Automatic generation of in-process models based on feature working step and feature cutter volume

ORIGINAL ARTICLE

Automatic generation of in-process models based on featureworking step and feature cutter volume

Jianxun Li & Zhuoning Chen & Xiaoguang Yan

Received: 21 May 2013 /Accepted: 17 November 2013 /Published online: 27 November 2013# Springer-Verlag London 2013

Abstract A 3D in-process model shows the intermediateshape of workpiece intuitively in different machining stages.Automatic generation of in-process models is one of the mostimportant key technologies in 3D computer aided processplanning (CAPP) system, which shortens modeling time andimproves the accuracy of model expression. This paper de-scribes implementation of an automatic generation of in-process models based on feature working steps (FWSs) andfeature cutter volumes (FCVs). The principle is proposed todemonstrate the relationship between machining process in-formation and in-process models. The expression and analysisof process route based on FWS is completed. And then twokinds of FCV constructions and instantiations are studied.One is parameterization for stable feature and the other islayering and reverse reconstruction for semi-stable feature.Two examples are presented to illustrate the approach.

Keywords Feature working step (FWS) . In-processmodel .

Machining feature (MF) . Feature cutter volume (FCV) .

Parameterization . Layering and reverse reconstruction

1 Introduction

Process planning is a bridge between design and manufactur-ing. 3D computer aided process planning (3D CAPP) shouldbe solved urgently for the application of full 3D productmodel, in which 3D machining process planning method

and technology need to be improved. One of the key technol-ogies in 3D machining process planning system is rapidforming 3D in-process model which will average up planningefficiency in 3D environment. A 3D in-process model showsthe changes of workpiece in different stages from blank to partto guide the processing intuitively [1].

Given part model/drawing and process route, researchersattempt to generate in-process models/drawings automaticallyassisted by computer. In 2D modeling environment, it is toodifficult to generate process drawings automatically for com-plex parts because of ambiguous 2D drawings [2] whichcontain lines only. Planners usually design process drawingsmanually or take part drawings with annotations as processfigures [3]. In this case, accurate intermediate shapes of work-pieces cannot be displayed by the process figures. The processfigures can be hardly utilized in CAM. While automaticgeneration of 3D in-process model is becoming possible be-cause of the unambiguous 3D part model and mature 3DCADtechnology. Unlike process drawings, 3D in-process modelsnot only present shapes of workpieces in stages, but also canbe utilized effectively in digital manufacturing such as ma-chining simulation [4], NC programming, inspection, machin-ing sequence verification, and 3D fixture design [5].

Much research work has been carried out for in-processmodel generation in sheet metal processing, and their appli-cations are more mature [6–8]. More relevant research isgoing on in machining process planning.

Taking machining operation sheets (Working Procedurelanguages and 2D drawings included) as inputs,an approachdriven by process planning course, machining semantics andmachining geometry to reconstruct incrementally the serial3D models for part's dynamic evolution is proposed by Zhanget al. [5]. This approach was fit for rotational parts for itscalculation method of position dimensions for machiningfeature (MF). As a more in-depth research, Huang et al. [9]proposed an approach based on subgraph isomorphism to

J. Li (*) : Z. Chen :X. YanSchool of Mechanical Science and Engineering, HuazhongUniversity of Science and Technology, Wu’han 430074, Chinae-mail: [email protected]

Z. Chen :X. YanWuhan Kaimu Information Technology Co. Ltd., Wu’han 430060,China

Int J Adv Manuf Technol (2014) 71:395–409DOI 10.1007/s00170-013-5507-7

solve the matching problem between precursory 3D processmodel and 2D working procedure drawings, fitting for moretypes of part. Fuh et al. [10] collected MFs at every stage ofoperation and generated in-process model by subtracting thesefeatures (plane and cylinder) from the CAD model of initialworkpiece. Gu et al. [11] got in-process model by drivingdesign features of part model. The driving dimensionswere concerned with part's design dimensions and processdimensions of each working step. This approach onlyworks with simple rotational parts. Mohan and Shunmugam[12] adopted CAD approach to simulate generation ofmachining process, generating intermediate models and contactlines simultaneously.

