Chapter 2 Cad Design

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DESIGN GUIDELINES 5

C omputer Aided Design, CA D, was intro-duced as a tool to aid designers in devel-oping part drawings as well as decreasing thetime necessary to draw the development onp a p e r. Over time it has become a much morepowerful tool enabling engineers to check form,fit, function and tolerancing of details or entire

assemblies prior to actual parts being built. Inthe time it takes to input data, the designer canhave a 3D visual model. As this process devel-oped, Computer Aided Manufacturing, CA M ,was introduced to the manufacturing environ-ment. This allowed for data to be input into aCAM system to create machine tool programs,thus automating many of the processing stepsthat were traditionally done manually.

OverviewAs CAD and CAM were developed, the

metalforming industry welcomed them withopen arms. Virtually all metalforming compa-nies today have some sort of CAM system with-in their engineering departments, drasticallyreducing the time required to produce a part.

The industry is demanding that this process betaken further by exchanging CAD files. Th i sallows for the customer to design parts on theirCAD system and exchange them with theirmetalforming suppliers. The goal for manycompanies is to create a part/assembly on acomputer screen and then to have it manufac-

tured without any paper drawings being creat-ed, reducing the overall time required fromdesign concept to completion of parts.

Traditionally, the process from concept to themanufacturing of parts was very time consum-ing. When a CAD model was completed, it wasturned over to a drafting department to createa typical orthographic drawing. The drawingwould be given to a metalforming companywho would recreate the part as a flat patterndevelopment in their CAD system. From thereit would be downloaded into a CAM systemto create a machine tool program. This processallowed for numerous opportunities for errors.Today there are many CAM systems on themarket that will actually take a CAD file andautomate the unfolding for you, creating a flat

CAD DESIGN

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pattern development, with little opportunity forerror.

One advantage of exchanging CAD files isthe ability to get your product design into thehands of the supplier prior to the design beingformally completed. Early supplier involve-ment in design reviews for manufacturability,tooling and manufacturing methods can bereviewed before changes are costly.

It should be noted that there are certain limi-tations to CAD file exchange. CAD files mustbe drawn to full scale. All objects within a filemust be put exactly where you want them. Thisis imperative for the simple reason that whenthe CAD file is imported into your supplierssystem and goes through the unfolding process

it will place all of your geometry exactly as youhave drawn it. If you have misplaced a hole,your final product will have that same hole mis-placed. Simply put, what you CAD is what youget. As CAD file exchange becomes fullyimplemented within the manufacturing envi-ronment and paper documents become obso-lete, the CAD file will become the master docu-ment for inspecting finished products.

Fi n a l l y, CAD files must be clean. There can-not be overlapping lines or lines that do notintersect. If these types of problems are con-

tained within the CAD file upon file exchange,then your supplier must take valuable time incleaning up your file. Lines that dont intersectcannot cleanly go through the unfolding process.

Overlapping lines that exist within the filecan create major problems in the machine toolp r o g r a m s. For example, if the part happens tobe run on a laser cutting machine, you will getholes or edges that are double burned thusdestroying the parts edge, causing a closely tol-eranced feature to be out of specification.These and many other problems can occur

when a CAD file is not clean.Within the metalforming community there

are many different types of CAD programs thatare available. Because of the variety ofCAD/CAM systems in use today, there are cer-tain guidelines that must be closely adhered to

when exchanging CAD files.

Guidelines for Designing in CADThis chapter is intended to help avoid diffi-

culties in exchanging files. Information willinclude proper part geometry, what should beand what should not be contained within thefile, different methods of file transfer, and mini-mum hardware requirements for CAD filee x c h a n g e. If these guidelines are followed youwill be able to exchange files, while avoidingmany of the major problems that have beenexperienced in the past, with virtually any com-pany with a CAD system.

In transferring the design of a sheet metal partor assembly via CA D, it is important that all nec-

essary information be communicated to assurethat the intended functionality will exist. Th i sinformation includes the CAD model, critical-to-function dimensions and non-geometrical infor-mation, such as metal type, and surface finish.

CAD Model DescriptionA CAD model is a collection of geometric

entities that describe the size and shape of a part.The entities may be 2-dimensional and show sev-eral orthographic views, or 3-dimensional andviewable from any orientation. 3-D, solid models

are preferred by most manufacturers becausethey are more versatile for programming and forgenerating additional documentation.

Rules for Designing Part FeaturesA sheet metal parts CAD model should be

composed of geometry that exactly describesthe intended design of the part or assemblywithout unnecessary complication. See Fi g u r e1. All geometry should be created at full-scaleusing nominal sizes. All edges, transitions andcross-sections that are represented in the model

should be represented by geometry that is freeof gaps, overlaps and duplication. See Figures 6and 7 for illustrations of common CAD errors.

