BACKGROUND ART Generally, prototypes of new products are manufactured in the early stage of design and development of the products for the sake of examining the design of the products and detecting any defects in the design. For this purpose, a rapid prototyping (RP) system has been applied in the process of design and development of new products. Such a rapid prototyping system enables rapid automatic creation of three- dimensional prototypes simply with a 3-D CAD (Computer Aided Design) and requires no separate process for setups and design of machine tools.

The conventional sheet-stacking rapid prototyping methods, which includes only deposition of the sheets in building the shape, involves dividing the whole building material into thin layers separated from each other at a predetermined distance and stacking the thin layers to form the whole shape. This inevitably results in a rough surface of the formed products with staircases, the removal of which requires a separate process.

The conventional rapid prototyping method divides a building material in one direction in bulk without consideration of the geometric feature of the material and the

use purposes of the shaped products, thus leading to a deterioration of accuracy and precision in building the shape.

Furthermore, the conventional rapid prototyping method divides the whole shape into thin layers having a predetermined thickness and requires a larger number of layers in building a complex geometric model. In this case, building the shape may be impossible.

DISCLOSURE OF THE INVENTION Accordingly, it is an object of the present invention to solve the above- mentioned problems related to the conventional rapid prototyping method performing deposition only and to provide a hybrid rapid prototyping method that avoids dividing the building material into thin layers, thus dramatically reducing the total building time, and processes a full-scale model of a large size at a time, which enables manufacture of prototype of various profiles having an even surface free from a staircase with high precision.

To achieve the above object, there is provided a rapid prototyping method performing both deposition and machining, the method including a main process cycle and an additional machining process, wherein the main process cycle comprises repetition of the steps of : (a) machining the backside of the sheet by roughing and fining; (b) reversing the backside-machined sheet by 180 degrees, applying an adhesive and depositing the sheet on another one; and (c) machining the front side of the sheet by roughing, fining, and boundary cutting, wherein the additional machining process comprises drilling, milling or grinding.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram illustrating a main process cycle and an additional machining process substituting a hybrid rapid prototyping method performing both deposition and machining in accordance with the present invention; Fig. 2 is a diagram illustrating the framework of the process planning system for extracting machining features, i. e., machining feature segment (MFS) and deposition feature segment (DFS) in the hybrid rapid prototyping method of the present invention; Fig. 3 is a diagram illustrating the hint of a hole MFS in the hybrid rapid prototyping method of the present invention; Fig. 4 is a diagram illustrating the data structure of the hole MFS; Fig. 5 is a diagram illustrating the hint of a slot MFS in the hybrid rapid prototyping method of the present invention; Fig. 6 is a diagram illustrating the hint of a pocket MFS in the hybrid rapid prototyping method of the present invention; Fig. 7 is a diagram for explaining potential problems in using thick sheets in the rapid prototyping method of the present invention, wherein Fig. 7a shows the thickness of stacked sheets in the prior art, Fig. 7b showing the thickness of stacked sheets in the present invention using thick sheets; Fig. 8 is a diagram for explaining the relationship between the access direction of a machine tool and manufacturable faces in a face division step for DFS generation in the rapid prototyping method of the present invention, wherein Fig. 8a shows an access of the machine tool to the front side of the sheet and Fig. 8b shows an access of the machine tool to the backside of the sheet;

Fig. 9 is a diagram illustrating the pseudo-DFS generating step for DFS generation in the rapid prototyping method of the present invention; Fig. 10 is a diagram illustrating the DFS subdivision step for DFS generation in the rapid prototyping method of the present invention; Fig. 11 is a diagram illustrating the data structure of a DFS finally generated through the four steps for DFS generation in the rapid prototyping method of the present invention; Fig. 12 is a diagram illustrating a main process cycle for DFS's and an additional machining process for MFS's in the rapid prototyping method of the present invention, which is applied to the cylinder block of a real engine; and Fig. 13 is a diagram showing that the number of required DFS's is reduced due to the MFS concept in the rapid prototyping method of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a detailed description will be given with reference to the accompanying drawings as to a hybrid rapid prototyping method performing both deposition and machining in accordance with the present invention.

