USH1769H

USH1769H - Optimized recursive foundry tooling fabrication method

Info

Publication number: USH1769H
Application number: US08/466,008
Authority: US
Inventors: Steven R. LeClair; Stephen C. Gregory; Benny L. Carreon; Yoh-Han Pao; Ron Cass; Kam Komeyli
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 1995-06-06
Filing date: 1995-06-06
Publication date: 1999-01-05

Abstract

A method for producing a pattern for making a cast part is described which comprises the steps of defining the structure of the part in terms of computer aided design system data, selecting a parting surface for the part to be cast; defining core requirements for the part by sweeping each positive feature of the part to the parting surface, subtracting the part from the projection, adding any remaining volume to the core, sweeping negative features away from the parting surface to the top or bottom of the mold and subtracting the negative features from the projection and intersecting the remainder of the part and adding any remaining volume to the core; repetitively generating alternative parting surfaces for the part and defining the corresponding core requirements whereby an optimum parting surface is defined for which the quantity and complexity of the corresponding core requirements are minimized, constructing core prints for each core requirement; constructing a pattern by adding the core prints to the part; and defining draft for the pattern surfaces perpendicular to the optimum parting surface.

Description

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.

BACKGROUND OF THE INVENTION

The present invention relates generally to metal casting methods, and more particularly to a method for efficiently producing a metal casting mold for a complex part by recursively identifying the cores for the casting and the molds for making the cores defining the complex part once a parting surface for the part is defined.

In metal casting discrete mechanical parts using sand molds, patterns are used in fabrication of the molds to ensure that the resulting cast parts have the correct geometry, or can be readily finished to the correct geometry. The pattern is a model of a part to form a mold cavity substantially defining the part shape, but is not simply a facsimile of the part because additional shapes (sprues, runners, gates, etc) are used to form channels for inserting molten metal, or shape modifications to provide taper (draft) on some surfaces of the part to facilitate withdrawal of the part from the mold. The principal molding material conventionally used in foundries is silica sand, which, when mixed with water and a binder (e.g. clay), can be formed to a complex geometry which retains its shape while being filled with metal and allowed to cool. The mold is usually destroyed when the casting is removed and must be recreated using the pattern for each cast part to be produced.

Mold design and fabrication are especially difficult if the cast part has sufficiently complex geometry or when the parting surface is defined such that the pattern cannot be withdrawn easily from the mold. In order to accommodate complex geometries by means of conventional casting methods, the pattern maker uses cores and loose pieces to ensure that those parts of the cavity which should be filled are filled. In standard practice, molds are often made up of two halves. The pattern is also made up of two parts mounted on the two sides of a board which represents the dividing (parting) surface (which may be more complex than a single plane) between the two mold halves. The two mold halves are formed by packing sand around each side of the pattern board, and subsequently combined to form the cavity left when the pattern is removed. The mold must therefore be made such that the pattern can be withdrawn from the mold. If the mold is made in two halves, each part of the pattern must be removable from the corresponding mold half. In order to define the casting pattern, the pattern maker modifies the pattern around the complex features of the part (to render it removable from the mold) using extra pieces of mold-like material, called cores or loose pieces, for filling extraneous spaces around the correct cavity shape for the cast part. The cores are generally made from bonded silica sand, and the molds used to make the cores, called core boxes, are permanent molds, usually made of wood or hardened epoxy.

In the practice of the invention, computer associative memories and feature-based computer aided design (CAD) are incorporated into a highly efficient and effective method for producing patterns and molds for casting substantially any complex part wherein withdrawal interferences of the pattern are defined for various parting planes or surfaces, and, once the parting surface is specified, the correct pattern structure is recursively defined.

It is a principal object of the invention to automate and optimize foundry tooling fabrication for metal casting.

It is another object of the invention to provide a method for producing a pattern for a part to be cast in a metal casting process.

It is another object of the invention to provide a method for sequentially drafting a pattern by part feature, core and rigging relative to a parting plane of a casting mold.

