Patent Grant
7047658
The present invention refers to an apparatus for the dimensional and form deviation checking of a crankpin of a crankshaft during orbital rotations about a main rotation axis on a numerical control grinding machine where it is worked, the grinding machine having a grinding-wheel slide carrying a grinding wheel and a worktable defining said main rotation axis, with a gauging head with a Vee-shaped reference device adapted to engage the crankpin to be checked, a feeler adapted to touch the surface of the crankpin to be checked, and a transducer adapted to provide signals indicative of the position of the feeler with respect to the Vee-shaped reference device, a support device, with mutually movable coupling elements, that movably supports the gauging head, a control device to control automatic displacements of the gauging head from a rest position to a checking position, and vice-versa, and processing and display devices connected to the gauging head adapted to receive and process said signals provided by the transducer.
The invention refers also to a method for checking the form deviation of a pin defining a geometrical symmetry axis, the pin orbitally moving about a main rotation axis parallel to and spaced apart from the symmetry axis.
Apparatuses having the above-mentioned features are shown in international patent application published with No. WO-A-9712724.
The embodiments described in such international application guarantee excellent metrological results and small forces of inertia and the standards of performance of the apparatuses with these characteristics, manufactured by the company applying for the present patent application, confirm the remarkable quality and the reliability of the applications.
In many numerical control grinding machines presently produced for working crankshafts, each piece to be worked is positioned on the worktable and rotated about its main rotation axis (i.e. the axis defined by the journal bearings), and during the rotation both journal bearings and crankpins are ground. As far as the crankpins are concerned, the proper working requires extremely accurate translation movements between the grinding-wheel slide and the worktable, synchronously with rotational movements of the shaft, under the control of the numerical control (NC) of the machine based on a proper working program that is the result of a numerical interpolation. Unavoidable imperfections in the dimensions or form deviation of the mechanical parts of the machine cause circularity or roundness deviations in the cylindrical surface of the ground workpiece. In order to correct such deviations (and considering that 2–3 μm is a typical value of tolerance for this kind of deviations, as required for crankshafts to be employed in cars), roundness of the worked crankpins must be checked, and the working program of the CN must be consequently corrected. Checking of the roundness of the crankpins is presently carried out by means of proper metrological apparatuses including a revolving table performing greatly accurate rotation movements, where the crankshaft is referred and fixed in such a way that the crankpin to be checked is substantially centred with respect to the rotation axis. A gauge having radial measuring axis detects the variations in correspondence of at least a transversal cross-section of the pin surface that is scanned in the course of a 360° rotation of the revolving table, with a proper sampling frequency. The detected variation values are processed to get the best-fit circumference, i.e. the circumference that best approximates the locus of the points corresponding to such values. Deviations of the detected values with respect to values of the best-fit circumference are calculated to define the roundness error of the checked surface, according to a well-known technique.
According to the presently used procedure, in order to check the roundness it is necessary to have a specific, costly and bulky apparatus, and to sequentially perform the following operations: remove the crankshaft to be checked from the grinding machine where it has been ground, position the crankshaft on the specific apparatus where careful operations are needed for a proper positioning and fixing on the revolving table, carry out the checking process, analyse the results, and manually correct the grinding program of the CN on the basis of such results. As a consequence, the involvement of properly instructed operators is needed to carry out the checking and the correction. Moreover, performing the above-mentioned operations negatively affects the working process, requiring not negligible interruptions, and appears in contrast with the even increasing requirements to continuously and timely check the production process.
An object of the present invention is to obtain a checking apparatus and a checking method allowing to carry out accurate and timely roundness or circularity checking of crankpins with the crankshaft still positioned on the grinding machine where it is worked.
Another object of the present invention is to obtain a checking apparatus and a checking method allowing to check both diametral dimensions of a crankpin that is orbitally rotating during its working on a grinding machine, and the roundness of the ground crankpin, during an additional orbital motion of the crankpin in the grinding machine.
These and other objects and advantages are obtained by means of a checking apparatus and a checking method according to, respectively, claims 1 and 13.
The invention is now described in more detail with reference to the enclosed drawings, showing preferred embodiments by way of illustration and not of limitation. In said drawings:
a, 5b, 5c and 5d schematically show the cross-section of a pin having an evident form error, and graphic representations of the profile of the pin detected with different apparatuses,
With reference to
The support block 19 further supports a guide device 21, that, according to the description of the above-mentioned international patent application published with No. WOA-9712724, serves to guide the reference device 20 to engage crankpin 18 and maintain contact with the crankpin 18 while the reference device 20 moves away from the crankpin, for limiting the rotation of the first 9 and of the second 12 coupling elements about the axes of rotation F, S defined by pins 6 and 10.
