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High-Precision Wire-EDM by Online Wire Positioning Control
High-Precision Wire-EDM by Online Wire Positioning Control Dr. ir. D. F. Dauw (21, Consultant; Dr. Phys. ETH I. Beltrarni, Vice-Director R & D AGIE, Losone, Switzerland Received on January 13,1994
The pa er deals with a technical realization to im rove the Wire EDM accuracy. The system whicg is readily available on commercial wire EgM machines is based on the on-line tracking and control of the wire position. The deviation of the wire position relative to the programmed wire path osition is continuously measured and corrections are bein made during the machine cutting. Ti% technique allows to cut complex sha es, arc paths andgcontours at a much faster cuttin speed as compared to conventional wire ED%l machines. Practical examples are discussed and t%e economical relevance is emphasized.
K
m
: Wire EDM, accuracy, precision
INTRODUCTION
A constant driving force for EDM machine tool builders is the challenge to design and manufacture wire EDM machines such that various market requirements, in particular those concerning part precision, can be met. Although WEDM is since recently and to some extent confronted with high spFed milling, it is pften.the sole alternative to finish the right part in the right time and at a justified customer cost. Yet, the ,s ecially desi ned wire EDM units, suited for the eolume Marfet" (i.e. machines to be sold at a reasonable customer price, but concentrating on a large potential marke!) will alwa s necessitate a high degree of .output precision, re ard;ess what market price will be demanded. In eed though, in various application areas no extreme accuracies are re uested, other growin application domains where hig precision is require evolve and feature an increasing market share.
i
a
8
Since the introduction of wire EDM on the market in 1969, 'overall performance has been increasing substantially. For instance, the cutting s eed has been doubled every four years, the obtaina le work iece surface finish has been improvin by a factor of &teen since the beginning of wire E D h , and the wire ta er an le has been enlarged, to some 40 degrees curreniy, [2fand [5].
g
However, wire. bending, which is a major cause of cutting imprecis!on, is still hampering the part accuracy for various applications. The fact that the wire itself is behavin like a metal string, straightened by two axial forces and deformed, by hydraulic forces in the ap, e ectro static forces acting on the wire and electro 8ynamic forces inherent to the spark generation, make the wire to lose its perfect straight position, Hence, when cutting out a curvature, the lag e fect of the wire creates a geometrical error on the workpiece to be machined. This error can be of the order of a hundred microns, which for some applications becomes unacceptable.
pu11in7
Figure 1 Schematic representation of an EDM wire during cutting. mathematically be modeled by the standard vibration equation for motion. It is assumed that the wire mass is uniformly spread along its considqred . length, i.e. between the wire guides. Where for stiff vibrating bars, stiffness is the most important propert , flexible strings or wires feature tension (mechanical road) as the most important Characteristic. An axial force F is applied to the wire. During machinin , other external forces are acting on the wire as hydfaulic forces, electro static forces, electro d namic forces, and pull-back forces [I], [3], [6], [7], [9], [lo]. Assuming an axial force applied to the wire, and that an external load q(z,t) varies as a function of time and space, then the general differential equation of motion for a stretched string of length L in a plane (along the z axis) can be written as:
[$,
F-
A hardware system with a proper control algorithm has been developed to remedy from this situation and will be dealt with in the next paragraphs.
WALYSIS OF WIRE DEFLECTION DUE TO PROCESS
lxmxs
One of the major geometrical inaccuracies of wire EDM eroded workpieces is due to the wire bending and deflection during the cutting. Figure I illustrates the wire deformation during a rough cut. The wire is fed through the gap by means of an unwinding system. The wire guides assure that the wire position is maintained properly but anyhow, various process forces acting on the wire make the wire bend. The wire can Annals of the ClRP Vol. 43/1/1994
a2y
F
~
E I a4Y , az -
= pS- a2Y at
+ c -ataY + q(z,t)
(1)
with F: axial force applied to the wire (N) E: Young's modulus (N/m2 I: moment of inertia = ar /4 (m4) p : wire density (k /mm3) S: wire section (mgL) y: wire deflection (m) c: damping coefficient (Ns/m) q (z,t): external load (N)
d
Because only static forces are considered, and assuming
193
that no time dependent phenomena are influencing the wire behavior, one can write equation (1) as:
This equation can be simplified if considering that wires used on wire EDM machines are hardly. sub'ected to bending moments. Indeed, the major loading orces are the axial pulling forces F and the gap forces. As will be shown later the latter ones are very small compared to the former ones. Hence:
f'
electro dynamical forces become dominant (I Iz). Results of test cuts in order to verify the theoretical model are shown in Figure 2 for workpiece thickness 50 and 150 mm. Note that the z-scale for the h = 150 m m curve has been reduced by a factor of 3 in order to compare the wire deflection y for both workpiece thickness values. One can observe that the larger the workpiece thickness, the larger the wire deflection in the middle of the workpiece, but the wire deflection D at the workpiece face remains practically constant as given by formula (4) for %.h = constant. An how, the wire tension force which can reduce the wire Lflection, cf. formula (4)? is limited b the maximu? yield strength of the wire electrode (320to 790 N/mm ).