It is another object of the invention to provide a method for producing casting patterns for complex cast parts which cannot be otherwise withdrawn from a mold without destroying the mold.

It is a yet further object of the invention to provide a solid modelling recursive molding procedure for defining casting pattern core and core box requirements.

These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.

SUMMARY OF THE INVENTION

In accordance with the foregoing principles and objects of the invention, a method for producing a pattern for making a cast part is described which comprises the steps of defining the structure of the part in terms of computer aided design system data, selecting a parting surface for the part to be cast; defining core requirements for the part by sweeping each positive feature of the part to the parting surface, subtracting the part from the projection, adding any remaining volume to the core, sweeping negative features away from the parting surface to the top or bottom of the mold and subtracting the negative features from the projection and intersecting the remainder of the part and adding any remaining volume to the core; repetitively generating alternative parting surfaces for the part and defining the corresponding core requirements whereby an optimum parting surface is defined for which the quantity and complexity of the corresponding core requirements are minimized, constructing core prints for each core requirement; constructing a pattern by adding the core prints to the part; and defining draft for the pattern surfaces perpendicular to the optimum parting surface.

DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the following detailed description of representative embodiments thereof read in conjunction with the accompanying drawings wherein:

FIGS. 1a-e show in a comprehensive fashion the foundry casting mold fabrication method of the invention with relation to a representative three dimensional part and associated parting surface, cores, core boxes and rigging which are defined in the practice of the invention;

FIG. 2 is a block diagram of the steps of the method for constructing a mold pattern according to the invention;

FIGS. 3a-g show the method of the invention by reference to a two-dimensional example;

FIG. 4 shows a perspective view of another example casting having complex features for illustrating the method of the invention;

FIG. 5 shows the location of a first parting surface which is automatically generated by the method of the invention for the FIG. 4 casting;

FIG. 6 shows the FIG. 4 casting as it would appear in the lower (drag) portion of the mold for the generated parting surface illustrated in FIG. 5;

FIG. 7 shows the FIG. 4 casting in a view from below as it would appear in the upper (cope) portion of the mold for the generated parting surface illustrated in FIG. 5;

FIG. 8 shows the identification of the volume of the mold, for the FIG. 4 casting and the FIG. 5 parting surface, in which a core is required;

FIG. 9 shows in isolation the core requirement identified in relation to FIG. 8;

FIG. 10 shows the FIG. 9 core requirement as viewed from below at a reverse angle;

FIG. 11 shows in isolation the FIG. 9 core requirement with core prints added to the core requirement to make up the finished core;

FIG. 12 shows the finished core of FIG. 11 as viewed from below at a reverse angle;

FIG. 13 shows the location a parting line as generated by the method of the invention for the finished core of FIG. 11;

FIG. 14 shows from below the finished FIG. 11 core with the parting line illustrated in FIG. 13;

FIG. 15 shows the loose piece requirement for the FIG. 11 core;

FIG. 16 shows the loose piece requirement identified in FIG. 15 as viewed from below at a reverse angle;

FIG. 17 shows the finished FIG. 11 core with the location of an alternative parting surface as generated by the method of the invention;

FIG. 18 is a view from below at a reverse angle of the finished FIG. 11 core with the alternative parting line illustrated in FIG. 17;

FIG. 19 is a view of the FIG. 11 core and alternative parting surface reverse of the FIG. 17 view;

FIG. 20 shows the loose piece requirement for the finished FIG. 11 core with the alternative parting surface location;

FIG. 21 shows the finished FIG. 11 core with the location of a second alternative parting surface generated by the method of the invention;

FIG. 22 is a view from a reverse angle of the finished FIG. 11 core with the second alternative parting surface illustrated in FIG. 19;

FIG. 23 is a view from below at a reverse angle of the finished FIG. 11 core with the second alternative parting line of FIG. 19;

FIG. 24 shows the FIG. 11 core with the lower half of the associated corebox for the second alternative parting surface illustrated in FIG. 19;

FIG. 25 shows the FIG. 11 core with the upper half of the associated corebox and second alternative parting surface as viewed from below and at an angle reverse of the FIG. 24 view;

FIG. 26 shows the FIG. 11 core and lower corebox half as viewed at an angle reverse of the FIG. 24 view;

FIG. 27 shows the finished pattern used to make the sand mold for the FIG. 4 casting and all the coreboxes required to make the cores for the mold;

FIG. 28 shows the lower half of the corebox required for the FIG. 27 pattern; and

FIG. 29 shows from below the upper half of the corebox required for the FIG. 27 pattern.

