Chapter 2 - DESIGNING IN CAD

Computer Aided Design, CAD, was introduced as a tool to aid designers in developing part drawings as well as decreasing the time necessary to draw the development on paper. Over the years it has become a much more powerful tool enabling engineers to check form, fit, function and tolerancing of details or entire assemblies prior to actual parts being built. In the time it takes to input data, the designer can have a 3D visual model. As this process developed, Computer Aided Manufac-turing, CAM, was introduced to the manufacturing environment. This allowed for data to be input into a CAM system to create machine tool programs, thus automating many of the processing steps that were traditionally done manually.

Overview

As CAD and CAM were developed, the metalforming industry welcomed them with open arms. Virtually all metalforming companies today have some sort of CAM system within their engineering departments, drastically reducing the time required to produce a part. Industry is demanding that this process be taken further by exchanging CAD files. This allows for the customer to design parts on their CAD system and exchange them with their metalforming suppliers. The goal for many companies is to create a part/assembly on a computer screen and then to have it manufactured without any paper drawings being created, reducing the overall time required from design concept to completion of parts.

Traditionally, the process from concept through manufacturing parts was very time consuming. When a CAD model was completed, it was turned over to a drafting department to create a typical orthographic drawing. The drawing would be given to a metalforming company who would recreate the part as a flat pattern development in their CAD system. From there it would be "downloaded" into a CAM system to create a machine tool program. This process allows numerous opportunities for errors. Today there are many CAM systems on the market that will actually take a CAD file and automate the unfolding for you, creating a flat pattern development, with little opportunity for errors.

One advantage of exchanging CAD files is the ability to get your product design into the hands of the supplier prior to the design being formally completed. Early supplier involvement in design reviews for manufacturability, tooling and manufacturing methods can be reviewed before changes are costly.

It should be noted that there are certain limitations to CAD file exchange. CAD files must be drawn to full scale. All objects within a file must be put exactly where you want them. This is imperative for the simple reason that when the CAD file is imported into your supplier's system and goes through the unfolding process it will place all of your geometry exactly as you have drawn it. If you have misplaced a hole, your final product will have that same hole misplaced. Simply put, what you CAD is what you get. As CAD file exchange becomes fully implemented within the manufacturing environment and paper documents become obsolete, the CAD file will become the master document for inspecting finished products.

Finally, CAD files must be clean. There cannot be overlapping lines or lines that do not intersect. If these types of problems are contained within the CAD file upon file exchange, then your supplier must take valuable time in cleaning up your file. Lines that don't intersect cannot cleanly go through the unfolding process.

Overlapping lines that exist within the file can create major problems in the machine tool programs. For example, if the part happens to be run on a laser cutting machine, you will get holes or edges that are double burned thus destroying the part's edge, causing a closely toleranced 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 that are available. In a recent survey, conducted by the Precision Metalforming Association, of 224 of its member companies, it was found that more than 50 CAD/CAM systems were in use. Because of the variety of systems there are certain guidelines that must be closely adhered to when exchanging CAD files.

Guidelines for Designing in CAD

This chapter is intended to help avoid difficulties in exchanging files. Information will include proper part geometry, what should be and what should not be contained within the file, different methods of file transfer, and minimum hardware requirements for CAD file exchange. If these guidelines are followed you will be able to exchange files, while avoiding many of the major problems that have been experienced in the past, with virtually any company with a CAD system.

In transferring the design of a sheet metal part or assembly via CAD, it is important that all necessary information be communicated to assure that the intended functionality will exist. This information includes the CAD model, critical-to-function dimensions and non-geometrical information.

CAD Model Description

A 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 several orthographic views, or 3-dimensional and viewable from any orientation. 3-D models are preferred by most manufacturers because they are more versatile for programming and for generating additional documentation.

There are many types of entities that can be used in CAD files, but only wireframe geometry should be included in the sheet metal part's CAD file. Other entity types, i.e. complex curves, surfaces and solids, etc. should be avoided because they do not translate well into most punching or laser CAM systems via current software methods. Some typical uses of wireframe entities:

• Lines are used to describe straight edges of material and connect the two surfaces of the material.
• Arcs, or partial circles, are used to describe rounded corners or bends in the material.
• Circles are used to describe round holes and any other circular features.
• Points are recognized as wireframe geometry, but should generally be avoided, since some CAD software will translate this as a hole.

