Enclosure arrangement for fixing workpieces to be processed and a process for manufacturing and separating the enclosure arrangement

A form-fitting cast-on enclosure arrangement has parallel chucking surfaces for the fixing of metallic workpieces with irregular contours to be processed. A process is provided for the manufacturing and separating of the enclosure arrangement. The enclosure arrangement is made of plastic material and consists of several spaced ribs which are connected by webs. The enclosure arrangement has the advantage that workpieces with irregular contours can be enclosed in a cost-effective and fast manner and can be finished or repaired.

BACKGROUND AND SUMMARY OF THE INVENTION
 The invention relates to a form-fitting cast-on enclosure arrangement
 having parallel chucking surfaces for the fixing of irregularly contoured
 metallic workpieces which are to be processed, as well as to a process for
 manufacturing and separating the enclosure arrangement.
 An enclosure arrangement or fixture of the above-mentioned type which is
 made of metal is known from the Great Britain patent document GB-21 66
 070, in which case a metallic high-temperature resistant workpiece with
 irregular contours, such as a power unit blade, is partially cast into a
 block of low-melting metal so that in the process parallel chucking
 surfaces are created and the further processing of the workpiece is
 therefore made possible. A disadvantage of this fixture is the fact that,
 during the cast-in operation, molten metal comes in contact with the
 workpiece surface so that there is at least the risk of contamination of
 the workpiece surface with foreign metals. In the high-temperature
 operation of the workpiece, an influencing of the stability
 characteristics in the area of the cast-on enclosure arrangement of the
 processed workpiece cannot be excluded. Another disadvantage is the
 high-expenditure chemical or chemical-physical cleaning of the surface of
 the workpiece when a contamination is to be removed. Disadvantageously,
 cracks may occur during the cooling of the molten bath of the enclosure
 arrangement material. Another disadvantage is the handling of the
 workpiece that is enclosed by metal because the cast-in block causes a
 considerable increase in weight. Finally, because of the softness of the
 low-melting metal with respect to the high-melting hard workpiece, a
 continuous cutting of the enclosure arrangement material and of the
 workpiece material is not possible.
 In addition, the process known from the Great Britain patent document GB-21
 66 070 is not economical, at least for repair work, because a metal
 melting energy must be applied and a complete metal casting process must
 be provided. This requires a correspondingly high-expenditure handling of
 molten baths and long cooling phases.
 It is an object of the invention to provide a enclosure arrangement of the
 above-mentioned type and a process for its manufacturing and separation.
 In such enclosure arrangement, a common cutting processing of enclosure
 arrangement material and workpiece material is possible and contamination
 of enclosure material that does not endanger the workpiece stability is
 acceptable without any cleaning after treatment. At the same time,
 however, the contamination is negligibly low. In addition, the enclosure
 arrangement must be free of material cracks, and the parallel chucking
 surfaces must permit a secure fixing. It is also an object of the
 invention to provide a process which is so reasonable with respect to cost
 that used and damaged components can be fixed by the enclosure arrangement
 for the purpose of being repaired.
 According to the present invention, the object is achieved by the fact that
 the enclosure arrangement consists of plastic material and has several
 spaced ribs which are connected by webs.
 The enclosure arrangement according to the present invention has the
 advantage that, at high operating temperatures of the workpiece, plastic
 material burns in the oxidizing atmosphere so that individual molecule
 chains which may possibly still contaminate the metallic surface after the
 processing and separating step, will disintegrate and will not negatively
 change the stability of the workpiece. Plastic material can easily be
 handled because of its low specific weight. Using conventional machines,
 plastic material can be cast onto the workpiece in a few seconds, and the
 ribbed structure of the parallel chucking surfaces ensures a crack-free
 casting-on of the enclosure material because plastic material, when it
 cools from the softened condition, can emit sufficient heat by way of the
 ribs and webs in order to uniformly cool and shrink. In addition, at room
 temperature, plastic material still has a sufficient plastic material
 flowability in order to shrink free of cracks.
 The preferred use of a thermoplastic material as the enclosure material
 results in special advantages. A thermoplastic material can be used
 several times because it is hard and tough at low temperatures and softens
 at higher temperatures. This permits an environmentally acceptable
 enclosure, because approximately up to 100% of the thermoplastic material
 can be reused without any cost-intensive processing steps.