DETAILED DESCRIPTION

Referring now to the drawings, FIGs 1a-e show in comprehensive fashion an overview of the rapid foundry tooling system and fabrication method of the invention with reference to a representative complex three dimensional part 10 intended to be cast. In accordance with a principle feature of the invention, a plurality of shape features (in selected sizes and locations), including bosses, disks, slots, shafts, blends and other simple shapes are used to define the structure of part 10. Therefore, part 10 may be defined by cylinder 11, disk 12, boss 13, (half) cylinder 14, block 15 and slot 16. The structure of part 10 is first defined based on data representing size and shape of each consitituent feature entered into a CAD system. The structure of part 10, having been defined in terms of CAD system data, may then be displayed in any representative view on the CAD system display. Once the structure of part 10 is defined as just described, an initial parting surface 17 for casting part 10 is then selected, and the associated sprues, runners, gates, risers, cores, core boxes and mold are then iteratively generated in order to optimize the design of the resultant pattern, pattern board and mold. For example, with reference to FIG. 1b showing part 10 from below, the initial parting surface 17 indicated in FIG. 1a suggests

volumes

18 and 19 of specified shapes as requiring cores in corresponding shapes and locations in a pattern for part 10. However, with reference to FIG. 1c, identification of

volumes

18 and 19 in FIG. 1b indicate an appropriate new parting surface 17' (with parting surface offsets) which eliminate the necessity of cores for

volumes

18,19. The automatic identification and generation of an appropriate offset parting surface 17', or other parting surface which results in minimum quantity and complexity of cores, is a critical feature of the invention. Once the offset parting line is generated, the invention specifies location of the appropriate rigging (sprues 20, gates 21, runners 22 and risers 23) for casting part 10 such as illustrated in FIG. 1d. Once the rigging for part 10 is specified, pattern board 25 (FIG. 1e) and mold configuration are automatically generated.

Referring now to FIG. 2, shown therein is a block diagram of the method steps for constructing a mold pattern according to the invention. The listing of a representative computer program useful in executing the algorithm for constructing the mold pattern, including identification of required cores, in the practice of the method of the invention, and used in demonstration of the invention, is presented in Appendix A hereto. As suggested in FIG. 2, and with reference to the computer listing in Appendix A, the geometry of the part to be cast is first identified and defined in terms of CAD) system data, and an appropriate parting surface for optimum orientation of the part within the mold is generated as at 26. (See Computer Graphics Handbook Geometry and Mathematics, by Michael E. Morrison, Industrial Press Inc. (1990), the entire teachings of which are incorporated by reference, particularly Part 10, "Transformations".) Geometry of the part to be cast may be defined in terms of any suitable CAD data system as would occur to the skilled artisan guided by these teachings, the software used to define the geometry of parts in demonstration of the invention being SHAPES (Release 1.5, XOX, Inc., Minneapolis MN (1995)), and is incorporated by reference herein.

Two solids, called mold blanks, which represent the volume of the mold on the upper (cope) side and lower (drag) side of the parting surface, are generated as at 27. Core requirements 28 for the mold blank defined with respect to the parting surface are then identified. Each positive feature of the part is swept to the parting surface and the part is subtracted from the projection, and any remaining volume is added to the core. Negative features are swept away from the parting surface to the top or bottom of the mold and subtracted from the projection, the remainder is intersected with the part; and any remaining volume is added to the core. Once core requirements, if any, are identified for the selected parting surface, optional new parting surfaces are successively generated at each combination of two or more vertices defining a unique new plane through the part. The optimum parting surface is selected by considering the number and complexity of the core requirements and the number of surfaces of the part to be drafted for each parting surface so generated and considered.