The axial displacements of transmission rod 16 with respect to a reference position are detected by means of a measurement transducer, fixed to tubular casing 15, for example a transducer 41 of the LVDT or HBT type (known per se), with fixed windings 40 and a ferromagnetic core 43 coupled to a movable element, or rod 42, movable with the transmission rod 16 (
The support block 19 is secured to guide casing 15 by means of pairs of screws 47 passing through slots 48 and supports reference device 20, consisting of two elements 31 with sloping surfaces, whereto there are secured two bars 32. The rest position of feeler 17 can be adjusted by means of screws 47 and slots 48. Transducer 41 of head 39 is connected to a processing and display device 22, the latter being on its turn connected to the numerical control (NC) 33 of the grinding machine.
The coupling elements 9 and 12 are basically linear arms with geometric axes lying in transversal planes with respect to the rotation axis O of the crankshaft and to the rotation axis M of grinding wheel 4. However, as schematically shown in
A control device includes a double-acting cylinder 28, for example of the hydraulic type. Cylinder 28 is supported by grinding-wheel slide 1 and comprises a movable element, in particular a rod 29, coupled to the piston of cylinder 28, carrying at the free end a cap 30. An arm 14 is fixed at an end to element 9 and carries, at the other end, an abutment with an idle wheel 26. When cylinder 28 is activated for displacing the piston and the rod 29 towards the right (with reference to
When, in order to permit displacement of the apparatus to the checking condition, rod 29 is retracted, and cap 30 disengages from the abutment, or idle wheel 26, support block 19 approaches the crankpin 18 through rotation of the coupling elements 9, 12, and the apparatus reaches and keeps the checking condition, substantially as described in detail in the above-mentioned international patent application published with No. WO-A-9712724.
The cooperation between crankpin 18 and reference device 20 is maintained thanks to the displacements of the components caused by the force of gravity. The action of the coil spring 27, the stretching of which increases with the lowering of the support block 19, partially and dynamically counterbalances the forces due to the inertia of the moving parts of the checking apparatus following the displacements of the crankpin 18. In such a way, it is possible, for example, to avoid over stresses between the reference device 20 and the crankpin 18, in correspondence of the lower position (shown in
The crankshaft 34 to be checked is positioned on the worktable 23, between a driving device with a spindle 36 and a tailstock 37, schematically shown in
During the checking phase, the transducer 41 of the gauging head 39 sends to the processing and display device 22 signals the values of which are indicative of the position of the feeler 17. The values of such signals can be processed and corrected, e.g. on the basis of compensation values or coefficients stored in the device 22, in order to obtain measurement signals the values of which are indicative of the diametral dimensions of the crankpin 18 that is ground. The measurement signals are used by the NC 33 to stop the working of the crankpin 18 when a predetermined diametral dimension is reached.
Afterwards, a checking relevant to the roundness of the crankpin surface is performed. In the roundness checking phase, the interpolated movements of the grinding machine parts (grinding-wheel slide, worktable) are controlled so that, during the orbital movement of the crankpin 18, the grinding-wheel 4 surface moves for keeping a negligible distance from the crankpin surface.
In the roundness checking phase the crankshaft 34 undergoes a 360° rotation, in the course of which the values of the signals outputted by the transducer 41 are detected and (after possible corrections as cited above) stored. Such values are detected at predetermined spaced out angular positions, e.g. every degree, under the control of the rotative transducer 35, to obtain a sequence of “rough” values rg(θ), where θ=0,1, . . . , 359. The signals of the transducer 41 can be detected in other suitable ways, e.g. through a time scanning at constant rotation speed of the crankshaft 43. The rough values rg(θ) refer to radial dimensions of crankpin 18 at predetermined angular positions θ of such crankpin 18, and include deviations due to some features of the checking apparatus. In particular, the rough values rg(θ) are affected both by reciprocal dynamical oscillations of the gauging head 39 in the course of the orbital movements of the crankpin 18, and by intermodulation of the form deviations of the surface of the crankpin 18 due to contact between the reference device 20 and such surface. The rough values rg(θ) are transmitted to the NC 33 to be processed—as specified in the description that follows—to obtain profile values r(φ) indicative of the actual crankpin profile, i.e. of variations of the radial dimensions of the crankpin 18 as a function of the angular position about the geometrical symmetry axis C. The profile values r(φ) can be directly used by the NC 33 to detect roundness errors—as can be done by the specific roundness checking apparatuses used in the known art—and to consequently correct the program that controls the working operations.