 A preferred material for the enclosure arrangement is polystyrene. This
 plastic material is hard and solid at room temperature and is capable of
 enclosing in ribs without any cracks an irregular workpiece contour after
 a casting-on at the softening temperature and cooling to room temperature.
 The stability is so high that a chucking of the ribs in conventional
 fixing tools is possible without any deformations. In addition, this
 plastic material can be processed in a cutting manner using the same
 water-cooled tools as the workpiece.
 In the case of a first embodiment of the enclosure arrangement, the ribs
 and the webs have a cross-sectional surface of approximately the same
 size. This has the advantage that shrinking strains at the transitions
 points between the ribs and webs are minimized and a crack-free enclosure
 is permitted.
 Another preferred embodiment of the process provides that, between the
 ribs, the workpiece is completely surrounded by the enclosure arrangement.
 As a result, on the one hand, the position of the ribs and the webs on the
 workpiece is additionally secured and, on the other hand, the workpiece
 can be metal-coated or processed electrochemically or chemical-physically
 in an advantageous manner on the exposed surfaces that are not covered.
 A preferred application of the enclosure arrangement is for the fixing of
 power unit blades, in which case the webs at least partially enclose the
 leading edge and the trailing edge of the blade. This results in the
 advantage that the leading edge and the trailing edge are protected from
 mechanical damage during the processing operation and, at the same time,
 the position of the ribs is fixed on the blade.
 The ribs are preferably arranged orthogonally or at an angle of between
 45.degree. and 90.degree. with respect to the blade axis. The ribs
 preferably completely enclose the blade. This has the advantage that the
 blade axis can be aligned more precisely and a secure fixing of the blade
 device is ensured.
 In the case of manufacturing and repair of a power unit blade, the blade is
 finished first. For this purpose, the blade is chucked in the area of the
 base or, when a cast-on shroud exists as an integral component, the blade
 is chucked on the shroud segment. For the finishing of the shroud segment
 and/or of the blade base, an enclosure is then cast around the blade in
 order to produce suitable parallel chucking surfaces. A preferred
 embodiment of the enclosure arrangement therefore provides that the
 distance between the ribs in the area of the blade base or the blade tip
 be smaller than in the blade center.
 In the blade center, the distance between the ribs is preferably larger
 than twice the rib thickness. With this arrangement of the ribs, the
 chucking forces may advantageously be distributed on the blade without the
 occurrence of deforming chucking forces, particularly since blades in
 aircraft engines have extremely thin wall thicknesses in the blade center.
 A preferred process for manufacturing an enclosure arrangement consists of
 fitting the workpiece into a receiving mold of an injection molding
 machine. The plastic material for the enclosure is then injection molded
 onto the workpiece. This has the advantage that, when polystyrene is used
 as the enclosure material, the areas to be processed, such as the blade
 base and the shroud segment, can also be cast in because a water-cooled
 cutting of both materials is advantageously possible using conventional
 tools. If the blade is completely surrounded by enclosure material, a
 cutting may advantageously be followed by a metal plating in the shroud
 area or in the blade area and/or electrochemical polishing or etching.
 Chemical-physical processing steps, such as vacuum metallizing or
 sputtering or chemical vapor depositing are also possible, in which case
 the finished blade surface is protected by a complete enclosure.
 For separating the workpiece from the enclosure, the enclosed workpiece is
 cooled to a temperature at which continuous shrinking cracks are formed in
 the enclosure. Since, at low temperatures, the plastic material becomes
 brittle and shrinks more than the metallic workpiece, continuous shrinking
 cracks are formed in the plastic material without any deformation of the
 workpiece. Advantageously, the enclosure chips-off the workpiece without
 any residues and can be reused in the case of a thermoplastic material and
 can again be supplied to an injection molding process.
