Assuring confident re-use ofproduction drawing informationby the NC programming activityby I. BlackHeriot-Watt University/Brown Brothers & Company Ltd. Teaching Company Programme

In the heavy/medium manufacturing environment, the generation ofNC part programs is a prolific and productive element of CAM. Withthe increasing use of computer-aided drafting a more effective andautomatic method for the generation and transcription ofmanufacturing instructions and information has become available.The creation and maintenance of a computer graphics database,containing master component definitions, can facilitate the automaticre-use of graphical information by NC part programmers. However,near automatic data re-use can only be accomplished by maintainingthe integrity of manufacturing information stored in the database,ensuring the correlation between such data and the associatedconventional presentation of manufacturing data as traditionalorthographic production drawings.

Introduction

Historically, the numerical control (NC)part programmer exercised controlover the geometric data used as thebasis for the generation of NC part pro-grams. This situation rang true for man-ual programming using language-assisted techniques (such as APT). Con-ventionally, the NC part programmerreceived an issued production drawing(the original master description of acomponent) and then interpreted thisdrawing and re-defined the geometryaccording to the component's machin-ing requirements. Unfortunately, thissituation led to the creation of a sep-arate source of manufacturing infor-mation, and any changes to the originaldrawing would have had to bepainstakingly communicated to all whowere affected, with subsequent del-eterious effects on productivity.

In order to reap the fullest benefitsavailable from the potential electroniclink between computer-aided design(CAD) and computer-aided manufac-turing (CAM), the master description ofa part has to be stored in the graphicsdatabase. It cannot be an uncontrol-lable, ad hoc reproduction of the origi-nal part geometry. In addition, anyderived production drawing shouldideally correlate with the source infor-mation heid in the graphics database.

Integrity of graphics data is of theutmost importance, and this conditioncannot be overemphasised if re-use ofmanufacturing information is to be bothpracticable and profitable.

This article proposes that, by imple-menting mechanisms and disciplines tofacilitate specific company require-ments, degrees of productive and cost-effective re-use of production drawinginformation are possible with a CADsystem. It is further contended thatwithin the heavy/medium mechanicalmanufacturing industry the linking ofCAD and CAM (more specifically pro-duction drafting and NC part program-ming), through the achievement andmaintenance of degrees of re-use, iscurrently also dependent on tailoringCAD system software to meet thosespecific needs.

Problems that arise

The conventional production drawing isstill widely employed as the prime car-rier of manufacturing information. WithCAD, orthographic plots of productiondrawings can be produced which mayact as such information carriers. Hencea dual database situation arises,whereby NC part programmers haveaccess to both computer graphics dataand the corresponding plotted produc-tion drawings, both purporting to

Computer-Aided Engineering Journal August 1986

convey the same manufacturinginformation.

For example, if the NC part programfor a component is to be manually gen-erated, it may be achieved throughinterpretation of a plotted productiondrawing which will have its source dataheld in the graphics database. At somelater stage, part programming on thesame component is to be accomplishedusing computer-aided programming(CAP) techniques. This will involvere-use of the source geometry held inthe graphics database. Hence theremust always be a high degree of asso-ciativity between plotted information(i.e. annotated dimensions) and thecorresponding stored data (i.e. geo-metric sizes).

Coherent and complete definition ofa part becomes mandatory with thepotential automatic and direct re-use ofthe geometric information defining thepart. All too often the geometry definedby the production drafting activity usinga CAD system is inadequate or inap-propriate for subsequent re-use by theNC programming activity.

With two-dimensional definition of athree-dimensional component, ge-ometry may be created which looksacceptable to the draftsman, but it isunsuitable for NC programming needs.Frequently, with CAP methods, the NCprogrammer must edit or re-create thesupplied information to provide a suit-able geometric definition for NC pro-gramming. The graphics database is ofno use if the geometric informationwithin it must be fundamentally modi-fied to accurately represent the part.The discipline of drafting for manufac-ture asssumes new importance with theautomated re-use of productioninformation.