N WORKPIECE ACCCXACY
The solution y = f(z) of this equation is a parabola "within the workpiece" if one assumes that the external load q(z) = qo is time independent and constant while the solution is a straight line 'lout of the workpiece", v.i.z. between the res ective wire guides.and the upper and lower workpiece .aces respectively since there q(z) = 0.The maximal wire deflection is given by solving equation (3) for z = U2,. where I is the distance between the upper and lower wire uides, h is the workpiece thickness, .H is the distance ?tween the. wire guide and the workpiece and D is the wire deflection at the upper and lower border of the workpiece, hence:
f!
%
+ D)
y (1/2) = - (- 'Oh2 8F
with D =
qo.h.H
2F
(4)
In order to have a good estimate of the w-ire deflection, the apload (z) must be known. Literature reveals data on t e gap oad [l], [7, [8]. However, today's wire EDM units feature much owerful pulse currents and gap flushing pressures ( = 78 bar), yieldin much more important gap loads on the wire too [5]. he load q(z) due to the spark collapse has been calculated in our research from the.measured wire deviations (= 300 pm at maximum cutting speed) and is found to be of the order of 9 N/m. It must be noticed that the load due to the spark formation will push the wire away from the workpiece zone, whereas the electromagnetic forces will tend to ull the wire towards the frontal workpiece area. A tota gap load qo of some 9 N/m is found to be real istic.
N
As long as a strai ht cut is being made [(zJ) or (z,y) plane], the wire. ag, and hence the geometrical error that yields from it have hardly any negative effect on the output results. However, in practice, contour aths are very .often executed. In such cases, the wire rag causes the wire to erode sharp corners and geometrical work iece information will be lost. F i y - e 3 depicts what is feine obtained when the wire guide position does not match with the real wire position. A part of a cut workpiece cross section is visualized in Figure 4..One can notice that the corner accuracy is not fully achieved, and that a geometrical error of some 50 pm is observed. TO remed from these errors, various means have been tried out y many researchers. For instance, increasing th? wire's tensile strength i n order to allow a larger wire tension force, or reducing the cutting speed when cutting comers possible with a combination of varying
B
g
a 4
9
P
X
WIRE GUIDE POSITION WIRE POSITION
Figure 3 Producing of geometrical errors due to the cor&ination of wire bending and contour EDM cutting. the pulse parameters to reduce the applied pulse ower, [3]. Because wire deflection cannot be avoi ed for "mechanical" reasons and the fact that reducing the cutting speed is not an economically justified approach, a real time correction of the cuttin path to be executed has been investigated and redzed, [4]. This is schematically represented in Figure 5. An optical wire positioning sensor, built in the upper wire guide, continuously measures the wire deflection relative to its theoretical position. The real cutting path is recalculated by the machine's Numeyical Controller (NC) and the necessary contour correction is carried out by the NC.
8
.
0
I
io
40
25 I
'
100 I
60
'
8'0
'
ioo
175 z(mm1 J(h= 150 mm)
Figure 2 Wire defection measurements for varying workpiece thickness. Worth to be noticed is also the fact that during wire EDM finishin the electro-static forces are important (= fI2?Fiereas for roughing regimes, the 194
HARDWARE CORRECTION
SOLUTION
FOR
CONTOUR
The sensor designed must be uaranteed to be immune a ainst electronic noise pertuiations generated by the ED process. Moisture splashing up out of the
%
work tank during machinin must not prevent the sensor from proper. working. TEerefore, an optical sensor s stem was finally selected. Jarious optical sensors final system was chosen. devices were considered.
surface. For our application, the length of the lens was chosen a multi le of the wave length to allow an outcoming paralye1 beam to be pro'ected onto the wire. This is also made clear by Figure 7!'
environment (humidity caused by the water dielectric, metallic particles disturbing the measurements and an extensive hydraulic pressure of the dielectric), this solution was disregarded: The. principle of the optical sensor we ended up with is outlined in Fipure 6.