As core requirements are identified for the optimum parting surface, the geometry of each core print (a separate part corresponding to the configuration of the associated core volume) is defined, and the core box for molding each defined core print is defined according to the recursive parting surface selection and core identification procedure just described for the original part to be cast including successive identification of any core requirements for each identified core as suggested at 29a in FIG. 2. For each core requirement, the core pattern, called a core print, is constructed by sweeping any vertical faces not flush with the part away from the body of the core piece. Distance of sweep is selected as one half the core depth in a direction normal to the surface being swept. If faces on opposite sides of the core are not being swept, the distance of sweep is selected equal to the depth of the core normal to the face being swept. These sweep distances are needed to maintain rigid positioning of the core print during metal pouring.

When all core requirements for the part (including core requirements for each core print) are identified and the corresponding core prints are defined by recursively defining as at 29b any required core prints (i.e. second or higher order core prints) for any cores defined at 29a, the pattern for the casting is defined by adding all the identified core prints to the part, adding draft to the pattern surfaces, and adding the appropriate rigging such as suggested for the example of FIG. 1d.

The hierarchical procedure for optimizing pattern construction according to the foregoing may be illustrated by reference to the two-dimensional example of FIGS. 3a-g . Consider the hook shaped member 30 to be cast which must be removed from the mold in a lateral direction in the plane of FIGS. 3a-g . First, a suitable parting surface 32 is defined (FIG. 3b). Parting surface 32 defines how the pattern will be oriented with respect to the mold, i.e., the pattern will be withdrawn from the two mold 31

portions

31a,33 perpendicularly to parting surface 32. Because of the complexity of mernber 30, namely the hook feature, the pattern cannot be removed from mold 31 without destroying mold portion 33. The volume where a core 34 will be used is therefore identified by cloaking that portion of the pattern which cannot be withdrawn from mold 31 with a core print of suitably simple geometry, such as a prismatic solid, to define an augmented pattern 35 which can be withdrawn without destroying the mold. Mold 31' formed to augment pattern 35 is called the first-level mold, and a core which will be inserted into mold 31' is called the first-level core (see FIG. 2 at 29a). Subtracting member 30 from core 34 volume defines core 34 (FIG. 3e) which must be cast each time a mold is made. The core box for casting the first-level core print is then fabricated. This mold is called the second-level mold. One second-level mold is required for each first-level core piece. In addition, some first-level cores are of such complexity as to also require cores, called second-level cores in the recursive procedure described herein (see FIG. 2 at 29b). Second-level molds and secondlevel cores may be constructed of suitable material to be reusable. Core 34 geometry may prevent its casting in a simple mold, and core 34 must therefore be cast in multiple (2 for core 34)

pieces

36,37 to be cast properly (FIGS. 3f,g).

Core pieces

36,37 may be connected in mold 31' as by positioning pins (not shown).

Referring now to FIGS. 4-29, shown therein are the steps defining the optimized recursive foundry tooling procedure outlined above and set out in the computer program listing of Appendix A in relation to a complex part to be cast. FIG. 4 depicts in perspective example part 40 to be cast in the recursive procedure. Example part 40 is first defined in terms of CAD system data as described above and has a base comprising an assemblage of a plurality of various

sized plate members

41,42, 43, upright cylindrical member 44, and a cavity 45 in cylindrical member 44 and cantilevered section 46 which renders the design of a pattern for making sand molds for part 40 a non-trivial procedure. FIG. 5 shows part 40 with one parting surface 48 generated at the upper surface of plate member 42 by the optimized recursive procedure of the invention. As suggested above in relation to FIG. 2, a plurality of parting surfaces may be generated for part 40, depending on its shape, as at any surface of the

plate members

41,42,43, but for clarity, discussion of the procedure related to part 40 will begin with reference to parting surface 48 illustrated in FIG. 5. It is noted, however, that for example part 40 the recursive procedure of the invention favors parting surfaces which are defined by the surfaces of

plate members

41,42,43. This constraint corresponds to the general objective of pattern-makers to have the majority of the volume of the casting in the lower half of the mold for optimum solidification of molten material. For clarity of the example, part 40 is shown in an orientation in the mold which is inverted to that which would normally be utilized.