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6 DESIGN GUIDELINES


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used to show that the circles are related.Additional lines that would appear in orthogo-nal views to show the extent of the hole aregenerally desirable. See Figure 4.

Other Fe a t u re s. Coined, drawn, formed,machined or rolled features as well asinstalled hardware should be represented bygeometry that details the edges, any transi-tions and cross-sections of the features orhardware. See Figure 5.

Figure 3. Preferred method for showing bends with and without radii.

8 DESIGN GUIDELINES

HOLES

DETAIL K

DETAIL M DETAIL N

CIRCULAR OBROUND SLOT

RECTANGULAR WITHIN BEND

DETAIL L

Figure 4. Preferred method of showing some

more common cutouts on drawings.

BENDS

DETAIL D DETAIL E

DETAIL G DETAIL H DETAIL J

90 180 HEM OFFSET

DETAIL F

CAD Design

FORMED FEATURES

COUNTERSINK EXTRUSION HALF-SHEAR

DETAIL P

DETAIL T

DIMPLE

DETAIL R

ENLARGED FOR CLARITY

DETAIL S

SHORTENED FOR CLARITY

DETAIL U

DETAIL Q

EMBOSS CARD GUIDE

Figure 5. Preferred method of showing other common features on drawings.


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CAD Design

DESIGN GUIDELINES 9

Figure 6. Some common CAD model errors illustrated in two views of a sheet metal part with a 90 bend.

INNER AND OUTERARCS DESCRIBING

BEND ARE NOTCONCENTRIC

DIMENSIONS THAT AREINACCURATEDO NOTMATCH CAD DATA

1.500

1.000

DUPLICATE ENTITIES ERRANT GEOMETRY

ENDPOINTS THATDO NOT MEET

VARYING MATERIALTHICKNESS

OK

NOT OK NOT OK

RADIUS SHOWN ON OUTSIDE OF BENDS BUTNOT ON INSIDECONSISTENTLY SHOW OR

DO NOT SHOW BOTH RADII

CONSISTENT APPROACH (NO RADII)BUT NO ALLOWANCE IS MADE FOR

MINIMUM BEND RADIUSTHISDESIGN IS NOT POSSIBLE AS

SHOWN WITH TWO 90 BENDS

OK

Figure 7. Two CAD model problems in sheet metal parts with offset bends shown both correctly and incorrectly.


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10 DESIGN GUIDELINES

Parts Separated By Layer

(all parts in the assembly are in one file)

Pros:+ requires only one file transfer+ all information kept in one place, nothing lost+ assembly information is defined with part

models+ view any combination of parts by choosing

layers+ file translation only needs to be done once

Cons:

file is larger and slower to manipulate file size may exceed CAD system limitations large file will need to be revised and exchanged

whenever a single component is revised layer names may change during file translation

Parts Separated By File

(multiple files, one part in each file)

Pros:+ revision level can be incorporated in file name+ customer only sends files for parts being revised

Cons:

file translation must be performed on each file

individually if an assembly model is desired, it must be

pulled together from all of the translated files

ALL LAYERS OR ALL FILES LAYER 1 OR FILE 1

LAYER 0 OR FILE 0 LAYER 2 OR FILE 2

Assemblies: Two MethodsAssemblies of sheet metal parts can be

described with CAD models using one of thesemethods:

1) a separate file for each component. SeeFigure 8.

2) one file which uses a separate layer foreach component.

There are distinct advantages and disadvan-tages to each of these methods, as detailed inTable 1.

Figure 8. Views showing an assembly CAD file and separation of components by layer or by file.

Table 1. Comparison of two methods of communicating assemblies.


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Critical-to-Function DimensionsIn the past, part designs were typically com-

municated by hand-drafted drawings, showingvarious views of the part with dimensions forevery detail and with all pertinent informationincluded. With CAD systems, some designershave stopped generating dimensioned drawingsof any kind, since dimensions can be extractedfrom the CAD model instead. Unfortunately, theresult is an incomplete hand-off of information.The designer still needs to communicate to themanufacturer other types of information: thedimensions that are critical to the success of the

design, tolerances and the other non-geometricalinformation that were included in the drawings.

Two-dimensional drawings are the best wayto communicate critical-to-function (CTF)d i m e n s i o n s. Figure 9 is an example of a CTFdrawing that includes critical dimensions andmost of the necessary non-geometrical informa-tion. In addition, this drawing contains enoughdimensions to completely form the describedpart. Without this information most manufac-turers would have to create an additional draw-ing to detail the formed part to the shop and forquality assurance records. This CTF drawing is

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DESIGN GUIDELINES 11

Figure 9. Features of critical-to-function drawings.