Generating an NC part program canbe achieved quickly and efficiently withCAP methods. The following figures,compiled from studies on actual livework within a mechanical manufactur-ing company, are indicative of the

159

to .the

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minimu^ti ..Cle^rdnice , •ipM &0;;)!-' '

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Fig. 1 Mid-size geometry definition

increase in product iv i ty:

Activity Productivity(CAP method:conventional)

5-7:1nesting andCNC flamecutting (up to 20:1 with

complex shapenesting)

CNC turning 3-5:1

However, the preparation of geometryfor subsequent NC programmingactivities can prove both time consum-ing and tricky if the requisite infor-mation inherits the previouslymentioned characteristics of inade-quate geometric definition. The extrac-tion of the necessary geometricinformation from the relevant graphicsfile can also prove to be tedious.Annotation such as centre-lines, hiddendetail, machining symbols etc. aretherefore unnecessary.

The above problems can stunt overallproductivity on NC tape generation andmay stimulate the creation of a stand-

alone computer graphics database forNC programming purposes only. This,as has been previously stated, is a highlyundesirable and unstable situation.

Achieving a degree of re-use

The problems that have been high-lighted can be alleviated to an extentwhere their effects on a downstreamactivity such as NC part programmingare insignificant. In the author's esti-mation, the degree of re-usability ofgraphical information contained withinthe graphics database and the tradi-tional paper database is critically depen-dent on the following factors:

• associativity of graphical infor-mation, which, for the purposes of thisarticle, expresses the correlationbetween annotated dimensions andassociated stored graphical data de-fining component size• definition and representation ofgraphical information, which can be

taken to convey the coherent and pre-cise incorporation of essential process-related geometric features within acomponent definition to accommodateautomatic re-use of this information bythe NC programming function• separation of graphical information,which can be interpreted.as the struc-turing of graphical information within aproduction drawing file to facilitate therapid and effective re-use of that infor-mation by the NC activity.

Associativity of graphicalinformation

The physical measurements of a com-ponent defined by a production draw-ing are intimated to manufacturing bythe inclusion of dimensions whichspecify the size limits of a particular fea-ture. With a plotted representation ofstored data held in the graphicsdatabase, the whole accuracy of anyplotted production drawing is a func-tion of the dimensioned geometry.With a CAD system, all geometry con-structed using interactive computergraphics methods should be definedfull size; i.e. accuracy of componentgeometry definition is functionallyindependent of any dimensional infor-mation. However, with plotted repre-sentations of stored data, associativitymust exist between the numericalvalues of the dimensioned sizes indi-cated on the drawing and the corre-sponding numerical data held in thegraphics database.

The specification of limits and fitsthrough specifically toleranced dimen-sions is critically dependent on the sizelimit (or fit) chosen for individual com-ponents. For the successful re-use ofgeometric information by CAP methodsit is expected that ideally all geometrywill be generated to the mid-size of thedetermined tolerance zone for a par-ticular class of fit specified for a compo-nent. Fig. 1 illustrates this point. Themid-size is the most preferable sincecompensation can be made for cutterwear, material deflection and toolingset-up inaccuracies.

*'" i

Tv'-T -' ' • 100.3913(stored)

* •• , ho warning of significantly different' p Hfor re-use •• in, this particular ease, upj

Fig. 2 Company requirements for specifically toleranced dimensions

160

Fig. 3 Problems with sizes which have a general tolerance

Computer-Aided Engineering Journal August 1986

To illustrate the importance of thepoints raised, this article presents anexplanation of the requirements of aparticular company — Brown Brothers& Company Ltd. — from its CAD systemto maintain associativity of informationbetween the computer graphicsdatabase and the corresponding plot-ted production drawings (or paperdatabase). For the sake of simplicity,consider a straight-line geometric ele-ment (see Fig. 2) which is to have aspecifically toleranced dimension. Forthis case, the associativity requirementsof the company were as follows:

• The geometric element is defined atsize A (full scale and mid-size).• The dimension is semi-automati-cally placed by the production drafts-person using a dimensioning facilityprovided with the CAD system thatprompts:

D 'find' X• 'find' Y, and (for a specific toler-

ance condition)• enter value B.

