GB2148755A - Method and apparatus for separating cast or extruded support blocks from workpieces - Google Patents

Abstract

Blocks of cast or extruded zinc, lead, antimony or tin alloys which surround portions of a workpiece, e.g. turbine blade blanks for convenient manipulation and support in a grinding machine are removed by cooling, e.g. by introduction into a liquid nitrogen bath. The material of the blocks is chosen to shrink at a higher rate and to extract a higher brittleness than that of the blades at the low temperature in the nitrogen bath. If the resulting internal stresses in the blocks do not suffice to effect their separation from the workpieces, the blocks are struck or scratched by a hammer or another suitable tool, either in or upon removal from the nitrogen bath. This initiates a disintegration of blocks without deformation or contamination of the workpieces. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Separating Cast or Extruded Blocks from Workpieces The present invention relates to a method and to an apparatus for separating cast or extruded blocks from workpieces. More particularly, the invention relates to improvements in a method and apparatus for separating a workpiece from a block-shaped support or holder in which the workpiece is partially embedded during treatment in a machine tool, such as a grinding machine.

It is already known to assemble a blank which is about to be converted into a finished workpiece with a suitable holder for convenient mounting in a machine tool and for more convenient manipulation of the blank. For example, it is known to utilize a block-shaped holder into which the workpiece is partially embedded so that only that portion of those portions of the workpiece which require treatment (for example, removal of material) are exposed. The block-shaped holder (hereinafter called block for short) forms with the workpiece a clampable or retainable unit which is installed in or on the customary work holder of a machine tool and is movable therewith and/or relative thereto so as to allow for more convenient and stress-free treatment of one or more selected portions of the workpiece.

As a rule, the thermal expansion coefficient of the material of the block is different from the thermal expansion coefficient of the material of the workpiece so as to allow for convenient separation of the two components of the unit from one another when the treatment of the workpiece in the machine tool is completed. As a rule, the thermal expansion coefficient of the material of the block is higher than the thermal expansion coefficient of the material of the workpiece. The material of the block is applied around one or more selected portions of the workpiece by casting or by resorting to an extrusion technique. In order to for a part of a unit, the workpiece is introduced into a casting mold and the liquefied material of the block is poured into the mold around the workpiece.Since the thermal expansion coefficient of the material of the block is higher than that of the material of the workpiece, cooling of liquefied material of the block in the mold results in shrinkage of the block around the selected portion or portions of the workpiece so that the latter is held in the block without play. When the hardening or setting of the material of the block is completed, the block allows access only to that portion or those portions of the workpiece which require treatment in one or more machine tools. A typical example of a workpiece which can be confined in a block in the aforedescribed manner is a blank which is to be converted into a turbine blade in a grinding machine.As a rule, the material of the blank which is to be converted into a turbine blade is a nickel alloy which satisfies the exacting requirements regarding the durability and/or other desirable characteristics of the finished product. The material of the block for such a workpiece is normally zinc or a zinc alloy, the thermal expansion coefficient of which is higher than that of the material (for example, the aforementioned nickel alloy) of the workpiece. This ensures that, during cooling, the block of zinc or zinc alloy shrinks relative to and closely follows the outline or outlines of one or more selected portions of the workpiece.

The heretofore known methods of embedding workpieces in blocks of the above outlined character exhibit the drawback that the removal of a block from the finished workpiece presents serious problems. German Auslegeschrift No. 28 22 828 proposes to impart to the block a shape which allows for its breakage or disintegration along predetermined rated break lines. For this purpose, it is necessary to apply a mechanical stress to preselected portions of the block so that the block falls apart into a number of components and exposes the treated workpiece. A drawback of this proposal is that the mechanical stressing or required intensity cannot be confined to the block alone, i.e., it cannot be avoided that some of such mechanical stresses are also applied to the workpiece which is confined in the block.Any mechanical stressing of the workpiece can entail undesirable deformation which renders the workpiece useless or requires a secondary treatment. This holds especially true for highly sensitive workpieces, such as the blades of turbines, when even minor deformations of the treated workpieces render the product useless for its intended purpose even if the deviation from the ideal shape is in the range of a few tenths of one thousandth of one millimeter. Furthermore, breaking apart of the block requires the application of a pressure which results in smearing of portions of the destroyed block upon the adjacent surface or surfaces of the workpiece.This necessitates a secondary treatment in order to clean the finished product and/or a more complex secondary treatment which involves removal of material, namely of the material of the destroyed block, which adheres to the surface or surfaces of the finished workpiece.

