are to be expected in cutting softer aluminium alloys, this work will examine only those changes that occurred in the kinematic
probe
frequency band and the high frequency band. In the present
whilst the workpiece was mounted on the machine tool by traversing the instrument over the surface using a precision stage, A total of 10000 points per surface scanned were
experiments, the feed per tooth was 0.15 mm. Thus we define the kinematic band by the spatial frequency range 2.5-17.0 mm-’ and the high frequency band by spatial frequencies greater than 17 mm - ’ .
Measurements
were obtained from the differential
obtained in this process. The measured differential profiles are shown in Fig. S(a) and Fig. S(b).
surface
It is not easy to derive such common surface profile parameters as R, from the differential profile as measured by the
fibre optic instrument but the characteristics of these profiles are readily available in the spatial frequency domain. In consequence this paper will examine the prolIes obtained from both
instruments in the frequency domain, specifically con-
The spatial frequency response of the differentiai optical probe is not flat. For spatial frequencies corresponding
exactly to the spot separation, 3 mm in the present case, there is a null response [ I3 1. However, for the higher spatial fre-
quencies of interest in the present context, the response is sufficiently unifomr when averaged over bands very much greater than l/3 mm - ‘,
P. Wilkhsnn tv al. / Wp~lr 212
(1W/I 22 I-228
225
Examination of Fig. 4(a) and Fig. 4(b) which show the absolute profiles allows the qualitative conclusion to be drawn that as insert damage occurred. the character of the profile changed markedly. Specifically, the high frequency
components have been attenuated whilst the waviness component in the profile has, as was expected, only changed marginally with tool damage. This is confirmedfrom conwiderationof Fig. 5 ( a) and Fig. S(b) which showthedifferential profiles obtained from the fibrc optic probe and from Figs. 6 and 7.
Distance
0.0
0.1
0.2
0.3
0.4
sconncd (mm )
0.5
0.6
0.7
0.8
0.9
1.0
Fig. 5. Differential profiles of fly-cu~ aluminium produced by and
(b)
damaged
fig. 7. SEM photognph of surface producedby damaged insert showing attenuation of high frequency
Distance scanned ( mm )
( a) new insert
waviness
structure
in presence
of marginallychanged
component.
insert.
0.6
5. Resultsand discussion
0.5
:i_.____. ..... Ii a 1....1..,..,:..........._..
0.4
Even in the absence of wear, the generation of milled surfaces includes a combination of dynamic and kinematic effects. The formerareassociated with the natural frequencies and dynamic Bexibilities of the machine and the latter are assaciated with the geometry of the cutting edge and, for milling, the feed per tooth. Long spatial wavelength variations due to machine vibrations are normally distinguished from the shorter wavelength variations associated with the tool profile and its movement relative to the cut surface.
0.5
c-i ... ....
1..
,
.
,;.
.
.
.
.
.
.
.
.
1 .o
1.5
Distance
scanned
.
.
.
.
.
.
Fig. 8. Filtered absolute surface
damagedinsert.
2.5
3.0
~,_.,_,.____._.i............,.~.,..,..........~...........~..~
Distance
Fig. 6. SEM photographof surfaceproducedby undamagedinsertshowing high frequencystructurein presenceof waviness.
2.0 (mm)
scanned
(mm)
profilesproducedby (a) new inscrland (h)
P. Wilkinson
226
ef al. / Wear
212
(I 997)
22 1-228
t ,o 0.a 0.6 0.1 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1 .a
0.0
0.1
0.2
0.3 0.4 0.5 0.6 0.7 Ohtanca scanned (mm)
0.8
0.9
0
1.a
20 spotid
0 Distctnce sconfwd
10
(mm)
20
10
insert. Fig. 9(a) and Fig. 9( b j show the corresponding filtereddifferentialdata. The filteredabsolute profile recordsproducedby the interference microscope contained 318 points and spatial frequency spectra of these profiles were obtained by processing all 3 18 points. Initially profiles from six locations on both of the specimen surfaces wea obtained so that rqeatability of the surface profiles could be investigated. Typical spectra are shown
in Fig. IO(a) for the surface produced
by the new
insert and Fig. 10(b) for that produced by the chipped insert. Subsequently profiles from a further sixteen locations (Fig. 11) on each specimen surface were measured and processed in a like manner. The sixteen profiles were made up of eight centre line measurements, four on the insert entry edge of the specimen and four on the exit edge. In order to demonstrate that a 1 mm scan length contains sufficient information to allow assessment of cutting edgecondition, alength of 1.Omm C1 I3 points) was extracted from the profile data and processed to produce spectral information.
-
1
40
*
1
50
60
(cycles/mm)
40
30
frequency
50
60
(cycl~s/mm)
Fig. 10. Spatial frequency spectra corresponding to the filtered absolute profiles produced by {a) new insert and (b) damaged insert.