Zhou et al. [3] used other manual methods to get in-processmodel such as deleting features from part model, modifyingfeatures on part model, and adding features to part model.Wang et al. [13] proposed to add volume to part modelincrementally in the order of reversed process route. Kimand Wang [14] offset the machining faces to obtain interme-diate models and blank model for casting part. This methodmight only be fit for simple part. But for complex parts andmass process steps, the reverse generation would be consid-erably less efficient.

Some CAD/CAM integrated systems provided powerfultool path generation and simulation functions [15–17]. Thosefunctions could be used to obtain in-processing models inCAM method. Some researchers took full advantages ofCAM and developed CAD/CAM integrated systems by usingfeature recognition technology. Jasthi et al. [1] presented aframework of CAPP and CAM integration for modeling ofremoving material. This technology was applied in rotationalparts. Gao et al. [18] extractedMFs including non-geometricalattributes for CAD/CAM integration. Hou and Faddis [19]proposed intelligent feature recognition and feature-based ma-chining simulation in CAD/CAPP/CAM integrated systems.The features only contained flat surface and hole which werethe limitation of those intelligent systems. Sheen and You [20]sliced the workpiece at assigned positions to obtain featureprofiles which are used to generate tool paths automatically inCAM. At last, in-process models were generated. However,little process planning is concerned in generation of in-processmodels, those methods concentrated in forming geometricshapes with the aid of CAM.

All the methods mentioned above revealed the short-comings which highlighted that process information wasinvalid in in-process model generation. On one hand, theaccuracy of models is guaranteed by planner rather thanprocess information directly. On the other hand, changes ofprocess route can hardly drive the alteration of in-processmodels. For reasons that process planning relies on experienceand the generation of in-process models is nonlinearity [4], itis hard to establish an appropriate relation between processdata and in-process models.

This paper adopts a CAD approach to generate in-process models automatically. To solve the above prob-lem, the feature working steps are used as the minimumcomponent unit to express part's machining processroute and then in-process model is generated according tothe process sequence.

2 Principles of automatic generation of in-process models

Taking part model and machining process route as inputs, howto obtain in-process models will be discussed in this paper.The in-process models are generated by removing volumefrom blank model to finished product model in order accordingto the process route. Two concepts are introduced: "Featureworking step" (FWS) and "feature cutter volume" (FCV).

Definition 1 Feature working step (FWS) is one step in afeature-process-route that needs to be sufferedfrom initial state to adequate accuracy such as"rough turning" or "finish turning" for outercylinder in IT (international tolerance) grade 7.

FWS builds a bridge between MF andmachining procedure. Generally, working stepis a process completed under the condition ofconstant machining surfaces, tools, spindlespeed and feed. A traditional working stepmay contain several FWSs so that the constantmachining surfaces may contain several MFs.

Definition 2 A feature cutter volume (FCV) correspondingto an MF is a volume used to simulate theremoving material process from blank model.FCVmodeling is the most important technologyto realize the automatic generation of in-processmodels.

Taking process route into consideration, in-process modelsare generated following these steps (depicted in Fig. 1):

(1) Taking part model as input, recognizing MFs and theirattributes.

(2) Based on feature's attributes, retrieving FWSs fromparameterized process knowledge database.

(3) Planning machining process manually by reorderingFWSs.

(4) Generating blank model according to part type andprocess information.

(5) Instantiating FCV to form FCV model through workingstep information. Different FCV construction methodsfor different types of MFs.

(6) Removing volume from blank model or previous processmodel gradually to form in-process model and linkingthe model to current machining process.

396 Int J Adv Manuf Technol (2014) 71:395–409

Actually, steps (5) and (6) are particularly emphasizedand while steps (1)–(4) will be introduced briefly in thispaper.

Supposing that P is a designmodel, B is a blank model,Wi

is the in-process model for current operation and fc is the FCVmodel, the method of in-process model generation in thispaper can be represented as Wi=Wi-1−FCi, as shown inFig. 2, where FCi={fci1,fci2,…,fcin} is a set of related FCVs

in a specified process and W0=B . So, Wi ¼ B− ∑p¼1

i∑q¼1

nfcpq .