SECTIONAL VIEW OF

FORMED FEATURE

SECTION AA

FORMING DIMENSIONS

REVISION INFORMATION

HIDDEN-LINE IMAGE

OF ISOMETRIC VIEW

TITLE BLOCK

NOTES:

1. _________________________

2. _________________________

3. _________________________


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simpler to produce than a complete fabricationdrawing because it has fewer dimensions.

A flat pattern view is acceptable and some-times very helpful. The manufacturer will usethese views mainly as a reference for the quot-ing process. If dimensions are included in anyunfolded views they should be for referenceonly, since the manufacturer will need flexibili-ty in order to meet the dimensions and toler-ances of the formed part.

Non-geometrical informationRequired information other than the wire-

frame geometry and CTF dimensions areknown as non-geometrical information. It is tex-tual information and most of it can be commu-

nicated in the CAD model or CTF drawing, butit can be separately enclosed in an ASCII textfile or on paper. Information regarding whom tocontact and the CAD media should be enclosedin a file elsewhere because that information willbe needed in case there are problems or ques-tions and to extract files from the media.

Checklist of non-geometrical information

which needs to be communicated

Design Engineer - name, phone #, e-mailaddress and fax #

Manufacturing Engineer - name, phone #,e-mail address and fax #

Buyer - name, phone #, e-mail address andfax #

CAD media information CD/e-mail/d i s k e t t e /tap e: commands requ ired toextract the files

File format and version number:IGES (.igs), STEP (.stp), ACIS (.sat),Parasolid (.x_t), Granite (.g)

Part number

Revision

Revision description

Part title

Estimated number of parts required peryear and part life time

Related CAD file name(s) or layername(s)

Material - thickness, type, hardness (ifapplicable), etc.

Punch or burr direction, material graindirection

Deburring instructions

Finish - plating instructions, paintinginstructions (i.e. mask, over spray, color),s p e c i f i c a t i o n s, camera ready art or digitalfile, etc.

Tolerances

Part marking information

Allowable bend radii

Allowable bend relief

Allowable corner radii

Allowable tooling holes

Hardware list - quantity, description, partnumber

Assembly instructions - welding, tapping,riveting, etc.

TolerancesCAD models define the dimensions of a part

c o m p l e t e l y, but generally do not describe thetolerances that should be maintained for eachdimension. Critical dimensions should be shownexplicitly in the CTF drawing with tolerances,but unless this is a complete fabrication draw-i n g, most of the remaining features are left undi-

mensioned and untoleranced. One solution is anote or tolerance block that defines the generalt o l e r a n c e s, not dependent on two- or three-place dimensions, but instead according to whattypes of features are being dimensioned.


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CAD Design

Example: Possible Tolerance Note

As specified by the critical-to-function draw-ing, standard tolerances will be the following:

Single-hit hole size

Edge or hole to edge or hole Edge or hole to form Form to form Form angle

The CAD model will contain all the nominaldimensions for a design, but tolerances need tobe explicitly communicated to the supplier in aCTF drawing or other specification document.Tolerances should be called out as bilateral tol-erances (i.e.: 2mm) so that nominal falls in themiddle of the tolerance band. Do not use uni-

lateral tolerances (i.e.: +0.010"/-000"). They willcause th e nominal dimension in the CA Dmodel to be at the edge of the tolerance band.

If the CAD model is used to program a CNCoperation, the computer-driven machine will tar-get the nominal dimension and operate at theedge of limit for acceptable product. The CNCprogrammer can intervene and manually edit theprogram to target the middle of the toleranceband, but then the process is no longer being dri-ven by customer data and errors can be made.

File Formats CAD Files. CAD software is developed byindependent companies, competing to be thefirst to market with the best combinations ofcapabilities and cost. CAD systems each usetheir own unique way of organizing and storingthe CAD data. Brand specific file formats areincompatible with each other. Part designs cre-ated by one CAD program are unreadable byothers unless a neutral file format is used whentransferring the CAD data.

Neutral file formates include I G E S ( . i g s,Initial Graphics Exchange Specification) andS T E P ( . s t p, S t a n d a rd for the Exchange ofP r o d u c t model data) are generally supported byall major solid modeling CAD programs.Neutral formats will strip away parmetric data

that created the original geometry.Industry standards have been developed to

give CAD programs a universal file format fortranslating CAD information from one compa-nys CAD format to another. Its official name isthe Initial Graphics Exchange Specification and often referred to as IGES. Files savedaccording to the IGES specification are identi-fied by the DOS file extension,.IGS.