Rules for Designing Part Features

A sheet metal part's CAD model should be composed of geometry that exactly describes the intended design of the part or assembly without unnecessary complication. All geometry should be created at full-scale using nominal sizes. All edges, transitions and cross-sections that are represented in the model should be represented by geometry that is free of gaps, overlaps and duplication.

Design Features

• Edges of the entire periphery of the sheet metal should be shown, with consistent separation equivalent to the metal's thickness. Connecting lines whose length is equal to the metal's thickness must be drawn along the periphery at every edge transition that occurs. 
• Bends in the material can be shown with or without bend radii. Bend radii, if shown, should be represented by pairs of concentric arcs with mold lines connecting inner and outer radii to show the extent of the bend. For simplicity, models with consistent bend radii can be represented with square corners as if the bend has no inside or outside radius. The actual radius will need to be allowed for in the design and communicated to the supplier. Bend reliefs, if required, should be shown. 
• Holes in a part should be detailed as described above for the periphery edges, including lines to connect the two surfaces. For circular holes, at least one line should be used to show that the circles are related. Additional lines that would appear in orthogonal views to show the extent of the hole are generally desirable. 
• Other Features. Coined, drawn, formed, machined or rolled features as well as installed hardware should be represented by geometry that details the edges, any transitions and cross-sections of the features or hardware.

Critical-to-Function Dimensions

In the past, part designs were typically communicated by hand-drafted drawings, showing various views of the part with dimensions for every detail and with all pertinent information included. With CAD systems, some designers have stopped generating dimensioned drawings of any kind, since dimensions can be extracted from the CAD model instead. Unfortunately, the result is an incomplete hand-off of information. The designer still needs to communicate to the manufacturer other types of information: the dimensions that are critical to the success of the design, tolerances and the other non-geometrical information that were included in the drawings.

Two-dimensional drawings are the best way to communicate critical-to-function (CTF) dimensions. An example of a CTF drawing includes critical dimensions and most of the necessary non-geometrical information. In addition, the drawing contains enough dimensions to completely form the described part. Without this information most manufacturers would have to create an additional drawing to detail the formed part to the shop and for quality assurance records. This CTF drawing is simpler to produce than a complete fabrication drawing because it has fewer dimensions.

A flat pattern view is acceptable and sometimes very helpful. The manufacturer will use these views mainly as a reference for the quoting process. If dimensions are included in any unfolded views they should be for reference only, since the manufacturer will need flexibility in order to meet the dimensions and tolerances of the formed part.

Non-geometrical Information

Required information other than the wireframe geometry and CTF dimensions are known as non-geometrical information. It is textual information and most of it can be communicated in the CAD model or CTF drawing, but it can be separately enclosed in an ASCII text file or on paper. Information regarding whom to contact and the CAD media should be enclosed in a file and elsewhere because that information will be needed in case there are problems or questions and to extract files from the media.

Checklist of non-geometrical information which needs to be communicated

• Design Engineer - name, phone #, E-mail address and fax #
• Manufacturing Engineer - name, phone #, E-mail address and fax #
• Buyer - name, phone #, E-mail address and fax #
• CAD media information diskette/tape: commands required to extract files- modem: any instructions necessary to communicate
• Part number
• Revision
• Revision description
• Part title
• Related CAD file name(s) or layer name(s)
• Material - thickness, type, hardness (if applicable), etc.
• Punch or burr direction, material grain direction
• Deburring instructions
• Finish - plating instructions, painting instructions (i.e. mask, over spray, color), specifications, etc.
• Tolerances
• Part marking information
• Allowable bend radii
• Allowable bend relief
• Allowable corner radii
• Allowable tooling holes
• Hardware list - quantity, description, part number
• Assembly instructions - welding, tapping, riveting, etc.

Tolerances

CAD models define the dimensions of a part completely, but generally do not describe the tolerances that should be maintained for each dimension. Critical dimensions should be shown explicitly in the CTF drawing with tolerances, but unless this is a complete fabrication drawing, most of the remaining features are left undimensioned and untoleranced. One solution is a note or tolerance block that defines the general tolerances, not dependent on two- or three-place dimensions, but instead according to what types of features are being dimensioned.

