GB2300918A

GB2300918A - Scalloped datum disc

Info

Publication number: GB2300918A
Application number: GB9510148A
Authority: GB
Inventors: Martin Stewart Suckling; John Edward Mutton
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1995-05-19
Filing date: 1995-05-19
Publication date: 1996-11-20
Anticipated expiration: 2015-05-19
Also published as: GB2300918B; GB9510148D0

Abstract

A rotatable datum structure (22), and a method, for use in measuring or machining rotatable structures which have a discontinuous outer surface, for example a bladed rotor structure found within a gas turbine engine. The datum structure (22) comprises a section (24) with features (28) forming a discontinuous outer circumferential surface which defines a reference datum (34). These features (28) of the datum structure (22), as detected by a probe used in the measurement or machining operation, resemble the features of the rotatable structure to be measured such that the probe signal produced is of a similar form for both structures. The datum structure (22) may additionally have a continuous outer surface section (26) which can be used conventionally.

Description

Measurement of Bladed Structures The present invention relates to the accurate radial measurement of any rotatable object which has a discontinuous outer surface, and the provision of datums for the these measurements. In particular it relates to measuring bladed rotatable structures. This measurement can occur either during a machining operation or separately.

During the final manufacture of compressor rotors for gas turbine engines, it is necessary to measure the blade lengths of the high pressure (HP), and intermediate pressure (IP) rotors. This measurement is carried out during the grinding of the blade tips to the correct radius to operate with the specified minimum clearances within the casing of a gas turbine engine. This machining is most effectively carried out using a computer controlled grinding machine.

Presently radial measurement of rotatable structures, for example compressor rotor sections of a gas turbine engine, is carried out using a measuring machine which measures the structure, while it is being rotated by the machine, with two solid datum discs mounted axially at each end of the structure. These two datum discs assist in mounting the rotatable structure concentric to, and aligned with the measuring machine axis. They also provide an accurate radial datum position from which the rotatable structures' radius can be measured. The measurement is carried out using a measurement probe that registers its close proximity to another object. By comparing the position when the probe registers against the datum disc and against the rotatable structure an indication of the object radius is produced.

However this method has been found to have certain problems, generally caused by the datum structure and the rotatable structure having significantly different forms.

Firstly the measurement probes react slightly differently when used on the datum disc and the rotatable structure, secondly the probe signal produced as the rotatable structure is detected, as it rotates past the measurement probe, is significantly different from that produced from the datum structure. Therefore the signal from the datum disc, used to produce the basis of the radial measurement, requires a different processing arrangement to that used by the signal from the rotatable structure.

Both of these problems can produce significant measurement errors and prevent the use of certain types of probes for measuring both the rotatable structure and datum discs.

The present invention seeks to provide a datum disc which has similar measurement characteristics, as seen by the probes, to the characteristics of the bladed rotor or the like being measured, whilst still being robust and easily mountable, allowing it to be used as a datum structure. Such a datum disc will eliminate or reduce the previously mentioned problems associated with detecting different measurement surfaces within one measurement operation and will enable more accurate and simplified measurement of rotor assemblies and other similar structures. Further benefits of such a disc, as compared with a solid disc will be apparent from the following description.

According to the present invention a rotatable datum structure comprises a section with features forming a discontinuous outer circumferential surface which defines a reference datum for measuring, or machining, a rotatable structure which also has features forming a discontinuous circumferential outer surface, whereby the features of the datum structure resemble, in so far as the circumferential surfaces and features are detected by a probe used in the measuring or machining of the rotatable structure, the discontinuous outer surface and features of the rotatable structure to be measured, such that a signal produced by the probe as it detects the surfaces of both the datum structure and the rotatable structure to be measured is of an similar form.

Preferably the rotatable datum structure additionally has a continuous outer surfaced section.