Fig. 1 is a diagram illustrating a main process cycle and an additional machining process that substitute the hybrid rapid prototyping method of the present invention.

As shown in Fig. 1, the hybrid rapid prototyping method of the present invention comprises a main cycle process and an additional machining process. The main cycle process is composed of backside machining step 11, sheet reversing and deposition step 12 and front side processing steps 13-16. The additional machining

process 17, which is to process small feature segments, comprises drilling, milling and grinding.

The backside machining step 11 involves roughing and fining the backside of the sheet.

Following the backside machining step 11, the sheet deposition step 12 reverses the sheet, applies an adhesive to the sheet, and combines the sheet with the previously processed one.

The front side processing steps 13-16 involves roughing and fining of the front side and contouring. That is, roughing 13, contouring 14, boundary cutting 15 and finishing 16 are carried out on the front side of the sheet.

According to the rapid prototyping method of the present invention, the main cycle process (i. e., backside machining, sheet reversing and deposition, and front side processing) is repeated periodically until the whole building process terminates.

In the meanwhile, the additional machining process composed of drilling, milling or grinding is carried out to process partially complex and small feature segments at any optimal points during the repetition of the main cycle process.

As described above with reference to Fig. 1, the hybrid rapid prototyping method of the present invention performs both deposition and machining and thus leaves no staircase on the final product's surface that is one of the problems of the conventional method simply performing deposition of the contoured sheets only.

Furthermore, this method simplifies processing of complex machining feature segments due to the additional machining process such as drilling, milling or grinding which can be carried out at any optimal points during the main cycle process of both deposition and cutting. Thus the present invention eliminates a need of dividing the

building material into thin layers to obtain complex and small feature segments, thereby reducing the total building time, and processes a full-scale model of a large size at a time, thus providing precision in building the shape.

A description will now be made as to machining feature segments (hereinafter, referred to as"MFS's") and deposition feature segments (hereinafter, refereed to as "DFS's") used in the rapid prototyping method of the present invention.

Fig. 2 shows the framework of the process planning system for extracting machining features, i. e., MFS and DFS in the rapid prototyping method of the present invention.

After completion of design of the product's feature, the designer converts a desired three-dimensional solid model to a physical file of STEP using a STEP AP203 converter supplied by the modeler. The designer extracts MFS's (Fig. 2b) from input geometric information (Fig. 2a) and removes the MFS's from the solid model to obtain a simplified shape (Fig. 2c). Based on the simple shape, the process planning system analyzes geometric information about all side faces of the solid model (Fig. 2d) and decomposes the solid model into minimum units, DFS's (Fig. 2e). These machining features MFS's and DFS's thus obtained form details of the process. It is thus possible to effectively arrange the whole process and provide information about the corresponding details of the process based on a feature concept.

A description will be first given to the MFS concept in the hybrid rapid prototyping method of the present invention.

The MFS is a machining feature segment accessible to a machine tool available. The individual machining feature segment must be machined in a single process. The MFS machining process is normally carried out after the completion of

deposition process and may be performed at any point optionally without respect to the deposition process according to circumstances. For example, the MFS machining process can be carried out prior to the deposition process when the MFS is inaccessible to the tools due to the obstructive upper layer deposited.

The present invention provides a fundamental geometric reasoning engine for extraction of all potential MFS's. The final MFS's are selected from the user's choice.

The invention also provides a manufacturability evaluation engine for the selected MFS's and machining information of the final MFS's.

The geometric reasoning engine is implemented based on a hint-based feature recognition approach. The term"hint"as used herein refers to the minimum criterion to determine the presence of a specific MFS. Even if many crosses occur between MFS's, the hint peculiar to a specific MFS substantially existing in the building material will not appear at all.

To implement the hint-based feature recognition approach, the system selects the type of a MFS to be extracted and provides the minimum hint for the MFS. Then, the system defines the MFS from the hint and forms the specific MFS through the geometric reasoning engine.

In the present invention method, MFS is categorized into four types of machining feature, i. e., hole, slot, pocket, and miscellaneous surface. The hole MFS is manufacturable by drilling, and the slot, pocket and miscellaneous surface MFS's are manufacturable by milling. These MFS's exist in the actual parts of the machine.