The last stage is checking whether the finishedW is adequatefor P.

For each single feature, in-process model is generatedthrough five steps, as shown in Fig. 3. The first step isanalyzing process route to get related MFs. The secondstep is matching FCVs with MFs. The third is translatingdesign dimensions and allowance into driven parametersfor FCVs. The fourth is instantiating FCVs by drivenparameters to form FCV models. The last step is insertingFCV models into last in-process model according to thelocation attributes of MFs. The kernels are how to constructFCVs and insert FCV models automatically.

The method mentioned in this paper satisfies the followingconstraints:

(1) In-processmodelsmust be associatedwith and synchronizewith process route.

(2) FCV could be driven by parameters related to attributesof MF and allowances for machining.

(3) The coordinate system of in-processmodel is the same asthat of part model.

3 Process route expression and analysis based on FWS

The expression and analysis of process route is one of the keytechnologies for in-process model generation. The expressionbuilds the relationship between process description and in-process model. The analysis reveals how the in-processmodels are driven by process description.

In semi-intelligent and intelligent CAPP system, MF-basedprocess planning is the main research direction. MF was animportant bridge to connect part models and process planning[21]. Zhang et al. [5] proposed a method that extracting

Fig. 1 Schematic diagram of in-process models automatic generation in CAPP

Int J Adv Manuf Technol (2014) 71:395–409 397

machining semantics based on process planning languageunderstanding to building relationship between machiningprocess and in-process model. Yifei et al. [22] mentionedthat typical features had corresponding typical processschemes and the process scheme could be retrieved andadapted in process planning for new part. Feature-process-route was predefined in the database and then selected fora given feature based on its machining accuracy [23]. Asimilar representation of process route based on FWS willbe proposed to build the relationship. In this paper, FWSis the minimal unit replacing traditional working step inprocess description.

It describes the generation of machining process routefor a part in Fig. 4. FWSs for each MF are retrieved inpredefined feature process knowledge database. When planninga process, FWSs are reordered and combined in operations

manually, while step descriptions are formed automaticallybased on machining process semantics and syntax.

A simple sample will be used to illustrate the generation ofmachining process. As for the part shown in Fig. 5a, theresults of feature recognition are shown in Fig. 5b and attri-butes of two MFs are specified. The main attributes containfeature type, annotated dimensions (for feature-process-routereasoning) and geometric parameters (for FCV instantiation).A mass of feature-process-routes are predefined in databaseaccording to these attributes. For a specified feature extractedfrom a part model, a corresponding feature-process-route canbe found. As shown in Fig. 5c, for outer cylinder 2, the processroute 2 is selected for the sample part. Inputting diametervalue (8 mm) of outer cylinder 2, reserved allowance valuefor each working step is worked out. When an FWS is select-ed, it is translated in step description in operation by specified

Fig. 2 A simple sample of in-process models generation

Fig. 3 Principle of in-process model generation for machining a single MF


rules shown in Fig. 5d. Finally, a set of operations areworked out.

By reverse analyses, the machining process contains severaloperations and an operation is constructed by several steps. A

step is linked to an FWS internally, and an FWS is correlatedwith an MF and its attributes. As shown in Fig. 6, an operationcontains several FWSs. So that the machining process can berepresented as:

Process ¼ Operation 1;Operation 2;…;Operation i;…;Operation mf g;where Operation i ¼ FWSi1;FWSi2;…; FWSij

� �;

so Process ¼ FWS11;FWS12;…; FWS1if g; FWS21;FWS22;…;FWS2if g;…; FWSm1;FWSm2;…ff FWSmngg:

As shown in Fig. 7, every FWS has attributes includingfeature type and reserved allowance value for next steps.The attributes will be used to instantiate FCV for everyworking step in operation. After analyses, a quantifiablerelationship between process route and FCV model hasbeen built.