In one embodiment the rotatable structure to be measured is a bladed structure for use within a gas turbine engine, and where the datum structure's discontinuous outer circumferential surface section is a simulation of the outer surface of the bladed structure to be measured.

Furthermore both the continuous outer surfaced section and the discontinuous outer circumferential surface section resembling the rotatable structure to be measured are structurally integral with each other.

In a further embodiment the datum structure forms a part of the rotatable structure to be measured and can be considered as one component with the datum structure becoming a datum feature of the rotatable structure being measured.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a sectional view of a gas turbine engine showing the location of the bladed rotor and case; Figure 2 is a representation of a datum disc in accordance with the present invention; Figure 3 is a view of the rotor assembly from a gas turbine engine and datum discs as mounted in the grinding/measuring machine; and Figure 4 is a pictorial view on A of a measurement probe measuring a blade.

Firstly a known method of machining and measuring an axial flow compressor rotor will be described with reference to figures 1,3 and 4. In this method the compressor rotor sections 12,14 of a gas turbine engine 10 are fully assembled with their associated blades 11.

The rotor 12,14 is mounted on a grinding machine 46 with two solid datum discs axially attached at each end, and the assembly rotated within the machine 46. By conventional use of a dial test indicator gauge, to measure the eccentricity of the mounting of the two datum discs, the axial alignment of the assembly is checked and adjusted so that the rotor assembly is concentric to, and aligned with the grinding/measuring machine axis 42.

Measurements of the datum disc radius are taken using a measurement probe 50, for example a spark probe, capacitance probe, optical probe or a laser probe. Each of these probes operates differently using different characteristics to detect their close proximity to another object: The spark probe detects the spark discharge when a spark jumps across the small distance between the probe tip and the object. The size of the small distance is considered to be consistent. Hence the position of the object relative to the known position of the probe can be calculated. However, when used in a rotating arrangement the probe can only be used to detect the extreme maximum radius. The probe cannot be brought in closer to identify the smaller radii of the object because, due to the size of the gap required, the probe will touch on the maximum radius of the object. If the object is not round or mounted eccentrically then the spark probe will not detect either of these conditions. The probe assumes that the entire rotating object is of the same single maximum radius.

The capacitance probe measures the capacitance of an air capacitor set up between the object and probe tip. As the distance between the object and the probe tip changes so does the capacitance, provided that the distance is not too great. Once this relationship is calibrated the position of the object relative to the probe can be found. This type of probe gives more information than the spark probe and when used in a rotating arrangement can detect any small changes in radius around the circumference of an object. The main problem is that the probe requires the object to have a discontinuous surface to produce an AC (alternating) probe signal. This type of signal is required by the processing electronics for this probe.

The optical probe passes a beam of light normal to the distance to be measured and records if the beam is broken by the object. By this means the edge of the object is found, with the shadow produced by the edge of the object being placed on a scale. This allows small distance changes of the edge of the object to be recorded. A problem found with this type of probe is that optical effects can occur at the edge of the object which can distort the shadow produced.

The laser probe directs a beam of laser light at the flat surface of the edge to be measured at a known angle.

The reflected light is detected and its position relative to the laser recorded. By careful triangulation of the relative angles of the emitted and reflected light beams the position of the object's edge, relative to the probe can be found.

The probe 50, of an appropriate type, is accurately mounted tangentially to the object 12,14 to be measured on a scaled slidable means 44 which can be adjusted axially and radially. The position when the probe 50 detects the circumference of the datum disc 22 establishes a known datum location. The probe 50 is then accurately moved axially and radially relative to this datum until the periphery of the blades 11 are encountered by the probe 50. The new position of the probe 50, as recorded by the scales on the axial and radial slides 44, relative to the datum position gives a comparison of the blade radius to the datum, and therefore, by reference to the datum dimensions, an indication of the absolute rotor 12,14 radius.