As previously cited, the rough values rg(θ) are affected by errors due to the reciprocal dynamical oscillations of the gauging head 39 on the crankpin surface. In fact, since the crankpin 18 rotates about a rotation axis (O) that is spaced apart of the eccentricity c from its own geometrical symmetry axis (C), during the above-mentioned controlled interpolated movements (according to which a negligible distance is maintained between the grinding-wheel 4 and the crankpin 18 surfaces), symmetry axis C oscillatory moves, with respect to the grinding wheel 4, following an arc of radius MC about axis M of the grinding wheel 4. Owing to kinematic and geometric features of the support device and of the head 39, defining the articulated quadrilateral MFSC, the Vee-shaped reference device 20 engages the crankpin 18 assuming an angular arrangement that, in general terms, varies during the orbital rotation of the crankpin.
As a consequence, there is not a full coincidence between the values of the increments of the angular arrangements θ of the crankshaft 34, as detected by the rotative transducer 35, and consequential increments values of angle φ, indicative of the position of the contact point P about symmetry axis C. The effect of the hunting of head 39 on crankpin 18 are alterations, or deviations of the rough values rg(θ) with respect to actual profile values, deviations that differently affect the rough values rg(θ) in different moments of the roundness checking phase. The method according to the present invention includes a first processing of the rough values rg(θ) in order to eliminate the above mentioned deviations due to the reciprocal dynamical oscillations of the gauging head 39 on the crankpin surface.
To this end, the following operations are performed for each value of angle θ comprised between 0° and 359°:
By repeating, as mentioned above, the operations for each of the 360 values of θ, it is possible to have a correlation function φ=φ(θ) allowing to correct (or “put in phase”) the sequence of rough values rg(θ) by means of well known numerical interpolation techniques, and to obtain a sequence of angularly compensated values rf(φ).
It is to be pointed out that the operations to get the correlation φ=φ(θ) must be performed only once, when the nominal dimensions of crankpin 18 to be checked or the geometric features of the apparatus (support device and head) vary.
As already cited in the present description, the sequence of angularly compensated values rf(φ), is still affected by further alterations, due to intermodulations of form deviations of crankpin 18 as a consequence of the fact that the position of the feeler 17 is detected making reference to the Vee-shaped device 20, the latter touching the surface to be checked of the crankpin 18.
In fact, contrary to what happens when measuring the crankpin 18 by means of a known roundness measuring apparatus, where the crankpin is fixed to a turning table precisely rotating about a reference axis (the accuracy of the rotation movement is about ten times better than the manufacturing tolerance), the head 39 includes a reference device 20 having surfaces of a Vee-shaped element resting upon portions of the crankpin 18 surface (indicated with points A and B in
According to the method of the present invention, the negative effects of the above-mentioned intermodulations of the form deviation errors of the crankpin 18 surface are compensated by performing a harmonic analysis of the angularly compensated values rf(φ).
Any periodic function, such as the detection of the pin profile according to the present invention, can be expressed as a Fourier series:
where the Ai, Bi represent the Cartesian projections X, Y of the ith harmonic component having amplitude Ci and phase φi:
Ci=√{square root over (Ai2+Bi2)}
In order to describe with sufficient approximation the profile of crankpin 18, it can be enough to calculate the first ten/fifteen harmonics, since further harmonics can give information about vary small surface imperfections, that cannot be defined as roundness errors, but give hints about roughness. It is pointed out that the harmonic analysis keeps separate but different harmonic components relevant to the form error, e.g. an ovality error (second harmonic) can be revealed only in its projections A2, B2, and in no harmonics of any other orders. It is possible to use this feature of the harmonic analysis to compensate for the harmonic modulation caused by the Vee-shaped reference device 20 of the head 39. In fact, each harmonic component is subject to an amplitude modulation and a phase displacement that only depend on the value of angle α between a side of the Vee and the straight line along which the feeler 17 moves, and on the harmonic order. As an example, the harmonic analysis relative to a Vee defining a symmetric angle of 80° (α=40°) gives rise to the compensation coefficients listed in the following table:
It is pointed out that angle α shall be chosen in such a way that the magnification coefficients Ki not be too much smaller than 1 (and in particular they shall not be null), at least as far as the harmonics of the actually interesting orders are involved.
After having calculated—once and for all for a given angle α—the values of the above table, it is possible to use the compensated values to obtain the “actual” profile of crankpin 18, i.e. the profile that is obtainable by means of the previously cited prior art roundness checking apparatuses.
To do so, the amplitude values Ci of the harmonic analysis must be divided by the corresponding magnification coefficient Ki, and the phase difference σi must be added to phase φi.
In substance, the method for the determination of the profile of the crankpin 18—in order to check its roundness—includes the following phases:
It is pointed out that some of the above-listed phases must not be repeated in case that the geometry of the apparatus and the nominal dimensions of the crankpin 18 do not change.
As a result, the “actual” profile r(φ) of crankpin 18 is obtained, and can be further processed, graphically represented (plotted), or used in other known ways.