In order to remove the effects of the low frequency components of the surface profiles from the spatial frequency spectra, the data from both tbe interference microscope and the differential probe were processed by two first order highpass digital filters both with a cut-off spatial frequency of 4 cycles mm - ’ . Fig. 8(a) and Fig. 8(b) show the ftlteredabsolute profiles produced by the new insert and the damaged
fi 30
frequency
Spatid
Fig. 9. Fittered differential surface profiles produced by [a) new insert and (b) damaged insert.
*
nun
Nonbock
Nocbsefthon15mm 10 edge
DD
cutzw\
4 qipoctd
xons
OII line 6 mm
fromMl side Fig. II. Diagram &owing approximate locations of scans of specimen surfaces.
Each filtered differential profile record consisted of 9998 points. To reduce the computational burden every second point was selected from these records resulting in data sets of 5000 points from which the middle batch of 4096 points was processed to produce a spatial frequency spectrum of the differential profile, The spatial frequency spectra thus obtained are shown in Fig. 12(a) and Fig. 12(b) . The spectra were integrated over the selected frequency bands shown in Figs. 10 and I2 and the resulting band energies defined a value of *integrated spectral content’ for each surface. Table 1 shows the integrated spectral content of the filtered data obtained from the interference microscope in the kinematic band and the high frequency band for the surface generated by a new insert and that generated by a damaged insert. The means and standard deviations obtained from the six
P. Wilkinson er ~4. /Wear 212 I lUY7) 221-22X
227
Table 3 One millinletre sari lengthintegratedspectralcontentof milled aluminium surfaces ( arbirrary units) from the interference microscope over 16 additionat sites Damaged insert
New inset-l 0.06
Kinematic band High frequency
L...
*
’
*
10
’
.
’
.
’
*
’
I 60
*
50
20 30 40 Spatial frequency (cycles/mm)
band
Meall
S.D.
Mean
S.D.
0.23
0.05
0.10
0.09
0.48
0.09
0.18
0.13
Table 4 Probabilities [hat data sets produced by different insert cutting edge condi-
tions arise from the same population at the 5% level Scan length
Kinematic band High frequency band
2.83 mm
l.Omm
3.2 x IO-” 4x lo-‘”
2.9 X 1O-5 1.2x Io-w
Table 5 Integrated spectr-I content of milled aluminium surfaces (arbitrary units) 0.02 0.00
from the fibre optic probe I.
0
I.
I.
I.
9
20
30
40
50
I.
10
Spatial frequency (cycles/mm)
I
Integrated spectmtcontentof milled aluminium surfaces(arbitrary units) from the interference microscope New inset7
Damaged insert
Mean
SD.
Mean
S.D.
Kinematic band
0.15
0.02
0.07
0.02
High frequencyband
0.26
0.02
0.13
0.03
Tabie 2 Full scan length integrated spectral content of milled aluminium surfaces
(arbitrary units) from the interference microscope over 16 additional sites New insert
Kinematic band High frequency band
Damaged insert
Kinematic band
0.86
0.59
High frequency band
I *OS
0.68
60
Fig. 12. Spatial frequency spectra corresponding to Lhe filtered differential profiles produced by ( a 1newinsert and (b 1damaged insert. Table
New insert
*
Darllaged inset-i
Mean
S.D.
Mean
S.D.
0.14 0.29
0.02 0.03
0.06 0.12
0.03 0.05
locations on the specimen surfaces are given in Table I ; it is clear from the small standard deviations that the data are representative of the entire milled surface. Table 2 shows the means and standard deviations in the spectral content obtained from the further sixteen sites on each test specimen for the full scan length of the interference microscope with results consistent with those of Table 1,
Table 3 shows the same information acquired from the 1 mm scan !ength. The use of a shorterscan length resuh carries the penalty of a diminished frequency discrimination in the associated spectra, This may be seen in the larger standard deviations of Table 3 as compared with those of Table 2. In order to establish that the data produced by the new insert and those produced by the damaged insert came from statistically different populations when viewed at the two scan lengths, a one-way analysis of variance was performed on the data sets, The re5ulLs of this analysis for both scan lengths and for both frequency bands are contained in Table 4. The values given are the probabilities that the data sets for the surface,produced by the new insert and those produced by the damaged insert come from the same population at the 5% level. We may conclude from these resufts that the data do represent changes observed in the specimen surfaces as a resuit of insert damage and further that a I mm scan length contains sufficient information to confirm such changes. In the kinematic band most of the energy is associated with the feed per tooth. This energy is observed to reduce sign& candy with insert damage, a trend that is in agreement with that observed in machining En24 steel following a chipping event [ IO]. Similarly the energy associated with the high frequency band is observed to decrease with damage. This is also in agreement with observations made in machining En24 [ 101. Table 5 shows the integratedspectral content in both
228
P. Wilkinson
etal. / Weur 212 (1997) 221-228 metric probe. Further, we have shown that, in the spatial frequency domain, the differential profiles measured in-situ contain similar tool wear information to the absolute prcfiles measured conventionally after removing the workpiece from the machine tool. Hence a differential surface profile obtained
by a non-contact optical method can be used as the basis for monitoring the condition of the culling tool insert by means of the spatial frequency spectrum. Acknowle&ements
This work was supported in part by the UK Engineering and Physical Sciences Research Council under its ACME programme, which the authors gratefully acknowledge, J. MuhI, Department of Mechanical Engineering, University of Edinburgh, is thanked for provision of the interference microscope facility.