Synchronisation of a series of measurements, taken at the same axial position, with the rotation of the rotor assembly allows individual measurement of each blade 11 radius within a plane normal to the machine and rotor axis 42. An iterative grinding operation of the blade tips, with continuous measurement of the blade 11 radii, produces a set of blades 11 of a suitable length to operate within an engine casing 20.

The use of a solid datum disc as the basis for measuring a bladed structure 12,14 causes significant errors. These are caused by the probe's measurement characteristic (spark length, capacitance, shadow, for example) being affected by the different exterior surfaces being measured within a single measurement procedure. The different exterior surfaces are a continuous, approximately constant radius surface of the datum disc and an intermittent surface of the actual rotor 12,14 to be measured, with a large radius at the blade tip and smaller radius in between blades 11.

The different surfaces also produce different signals from the probes 50. A time invariant (DC) signal is produced from the continuous datum surface and a periodic (AC) signal from the bladed rotor 12,14, from which the peaks are taken as the blade radius. Both of these signals are processed by different means and are subsequently compared in order to measure the radius of the rotor 12,14. This inescapably introduces processing errors into the measurement system. The presence of a DC datum signal also makes the signal processing more troublesome and precludes the use of some measurement probes.

In order to improve the measurement of the radius of a rotatable object, for example a bladed compressor rotor 12,14, using an electronic measurement machine, either as part of a machining device 46 or as a stand alone device, a datum disc 22 as shown in figure 2 is provided. This datum disc 22, in a similar way to the known practice, aids in mounting the rotor 12,14 concentrically with the machine axis 42 and establishes set datums for the subsequent measurements.

The datum disc 22 is composed of two sections: a section that is peripherally continuous 26 as on known datum discs; and a scalloped section 24 that, on the outside surface resembles the peripherally discontinuous features of the rotatable structure being measured, for example the tips of rotor blades 11.

The radius 34 of the blade-like scalloped section 24 is the same as the radius of the adjacent continuous surfaced section 26 and is comparable to the radii of the rotor 12,14 being measured. The width 32 of the continuous surfaced section 26 is such that a Dial Test Indicator (DTI) gauge can be used on it to align the datum discs 22 concentrically within the grinding machine 46 as shown in figure 3. This section 26 also provides sufficient support for the scalloped section 24 and provides the necessary rigidity to the whole datum structure 22. The scalloped section 24 is of suitable dimensions 30,36,40 such that the measurement probes 5C cannot distinguish between it and the real features 11 (blades in this embodiment) to be measured.Furthermore the number of replica features 28 forming the scalloped section 24 of the datum disc 22 is approximately equal to the average number of real features 11 on the rotor 12,14 or object being measured. To reduce the errors due to thermal effects the entire disc 22 is fabricated from similar materials to that of the rotor 12,14 being measured, and is preferably manufactured from a single piece of material; eg by casting or machining.

Once made the datum discs 22 are independently and accurately measured using, for example, a co-ordinate measuring machine which measures the discs 22 when static, and records their dimensions 34. These reference dimensions are used to establish the datum points marked out by the circumference of the datum disc.

The datum discs have conventional mounting features 48 that allow them to be easily and securely mounted onto the rotatable structure being measured 12,14. In the present embodiment two such datum discs 22 are mounted to each end of a rotor 12,14, in a similar way to the mounting of conventional datum discs, as shown in figure 3. Indeed the datum discs 22 can be used in an identical way to the use of conventional datum discs by utilising the continuous surfaced section 26 on its own.

The scalloped section 24 of the datum disc 22 is used in a way analogous to the use of a conventional non-scalloped datum disc, except that the probe 50 will produce a periodic signal corresponding to the features 28 of the scalloped section of the datum disc 22 rotating past it. The periodicity of this signal has significant advantages in that it allows the use of differing types of probes 50.