One feature of the present invention resides in the provision of a method of separating a workpiece whose material has a first thermal expansion coefficient from a block whose material has a second thermal expansion coefficient and which is cast or extruded around one or more selected portions of the workpiece to constitute therewith a unit which can be more readily manipulated in a machine tool (especially in a grinding machine) than the workpiece along during treatment of one or more exposed portions of the workpiece (e.g., during removal of material from that portion or those portions of the workpiece which remain exposed subsequent to the casting or extrusion of the block therearound).The method comprises the steps of making the block from a material which becomes brittle in response to cooling to a predetermined temperature at which the brittleness of the material of the workpiece is less pronounced and which shrinks as a result of cooling at a rate exceeding the rate of shrinkage of the material of the workpiece with attendant development of internal stresses in the block, and cooling the unit to the predetermined temperature upon completed treatment of the workpiece to thereby initiate the generation of the aforementioned internal stresses in the block and the tendency of the block to crack off the workpiece.

The method can comprise the additional step of subjecting the block to the action of external mechanical separation promoting stresses subsequent to or simultaneously with the cooling step so as to initiate or promote separation of the material of the block from the workpiece. Such additional step can include applying to the block an abrupt blow or shock in the form of one or more impulses. Alternatively or in addition to the application of a blow or shock, the additional step can include scratching the block.

The material of the block can contain zinc.

The cooling step can include reducing the temperature of the unit to approximately -200"C., i.e., to the liquefaction temperature of nitrogen.

In order to automate the operation, the method can comprise the steps of automatically transporting the unit, together with a series of additional units, one after the other from the machine tool and along a first predetermined path into a deep cooling bath, carrying out the cooling step upon successive units in the cooling bath (e.g., in liquid nitrogen), and automatically advancing successive workpieces from the bath and along a second predetermined path, particularly to a further processing machine. Separation of blocks from the respective workpieces can take place in the bath or during movement of the workpieces and of the corresponding blocks or portions of blocks along the second path.

Another feature of the invention resides in the provision of an apparatus for separating a workpiece whose material has a first thermal expansion coefficient from a block whose material has a different second thermal expansion coefficient, which becomes brittle in response to cooling to a predetermined temperature at which its brittleness exceeds that of the material of the workpiece, which shrinks as a result of cooling at a rate higherthan that of the material of the workpiece with attendant development of internal stresses therein, and which is cast or extruded around one or more selected portions of the workpiece to constitute with the latter a unit which can be manipulated in a machine tool (such as a grinding machine) for the treatment of one or more exposed portions of the workpiece with greater ease than the workpiece alone.The apparatus comprises means for cooling the unit to the predetermined temperature, and transporting means including means for conveying the unit along a first path into the cooling means (whereby the material of the block becomes brittle and the internal stresses which develop therein at least tend to effect a separation of the block from the workpiece) and means for conveying the workpiece from the cooling means along a second path, e.g., to a processing station where the workpiece is subjected to an additional treatment.

The apparatus can further comprise separation promoting means for subjecting the block downstream of the first path to at least one externally applied mechanical stress to thereby promote the separation of the block from the workpiece. Such separation promoting means can be installed in the cooling means, it can be adjacent to the second path or it can comprise two sections one of which is disposed in the cooling means and the other of which is adjacent to the second path.

The separation promoting means can comprise a tool (e.g., a hammer) and means (e.g., an electromagnet) for causing the tool to strike against the block. The tool can have at least one edge which strikes against the block in the cooling means or while the block advances along the second path.

Furthermore, and in addition to or in lieu of the aforediscussed tool, the separation promoting means can comprise means for scratching the block.