4 FCV parameterization for stable feature

As for the first type of MFs, which are stable in topology suchas outer cylinder, hole, and key seat, their removing volumesin working process also have a stable topology structure.Through CAD technology, their FCVs can be predefined inparameterization (as shown in Fig. 8) and instantiated by

parameters including shape parameters and location parametersin in-process model generation (as shown in Fig. 9).

4.1 Creating FCV library

Based on the principles shown in Fig. 1, all FCVs for this typeof MFs need to be predefined in library. For eliminating illeffects from changes of follow-up in-process model in topol-ogy and guaranteeing correct shapes and positions for theprocessed MFs, the following constraints must be satisfied:

(1) Shape and location of FCV can be driven by parametersand location references are independent.

(2) FCV is independent and has Boolean "subtract" functionor can be subtracted.

Fig. 4 Method of machining process route generation


Fig. 5 A sample of machining process route generation


Some FCV samples are present in Fig. 8. FCVs are definedin following steps:

(1) Create a model for the shape of FCV.(2) Define shape parameters which are driving parameters

for forming a shape model.(3) Define location references and parameters for placing

FCV model.(4) Link shape model to MF.

4.2 Instantiating FCV

Generally, machining process removes materials from blank.The volume of removed materials is composed of concernedFCVmodels in CAD. The FCVmodels are instantiated by thevalues of shape parameters and location parameters. Theshape parameter values are calculated through relationshipbetween design dimensions and allowances. The locationparameter values are obtained from the location attributes of

Fig. 6 Relationship betweenprocess expression and FCV

Fig. 7 An instance of process expression


MF. Supposing the modeling coordinate system (CSm) inCAD is the same with absolute coordinate system (CSa),attributes of MF are extracted from part model in absolutecoordinate system (CSap), and its location references are alsodefined in absolute coordinate system (CSap) of the partmodel, then CSap=CSa. Meanwhile, the in-process modelsare formed in CSm, then CSap=CSm=CSa, which meansthere is no need to transform coordinates. So the locationposition of FCV models can be inferred from the locationattributes of MF.

Taking an outer cylinder as example, the in-process modelfor rough turning process is generated automatically in theorders described in Fig. 9.

It is seen that some values of driving parameters are accu-rate while some are not. As a general rule, the parametervalues used to form machining surface (such as bore diameter"Ø8.5") must be guaranteed and the others (such as diameter"Ø20" and length "18") are greater than that of working blank.As the Boolean operation is: W −∑W∩FCVi, as shown inFig. 10, the correct will produce correct in-process model.

5 Layering and reverse reconstruction for semi-stablefeature

Another kind of MF discussed in this paper is 2.5D pocketfeature which is semi-structured, such as the pocket in Fig. 11.

All the side surfaces of the feature are parallel to a refer line(axis line of milling cutter for pocketing) and the layer planesare vertical to it.

The area of 2.5D pocket feature in height direction iscontrolled by top interface (f t) and bottom interface (fb) andthe altitude is h t and hb, respectively. Supposing the heightarea is H , then H =[hb,h t]. So, the area of each side surface inheight direction falls inside the area of H partly or totally.

The principles of forming in-process model in 2.5D pocketmilling will be discussed next.

5.1 Layering and obtaining section boundaries

The aim of the method of layering and reconstructing FCV isto obtain the shape (can be driven by parameters) of FCV fromthe original pocket feature and then subtracting its FCVmodelfrom blank model.

In the vertical direction, 2.5D pocket feature can be dividedinto several layers by layer planes. The layers are formed byevery two neighboring layer planes. So the FCV can beobtained by overlying cutter volumes of layer (LCV). Thesection of LCV is obtained using the intersecting line method,and the height is the distance between two neighboring layerplanes. When forming the pocket feature in in-process model,LCVs are instantiated and subtracted from blank model fromtop to down, like the process of milling layer first.

Fig. 8 Feature cutter volumeslibrary


Taking the closed pocket in Fig. 11 as example, there arefive layer planes and the height order is h0>h1>h2>h3>h4=0.