Ugbt Emitter Configuration
Imagine the two boxes on the ri ht side of the wire, F i p r e 6. Each of them emits a Eeam of uniform and parallel li ht towards the wire along the perpendicular axes x an y. Imagine two detectors at the opposite side of the light emitters, each of them composed of two triangular shaped lenses. The wire shadow rojected on both x and y detectors is re resentative or the wire dis lacements in the x and y irection respectively. The ligk intensity difference. registered by the u per and lower triangular detector is a linear function o the wire displacement along the, perpendicular axes, whereas the sum of both light intensity si nals is dependin on the overall intensity which might e affected by a c ange in transparency of the dielectric around the wire. The practical realization of the sensors described above is
8.
P
b!
P fl
%
f
><
AGIECUT
To light emitting diodes are interfaced to two fiber optics (0,25 mm diameter) to the two SELFOP lenses (x, and ). The light beams are directed towards the wire. or reasons that the entering light beams are parallel for each respective axis x and y and that the wire's relative position will not disturb the parallelism of the light beams, wire .positioning measurements can be carried out relatively simple, Figure 6 and 7.
J
Light sensor confiruration Two trian ular lenses are faced to each other. Both triangular fenses have each there own focal center, and both foci are distinct and effective. As shown in Figure 6, the rojected wire shadow area is now measured with a doub e sensitivity for each measuring axis x and y.
-
P
The construction of such a bi-trian ular o tical sensor is made b facing two ground SEL$OC@ Yenses to each other. $deed, by removing (i.e. grinding) the dotted area of the auto focusing lens, without removing or disturbing the focal axis, one can manufacture a triangular .longitudinal lens which still features the same characteristics as the standard SELFOP lens, Figure 8.
j Figure 7 Principle of a SELFOP lens configuration. Hence, the complete structure of a the light sensor either x or y axis) consist of two triangular autoocusin lenses faced to each other such that both focal axes o?the triangular lenses are parallel to each other. Both lenses are each interfaced with two fiber optics (0.25 mm in diameter connected to the electronic circuitry for analysis an processing.
f
d'
Figure 5 Princgle of the. path correction applied in Wire DM machining.
carried out by usin s ecial optical "self focusing" lenses named "SELEO&'". They feature the same optical functions as standard spherical lenses but in addition,, both end surfaces are flat. This allows interfacin with optical fibers with ease. The "SELFO&" micro lens is a cylindrical lens, in which light of a uniform wavelength travels sinusoidally. By va ing the pitch length .of the lens, light can be cozmated, focused or divergent at the output flat
Figure 8 nYo ground self focusing lenses assembled together make a one axis sensor.
195
Practical built in solution on the wire EDM unit The wire positioning sensor is. built in and positioned in the upper wire lane (u,v) just undqneath the upper wire guide. Tfe closer the device is positioned to the workpiece, the more accurate the measurements are.. Fieure 6 depicts the principles how the sensor I S built in. Figure 9 depicts the real configuration.
c N
m
t
D P
CONTOUR CHARACTERISTICS
- CONNECTION RADII: 0.3 MM - TOTAL CONTOUR LENGTH: 82.1 M M - LENGTH OF STRAIGHT LINES: 72.2 M M - LENGTH OF CONNECTIONS: 9.9 M M 12 % - AMOUNT OF RADII:
af
Figure 9 Illustration o the assembled and built in wire positioning evice.
RESULTS OF REAL TIME W IRE POSITIONING DETECTION
STEEL WORKPIECE WORKPIECE THICKNESS: 5 0 MM CUTTING SPEED: 300 M M ~ ~ M I N
Fizure 10 a
Before, discusing the obtained results in terms of machining time and part precision, some principles of the built in control strategy will be explained. In addition, an idea of the problems and measurement thresholds are given.
Geometry of ihe standard test cut piece.
CUTTING TIME: 14.8 MIN (51 %f WITHOUT INTERVENTION
In order to correct the wire path the wire deflection is measured at a sampling eriod of 10 ms. At every measurement event, the rea direction vector "13" of the wire guides is known. A Correction vector "C" is computed based on the difference between the theoretical vector 'ID"and the real osition of the wire, measured at the sensor height. In Pact, a compensating cutting path is su erimposed and executed, yielding a correct cutting pat at the workpiece itself.