The cooling means can comprise a liquid nitrogen bath.

The first and second conveyors can form part of a single conveyor which serves to advance units from the machine tool into the cooling means, to advance at least the workpiece of each unit through the cooling means, and to advance at least the workpiece of each unit along the second path.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

Fig. 1 is a schematic elevational view of a machine tool for treatment of workpieces which are confined in blocks and a schematic elevational view of an apparatus which embodies one form of the present invention; and Fig. 2 is a perspective view of a unit including a workpiece and a block which confines selected portions of the workpiece.

Referring first to Fig. 1, the reference character 1 denotes a machine tool which constitutes a grinding machine having a base 2 supporting a table 3 which is reciprocable in the X-direction. The table 3 carries a second table 4 which is reciprocable in the Z-direction. The second table 4 supports a unit 6 which may be of the type shown in Fig. 2 and includes a workpiece 8 as well as a block 7. The exposed portion or portions of the workpiece 8 forming part of the unit 6 on the table 4 can be treated by one or more grinding wheels 11 on a horizontal spindle 10 which is mounted for movement in the direction of the Y-axis along an upright column 9 of the grinding machine 1. The holder for the unit 6 is not specifically shown in Fig.

1; such holder can be rigidly secured to the table 4 or it can be moved with reference thereto in one or more directions, depending on the nature of the treatment to which the exposed or accessible portions of the workpiece 8 forming part of the unit 6 must be subjected.

The improved apparatus comprises a cooling means 12 which is a bath containing a supply of liquid nitrogen, and a transporting system 13 including a first conveyor 13a serving to transport successive units 6 along a first path from the table 4 of the grinding machine 1 into the cooling means 12, a second conveyor 13b which transports at least the workpieces 8 of successive units through the cooling means 12, and a third conveyor 13e which transports the workpieces 8 or workpieces 8 and the corresponding blocks 7 along a further path past a separation promoting device which is denoted by the reference character 18.The separation promoting device 18 which is shown in Fig. 1 comprises a hammer or an analogous tool 14 having an elongated edge 14a (shown by broken lines) which can be caused to strike against the block 7 if such block is not separated from the workpiece 8 prior to entering the path which is defined by the conveyor 13c. in the illustrated embodiment, the tool 14is mounted on one arm 16 of a bell crank lever the other arm 19 of which is articulately connected to the armature 21 of an electromagnet 22. The lever including the arms 16, 19 is pivotable about the horizontal axis of a shaft 17 which is denoted by a phantom line.

The unit 6 including a workpiece 8 and a block 7 is shown in Fig. 2. The illustrated block 7 is a rather complex body. However, it is clearly within the purview of the invention to make much simpler blocks. The configuration and the dimensions of the block 7 depend upon the extent to which such block should confine selected portion or portions of the workpiece 8. In the illustrated embodiment, the workpiece 8 is a blank of a turbine blade which is to be treated in the grinding machine 1. As a rule, the block 7 will be made by casting in a suitable form into which the workpiece 8 is inserted before the liquefied material of the block is poured into the mold. The thermal expansion coefficient of the material of the block 7 is higher than the thermal expansion coefficient of the material of the workpiece 8.Consequently, when the liquefied material of the block 7 is caused or allowed to cool in the interior of the mold, the block shrinks around the selected portion or portions of the workpiece 8 and forms therewith a more or less integral unit 6.

The material of the block 7 is preferably selected in such a way that, when the cooling from casting temperature to room temperature is completed, the resulting solid block has shrunk around the selected portion or portions of the workpiece with the required force which allows for predictable and convenient manipulation of the resulting unit 6 in the grinding machine 1.