(A + B) and (A — B) are evaluated by thecomputer. Note that 'find' is a precisefunction.• The drawing is plotted as seen on aCAD workstation graphics displayscreen and then subsequently checked,in the conventional way.• The result is that one check ensuresthe functional accuracy of the dimen-sion and, assuming that the productiondraftsperson has used the 'find' facilitycorrectly, the data held within thegraphics database corresponds to theplotted graphics and is therefore avail-able for automatic re-use.

However, the above requirements werecomplicated by the following problems,which relate to both specific and gen-eral tolerance cases:

• a problem related to the dimension-ing facilities available to a draftsmanusing the CAD system, concerning thepossibility of typing sizes .for dimen-sions in preference to 'find' operations,and hence supplying biasedinformation• a problem related to the way thehost computer of the CAD systemstores and manipulates real numbers,which occurs because of inevitable vari-ances that arise with real-number tobinary-number conversion, manipula-tion in binary and re-conversion to real-number form.

A comprehensive solution to bothproblems was essential to guaranteegraphics database integrity.

The first problem was satisfactorily

Fig. 4 Company requirements for dimensions to which a general tolerance applies

Fig. 5 Example of correct component definition for successful re-use

addressed by in-house CAD appli-cations software tailoring to restrictspecific user freedoms. A furtherinsight into the second problem is givenin order to highlight the difficulties aris-ing from this situation. The followingprocedure is typical of what couldoccur:

• A production draftsperson couldgenerate a geometric size, for example100 mm, say, to zero-decimal-placeaccuracy, which would imply (to thedraftsperson) that 100.0000. . . was thesize stored by the computer.• The host computer would store thisnumber in binary form according to thefloating-point arithmetic system used(in this particular case single precision).This would incur an inevitable mis-match between the real form of thenumber and the binary representation;for example the host computer maystore 100.0014 expressed in real form. Inaddition, any subsequent manipulationof graphics, for example moving, copy-ing, mirroring etc., could result in fur-ther cumulative mismatch.• Using an interrogation facility avail-able with the CAD system, a checkerwould still see 100 on the plotted pro-duction drawing, or workstationgraphics display screen, when hechecked this size to the required deci-mal-place accuracy.• However, the CAP applications soft-ware would use 100.0014 when inreceipt of the geometric data, and with-out careful checking (which woulddramatically impair productivity) thiscould go un-noticed.

Computer-Aided Engineering Journal August 1986

• Now, the importance of the vari-ance (or error) present between thestored and displayed numbers (sizes) isentirely related to the tolerance asso-ciated with the dimension. For exampleif the draftsperson intended to dimen-sion a size with a tolerance band of ± B,where B = 1, 0.1 or even 0.01, then anerrorof 0.0014 in the mid-size definitionmay be ruled to be insignificant.• Despite this, the point was that, ifthe CAD system could not automaticallycheck for the significance of any vari-ance caused by the host computerwhenever data was interrogated or visu-alised, then mid-size geometry defi-nition on the CAD system could notconfidently ensure re-usability of geo-metric data by the NC programmingactivity.• In addition, it was envisaged thattoleranced dimensions on either largeor precision components manufacturedby the company could affecf the signifi-cance of any variance created by thehost computer, operating with single-precision floating-point arithmetic(real-number representation up to amaximum of seven significant figures).

To alleviate the above difficulties, theCAD applications software vendor wasasked to provide a facility to automati-cally check for the significance of anyvariance occurring whenever data wasinterrogated or visualised.

However, the facility to check vari-ance, as supplied, was not effectivewhen a production draftsperson wasemploying a general tolerance stated ona drawing (see Fig. 3).

161

Hence an additional method ofensuring that data had been correctlydefined was required and proposed.The proposal simply consisted of avisual check, whereby all dimensionedsizes are displayed to a minimum deci-mal-place accuracy. If this proposalwere adopted, then any error in datadefinition would not significantly affectthe relevant decimal place of a dis-played number (size). Together with theautomatic checking facility provided,there would now exist mechanismswhich could ensure that all geometrywas defined to an acceptable degree ofaccuracy for the purposes of confidentre-use.