P
CUTTING TIME: 29.3 MIN (100 K) WITH CORNER STRATEGY
R
The fact that the maximum error alwa s occurs in the middle of the workpiece thickness, cf. Tormula (4), and the measurement is executed underneath the up er wire guide, an additional correction must be providedq Fi_ure 10 a outlines a test cut of a steel workpiece which is used for comparison purposes. A combination of straight cuts and curvatures allows to distinguish resu!ts when either a wire positioning correction is applied or no strategy at all. An enlarged view of the marked area in Figure 10 a (lower left part) is depicted in Figure 10 b. As already mentioned before, the wire deflection has a major effect when contours are being cut. Therefore, in Fipure I0 b three conditions are compared, where each time the cuttin conditions are different but the contour is idktical. The upper graph of this picture shows t e results r a rou h cut at a reference-cutting speed of 300 mmffimin. f i e corner accuracy is rater oor. In the middle figure, a cutting corner strategy, f)orrnerly applied on standard machines was selected. This strate y is based on pulse power reduction as a function otthe contour eomet . Already a substantial improvement is achieves thoug? the cuttin speed was substantially reduced. The last picture of Figure 10 b
geometX
196
CUTTING TIME: 15.6 MIN (53 %) WITH OPTICAL DETECTION
Figure 10 b
Enlarged view of right kand side of figure 10 a , when various straiegies are applied.
s
illustrates the results yieldin when the optical sensor detection technique is app ied [I/. A remarkable improvement of corner precision is obtained while maintaining the maximum cuttin s eed. Indeed, a ain in cutting time reductio of 5f was obtainecf i n comparison with the standard corner strategy.
Cuttin results with either one of. the strategies applief were Compared. Substantial machining time gains and corner accuracy improvements were noticed, when optical wire positioning and path correction was made.
k
Fipure I 1 is a similar test cut for a square wave cutting shape. Here similar results as prio discussed are observed. In Fif4t-e 12, an evolution oythe cutting time reduction is rawn. versus the relative amount. of connection radii. With increasing contour complexity,
REFERENCES [ 11
Balk s F . , 1977, "Removal Rate versus Accurac in d i r e EDM", Proceedin s of the ISEM { Switzerland, Wolfsberg, pp. 57-159.
B
[2] Beltrami I . , "AGIEPILOT", 1992, Internal report AGIE.
131 Beltrami I., 1989, "I1 Sensore Ottico", Internal report AGIE, pp. 1-8. [4] Beltrami I. et al., 1991, "Electro erosion machine featurine Dhotoekctric sensing means for measurin wire electrode deflecti&", U.S. patent # 5,057,[62. V
2
1
3
1
CUTTING TIME: 8.9 MIN (40 %) WITHOUT INTERVENTION
2
CUTTING TIME: 22.1 MIN (100 %) WITH CORNER STRATEGY
3
[5] Dauw D.F., Albert L., 1992, "About the Evolution of the Wire Tool performance in Wire EDM", Annals of the CIRP Vol. 41/1/1992, pp. 22 1 -225.
[6] Dauw D.F., Sthioul H., Delpretti R., Tricarico, Wire Analysis and Control for Precision EDM Cuttin Trondheim 1989, Annals of the CIRP Vol. 3%)/1989, pp. 191-194.
CUTTING TIME: 9.2 MIN (42 %) WITH OPTICAL DETECTION
Figure 12 Comparison of gain in cutting time as a function of workpiece complexiry. one can observe that the cutt.ing time reduction. is substantial when the optical sensing and path correction is carried out. 125
RELATIVE CUTTING TIME
(%I
[7] Deke ser W., 1988, "Knowledge-Based System for &ire-EDM", Ph.D. Thesis, K.U. Leuven, Be1 ium, 88D2, UDC 681.3*12:621.9.048.4. pp. 8-25.
[8] Hensgen
G., 1984, "Werkzeugspezifische Einflusse beim funkenerosiven Schneiden mit ablaufender Drahtelektrode" , Ph.D. Thesis, T.H. Aachen, pp. 36-52.
[9] Kinoshita N., Fukui M., Kimura Y., 1984, "Study on the Wire-EDM: In Process Measurement of Mechanical Behaviour of Electrode-Wire", Annals of the CIRP Vol. 33/1/1984, pp. 89-92.
\WITHOUT OPTICAL DETECTION. ELIT CGRNEG STRATEG'f ' N W K P l t G t MATERIALS - T K WORKPIECE THICKNESS 50 tvlM
[lo] Panschow R . , 1984 "Ueber die Krafte und ihre Wirkungen beim eiektroerosiven Schneiden mit ablaufender Draht-elektrode", Ph.D Thesis, T.H. Aachen , Germany.
50----
WITV OPTICAL/
25
.
STPATEGY -- DETECTION ____
0 0
5 10 15 20 25 30 % OF CONNECTION RADII ( d 0.7 MM ON PATH)
35
Figure 1 I Cutting results for a square wave contour path.
!22wwmM An introduction on wire deflection analysis was presented. It was shown that the wire deflection in wire EDM can be. described as a bending string, featuring a parabolic deflection. An ori inal solution was discussed about the real time 6tection and compensation of the wire deflection during wire EDM cutting. The description of an optical detector was iven as well as the. strategy applied to remedy from corner imprecision.