Referring now to FIG. 6, shown therein is part 40 as its casting would appear in the lower (drag) portion 51 of the mold for the generated parting surface 48 of FIG. 5. FIG. 7 shows the casting of part 40 as it would appear in the upper (cope) portion 52 of the mold for the same parting surface 48. As suggested above in relation to FIG. 2 and the program listing of Appendix A, once the structure of part 40 is defined and parting surface 48 is selected, volume 55 of the mold in which a core is required (core requirement) is identified as depicted in FIG. 8. FIG. 9 shows in isolation the core 56 requirement identified in relation to FIG. 8, and FIG. 10 shows the core 56 requirement viewed from below. In the FIG. 10 view, tab 58 corresponding to cavity 45 in cylindrical member 44 of part 40 is revealed.

Because the core 56 requirement identified in FIG. 8 itself does not constitute the entire core which the foundryman would insert into the mold, structures must be added to the core requirement which allow it to be mounted securely inside the mold. These structures, called core prints, are generated as described above and are added to the core requirement to make up finished core 60, as shown in FIG. 11, and are added to the pattern to create the cavities in the mold in which core 60 is mounted. As such, the core prints must also be removable from the mold. The recursive procedure of the invention automatically adds correct core prints to a core requirement to complete the core design. In FIG. 11, core prints are shown added to the sides of the core and comprise structures which are not merely extensions of the exposed sides of the core, but extend to parting surface 48 to ensure proper positioning of the core print within the mold. FIG. 12 shows the finished core 60 of FIG. 11 as viewed from below at a reverse angle. The extension of the core prints to parting surface 48 is illustrated. If the prints were only extensions of the exposed surfaces of the core requirement then a volume of the mold would be trapped between the core prints and the large plate member 42 of the casting defining part 40.

Because all cores must be constructed each time a new part is cast, the most efficient way to construct cores is by molding in permanent molds called coreboxes. Coreboxes, like molds for the casting, must be constructed so that the core can be removed from the corebox in a nondestructive manner. The recursive procedure of the invention efficiently designs the structure for the coreboxes for all cores using the same procedure as that used to construct the mold of the casting. Specifically, the recursive procedure of the invention generates the appropriate parting surfaces for the core, and, for each generated parting surface, identifies any trapped volumes in the corebox (called loose piece requirements rather than core requirements for clarity), and specifies the structure of the corresponding coreboxes. FIG. 13 shows parting surface 48' for the FIG. 11 core 60 which is generated by tie procedure and which is flush to the bottom of core 60. FIG. 14 shows core 60 and parting surface 48' from below, and reveals trapped volume 61 between parting surface 48' and core 60. The geometry of trapped volume 61 between core 60 and parting line 48', seen in FIG. 15, is then defined in order to identify a corresponding loose piece requirement for core 60. FIG. 16 shows the loose piece 63 requirement in a view reverse of FIG. 15.

Because the existence of a loose piece 63 requirement identified in relation to parting surface 48' generated as shown in FIG. 13 may not be the optimum configuration for the finished pattern, the recursive nature of the procedure generates second and successive parting surfaces and identifies the associated core and loose piece requirements in order to arrive at the optimum configuration (FIG. 2 at 27). FIGS. 17 and 18 are respective views from the top and bottom of the FIG. 11 core 60 with an alternative parting surface 65. Note that, in accordance with the general scheme of the algorithm of the invention to generate parting surfaces at vertices of the part geometry, the alternative parting surface is flush with the top of the impression in the core corresponding to the smaller plate member 43 of part 40. FIG. 19 is a view reverse of the view of FIG. 17 showing core 60 and alternative parting surface 65, and reveals trapped volume 67 between tab 58 on core 60 and alternative parting surface 65. Trapped volume 67 identified in FIG. 19 then defines the geometry of a loose piece requirement associated with alternative parting surface 65.