Fig. 2 shows that the foot and head portions of the turbine blade which constitutes the workpiece 8 are exposed. The grinding machine 1 can be designed to treat either of or both such exposed portions of the workpiece 8. When the treatment of the workpiece is completed, the conveyor 13a of the transporting system 13 automatically advances the unit 6 from the grinding machine 1 into the liquid nitrogen bath which fills or partially fills the cooling means 12. The boiling point of liquid nitrogen is at -196 C. Introduction of the unit 6 into the body of liquid nitrogen in the cooling means 12 entails a pronounced cooling of the block 7 as well as of the workpiece 8.However, since the brittleness of the material of the block7 at -196 C is much more pronounced than the brittleness of the finished workpiece 8, the material of the block 7 develops internal stresses which normally or often suffice to entail a cracking off and resulting separation of the components 7 and 8 of the unit 6. Furthermore, and since the material of the block 7 tends to contract more rapidly than the material of the workpiece 8, the internal stresses which already exist between the parts 7 and 8 increase as a result of such cooling and this also contributes to more predictable and more likely separation of the material of the block 7 from the workpiece 8.The selection of the material of the block and the configuration of the block contribute or can contribute to more ready separation of its material from the workpiece as a result of introduction into the liquid nitrogen bath in the cooling means 12. If the separation takes place and is completed in the interior of the cooling means 12, the conveyor 1 3c transports only the liberated workpiece 8 which is advanced to the next processing station P. The exact nature of the treatment at the station P forms no part of the present invention. The fragments of the separated block 7 remain in the bath in the interior of the cooling means 12 and are removed by a separate conveyor or in any other suitable way forming no part of the present invention.It suffices to say that the fragments of a separated block 7 need not necessarily be transported by the conveyor 1 3c of the transporting system 13.

If the introduction of a unit 6 into the cooling means 12 does not suffice to effect full or even partial separation of the block 7 from the workpiece 8, the separation promoting device 18 becomes effective while the unit 6 is being transported by the conveyor 13c. The arrangement may be such that an optical monitoring device 25 monitors the path which is defined by the conveyor 13c in order to ascertain whether the conveyor 1 3c transports only a workpiece 8 or a complete unit 6 including a workpiece 8 and a block 7 or a fragment of a unit including a complete workpiece 8 and a small or a substantial portion of the corresponding block 7.

The monitoring device 25 then transmits a signal to the electromagnet 22 which is energized and causes its armature 21 to pivot the level 16, 19 in a direction to cause the edge 14a of the hammer 14 to strike against a selected portion of the block 7 in the path which is defined by the conveyor 13c. Such externally applied mechanical blow, impact or shock suffices to assist the internal stresses in the block 7 to cause it to break up into several pieces and to become separated from the workpiece 8. It has been found that a very slight impact of the hammer 14 upon the block 7 in the path which is defined by the conveyor 13c suffices to ensure that such block becomes fully separated from the workpiece.The arrangement may be such that the edge 14a merely scratches the surface of the block 7, and such scratching alone suffices to assist the internal stresses in effecting a complete separation of the blockfrom the workpiece. Separation of the material of the block 7 from the workpiece 8 in the interior of the cooling means 12 or along the path which is defined by the conveyor 13c is so completethatthe workpiece 8 need not be subjected to any secondary treatment for the purpose of removing traces of the material of the block.Furthermore, the extent of separation in the liquid bath or during travel with the conveyor 1 3c is such that the workpiece 8 is not subjected to any deforming stresses which would require discarding of the workpiece or a secondary treatmentforthe purpose of compensating for its deformation.

An important advantage of the improved method and apparatus is that complete separation of the block 7 from the workpiece 8 can be accomplished without any undesirable deformation of the workpiece 8 and/or without contamination of the workpiece by remnants of the block 7. In many or in most instances, separation of the block 7 from the workpiece 8 will be completed in the liquid bath which is contained in the cooling means 12 of Fig. 1 so that the separation promoting device 18 need not be actuated at all.In other words, the difference between the thermal expansion coefficients of the materials of the workpiece 8 and block 7 can be readily selected in such a way that the internal stresses which develop in the block 7 as a result of introduction into the cooling means 12 suffice to cause the block to break up into several pieces which remain in the cooling means 12 while the conveyor 13b transfers the thus liberated workpiece 8 into the range of the conveyor 13c. Since the shrinkage of the material of the block 7 in response to cooling is much more pronounced than the shrinkage of the material of the workpiece 8, the internal mechanical stresses which develop in the interior of the cooled block are normally amply sufficient to ensure residue-free or remnant-free crackling of the material of the block off the adjacent surfaces of the workpiece.