As with most standards, the capabilities ofthe universal IGES format follow the industryit supports. The IGES standard is updated tosupport the new capabilities designed intoCAD systems, but there is a time delay. Today,IGES captures 3-D model information, surfacesand wireframes. It does not include 3-D solids,

parametrics or certain complex curve functions.CAD software companies take responsibilityfor how their CAD information is translated toand from the IGES format. Some CAD pro-grams allow the designer to save a design direct-ly to an .IGS file. Others require that you savethe design in the CAD systems native file for-mat, and then run a separate program to con-vert it to an .IGS file. In either case, it is impor-tant to use the most current revision of theIGES translator so your .IGS files can be under-stood by CAD systems at other companies.

A word of caution in using IGES. There areseveral pitfalls that can make it very difficult touse IGES effectively:

CAD systems (and even IGES) do not sup-port all of the geometric shapes used in theCAD design world. The root of most transla-tion problems lies in the basic differences in theway CAD systems store design information.CAD systems may describe common geometricshapes in incompatible ways.

While one CAD system may not recognize a

circle (but represent it with a 90 ellipse) anoth-er system may not recognize an ellipse (but rep-resent one with polyline arcs). Translating adesign through this combination turns circlesinto polyline arcsthe polyline arcs may not beunderstood when the design is translated back



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CAD Design


to the CAD system used by the original design-er. And that designer will not understand whythe circles were deleted from the design with-out authorization. Each translation is an oppor-tunity for creating errors.

The IGES and STEP translator for yourCAD system may be poorly written. They areoften written by third party services who may notunderstand all the hidden incompatibilities. Ifyour CAD system uses a shape, a color, a linewidth, or other feature that is not supported byIGES and STEP, the translator will determinewhether or not the entity gets written to the IGESor STEP file, and what it will be translated as.

Your IGES or STEP translator may not be acurrent revision. The latest IGES and STEPtranslator will typically convert an old designf i l e. But an old translator will not recognize theformat of a new IGES or STEP file and may dis-card data without telling you or create a file thatis unopenable on the receiving CAD system.

Pitfalls are common in todays world andmake it very difficult for a good supplier tointerpret a good CAD file. To minimizeproblems, test the compatibility between CADs y s t e m s. Then expect to check all translated

designs carefully on an ongoing basis.Kernal specific file formats include, AC I S

(.sat, Spati al Te ch n o l o g y), Parasolid (.x_t,Unigraphics Solutions), and Granite (.g,Parametric Te ch n o l o g i e s). These file formatswill provide a better level of compatibility andare recommended over Neutral file formats, ifavailable. Kernal specific file formats, like neu-tral formats, will strip away parametric datathat created the original geometry.

Product specific file formats are the nativefile format of the creating CAD software. This

is always the best option for moving CAD dataif your fabricator supports compatible software.It is recommended to check with your fabrica-tor on software type, file format and transfermedia before sending any CAD file.

While IGES and STEP are the standard for-mat for CAD geometry, there are other file for-mats that have become defacto standards forexchanging drawings and text. (IGES will han-dle drawings and text, too, however the transla-tors available today do an unreliable job oftranslating them.)

D r awing Files. Though drawings can beincluded in an .IGS file, this guideline recom-mends two formats for drawings, HPGL(Hewlett Packard Graphics Language) andDXF (Drawing Interchange Fi l e, a formatdeveloped for Au t o CAD and commonly usedby 2-D CAD systems).

H P G L is a printing format that computersuse for telling a plotter how to plot a drawing. Tosave an HPGL file, one tells the CAD software it

should plot to a plotter, but captures the instruc-tions to a disk file instead. In order to print thefile later, one copies the disk file to an HPGLdevicea plotter or printer. This capability isavailable on most CAD software packages.

The HPGL formats key strength is that alldrawing information is reliably captured in theelectronic file and can be printed on a widerange of plotters and printers. The file formathas two drawbacks. First, the file will have thedrawings size coded into it when the file is cre-ated. Secondly, the file is a set of plottinginstructions. It is no longer a CAD design andcannot be revised with most CAD software sys-tems. HPGL files do not keep track of attributeinformation or drawing layers. It is essentiallyan electronic version of a plotted drawing.

Test the compatibility betweenCAD systems.

Create a test file that includes each of the entities

supported by your CAD system.

Translate the file into the target CAD system.

Compare each entity.

Do this both ways between customer and supplier.