The following example describes a possible tolerance note:

Except as specified by the critical-to-function drawing, standard tolerances will be:

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 nominal dimensions for a design, but tolerances need to be explicitly communicated to the supplier in a CTF drawing or other specification document. Tolerances should be called out as bilateral tolerances (i.e.: ±2mm) so that nominal falls in the middle of the tolerance band. Do not use unilateral tolerances (i.e.: +0.010"/-000"). They will cause the nominal dimension in the CAD model to be at the edge of the tolerance band.

If the CAD model is used to program a CNC operation, the computer-driven machine will target the nominal dimension and operate at the edge of limit for acceptable product. The CNC programmer can intervene and manually edit the program to target the middle of the tolerance band, but then the process is no longer being driven by customer data and errors can be made.

File Names and Revision Control

In recognition that DOS is the standard operating system with the majority of CAD/CAM users, it is important to meet its more restrictive file naming convention for consistency between systems. Therefore, file names should be limited to eight alphanumeric characters, a period and a three-character extension. The extension is often pre-determined according to the type of file enclosed.

It is very helpful to develop a consistent method of using the eight-character name that is based on the part number and revision level of the part or assembly in the file. For example, assume your company numbers parts according to the following scheme:

123-4567-89 revision A

and the last six numbers and the revision letter are all that normally change. If you are naming an IGES format CAD file, you might want to standardize on a naming method that would result in:

456789A.IGS

The .IGS extension allows for this file to be translated directly into most CAD or CAM systems without any need for renaming. This scheme also allows for one additional character to be used, perhaps to differentiate between several IGES files that might be related to the same part and revision.

In case multiple components of an assembly are described in a single file, it is important that future releases of this assembly include all of the components from the previous release, so that no obsolete components can accidentally be accessed.

File Formats

• CAD Files. CAD software is developed by independent companies, competing to be the first to market with the best combinations of capabilities and cost. CAD systems each use their own unique way of organizing and storing the CAD data. File formats are incompatible with each other. Part designs created by one CAD program are unreadable by others unless a common language can be found for translating the design into the other CAD system's format.
  Industry standards have been developed to give CAD programs a universal file format for translating CAD information from one company's CAD format to another. Its official name is the Initial Graphics Exchange Specification -- and often referred to as IGES. Files saved according to the IGES specification are identified by the DOS file extension,".IGS."
  As with most standards, the capabilities of the universal IGES format follow the industry it supports. The IGES standard is updated to support the new capabilities designed into CAD systems, but there is a time delay. Today, IGES captures 3-D model information, surfaces and wireframes. It does not include 3-D solids, parametrics or certain complex curve functions.
  CAD software companies take responsibility for how their CAD information is translated to and from the IGES format. Some CAD programs allow the designer to save a design directly to an .IGS file. Others require that you save the design in the CAD system's native file format, and then run a separate program to convert it to an .IGS file. In either case, it is important to use the most current revision of the IGES translator so your .IGS files can be understood by CAD systems at other companies.
  A word of caution in using IGES. There are several pitfalls that can make it very difficult to use IGES effectively:
• CAD systems (and even IGES) do not support all of the geometric shapes used in the CAD design world. The root of most translation problems lies in the basic differences in the way CAD systems store design information. CAD systems may describe common geometric shapes in incompatible ways.
  While one CAD system may not recognize a circle (but represent it with a 90° ellipse) another system may not recognize an ellipse (but represent one with polyline arcs). Translating a design through this combination turns circles into polyline arcs -- the polyline arcs may not be understood when the design is translated back to the CAD system used by the original designer. And that designer will not understand why the circles were "deleted" from the design without authorization. Each translation is an opportunity for creating errors.
• The IGES translator for your CAD system may be poorly written. They are often written by third party services who may not understand all the hidden incompatibilities. If your CAD system uses a shape, a color, a line width, or other feature that is not supported by IGES, the translator will determine whether or not the entity gets written to the IGES file, and what it will be translated as.