The impulses which are applied by the edge or edges 14a ofthetool 14to a block7 on the conveyor 13 in the event that such block continues to adhere to the workpiece while the latter is being advanced by the conveyor 1 3c are relatively light, and a single impulse-like impact normally suffices to trigger the separation which is thereupon completed practically instantaneously. As mentioned above, the edge or edges 14a of the tool 14 can also be designed to merely scratch selected portions of the block in the path which is defined by the conveyor 13c in order to trigger the separation of the material of the block from the finished workpiece.

As also mentioned above, shrinkage of the material of the block 7 in response to cooling is more pronounced than shrinkage of the material of the workpiece 8. This does not create any problems as regards the application of stresses to the workpiece because the application of stresses is uniform in all regions where the material of the block contacts the material of the workpiece. It has been found that the improved method and apparatus can be used for complete remnant-free segregation of the entire block 7 from the workpiece 8 so that the workpiece does not require any secondary treatment for the purpose of removing traces of the material of the block.

At the present time, the block 7 is preferably made of zinc or zinc alloys. However it is also possible to employ many other types of alloys, such as alloys of lead, antimony or tin. All that counts is to ensure that, when the unit 6 is cooled, shrinkage of the material of the block 7 is more pronounced than shrinkage of the material of the workpiece 8 and also that, when the cooling of the block 7 to a particular temperature is completed, the brittleness of the material of the block is much more pronounced than the brittleness of the material of the workpiece. As mentioned above, if the workpiece 8 is a turbine blade, it is normally or often made of nickel or nickel alloy.An advantage of such materials is that they do not become brittle in response to cooling to temperatures corresponding to those which are required to liquefy nitrogen and which are sufficiently low to ensure adequate brittleness of and development of adequate internal mechanical stresses in the block 7 if the block consists of one of the aforediscussed materials or an analogous material.

It is clear that the operation of the structure which is shown in Fig. 1 can be automated to any desired extent. For example, a machine which is disposed to the left of the grinding machine 1 and is not shown in Fig. 1 can turn out a succession of units 6 which are delivered to the table 4 by a suitable conveyor so that the exposed portion or portions of successive workpieces 8 can be treated by the grinding wheel or wheels 11 on the spindle 10 of the grinding machine 1. The transporting system 13 automatically advances successive units 6 from the range of the grinding wheel or grinding wheels 11 into the cooling means 12 and advances the liberated workpieces 8 or the units 6 along the path which is defined by the conveyor 13c.

The improved apparatus is susceptible of many additional modifications. For example, the tool 14 can be positioned in such a way that it has an edge 14a continuously extending into the path of movement of a block 7 if such block is still connected with the workpiece 8 which is being transported by the conveyor 13c. If the block became separated from the workpiece in the interior of the cooling means 12, the workpiece 8 simply bypasses the stationary edge 14a of the thus positioned tool 14. In such apparatus, the electromagnet 22 can be omitted or it can remain inactive provided that the cutting edge 14a is properly positioned so that it cannot strike against an oncoming workpiece 8.

It is also possible to utilize a second separation promoting device 18' which is installed in the interior of the cooling means 12 to strike against the block 7 as soon as the latter is cooled to the temperature of approximately -200 C which is the range of liquefaction temperatures of nitrogen. Fig.

1 merely shows the hammer 14' and the edge 14a', of the second separation promoting device 18'. It is also possible to utilize two separation promoting devices, namely one in the cooling means 12 and the other along the path which is defined by the conveyor 13c. The first separation promoting device 18' can be actuated whenever a unit 6 enters the cooling means 12, and the separation promoting device 18 is utilized only if the monitoring means 25 detects that the conveyor 1 3c transports a block 7 beyond the cooling means 12.

An important advantage of the improved method and apparatus is that the separation of blocks 7 from the respective workpieces 8 is complete and that such separation does not entail any damage to or deformation of the workpieces. Another important advantage of the improved method and apparatus is thatsegregation of the material of the blocks 7 from the respective workpieces 8 is complete so that the workpieces need not be treated for removal of remnants of the material of the respective blocks.