If any variance between stored anddisplayed sizes proved significant, thenthe geometry would have to be modi-fied in order to closely associate withthe displayed dimension desired by theproduction draftsman (see Fig. 4). Thisstate of affairs would ensure that asso-ciativity of graphical information couldbe more successfully guaranteed, withonly manual checking of the dimen-sioned information contained within aproduction drawing being necessary.

Definition and representation ofgraphical information

As has been stated previously, for NCpart programming employing CAP tech-

Fig. 6 Parametric programming canpositively affect geometric definition

niques the requirement for correctcomponent definition and representa-tion on mechanical production draw-ings has become mandatory. Theproductive and automatic re-use of partgeometry by the NC programming func-

tion has dictated that requirement.Fig. 5 shows the semi-profile of a

turned piece-part. For the geometricinformation contained in this exampleto be successfully re-used, the piece-part geometry must be correctlydefined; for example radii on internalcorners and on external sharp edges areincorporated on the finished produc-tion drawing. In this specific exampleincorporation of such details helpstowards automatic re-use of geometricinformation as the NC programmerneed not modify the existing part ge-ometry to accommodate either toolingrequirements (for internal corners), orvalidate drawing annotation require-ments (i.e. remove sharp edges),respectively.

In some cases the clear definition andrepresentation of components can con-flict with the requirements of asso-ciativity. In this case, a degree ofdrafting licence must be invoked toensure that there are no ambiguities tointerpretation of the plotted productiondrawing. This can be accomplishedthrough the inclusion of scrap views orby selecting suitable pen widths for par-ticular line definitions whereapplicable.

An example of correct componentrepresentation is the drilled and tappedhole shown in Fig. 6. In this case the keyparameters have been precisely and

LEVEL

1-9

10-19

PRESCRIBED DATA MECHANISM

any two-dimensional module placement is theviews that adjoin or responsibility ofoverlap should be separated the userby level

any three-dimensional not yet usedmodule views by the company

reserved exclusively for use by CNC users

not usedall dimensions— _ automatically placedannotationt automatically placed by a two-dimensionalnot used by annotation menu module parameters

programs file settingnot usedall textual annotation§ automatically placed

by a two-dimensionalmodule parameterssetting

reserved for future use

not usedborder text automatically placedborder automatically placed by annotation menunot used by BORDERS program programreserved for constructions

machining symbols, geometric tolerancing etc. § excluding border text

Fig. 7 Representative company level structure

162 Computer-Aided Engineering Journal August 1986

accurately defined and represented.Note also that the geometry has beendefined with the aid of a two-dimen-sional parametric program.

Parametric programming can assist inthe achievement of correct componentdefinition and representation. By mak-ing the generation of such informationnear-automatic and consistent, con-fident and productive re-use of thatinformation can be assured. A coherentand established range of drafting stan-dards is invaluable in assisting with theparametric description of correct com-ponent geometry. Two-dimensionalparametric programming has the poten-tial for providing, to a noticeable extent,mechanisms to achieve, and enforce,the discipline of correct componentdefinition.

Separation of graphical information

The level (or layer) facility available withmost CAD systems can be employed toadvantage for the separation of infor-mation within a production drawingfile. By adopting a level structure whichcontrols the placement of graphics,subsequent isolation of geometry forNC part programming purposes can beeffected in an easy and rapid manner.

Ultimately the placement of graphicswithin a production drawing is theresponsibility of the production drafts-person. However, the CAD applicationssoftware can positively assist this pro-cess by automatically placing identifiedgraphics (such as component dimen-sioning, notes, drawing frame etc.) onpre-selected levels. Fig. 7 shows a typi-cal level structure that has beenadopted to facilitate the separation ofgraphical information, while Fig. 8 dem-onstrates how certain production draw-ing file information can be decomposedinto constituent levels, each containingpertinent graphical information. Whena production drawing file has its con-stituent information separated in such amanner, it becomes a relatively straight-forward task, upon re-use, for the NCpart programmer to selectively andrapidly strip the file of unwanted infor-mation and isolate the desired compo-nent geometry.