For example, a capacitance type probe, which could not before be used on the datum structure, can now be used to measure both the datum disc 22 and the rotor 12,14. As has been described, this particular type of probe 50 not only registers proximity to an object, but also gives a value of the small distance 52 between the object and probe 50. This gives a more accurate and detailed measurement of the position of the exterior circumference and identifies the actual radius of a series of points around the circumference. The capacitance probe can now be used on its own to measure both rotor 12,14 and datum structure 22, whereas previously another type of probe had to be used to measure the datum structure 22, which could add errors.

The more detailed measurement of the datum disc 22 produced can be compared with the accurate measurements of the datum disc 22 recorded during its manufacture, or when the structure was calibrated, by a co-ordinate measuring machine. Any differences in these readings, excepting measurement errors, can be attributed to mounting errors. Using these detailed error readings, recorded for a number of circumferential points around both discs, the actual misalignment of the rotating assembly, i.e. datum discs 22 and rotor 12,14, is calculated. Analysis of this misalignment enables any subsequent error to be removed from the measurements of the rotor 12,14, to give a more accurate measurement.The accuracy and ease of identifying these errors also allows the rotor 12,14 and datum structures 22 to be more accurately mounted concentrically within the measuring machine 46 in the first place.

The frequency content of the probe 50 signal when sensing the blade features 28 of the scalloped datum disc 22 is similar to that produced by the real rotor blades 11 on the rotor being measured, and hence the same or similar processing electronics can be used to process the probe sensing signal from both the datum disc 22 and rotor 12,14. This simplifies the signal processing and further aids the overall accuracy of the method.

The similarity of the features 28 of the scalloped section 24 of the datum disc 22 to the features 11 of the rotor 12,14 being measured means that any systematic effects produced by the measurement device due to measuring this type of feature, that can lead to errors, are common to measuring the datum structure and the rotor. Therefore the systematic effects can be easily accommodated and corrected for during calibration of the measurement system so eliminating any induced errors.

Datum discs within the scope of the present invention may be used to measure different types of rotatable components, for example gear wheels, using similar types of electronic measurement devices.

Also within the scope of the present invention is that a datum structure, as described, can be built into the actual rotor and form part of its structure. In this arrangement any errors induced by mounting the datum structures to the rotor will be eliminated. The datum structure on the rotor itself will provide the datum for measuring the bladed features of the rotor in a similar way to that described for independent datum discs.

Claims

1. A rotatable datum structure comprising a section with features forming a discontinuous outer circumferential surface which defines a reference datum for measuring, or machining, a rotatable structure which also has features forming a discontinuous circumferential outer surface, whereby the features of the datum structure resemble, in so far as the circumferential surfaces and features are detected by a probe used in the measuring or machining of the rotatable structure, the discontinuous outer surface and features of the rotatable structure to be measured, such that a signal produced by the probe as it detects the surfaces of both the datum structure and the rotatable structure to be measured is of an similar form.

2. A rotatable datum structure as claimed in claim 1 which additionally has a continuous outer surfaced section.

3. A rotatable datum structure as claimed in claim 1 or 2 in which the rotatable structure to be measured is a bladed structure for use within a gas turbine engine, and where the datum structure's discontinuous outer circumferential surface section is a simulation of the outer surface of the bladed structure to be measured.

4. A rotatable datum structure as claimed in claim 2 in which both the continuous outer surfaced section and the discontinuous outer circumferential surface section resembling the rotatable structure to be measured are structurally integral with each other.

5. A rotatable datum structure as claimed in any of the preceding claims where the datum structure forms a part of the rotatable structure to be measured and can be considered as one component with the datum structure becoming a datum feature of the rotatable structure being measured.

6. A rotatable datum structure comprising, a section that has a continuous outer surface; and a scalloped section 24 that, on the outside surface resembles the circumferential discontinuous features of the rotatable structure being measured and which simulate a bladed rotor in the effect the features have upon a proximity probe held adjacent thereto.

7. A rotatable datum structure and method of use as hereinbefore described with reference to and as shown in figures 2.