Selectively removing the MFS's from the model results in a reduction of the total number of MFS's to be stacked, shortening the total building time.

Fig. 3 shows the hint of a hole MFS Mh. l. in the hybrid rapid prototyping

method of the present invention, Fig. 4 a data structure of the hole MFS Mhole.

As shown in Fig. 3, the hint hole ouf the hole Mhole is a wall surface consisting of a cylindrical surface. The data structure of the hole MFS Mhole can be divided, as shown in Fig. 4, into geometric information and manufacturability information, both of which will be eventually used as data in planning the process.

Fig. 5 shows the hint of a slot MFS Mulot in the hybrid rapid prototyping method of the present invention. As shown in Fig. 5, the hint hMSlot of the slot Mulot is a pair of parallel wall surfaces opposing each other. The final component of the slot, i. e., bottom surface can be found based on the pair of parallel wall surfaces. Finding a pair of parallel wall surfaces and a bottom surface leads to extraction of variables required to define the slot MFS Mslot and thereby generate an instance corresponding to the individual slot MFS Mulot. For example, the model shown in Fig. 5 consists of 5 slots M510t. The basic data structure for slot M,,., is similar to that of hole Mhole. Eventually, these data structures are also used as geometric information for drawing up a process of plan.

Fig. 6 shows the hint of a pocket MFS Mpocket in the hybrid rapid prototyping method of the present invention. As shown in Fig 6, the pocket MFS Mpocket is composed of a bottom surface having the same shape as a cross section, and a group of wall surfaces formed from the bottom surface vertically sweeping and surrounding the pocket. The pocket MFS Pocket is not easy to extract in an automatic manner because the whole shape of a pocket is much variable depending on the shape of the cross section and the sweeping direction. It is widely used a pocket MFS extraction approach using geometric information about the cross section which formed on a plane perpendicular to the access direction of a machine tool.

However, such a general approach for pocket MFS extraction is impossible to implement if the pocket is accessible to a machine tool in all directions of the hemisphere. In this case, the pocket MFS extraction largely depends on the user's selection for the sake of higher accuracy and efficiency in machining.

The typical pocket MFS Mpocket usually selected by a process designer are relatively small-sized pockets that lie in the lateral side of the building material. Thus the designer has only to select one of the components of pocket MFS Mpocket to be extracted in order to generate the instances of the pocket MFS Mpocket.

Fig. 6 illustrates an example of the pocket MFS Mpocket that can be typically selected by the process designer in the hybrid rapid prototyping method of the present invention. Referring to Fig. 6, A, B and C denotes relatively small-sized pockets selected as pocket MFS's Mpocket and D is not a pocket MFS Mpocket despite its pocket- like shape. D can be manufactured in the deposition process cycle. The basic data structure of pocket Mpocket is similar to that of the other MFS's.

Most important is manufacturability in the MFS extraction. So, a MFS first extracted must be examined whether it is actually manufacturable, and only the manufacturable MFS is eventually considered worthy in the prototyping system. In the present invention, the potential MFS's first extracted are evaluated in regard to manufacturability according to the following three criteria and only those that satisfy all the three criteria are selected as actual MFS's.

First of all, the MFS must be manufacturable using the presently available machine tools. If information about the available machine tools is stored in the form of a table, manufacturability of the MFS can be determined through comparison of the content of the table with information about the MFS to be machined.

Second, the MFS must be accessible to the machine tools in an available direction. If using a machine tool manipulatable in five directions with a single setup operation, all MFS's are manufacturable theoretically in the respect that the MFS's are accessible to the machine tool in all directions. It is thus necessary to determine whether the individual MFS is accessible to the machine tool at a certain point of time during the machining process.

Third, the MFS must be manufacturable without respect to the shape of the building material, i. e., it is determined whether the machine tool has an inference with the shape of the building material during the cutting process. This third evaluation of manufacturability includes alteration of processing time as well as examination of manufacturability. That is, if a given MFS is measurable with an available machine tool in an available direction but inaccessible to the machine tool due to an interference of the peripheral material whose shape is obstructive to an access of the machine tool, the machining process must be performed prior to the deposition process during which the MFS is accessible to the machine tool without any interference, or the MFS must be omitted from the list of potential MFS's in the worse cases.