References [ I 1D.J. Whitehvuse. Handbook of Surface Metrology, Institute of Physics,
Fig*14sEMphoro%rophofstufacepmducedby smearing of work-hardened material0-m surface.
insert hewing
the kinematic frequency band and the high irequency band obtained fromtheoptical fibreprobeover a I mm scan length. The trend of reducing spectralcontent is readily seen in this table, thus confirming the systematic results obtained from the interference microscope. The reduction is less marked than that observed in the absolute profile (Table 3), which is to be expected from the presence of the nulls in the response spectrum of the differential probe. The reason for the reduced spectral content with insert damage most probably lies in the non-ideal chip formation characteristicsassociated with the damaged insert. This gives rise to a cut surfacewhere work hardenedmaterialis smeared over the surface during passage of the cutting edge and wiping flat. Such un effect can be seen by comparing magnified images of the cut surfaces as shown in Figs. 13 and 14.
We have demonstrated that energies in two spatial frequency bands (kinematic frequencies and high frequencies)
of the machined surfaces decrease significantly with insert damage for single point face milling of aluminium alloy. We have also demonstratedthat differential surface profile measurements can be made in-situ using a fibre optic jnterfero-
199J, pp. 732-738. [2] J.M. Bennett, Recent developnffnts in surface roughness characterization. Meas. Sci. Technal. 3 ( 1992) I t 19-I 127. I3 1 EN Diei. D.A. Domfeld.Acoustic emission sensing of tool wem in facemilling. Trans. ASMF 1. Eng. ind. 109 ( 1987) 234L240. [4] SX. Chwdhury, S. Ramesh, On-line tool wear sensing and cotnpensatioc in turning. J. Mater. Process. Technol. 49 ( 1995 ) 247254. [ 5 1 S.A. Coker, C.S. Yung. In-process control of surface roughness due to tool wear using a new ultrasonic system. Int. J. Mach. Tools Mzuluf. 36 t 19%) 411422. 161 GM. zhang, S. Ycmmareddy,S.M. Lee. SC-Y. la, Simuhuion of s~rfacctopography fbrmed during the intermittent turning process. Trans. ASME 3. Dyne Sys. 113 I 1991) 27>239. [ 7 1 GM. Bang. S.G. K.av+ Dynamic generation of machined surfaces, part 1: description of 9 random ercitation system, Trans. ASME J. Eng. Ittd, 113 ( 1991) 137-144. I8 1 G.M. Zhang. S.G. Kapow, Dynamic generation of machined surfaces. part 2: construction of surface topography, Trans. ASME J. Eng. Ind, I13 ( 1991)145-153. 191 K.S. Moon, J.W. Sutherland. The origin and interpretation of spatial frccpenciesin a turned surface profile, Trans. ASME 3. Eng. Ind. 116 ( 1994) 340-347. 1101P. Wilkinm. R.L. Reuben,J.D.C. Jones, J.S. Barton, D.P. Hand,T.h. Carolan. S.R. Kidd. :: *ace finish parameters as diagnostics of tool wear in face milling, Wear ( 19961, in press. I 111 T. Connolly, Scanning interferometer characterizes surfaces, Laser Focus World 3 1 ( 19%) 85-87. 1121 D.P. Hand. T.C. Carolan. J.S. Barton. J.D.C. Jones, Profile measuremen of optically rough surface by fibre-optic interferometry, Opt. htt. IS (1993) 1361-1363. II3 1S.R. Kiti. D-P. Hand. T.A. Carolan. J-S. Barton. j.D.C. Jones, Measurement of aspects of surface form using an optica; differential height measurement technique, Meas. Sci. Technol. 7 ( 1996) 15791582. I 141 S.J. Wilcox, W.K.D. Botthwick, R.L. Reuben, An investigation of the influence of machine/fixture stiffness in AE and workpiece vibration generated during face milling on bright mild steel. 4th Int. Conf. on Condition Monitoring and Diagnostic Engtneering Management COMADEM ‘92. Sanlis. France. July 1992.