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.DXF is another standard CAD design fileformat. It is commonly used by 2-D CAD pro-grams, but is 3-D capable. (Your CAD manualwill explain the process for saving a .DXF file.)The .DXF file can be revised and plotted. It issimpler and 2-D drawings are more reliablyinterpreted than drawings from an IGES file.Drawbacks are that it will be a bigger drawingfile than an HPGL file.

Text files. Text files are very useful fordescribing non-graphical information. Th e ymay be saved on the same e-mail or disk asCAD files. Text files can be in a variety of for-mates including Microsoft Word and WordPad.

File ContentsUntil there is greater standardization in thei n d u s t r y, transferring design information fromone CAD system to another will be unreliable.

To simplify matters, we recommend thatcompanies use each of the file types for the par-ticular job they do best:

Use native CAD files, if supported, first. Use Kernal specific file as a second choice. Use .IGS or .STP as a last resort Use .IGS for design models.

Use .DXF or HPGL for drawings. Use .DOC or .TXT files for text informa-

tion.

File PreparationWe recommend that all files be compressed

using a compression utility such a WinZip orStuffit. This reduces e-mail transmission timesand archives all files into a single file. If the filesare coming from a Macintosh, include theDOS 3 character extension to all files to allowfor safe transfer to Windows systems.

File Transfer E-mail attachments are the simplest way of

transferring the CAD data and accompanyingf i l e s. This usually has a 2 meg file size limita-tion. Check with your fabricator regardingmailbox size limitations.

Your fabricator may have an FTP site

which allows for peer-to-peer transfers. Usuallylarger files can be transmitted using thisapproach and the transfer is more secure.

Disk Transfer. Files can be saved to a CD,floppy disk or Zip disk and sent via overnightmail. Unless otherwise arranged, the diskshould be a DOS format

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Save design as anIGES wireframe.Strip out solidsand surfaces.

Verify part numbers andrevision levelsare correct.

Flowchart for Exchanging CAD Files

Complete designand save file.

Fax a copy of theagreement form

to the other party.

Plot drawings toHPGL files.

Create text filesas desired.

Follow instructionsfor file transfer.

Copy file to 1.44MB3-1/2 diskette.

E-mail or fax a copy ofthe agreement formto the other party.

Save the design in thenative file format for your

CAD systemper

agreement byboth parties.

Set communicationssoftware to (9600,N,8,1)

or faster. Dial andconnect with remote

host computer.

Archive the filestogether and create

a self-extracting.EXE file.

Isother party

using the sameCAD software

?

Mail to other partywith a copy of theagreement form.

Attach file toE-mail message.

Send E-mailmessage toother party.

Methodof transfer

?

Yes

No

Modem DisketteE-mail



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Floppy Disk Drive

3-1/2" diskette

1.44MB able to read DOS format

CAD File Transfers

Minimum RequirementsHardware & Software

Preferred File Transfer Methods

modem upload/download

3-1/2" DOS diskette

Optional File Transfer Methods(only when prearranged between

customer and supplier)

Internet e-mail

magnetic tape

5-1/4" diskette

Format of Transferred File(s)

file shall be compressed and archived

in a self-extracting .EXE file.

.EXE file may include:

.IGS 3 dimensional model

.DXF drawings

.TXT files containing text

HPGL plotter files

CAD model shall not include solids

or surfaces.

optional file formats, solids, and

surfaces may be used if

prearranged between

customer and supplier.

Modem

9600bps (or faster)

v.42 bis (or better)

Computer Software

communications software with

host capability.

file compression software

software for creating self-extracting

.EXE files.

File Compatibility

able to read:

IGS files

DXF files

HPGL/HPGL2 files


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Date:_________________________

Customer/Supplier CAD Agreement

Company Name:_______________________ Contact Name: ______________________

Project Name:_______________________ Title: ______________________

Phone: ______________________Part Number(s):_______________________ Fax: ______________________

Revision Level:_______________________ E-mail: ______________________

Action Requested CAD Media:

Quote Disk

Prototype Modem

Production E-mail

Other

Deviations allowed:

Material substitutions Others

Hardware substitutions

Tolerances

Redesign for manufacturability

Other docs required: Types of files included:

Customer standards .IGS model

Other .DXF model/plots HPGL/HPGL2 plots

Controlling document is .TXT docs

CAD model Material/hardware list

Plot files Other

Hardcopy drawings

Other__________________________ CAD software used?______________

Command required to extract files

All nominal dimensions for prototypes and production parts will be taken from the

CAD modelThe customer agrees that the CAD model will be used to program computer

aided manufacturing (CAM) processes.

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Chapter 2 Cad Design

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