197
The pa er deals with a technical realization to im rove the Wire EDM accuracy. The system whicg is readily available on commercial wire EgM machines is based on the on-line tracking and control of the wire position. The deviation of the wire position relative to the programmed wire path osition is continuously measured and corrections are bein made during the machine cutting. Ti% technique allows to cut complex sha es, arc paths andgcontours at a much faster cuttin speed as compared to conventional wire ED%l machines. Practical examples are discussed and t%e economical relevance is emphasized.
K
m
: Wire EDM, accuracy, precision
INTRODUCTION
A constant driving force for EDM machine tool builders is the challenge to design and manufacture wire EDM machines such that various market requirements, in particular those concerning part precision, can be met. Although WEDM is since recently and to some extent confronted with high spFed milling, it is pften.the sole alternative to finish the right part in the right time and at a justified customer cost. Yet, the ,s ecially desi ned wire EDM units, suited for the eolume Marfet" (i.e. machines to be sold at a reasonable customer price, but concentrating on a large potential marke!) will alwa s necessitate a high degree of .output precision, re ard;ess what market price will be demanded. In eed though, in various application areas no extreme accuracies are re uested, other growin application domains where hig precision is require evolve and feature an increasing market share.
i
a
8
Since the introduction of wire EDM on the market in 1969, 'overall performance has been increasing substantially. For instance, the cutting s eed has been doubled every four years, the obtaina le work iece surface finish has been improvin by a factor of &teen since the beginning of wire E D h , and the wire ta er an le has been enlarged, to some 40 degrees curreniy, [2fand [5].
g
However, wire. bending, which is a major cause of cutting imprecis!on, is still hampering the part accuracy for various applications. The fact that the wire itself is behavin like a metal string, straightened by two axial forces and deformed, by hydraulic forces in the ap, e ectro static forces acting on the wire and electro 8ynamic forces inherent to the spark generation, make the wire to lose its perfect straight position, Hence, when cutting out a curvature, the lag e fect of the wire creates a geometrical error on the workpiece to be machined. This error can be of the order of a hundred microns, which for some applications becomes unacceptable.
pu11in7
Figure 1 Schematic representation of an EDM wire during cutting. mathematically be modeled by the standard vibration equation for motion. It is assumed that the wire mass is uniformly spread along its considqred . length, i.e. between the wire guides. Where for stiff vibrating bars, stiffness is the most important propert , flexible strings or wires feature tension (mechanical road) as the most important Characteristic. An axial force F is applied to the wire. During machinin , other external forces are acting on the wire as hydfaulic forces, electro static forces, electro d namic forces, and pull-back forces [I], [3], [6], [7], [9], [lo]. Assuming an axial force applied to the wire, and that an external load q(z,t) varies as a function of time and space, then the general differential equation of motion for a stretched string of length L in a plane (along the z axis) can be written as:
[$,
F-
A hardware system with a proper control algorithm has been developed to remedy from this situation and will be dealt with in the next paragraphs.
WALYSIS OF WIRE DEFLECTION DUE TO PROCESS
lxmxs
One of the major geometrical inaccuracies of wire EDM eroded workpieces is due to the wire bending and deflection during the cutting. Figure I illustrates the wire deformation during a rough cut. The wire is fed through the gap by means of an unwinding system. The wire guides assure that the wire position is maintained properly but anyhow, various process forces acting on the wire make the wire bend. The wire can Annals of the ClRP Vol. 43/1/1994
a2y
F
~
E I a4Y , az -
= pS- a2Y at
+ c -ataY + q(z,t)
(1)
with F: axial force applied to the wire (N) E: Young's modulus (N/m2 I: moment of inertia = ar /4 (m4) p : wire density (k /mm3) S: wire section (mgL) y: wire deflection (m) c: damping coefficient (Ns/m) q (z,t): external load (N)
d
Because only static forces are considered, and assuming
193
that no time dependent phenomena are influencing the wire behavior, one can write equation (1) as:
This equation can be simplified if considering that wires used on wire EDM machines are hardly. sub'ected to bending moments. Indeed, the major loading orces are the axial pulling forces F and the gap forces. As will be shown later the latter ones are very small compared to the former ones. Hence:
f'
electro dynamical forces become dominant (I Iz). Results of test cuts in order to verify the theoretical model are shown in Figure 2 for workpiece thickness 50 and 150 mm. Note that the z-scale for the h = 150 m m curve has been reduced by a factor of 3 in order to compare the wire deflection y for both workpiece thickness values. One can observe that the larger the workpiece thickness, the larger the wire deflection in the middle of the workpiece, but the wire deflection D at the workpiece face remains practically constant as given by formula (4) for %.h = constant. An how, the wire tension force which can reduce the wire Lflection, cf. formula (4)? is limited b the maximu? yield strength of the wire electrode (320to 790 N/mm ).