Outcomes and benefits of assuredre-use

The maintenance of dual databases ofgraphical information (i.e. thecomputer graphics database and thecorresponding paper database) is a situ-ation which has existed, and will con-tinue to exist, within the mechanicalmanufacturing industry for some time.It is hoped that this article has made itclear that the electronic linking of CAD

piece-part geometrydisplayed only on aparticular level

in—

is

geometry with dimensionswhich are automatically Jplaced on a distinct level fl

Lii

Fig. 8 Drawing information may be stored on separate levels

(specifically production drafting) withCAM (specifically NC part program-ming) can only be achieved in a produc-tive and cost-effective manner throughestablishing and maintaining a graphicsdatabase which contains informationthat can be confidently and automati-cally re-used.

Further to this point, wheneverre-use of geometrical data is con-templated, it must be recognised thatpiece-parts having a two-dimensionaldefinition on a CAD system should beconsidered purely as geometric shapes,not as process-specific items (such as acasting or as a turned component). Anygeometry defined can, and may, be sub-sequently re-used for defining othercomponent shapes requiring differingmanufacturing methods to producethem. This condition entails accuratecomponent definition (to a predeter-mined degree of accuracy) by the pro-duction draftsperson at all times. Inaddition, the degree of associativity ofgraphical information can be influ-enced by the manufacturing environ-ment, i.e. the physical size ofcomponents and the accuracy to whichthey are produced.

From the author's experience, themechanisms and disciplines describedcan contribute significantly to theimprovement of graphics databaseintegrity. These mechanisms and disci-plines have been endorsed by one com-pany through the establishment ofrecognised procedures on definition,representation and separation of graph-

ical information. It is also notable that,with these mechanisms and disciplines,the integrity of the graphics databasecan be verified through relativelysimple manual checking of the derivedpaper database information, i.e. thecorresponding plotted productiondrawings.

In summary, it is the author's viewthat disciplines and mechanisms toensure associativity, definition, repre-sentation and separation of graphicalinformation are essential when attempt-ingto link CAD with CAM while utilisingand propagating dual databases ofmanufacturing information. This view isfurtherenhanced by the probability thatthis dual-database situation will con-tinue within most heavy/medium mech-anical manufacturing industries forsome time to come.

Acknowledgments

The work described in this article wasundertaken by the author whileemployed as a Teaching CompanyAssociate with the Heriot-Watt Univer-sity/Brown Brothers & Company Ltd.Teaching Company Programme. On apersonal basis, the author is indebted toProf. J. L. Murray (Head of Departmentof Mechanical Engineering, Heriot-WattUniversity), Mr. P. M. Wilson (Engin-eering Manager, Brown Brothers &Company Ltd.) and to Mr. E. Strachan(CAE Systems Supervisor, BrownBrothers & Company Ltd.) for their con-structive assistance and guidance.

Bibliography

1 MEISTER, A. E.: 'Ironing out the rough spots between CAD and CAM', CAD/CAM Technol-ogy, 1984, 3, (3), pp. 31-34

2 SMITH, W. A. (Ed.): 'A guide to CADCAM' (Institution of Production Engineers, 1983)3 BLACK, I.: 'Applicationsand influencesof interactivecomputergraphics within mechanical

product design: a critical appraisal and evaluation'. M.Sc. Thesis, Heriot-Watt University,Edinburgh, Scotland, Nov. 1986 (to be published)

4 BEEBY, W.: 'The future of integrated CAD/CAM systems: the Boeing perspective', IEEEComputer Graphics & Applications, 1982, 2, (1), pp. 51-56

I. Black is a Teaching Company Associate with the Heriot-Watt University/Brown Brothers &Company Ltd. Teaching Company Programme, located at Brown Brothers & Company Ltd.,Broughton Road, Edinburgh EH7 4LF, Scotland.

Computer-Aided Engineering Journal August 1986 163