In a manner like that for generation of first alternative parting surface 65, because of the identification of a loose piece requirement for trapped volume 67 of FIG. 20, the procedure of the invention recursively generates second parting surface 70 such as shown in FIGS. 21,22,23. Second alternative parting surface 70 is flush with the bottom of tab 58 which corresponds to cavity 45 in cylindrical mem-ber 44 of part 40 (FIG. 4). It is noted that no trapped volume exists between any portion of core 60 and second alternative parting line 70, so that the procedure of the invention has successfully identified a parting surface 70 and associated core requirements for which no loose piece is required.

Having identified optimum parting surface 70 for core 60, the associated core box for casting the core is constructed from two rectangular prisms, one on each side of the parting surface. FIGs 24 and 26 show two views of core 60 and lower portion 72 of the corebox, and FIG. 25 shows core 60 and upper portion 73 of the corebox.

Once the core and loose piece requirements are identified and defined, the finished pattern 75 is needed to make the sand mold for casting part 40 and all the coreboxes required to make the cores for the mold comprise the parts required to make sand molds of the casting. The finished pattern is constructed by adding the core prints to the part pattern. FIG. 27 shows final pattern 75 for casting part 40 with the features (rigging) used to convey metal into the mold and reservoirs for holding the metal being omitted for clarity. FIGS. 28 and 29 show respective lower and

upper portions

77,78 of the corebox for the core (FIG. 11).

The permanent components used in the recursive molding process of the invention may be fabricated using virtual reality based rapid prototyping technology, such as stereolithography, and feature-based CAD solid modelling software and associative memory.

The invention therefore provides a novel method for efficiently producing a metal casting mold for a complex part. It is understood that modifications to the invention may be made as might occur to one with skill in the field of the invention within the scope of the appended claims. All embodiments contemplated hereunder which achieve the objects of the invention have therefore not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims. ##SPC1##

Claims

We claim:

1. A method for producing a mold pattern for making a cast part, comprising the steps of:

(a) defining the structure of a part to be cast in terms of computer aided design system data;

(b) selecting a parting surface for said part defined by said computer aided design system data, said parting surface defining a selected orientation of said part within a mold;

(c) defining core requirements for said part to be cast by first sweeping each positive feature of the part to said parting surface, subtracting said part from the projection on said parting surface and adding any remaining volume to the core, and then by sweeping negative features of said part away from said parting surface to the top or bottom of said mold, subtracting said negative features from said projection and intersecting the remainder of said part and adding any remaining volume to said core;

(d) successively selecting alternative parting surfaces for said part and defining the corresponding core requirements whereby an optimum parting surface having minimum core requirements is defined, (e) molding core pieces defined by said core requirements defined at said optimum parting surface for said part to be cast; and

(f) assembling said core pieces in a mold box to define the mold pattern for said part.

2. A method for producing a mold pattern for making a cast part, comprising the steps of:

(c) defining first core requirements for said part by first sweeping each positive feature of said part to said parting surface, subtracting said part from said projection on said parting surface and adding any remaining volume to the core, and then sweeping negative features away from said parting surface to the top or bottom of said mold, subtracting the negative features from said projection and intersecting the remainder of said part and adding any remaining volume to the core;

(d) successively selecting alternative parting surfaces for said part and defining the corresponding core requirements whereby an optimum parting surface having minimum core requirements is defined;

(e) defining second core requirements for each first core requirement defined for said part for said optimum parting surface by repeating step (c) for each said first core requirement defined at said optimum parting surface;

(f) molding core pieces defined by said first and second core requirements defined at said optimum parting surface; and

(g) assembling said core pieces in a mold box to define the mold pattern for said part.