N WORKPIECE ACCCXACY
The solution y = f(z) of this equation is a parabola "within the workpiece" if one assumes that the external load q(z) = qo is time independent and constant while the solution is a straight line 'lout of the workpiece", v.i.z. between the res ective wire guides.and the upper and lower workpiece .aces respectively since there q(z) = 0.The maximal wire deflection is given by solving equation (3) for z = U2,. where I is the distance between the upper and lower wire uides, h is the workpiece thickness, .H is the distance ?tween the. wire guide and the workpiece and D is the wire deflection at the upper and lower border of the workpiece, hence:
f!
%
+ D)
y (1/2) = - (- 'Oh2 8F
with D =
qo.h.H
2F
(4)
In order to have a good estimate of the w-ire deflection, the apload (z) must be known. Literature reveals data on t e gap oad [l], [7, [8]. However, today's wire EDM units feature much owerful pulse currents and gap flushing pressures ( = 78 bar), yieldin much more important gap loads on the wire too [5]. he load q(z) due to the spark collapse has been calculated in our research from the.measured wire deviations (= 300 pm at maximum cutting speed) and is found to be of the order of 9 N/m. It must be noticed that the load due to the spark formation will push the wire away from the workpiece zone, whereas the electromagnetic forces will tend to ull the wire towards the frontal workpiece area. A tota gap load qo of some 9 N/m is found to be real istic.
N
As long as a strai ht cut is being made [(zJ) or (z,y) plane], the wire. ag, and hence the geometrical error that yields from it have hardly any negative effect on the output results. However, in practice, contour aths are very .often executed. In such cases, the wire rag causes the wire to erode sharp corners and geometrical work iece information will be lost. F i y - e 3 depicts what is feine obtained when the wire guide position does not match with the real wire position. A part of a cut workpiece cross section is visualized in Figure 4..One can notice that the corner accuracy is not fully achieved, and that a geometrical error of some 50 pm is observed. TO remed from these errors, various means have been tried out y many researchers. For instance, increasing th? wire's tensile strength i n order to allow a larger wire tension force, or reducing the cutting speed when cutting comers possible with a combination of varying
B
g
a 4
9
P
X
WIRE GUIDE POSITION WIRE POSITION
Figure 3 Producing of geometrical errors due to the cor&ination of wire bending and contour EDM cutting. the pulse parameters to reduce the applied pulse ower, [3]. Because wire deflection cannot be avoi ed for "mechanical" reasons and the fact that reducing the cutting speed is not an economically justified approach, a real time correction of the cuttin path to be executed has been investigated and redzed, [4]. This is schematically represented in Figure 5. An optical wire positioning sensor, built in the upper wire guide, continuously measures the wire deflection relative to its theoretical position. The real cutting path is recalculated by the machine's Numeyical Controller (NC) and the necessary contour correction is carried out by the NC.
8
.
0
I
io
40
25 I
'
100 I
60
'
8'0
'
ioo
175 z(mm1 J(h= 150 mm)
Figure 2 Wire defection measurements for varying workpiece thickness. Worth to be noticed is also the fact that during wire EDM finishin the electro-static forces are important (= fI2?Fiereas for roughing regimes, the 194
HARDWARE CORRECTION
SOLUTION
FOR
CONTOUR
The sensor designed must be uaranteed to be immune a ainst electronic noise pertuiations generated by the ED process. Moisture splashing up out of the
%
work tank during machinin must not prevent the sensor from proper. working. TEerefore, an optical sensor s stem was finally selected. Jarious optical sensors final system was chosen. devices were considered.
surface. For our application, the length of the lens was chosen a multi le of the wave length to allow an outcoming paralye1 beam to be pro'ected onto the wire. This is also made clear by Figure 7!'
environment (humidity caused by the water dielectric, metallic particles disturbing the measurements and an extensive hydraulic pressure of the dielectric), this solution was disregarded: The. principle of the optical sensor we ended up with is outlined in Fipure 6.