The pocket is divided into four layers: {[f0, f1], [f1, f2], [f2, f3],[f3, f4]}. The first step is creating four datum planes on the layer

Fig. 9 Process of in-process model generation for an outer cylinder

Fig. 10 Comparison betweentwo FCVs with differentconstraints


planes {f1, f2, f3, f4} and intersecting themwith side surfaces toform boundary of four sections {S1, S2, S3, S4}.The closedboundaries of sections will be used in FCV instantiationfor pocket.

5.2 Reconstructing and instantiating

For the corresponding pocket, its FCV model is recon-structed by instantiating section boundaries with allow-ance taken into account and extruding boundary for eachlayer. Considering allowance, FCV model in rough-milling is different from that in finish-milling in volume.Allowance is divided into two parts: side allowance andlayer allowance. Side allowance is used to control theoffset of section boundary, affecting open lines only.Layer allowance is used to control the starting positionfor extrude.

As shown in Fig. 11, extruding S i from f i to f i+1 insequence to form four LCVs ({L1, L2, L3, L4}) and thenFCV model of the pocket is reconstructed by overlying thefour LCVs.

5.3 Inserting FCV

Every section has a local coordinate system (CS) relatedto absolute CS of part model. However, the absolute CSof blank model is equal to that of part. So, locating theposition of FCV model is easy to calculate by therelationship between local CS of section and absoluteCS of blank. And then subtracting the FCV model fromblank model layer by layer from top to down, as shownin Fig. 12.

Fig. 11 Layering and reconstructing FCV for a pocket

Fig. 12 Process of in-processmodel generation for a pocket


5.4 Special fix tool for section boundary

For the semi-enclosed pocket in Fig. 13, its FCV can be alsoobtained using the same method, although it will be morecomplicated since the boundaries combined by intersectionlines for sections are not closed sometimes. And the unclosedboundary cannot be extruded to form LCV. A tool fix isintroduced in this paper to make up boundaries.

As shown in Fig. 13a, the red boundaries for the threesections are not closed. There is a pair of breakpoints (BPs)at each boundary gap (see Fig. 13b), which need to be con-nected reasonable by the tool fix.When all BPs are connected,the boundary is closed.

Generally, in 2.5D pocket milling, layer with higher alti-tude should be machined first; otherwise the lower layer cannot be touched by tool. According to the machining logic, the

boundary of upper section contains that of lower section(except the layer is machined by T-cutter). So the upperboundary can be fixed by inheriting the lower boundary. Thecore of fixing boundary is overlaying loops. The sectionboundary of each layer is made up in following method (asshown in Fig. 11), from bottom to top.

Step 1: Overlaying previous loop on current boundaryThe lower boundary is called previous loop if it is

closed. Overlapping current boundary with previousloop to ensure the area of current section containsthat of the lower section. If the section is at thebottom, go to step 2 for there is no previous loop.

Step 2: Projecting edges on current sectionPart of the rest of BPs can be connected by

projection lines. First of all, edges that need to

Fig. 13 Fixing section boundaryfor semi-enclosed pocket


be projected should be determined. In 2.5Dpocket, the vertex closing to the BP is sharedby three edges. And two of them, which are inthe boundary of layer surface, are called alternativetrack edges (ATEs). Projected edges tracking algorithmis as follows:

(1) Track from the first BP, checking its ATE.(2) If the ATE overlaps a boundary line, give it up. If not,

project it on current section, creating a new BP if theprojection line does not connect to an existing BP.

(3) Then keep tracking from the new BP until anexisting BP is connected by projection line.

Step 3: Adding lines based on blank model projectionboundary

Some boundaries are available or applicable to befixed by projection, like the boundary of the sectionsin Fig.14a and c. It can be fixed with the aid of blanksection boundary. First, obtain the projection regionof blank model relative to current section and obtainthe boundary. Second, project the other BPs to thenearest boundary lines and connect the BP and its

Fig. 14 Fixing section boundary based on blank projection boundary

Fig. 15 Instantiating sectionboundary based on open lines


Fig. 16 Process planning and in-process model generation for a shaft


projection point. A projection point divides a bound-ary line into two line segments. One segment is inentity region and the other is in open region. Third,keep track of the line segments in open region untilthe next projection point is found. Keep the linesegments which are tracked and delete the otherline segments; the rest is a closed boundary andthe fix is completed as shown in Fig. 14b and d.