Through extraction and removal of the MFS's, many details in the original solid model can be ignored and the original model is much simplified. However, it is impossible to eliminate the extracted MFS's completely from the original model because the geometric information of the model stored in the STEP AP203 converter does not include information about the feature segments.

Thus, the present invention conceptually removes the extracted MFS's from the CAD model in such a case. That is, conceptual tags are assigned to the side faces defining the individual MFS's extracted so that the tagged side faces can be excluded

from the list of parts under examination in deriving DFS's after extraction of the MFS's. The purpose of removing the extracted MFS's from the original model is to reduce the resultant number of DFS's that may be unnecessarily generated due to the existence of the MFS's in the model. The resultant DFS's are not actually affected by adopting the concept of MFS.

In contrast to the conventional rapid prototyping method that divides the building material into very thin layers, the hybrid rapid prototyping method of the present invention uses much thick sheets as a building material. The conventional method is inapplicable to such a building material, i. e., thick sheets in building a shape.

However, using the thick sheets as in the present invention may also present some problems.

Fig. 7 is a diagram for explaining potential problems in using thick sheets in the sheet-stacking rapid prototyping method of the present invention. Fig. 7a shows the thickness of stacked layers in the conventional method, Fig. 7b showing the thickness of stacked layers in the present invention method using thick sheets. The circled part shown in Fig. 7b indicates a part inaccessible to the machine tool from the top or bottom sides of one layer in the conventional method.

As shown in Fig. 7, it is required to decompose the building material into DFS's based on the geometric information of the building material for the sake of building a desired shape by stacking thick sheets.

Extraction of DFS's is performed after the completion of the above-stated extraction of MFS's and the shape factors of the extracted MFS's are excluded from the list of geometric information to be extracted for generation of DFS's. The DFS's as well as the MFS's determine a plan of the whole process and a series of machining

processes are planned through evaluation of the whole process.

For this purpose, an analysis is performed in regard to the geometric information about all side faces constituting the model whose shape has been simplified through removal of the extracted MFS's from the original solid model and the model is decomposed into units of the laminated material, i. e., DFS's.

All DFS's have to meet the following requirements, i. e., they must have (1) the shape manufacturable with at most two setups, and (2) the thickness not larger than the maximum height of the sheet used.

Being manufacturable with at most two setup operations means that all side faces (excepting vertical faces) constituting a single DFS are visible from either side of top or bottom of the DFS. This requirement is originated from the sheet reversing process included in the rapid prototyping method of the present invention.

In generation of DFS's, the building material is decomposed based on the geometric information solely and the respective DFS's are then reconstructed within the range of the maximum thickness of the actually usable sheet material. It is ideal to perform deposition of actual sheet materials immediately after generation of DFS's.

But, the maximum sheet thickness is determined based on hardware architecture and limitation on the sheet's thickness.

The process for generating DFS's is largely divided into four steps, i. e., face division, construction of pseudo-DFS's, space division, and subdivision/mergence of DFS's.

Based on geometric information and MFS information, the silhouettes of all side faces constituting the building material and additional connection curves associating the silhouettes are generated and used as boundaries between

manufacturable faces in the front side and backside machining steps. The silhouettes of the side faces out of the existing edges of the material generate a new edge that divides the faces. The normal vectors of the individual divided faces have the same sign with respect to a stacking direction on the same plane. Such faces are grouped according to the rules that will be described later, to form a set of pseudo-DFS's.

Based on the boundaries between the pseudo-DFS's, the original model is spatially divided into a set of DFS's. If the height of a generated pseudo-DFS is larger than the maximum sheet thickness, the pseudo-DFS is subdivided and merged with the other adjacent pseudo-DFS's. After the completion of DFS generation, the machining information is stored.

A detailed description will now be made as to the above four steps for DFS generation.