Ugbt Emitter Configuration
Imagine the two boxes on the ri ht side of the wire, F i p r e 6. Each of them emits a Eeam of uniform and parallel li ht towards the wire along the perpendicular axes x an y. Imagine two detectors at the opposite side of the light emitters, each of them composed of two triangular shaped lenses. The wire shadow rojected on both x and y detectors is re resentative or the wire dis lacements in the x and y irection respectively. The ligk intensity difference. registered by the u per and lower triangular detector is a linear function o the wire displacement along the, perpendicular axes, whereas the sum of both light intensity si nals is dependin on the overall intensity which might e affected by a c ange in transparency of the dielectric around the wire. The practical realization of the sensors described above is
8.
P
b!
P fl
%
f
><
AGIECUT
To light emitting diodes are interfaced to two fiber optics (0,25 mm diameter) to the two SELFOP lenses (x, and ). The light beams are directed towards the wire. or reasons that the entering light beams are parallel for each respective axis x and y and that the wire's relative position will not disturb the parallelism of the light beams, wire .positioning measurements can be carried out relatively simple, Figure 6 and 7.
J
Light sensor confiruration Two trian ular lenses are faced to each other. Both triangular fenses have each there own focal center, and both foci are distinct and effective. As shown in Figure 6, the rojected wire shadow area is now measured with a doub e sensitivity for each measuring axis x and y.
-
P
The construction of such a bi-trian ular o tical sensor is made b facing two ground SEL$OC@ Yenses to each other. $deed, by removing (i.e. grinding) the dotted area of the auto focusing lens, without removing or disturbing the focal axis, one can manufacture a triangular .longitudinal lens which still features the same characteristics as the standard SELFOP lens, Figure 8.
j Figure 7 Principle of a SELFOP lens configuration. Hence, the complete structure of a the light sensor either x or y axis) consist of two triangular autoocusin lenses faced to each other such that both focal axes o?the triangular lenses are parallel to each other. Both lenses are each interfaced with two fiber optics (0.25 mm in diameter connected to the electronic circuitry for analysis an processing.
f
d'
Figure 5 Princgle of the. path correction applied in Wire DM machining.
carried out by usin s ecial optical "self focusing" lenses named "SELEO&'". They feature the same optical functions as standard spherical lenses but in addition,, both end surfaces are flat. This allows interfacin with optical fibers with ease. The "SELFO&" micro lens is a cylindrical lens, in which light of a uniform wavelength travels sinusoidally. By va ing the pitch length .of the lens, light can be cozmated, focused or divergent at the output flat
Figure 8 nYo ground self focusing lenses assembled together make a one axis sensor.
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Practical built in solution on the wire EDM unit The wire positioning sensor is. built in and positioned in the upper wire lane (u,v) just undqneath the upper wire guide. Tfe closer the device is positioned to the workpiece, the more accurate the measurements are.. Fieure 6 depicts the principles how the sensor I S built in. Figure 9 depicts the real configuration.
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m
t
D P
CONTOUR CHARACTERISTICS
- CONNECTION RADII: 0.3 MM - TOTAL CONTOUR LENGTH: 82.1 M M - LENGTH OF STRAIGHT LINES: 72.2 M M - LENGTH OF CONNECTIONS: 9.9 M M 12 % - AMOUNT OF RADII:
af
Figure 9 Illustration o the assembled and built in wire positioning evice.
RESULTS OF REAL TIME W IRE POSITIONING DETECTION
STEEL WORKPIECE WORKPIECE THICKNESS: 5 0 MM CUTTING SPEED: 300 M M ~ ~ M I N
Fizure 10 a
Before, discusing the obtained results in terms of machining time and part precision, some principles of the built in control strategy will be explained. In addition, an idea of the problems and measurement thresholds are given.
Geometry of ihe standard test cut piece.
CUTTING TIME: 14.8 MIN (51 %f WITHOUT INTERVENTION
In order to correct the wire path the wire deflection is measured at a sampling eriod of 10 ms. At every measurement event, the rea direction vector "13" of the wire guides is known. A Correction vector "C" is computed based on the difference between the theoretical vector 'ID"and the real osition of the wire, measured at the sensor height. In Pact, a compensating cutting path is su erimposed and executed, yielding a correct cutting pat at the workpiece itself.