The flow chart of
The blocks of the flow chart have the following meaning:
60—start
61—the crankshaft 34 is positioned and connected to the worktable 23 and rotated about axis O, and the NC
33—controls movements of the grinding-wheel slide 1;
62—under the control of the NC 33, the double-acting cylinder 28 is activated to bring the head 39 to the checking condition, i.e. to bring the Vee-shaped reference device 20 into engagement with the crankpin 18 surface during the orbital motion of the latter;
63—the working of the crankpin 18 is performed until a proper measuring signal relevant to the diametral dimensions of the crankpin 18 is provided by the transducer 41 and detected by NC 33;
64—in case that the roundness checking is not required, the cycle ends (block 73);
65—rough values rg(θ) are stored during a further orbital rotation of the crankpin 18;
66—it is checked whether a new correlation function φ=φ(θ) must be calculated, e.g. in case it has never been calculated or if the geometrical features of the grinding machine and of the checking apparatus, and/or the nominal dimensions of the crankpin were changed;
67—a (new) correlation function φ=φ(θ) is calculated;
68—the rough values rg(θ) are compensated based on the correlation function φ=φ(θ) to obtain angularly compensated values rf(φ) relevant to an “apparent” profile rf(φ) of the crankpin 18;
69—the harmonic analysis of the “apparent” profile rf(φ) is performed, and amplitudes (Ci) and phase (φi) values of the n harmonics are calculated;
70—it is checked whether a proper table of sensitivity and phase difference values in connection with the particular Vee-shaped device 20 and relevant angle α is available;
71—a (new) table of sensitivity and phase difference values is obtained;
72—the values of the amplitudes and phases of the n harmonics are corrected on the basis of the contents of the table, and the actual profile r(φ) of the crankpin 18 is obtained;
73—the cycle ends.
It is pointed out that the flow chart of
It is pointed out what follows. In case that the dimensions and mutual arrangement of the grinding machine, the checking apparatus and the crankshaft are chosen so that, making reference to
The checking apparatus according to the present invention can include a Vee-shaped reference device 20′ having a Vee surface asymmetric with respect to the translation direction of feeler 17. A gauging head 39′ including the device 20′ is shown in
By employing the asymmetric device 20′ it is possible to improve the accuracy of the roundness checking, by increasing the sensitivity of the apparatus to errors corresponding to harmonic in a range of orders that is wider than the range that can be covered by means of the gauging head 39. In fact, the compensation table corresponding to reference device 20′ is as follows:
By comparing the contents of the tables relevant to reference devices 20 and 20′, it is evident that the values of the magnification coefficients Ki are far better in the latter case. In fact, as far as the order range 2–15 is concerned, in the latter case just three out of fourteen coefficients have value lower than 1 (in the former case only eight coefficients reached such value). Moreover, the lower value of Ki with the asymmetric device 20′ is not so far from 1 (i.e. 0,807), and is greater than six of the fourteen coefficients relevant to the symmetric device 20 (in the “symmetric case” the lower value is 0,192).
It is to be noted that the particular roundness checking cycle, involving the mutual movements of the grinding-wheel slide and worktable substantially simulating a working cycle (but without contact taking place between the grinding wheel and the crankpin to be checked) is particularly advantageous. In fact, in such a cycle the support device undergoes limited displacements, limiting in such a way the reciprocal dynamical oscillations of the gauging head 39 (or 39′) on the crankpin surface. In this way, the deviations that such oscillation causes in the rough values rg(θ) are reduced, and it results easier to compensate for such deviations with a method according to the present invention. Moreover, the layout of the same support device can be compact, since wide movements of the gauging head 39 (or 39′) to follow the crankpin 18 are not required.
By means of a checking apparatus and method according to the invention it is possible to accurately perform in-process dimensional checking of the crankpin 18 as well as roundness checking of the same crankpin 18 in a particularly simple and quick way, without the need of additional costly metrological devices.
Apparatuses according to the present invention can include features differing from what is described above and shown in the drawings. As an example, the components of the support device can have different shape and/or arrangement, and, at least one of them, can be translatable and not rotatable. Other possible differences can involve the guide device 21, that can be omitted or replaced by a different device, having guiding surfaces touching portions of the connecting elements (9 or 12) or other parts of the apparatus, instead of touching the crankpin 18 surface.
Moreover, the support device can be connected to a different part of the grinding machine, e.g. to a basement or to another part fixed with respect to the grinding-wheel slide.
The sampling frequency in the acquisition phase of the rough values rg(θ) can be different with respect to what is described above, and the activities of the processing and display device 22 can be performed by any processing means having the proper features, e.g. by a commercially available personal computer.
| Number | Date | Country | Kind |
|---|---|---|---|
| BO2000A0112 | Mar 2000 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP01/00596 | 1/19/2001 | WO | 00 | 8/29/2002 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO01/66306 | 9/13/2001 | WO | A |
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| 20030056386 A1 | Mar 2003 | US |