Fig. 8 shows the relationship between the machine tool's access direction and manufacturable faces in the face division step for generating DFS's in the rapid prototyping method of the present invention. Fig. 8a is an exemplary view for an access of the machine tool to the front side of the sheet and Fig. 8b is an exemplary view for an access of the machine tool to the backside of the sheet.

Here, the faces accessible to the machine tool and thus manufacturable are confined to those visible from the access direction of the machine tool. That is, assuming that the sheet is pierced with an axis defining the stacking direction, all side faces constituting the DFS must be visible from the top and bottom sides of the sheet.

The boundary between the DFS's generated is a baseline on which the axial component of the normal vector in the stacking direction has the sign switching between positive (--) and negative (-). A curve defining the baseline in an aspect of

geometry is called"silhouette".

Once silhouettes are defined for all faces constituting the building material, additional connection curves associating the silhouettes are generated. Face division is implemented based on the silhouettes and the additional connection curves.

The reason why the face division is based on the silhouettes in the present invention lies in that all normal vectors in the individual face are identical in a sign of the Z coordinate value and the faces are grouped into [-]-z-signed face set and [+]-z- signed face set so as to form a set of pseudo-DFS's. Particularly, the faces divided by the edges additionally generated have their own marks stored in the data structure and indicate that they are the last faces for constructing the pseudo-DFSs.

Fig. 9 illustrates an example of the pseudo-DFS generating step for DFS generation in the rapid prototyping method of the present invention.

Once the faces are divided in the face division step and all normal vectors in the same plane are identical in a sign of the Z coordinate value, a sequential grouping of [-]-z-signed face set and [+]-z-signed face set forms a set of pseudo-DFS's.

Consequently, each pseudo-DFS is not a closed solid but a set of side faces with openings at the top and bottom. If a new side face is formed at the boundary to be cut off from the building material, the pseudo-DFS has a closed solid form.

Face grouping for constructing a set of pseudo-DFS's is performed as follows.

The individual face has its own data structure configured to store the sign of the Z coordinates of every normal vector, marks indicating that the face has been cut off by the additional connection curve linking the silhouettes, and the ID number of the corresponding pseudo-DFS. Once the stacking direction is determined, the [+]-z- signed face set and the [-]-z-signed face set form the top and bottom sides of each

pseudo-DFS.

To construct such a pseudo-DFS, a set of [+]-z-signed faces must be combined with a set of [+]-z-signed faces on the silhouette. That is, the pseudo-DFS can be formed if tracing a propagating path from one negative (-) face to one positive (+) face allows only one transition from [-]-z-signed face set to [+]-z-signed face set.

The transition from [-]-z-signed face set to [+]-z-signed face set occurs in every face propagation path. Following the first sign transition, another sign transition occurs into a new side face or a face divided from the previous one by an additional connection curve linking the silhouettes, which results in a single pseudo-DFS.

Ignoring vertical faces or those faces defining the MFS, the face propagation proceeds to the next side face.

Fig. 9 shows an example of the pseudo-DFS generating algorithm described above. The model shown in Fig. 9 is composed of two pseudo-DFS's, with one slot MFS and one hole MFS. Once the face propagation for forming a pseudo-DFS according to the above-described rules proceeds to a face defining MFS's, the system stores the ID of the pseudo-DFS and continues the face propagation to the next face.

Assuming that all faces propagated for generation of one pseudo-DFS are denoted in terms of a chain of signs. If the starting negative (-) sign is continued for a while and once changed to the positive (+) one, then the positive (+) sign has to continue without inversion to the negative (-) one.

The resultant pseudo-DFS has an opening at the boundary with another pseudo-DFS, as shown in the last figure of Fig. 9. It is thus required to close the opening at the boundary between the pseudo-DFS's with a new side face for the sake of generating a machining path for the generated DFS by means of an external CAD

software. This space division leads to complete generation of one DFS from the pseudo-DFS.

Next, a detailed description will be made to the space division step for generating DFS's in the rapid prototyping method of the present invention.

Space division is a process for constructing the original model with a set of DFS's based on the boundaries between the above-generated pseudo-DFS's. In the space division step, the outer edges on the top of the pseudo-DFS are constructed as a loop to form faces filling the inside of the loop and separate the DFS from the original model. The new side faces are generated on the boundary between the DFS's. Once a DFS is completed, the bottom side of the next DFS is derived from that of the previous DFS separated from the original model. These serial processes can be implemented using various modeling functions supported by a commercially available kernel (Parasolid from Unigraphics Solution. Inc.).