Step 4: Sorting line segmentsWhen taking allowance into account in FCV

instantiation, the section boundaries must beoffset along the direction of leaving solid, and offsetvalue is equal to allowance value.

The boundaries are combined by several linesegments. In these segments, some are intersectionlines, and others are addition lines made by tool fix.In this paper, all intersection lines and addition lineswhich are between two intersection lines are "non-open lines" which must be offset in rough-milling,and the other addition lines are "open lines" whichneed no offset as shown in Fig. 15.

6 Case study

A module called In-process Model Auto-generation is devel-oped by the authors in a 3D CAPP system for machining

(KM3DCAPP-M). In this CAPP system, two kinds of 3DCAD software (Pro/Engineering from PTC and UG/NX fromSiemens) are taken as virtual platforms.

FCVs are predefined by using the technology of UDF(user-defined feature) supplied by the CAD software. UDFtechnology allows users to combine several existing featuresinto a UDF which is saved in a separate file. Next, theparameters of UDF will be inherited from the existing fea-tures. Then an instantiated UDF can be inserted in a targetmodel. So, an FCV library can be created by UDF through thefollowing steps:

(1) Predefine features in CAD.(2) Define the UDF for each type of MF which has stable

topology and select driven parameters, respectively.(3) Save UDFs into a database.

Next, two cases will be studied to show the achievementsof in-processmodel auto-generation. The first sample is takinga shaft part as example and the other is for shell part.

As shown in Fig. 16, after geometrical information andnon-geometrical information have been extracted from partmodel and reconstructed for MFs, machining process isplanned semi-automatically. Each operation contains severalFWSs. When generating an in-process model for each opera-tion, related features and FWSs should be determined. Then,retrieve corresponding attributes values and FCV files, and

Fig. 17 In-process modelgeneration for machining a pocketin a shell


instantiate FCVs and insert them one by one. Finally,in-process models have been achieved.

Generally, shell parts are composed of pockets, holes, flatplanes, etc. When machining holes and flat planes, the in-process models can be generated by the method mentionedabove. And pockets are generated in another way which useslayering and reverse reconstruction. For a pocket as shown inFig. 17, obtain intersection curves and fix them, then save themin a sketch file for each layer first. When milling the pocket,retrieve sketch files and instantiate the section boundary. Takethe section boundary of each layer as sketch, and extrude andremove them from top to bottom. For the shell shown in Fig. 17,each in-processmodel is generated from blankmodel to finishedmodel in the order of the operation and step sequence.

7 Conclusions and future work

This paper has introduced a new representation of processroute based on FWS which builds the driven relationshipbetween process data and in-process model. And then in-process models are generated by taking advantage of FCVs,synchronized with process route. Two constructions of FCVsare proposed for different MFs. To verify those methods,automatic generation of in-process models is demonstratedby two typical parts.

Some of the benefits of the proposed approach are as follows.

(1) After machining process had been planned on method ofreorder FWSs, in-process models are generated automati-cally, reducing planning time on modeling and displayingintermediate state of workpieces visually.

(2) Taking in-process models as inputs, CAMwill avoid toolpath planning of empty feeding for workpiece and willbe more accurate and output more efficient NC code.

The method in this paper is appropriate for most parts. Thecomplexity of a part is evaluated by the complexity of MF inthis paper. The more complex the MF is, and the more com-plex the part becomes. In other words, if a part contains plentyof features and the MFs are in research range, the method isadaptable to the part. In general, this approach is mainlyadapted to the MFs not larger than 2.5D. For most parts, itcan work very well, but still needs to be improved:

(1) When the parts include freeform surfaces, it is necessaryto find a newmethod tomodel FCV for freeform surface.

(2) When the part models are annotated with dimensionswhich have asymmetrical tolerance distribution, it is hardto use in CAM, because CAM is on the basis ofgeometry at present rather than semantic annotation.Correct in-process models are needed to be generatedbased on annotation.

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Automatic generation of in-process models based on feature working step and feature cutter volume