 Other objects, advantages and novel features of the present invention will
 become apparent from the following detailed description of the invention
 when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS
 FIG. 1 illustrates a cast-on enclosure 1 for a workpiece 2 to be processed,
 such as a blade for gas turbine engines before the finishing. The
 workpiece 2 has irregular contours. With the exception of a few
 positioning surfaces 4, the workpiece 2 had been completely surrounded by
 an injection molding technique with an enclosure material made of plastic
 material. In this example, the plastic material is polystyrene, a
 thermoplastic material whose softening temperature is between 80.degree.
 and 150.degree. C. and which has sufficient stability at room temperature
 in order to absorb chucking forces by way of chucking surfaces 5 which are
 arranged in a plane-parallel manner. These chucking surfaces 5 are
 arranged on ribs 6 which are kept at a distance from one another by webs 7
 and 8. This distance decreases in the direction of the surfaces 9 and 10
 to be processed. Thus, a secure chucking is ensured in the proximity of
 the processing areas.
 In the case of a blade, the surfaces to be processed are situated in the
 area of the blade base 11 and in the area of a shroud segment 12. The
 blade base 11 and the shroud segment 12 are integral components of a blade
 device. Between these, the blade vane 13 is arranged. The blade vane 13
 already has its final contour before the enclosing. In this case, the web
 7 at the same time covers the leading edge of the blade vane 13, and the
 web 8 covers the trailing edge so that the blade vane 13 is protected from
 mechanical impairment. The cross-sectional surfaces of the webs 7 and 8
 have approximately the same size as the cross-sectional surfaces of the
 ribs 6.
 The blade surface between the ribs 6 and the webs 7 and 8 of the enclosure
 arrangement 1 may also be enclosed by a thin plastic layer. The blade
 surface will therefore be protected in the case of metalplating steps
 and/or electrochemical polishing or etching or in the case of
 chemical-physical process steps, such as vacuum metallizing, sputtering or
 chemical vapor depositing in the shroud area or in the blade base area.
 As in the example shown, the plastic enclosure will then cover the whole
 blade device with the exception of a few positioning surfaces 4. During
 the injection molding, the positioning surfaces 4 are kept free of
 enclosure material in order to, for example, during the milling of
 plane-parallel surfaces 5, indicate on the ribs 6 of the enclosure 1
 suitable reference surfaces for the adjusting of the workpiece 2.
 During the finishing, the plastic material of the enclosure arrangement 1
 can be cut without any additional special tools together with the
 workpiece 2 in a water-cooled manner.
 FIG. 2 illustrates a lateral view of a cast-on form-fitting enclosure 1 for
 a workpiece 2. After the injection molding of the enclosure arrangement 1,
 forming the ribs 6 and webs 7, the chucking surfaces 5 and 14 on the ribs
 were milled in a plane-parallel manner after the adjusting with respect to
 the positioning surfaces 4. This milling step will not be required when
 the enclosure 1 has precisely positioned plane-parallel chucking surfaces
 5 and 14 on the ribs 6 after the injection molding.
 FIG. 3 shows an enclosed workpiece 2 after finishing. In this case,
 together with the finishing of the lower 15 and upper 16 workpiece areas,
 the enclosure 1 made of plastic material was also worked off. In the case
 of the finishing of a blade device, the blade base 11 and a shroud segment
 12 with sealing tips 17 were processed in geometrical reference to the
 chucking surface 5 so that the enclosure 1 with its plane-parallel
 chucking surfaces 5 has stopped in the area of the blade vane 13. Using a
 cooling operation, the remainder of the enclosure 1 is separated from the
 blade vane 13 after the finishing. For this purpose, the enclosed
 workpiece 1 is cooled to a temperature at which continuous shrinking
 cracks are formed in the enclosure and the enclosure 1 chips off the
 workpiece 2 in fragments without any residue 2.
 The manufacturing of a form-fitting enclosure 1 in the injection molding
 operation according to the invention requires only seconds. Likewise, the
 no-residue separating of the enclosure 1 from the workpiece 2 by means of
 a cooling step also requires only seconds. The plastic fragments obtained
 from the separation step can be reused. Thus, this enclosure arrangement 1
 makes available a cost-effective device which permits processing steps on
 workpieces 2 with irregular contours and repairs of such workpieces 2 in a
 cost-effective and fast manner. The invention even allows the
 implementation of repair work in an economical fashion.
 Although the invention has been described and illustrated in detail, it is
 to be clearly understood that the same is by way of illustration and
 example, and is not to be taken by way of limitation. The spirit and scope
 of the present invention are to be limited only by the terms of the
 appended claims.