Next, a detailed description will be made to the DFS subdivision and mergence step for generating DFS's in the rapid prototyping method of the present invention.

If the height of a generated pseudo-DFS is larger than the maximum thickness of a sheet available, it is unavoidable to subdivide the pseudo-DFS and, according to circumstances, merge the subdivided pseudo-DFS with other adjacent ones.

In the present invention, the DFS is divided not at the maximum height of the sheet available but at the position of the silhouette located right under the maximum height. This is to increase efficiency of the whole process and minimize the time required for the sheet reverse step in the rapid prototyping method of the present invention.

As shown in Fig. 10, separating the DFS from the silhouette reduces the required number of setups.

The subdivided DFS's can be merged with the upper DFS in two cases. The one case is where all faces constituting the separated DFS have a positive (+) sign. In this case, the bottom of the upper DFS to be stacked is separated and emerged with the subdivided DFS's only when the upper DFS consists of vertical faces solely. The other case is where the separated DFS has only vertical faces and the subdivided DFS's can be merged with the upper DFS in all cases. This DFS subdivision process also reduces the number of unit process and hence the total building time.

Fig. 11 illustrates the data structure of a DFS generated through the four steps for DFS generation in the rapid prototyping method of the present invention.

In the present invention, the DFS means a unit process as well as geometric information. That is, one DFS includes one main process cycle composed of a backside machining step, a sheet reversing and deposition step and front side processing steps in the rapid prototyping method. So, the process designer has to designate machining information related to the corresponding processes for the individual DFS's, in addition to the geometric information about the DFS's. The machining and geometric information about the DFS's are used in the subsequent processes for generation of machining path and evaluation of the whole process.

As shown in Fig. 11, nodes for MFS's in the data structure of a DFS are data factors for storing the sequence of MFS machining steps. As the machining process of the MFS's is performed at any time for efficiency of the process, the MFS nodes also provide information about the time points of the machining steps for a specified MFS in the main process cycle.

Fig. 12 shows a main process cycle for DFS's and an additional machining process for MFS's in the sheet-stacking rapid prototyping method of the present invention applied to the cylinder block of a real engine. The engine cylinder block consists of 5 DFS's with 43 holes, 15 slots, 4 pockets, and 3 miscellaneous surfaces.

As shown in Fig. 12, the stacking unit of the present invention rapid prototyping method has three-dimensional layers completely different from those of the conventional method. Also, the actually used building materials have such a complex profile as to generate an extremely large number of DFS's, in which case building of the shape is impossible without using the additional machining process in the present invention method.

As described above, the rapid prototyping method performing both deposition and machining according to the present invention includes a main process cycle, which is composed of backside machining step, sheet reversing and deposition step and front side processing steps, and an additional machining process 17, which is to process small feature segments and composed of drilling, milling or grinding. The present invention method decomposes the whole building material into three-dimensional segments to dramatically reduce the total building time, while the conventional method divides the whole building material into thin layers of a predetermined thickness.

Fig. 13 is a diagram showing that the number of required DFS's is reduced due to the MFS concept in the rapid prototyping method of the present invention. Fig. 13a shows that 7 DFS's are needed in the conventional method not based on the MFS concept. On the contrary, Fig. 13b shows that two DFS's are finally generated by removing hole and slot MFS's from the original model prior to decomposition of the building material by means of the MFS concept in the present invention method. As

the principal processes are carried out in the unit of DFS, a smaller number of DFS's results in a reduction of the total building number.

Furthermore, with a view to maximizing the advantages of machining performed together with deposition, the present invention extracts the MFS's in the course of planning the whole process and handles the extracted MFS's independent of the DFS's, thereby enhancing the functional characteristics of the final products and process efficiency.

It is to be noted that like reference numerals denote the same components in the drawings, and a detailed description of generally known function and structure of the present invention will be avoided lest it should obscure the subject matter of the present invention.