This entails substantial savings in the overall cost of the treatment of workpieces. The material of the separated blocks 7 can be reused for the making of fresh blocks.

Still another important advantage of the improved method and apparatus is that the method can be practiced by resorting to an extremely simple, compact and relatively inexpensive apparatus.

Furthermore, the operation of the apparatus can be automated to any desired extent.

Claims (19)

1. Amethod of separating a workpiece whose material has a first thermal expansion coefficient from a block whose material has a second thermal expansion coefficient and which is cast or extruded around one or more selected portions of the workpiece to form therewith a unit which can be manipulated in a machine tool, such as a grinding machine, during treatment of the exposed portion or portions of the workpiece, comprising the steps of making the block from a material which becomes brittle in response to cooling to a predetermined temperature at which the brittleness of the material of the workpiece is less pronounced and which shrinks as a result of cooling at a rate higher than the material of the workpiece with attendant development of internal stresses in the block; and cooling the unit to said predetermined temperature to thereby initiate the generation of said internal stresses in the block and the tendency of the block to crack off the workpiece.

2. The method of claim 1, comprising the additional step of subjecting the block to the action of external mechanical stresses subsequent to or simultaneously with said cooling step so as to initiate or promote the separation of the material of the block from the workpiece.

3. The method of claim 2, wherein said additional step includes applying to the block an abrupt blow or shock.

4. The method of claim 2, wherein said additional step includes scratching the block.

5. The method of claim 1, wherein the material of the block contains zinc.

6. The method of claim 1, wherein said cooling step includes reducing the temperature of the unit to be liquefaction temperature of nitrogen.

7. The method of claim 6, wherein said temperature is approximately - 200"C.

8. The method of claim 1, further comprising the steps of automatically transporting the unit, together with a series of additional units, one after the other from the machine tool and along a first predetermined path into a deep cooling bath, carrying out said cooling step upon successive units in the cooling bath, and automatically advancing successive workpieces from the bath and along a second predetermined path, particularly to a further processing station.

9. Apparatus for separating a workpiece whose material has a first thermal expansion coefficient from a block whose material has a different second thermal expansion coefficient, which becomes brittle in response to cooling to a predetermined temperature at which its brittleness exceeds that of the material of the workpiece, which shrinks as a result of cooling at a rate higher than that of the material of the workpiece with attendant development of internal stresses therein, and which is cast or extruded around one or more selected portions of the workpiece to form therewith a unit which can be manipulated in a machine tool, particularly in a grinding machine, for the treatment of the cxposed portion or portions of the workpiece, comprising means for cooling the unit to said predetermined temperature; and transporting means including means for conveying the unit along a first path into said cooling means whereby the material of the block becomes brittle and the internal stresses which develop therein at least tend to effect a separation of the block from the workpiece, and means for conveying the workpiece from said cooling means along a second path.

10. The apparatus of claim 9, further comprising separation promoting means for subjecting the block downstream of said first path to at least one externally applied mechanical stress to thereby promote the separation of the block from the workpiece.

11. The apparatus of claim 10, wherein said separation promoting means is adjacent to said second path.

12. The apparatus of claim 10, wherein said separation promoting means includes a tool and means for causing said tool to strike against the block.

13. The apparatus of claim 10, wherein said tool has at least one edge which is arranged to strike against the block in said cooling means or while the block is located in said second path.

14. The apparatus of claim 10, wherein said separation promoting means includes means for scratching the block.

15. The apparatus of claim 9, wherein said cooling means comprises a liquid nitrogen bath.

16. The apparatus of claim 9, wherein at least a portion of said separation promoting means is installed in said cooling means.

17. The apparatus of claim 9, wherein said first and second conveyors form part of a single conveyor which is arranged to advance the unit into said cooling means and to advance at least the workpiece through the cooling means and into said second path.

18. A method of separating a workpiece substantially as herein described with reference to the accompanying drawings.

19. Apparatus for separating a workpiece substantially as herein described with reference to the accompanying drawings.