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CUTTING TIME: 29.3 MIN (100 K) WITH CORNER STRATEGY
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The fact that the maximum error alwa s occurs in the middle of the workpiece thickness, cf. Tormula (4), and the measurement is executed underneath the up er wire guide, an additional correction must be providedq Fi_ure 10 a outlines a test cut of a steel workpiece which is used for comparison purposes. A combination of straight cuts and curvatures allows to distinguish resu!ts when either a wire positioning correction is applied or no strategy at all. An enlarged view of the marked area in Figure 10 a (lower left part) is depicted in Figure 10 b. As already mentioned before, the wire deflection has a major effect when contours are being cut. Therefore, in Fipure I0 b three conditions are compared, where each time the cuttin conditions are different but the contour is idktical. The upper graph of this picture shows t e results r a rou h cut at a reference-cutting speed of 300 mmffimin. f i e corner accuracy is rater oor. In the middle figure, a cutting corner strategy, f)orrnerly applied on standard machines was selected. This strate y is based on pulse power reduction as a function otthe contour eomet . Already a substantial improvement is achieves thoug? the cuttin speed was substantially reduced. The last picture of Figure 10 b
geometX
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CUTTING TIME: 15.6 MIN (53 %) WITH OPTICAL DETECTION
Figure 10 b
Enlarged view of right kand side of figure 10 a , when various straiegies are applied.
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illustrates the results yieldin when the optical sensor detection technique is app ied [I/. A remarkable improvement of corner precision is obtained while maintaining the maximum cuttin s eed. Indeed, a ain in cutting time reductio of 5f was obtainecf i n comparison with the standard corner strategy.
Cuttin results with either one of. the strategies applief were Compared. Substantial machining time gains and corner accuracy improvements were noticed, when optical wire positioning and path correction was made.
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Fipure I 1 is a similar test cut for a square wave cutting shape. Here similar results as prio discussed are observed. In Fif4t-e 12, an evolution oythe cutting time reduction is rawn. versus the relative amount. of connection radii. With increasing contour complexity,
REFERENCES [ 11
Balk s F . , 1977, "Removal Rate versus Accurac in d i r e EDM", Proceedin s of the ISEM { Switzerland, Wolfsberg, pp. 57-159.
B
[2] Beltrami I . , "AGIEPILOT", 1992, Internal report AGIE.
131 Beltrami I., 1989, "I1 Sensore Ottico", Internal report AGIE, pp. 1-8. [4] Beltrami I. et al., 1991, "Electro erosion machine featurine Dhotoekctric sensing means for measurin wire electrode deflecti&", U.S. patent # 5,057,[62. V
2
1
3
1
CUTTING TIME: 8.9 MIN (40 %) WITHOUT INTERVENTION
2
CUTTING TIME: 22.1 MIN (100 %) WITH CORNER STRATEGY
3
[5] Dauw D.F., Albert L., 1992, "About the Evolution of the Wire Tool performance in Wire EDM", Annals of the CIRP Vol. 41/1/1992, pp. 22 1 -225.
[6] Dauw D.F., Sthioul H., Delpretti R., Tricarico, Wire Analysis and Control for Precision EDM Cuttin Trondheim 1989, Annals of the CIRP Vol. 3%)/1989, pp. 191-194.
CUTTING TIME: 9.2 MIN (42 %) WITH OPTICAL DETECTION
Figure 12 Comparison of gain in cutting time as a function of workpiece complexiry. one can observe that the cutt.ing time reduction. is substantial when the optical sensing and path correction is carried out. 125
RELATIVE CUTTING TIME
(%I
[7] Deke ser W., 1988, "Knowledge-Based System for &ire-EDM", Ph.D. Thesis, K.U. Leuven, Be1 ium, 88D2, UDC 681.3*12:621.9.048.4. pp. 8-25.
[8] Hensgen
G., 1984, "Werkzeugspezifische Einflusse beim funkenerosiven Schneiden mit ablaufender Drahtelektrode" , Ph.D. Thesis, T.H. Aachen, pp. 36-52.
[9] Kinoshita N., Fukui M., Kimura Y., 1984, "Study on the Wire-EDM: In Process Measurement of Mechanical Behaviour of Electrode-Wire", Annals of the CIRP Vol. 33/1/1984, pp. 89-92.
\WITHOUT OPTICAL DETECTION. ELIT CGRNEG STRATEG'f ' N W K P l t G t MATERIALS - T K WORKPIECE THICKNESS 50 tvlM
[lo] Panschow R . , 1984 "Ueber die Krafte und ihre Wirkungen beim eiektroerosiven Schneiden mit ablaufender Draht-elektrode", Ph.D Thesis, T.H. Aachen , Germany.
50----
WITV OPTICAL/
25
.
STPATEGY -- DETECTION ____
0 0
5 10 15 20 25 30 % OF CONNECTION RADII ( d 0.7 MM ON PATH)
35
Figure 1 I Cutting results for a square wave contour path.
!22wwmM An introduction on wire deflection analysis was presented. It was shown that the wire deflection in wire EDM can be. described as a bending string, featuring a parabolic deflection. An ori inal solution was discussed about the real time 6tection and compensation of the wire deflection during wire EDM cutting. The description of an optical detector was iven as well as the. strategy applied to remedy from corner imprecision.
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