• Your IGES translator may not be a current revision. The IGES file format itself is revised and improved. There have been five versions of IGES since 1981. The latest IGES translator will typically convert an old design file. But an old translator will not recognize the format of a new IGES file and may discard data without telling you.
  Pitfalls are common in today's world and make it very difficult for a "good" supplier to interpret a "good" CAD file. To minimize problems, test the compatibility between CAD systems. Then expect to check all translated designs carefully on an ongoing basis.
  There are other CAD file standards. The German VDA standard is commonly used in Europe, and the International Organization for Standardization (ISO) is developing the Standard for Exchange of Product Model Data (STEP). However at this time, IGES is the standard format for design geometry in the United States.
  While IGES is the standard format for CAD graphics, there are other file formats that have become defacto standards for exchanging drawings and text. (IGES will handle drawings and text, too, however the translators available today do an unreliable job of translating them.)

• Drawing Files. Though drawings can be included in an .IGS file, this guideline recommends two formats for drawings, HPGL (Hewlett Packard Graphics Language) and DXF (Drawing Interchange File, a format developed for AutoCAD and commonly used by 2-D CAD systems).
  HPGL is a printing format that computers use for telling a plotter how to plot a drawing. To save an HPGL file, one tells the CAD software it should plot to a plotter, but captures the instructions to a disk file instead. In order to print the file later, one copies the disk file to an HPGL device -- a plotter or printer. This capability is available on most CAD software packages.
  The HPGL format's key strength is that all drawing information is reliably captured in the electronic file and can be printed on a wide range of plotters and printers. The file format has two drawbacks. First, the file will have the drawing's size coded into it when the file is created. Secondly, the file is a set of plotting instructions. It is no longer a CAD design and cannot be revised with most CAD software systems. HPGL files do not keep track of attribute information or drawing layers. It is essentially an electronic version of a plotted drawing.
  .DXF is another standard CAD design file format. It is commonly used by 2-D CAD programs, but is 3-D capable. (Your CAD manual will explain the process for saving a .DXF file.) The .DXF file can be revised and plotted. It is simpler and 2-D drawings are more reliably interpreted than drawings from an IGES file. Drawbacks are that it will be a bigger drawing file than an HPGL file and it is not as capable a method for exchanging 3-D designs as IGES.
• Text files. Text files are very useful for describing non-graphical information. They may be saved on the same disk as graphics files. Text files should be ASCII text and given a .TXT file extension. Lines of text should be no longer than 80 columns and each line should be terminated with a carriage return or break.

File Contents

Until there is greater standardization in the industry, transferring design information from one CAD system to another will be unreliable.

To simplify matters, we recommend that companies use each of the file types for the particular job they do best:

• Use .IGS for design models.
• Use .DXF or HPGL for drawings.
• Use .TXT files for text information.

• IGES File -- a 3-D model of the design. A wireframe model is preferred. Do not save the 3-D model with surfaces or solids information. Wireframe files will be smaller than files with surfaces and can be read reliably by most 3-D CAD systems. A model with surfaces included will make the file 5-10 times larger, and the surfaces may be unreadable by the other CAD system. Drawings and text may be included in an IGES file, but are not translated consistently between different CAD packages. IGES will take up considerably more disk space (and modem time) than alternatives that are better suited. Drawings that look correct on your CAD system will often be very confusing and incomplete when translated through IGES and viewed with a different CAD system.
• .DXF and HPGL Files -- drawings. Both formats are well defined standards that can be read with inexpensive software and hardware. HPGL files should not be plotted for drawings larger than D-size, unless agreed upon with the company that will plot the file.
• Text Files--nongeometrical information. This is an excellent way for documenting information detailed in the checklist on page 12. It is compact and readable without CAD software.

File Preparation

The CAD File Transfer diagram outlines a minimum suggested computer hardware configuration and software that will be capable of doing the steps described in this section. The computer processor type and processor speed do not limit your ability to do these functions, nor do they have a significant effect on speed. In most cases, the quality of the telephone line will be the limiting factor for how fast a file can be transferred.

The most effective way to exchange a group of files is to compress them and archive them together into a single file. Then convert the archive file into a self-extracting .EXE file--an executable program file. The process is much simpler than it sounds.

This preparation process is strongly recommended. It offers advantages to both the customer and the supplier:

advantages of archiving:

• all the files are bundled in a single file so that none of them can be lost.
• if files are being sent by modem, only one file transfer is required.

advantages of compressing:

• the files are compressed and are more likely to fit on a single diskette.
• if the files are to be transferred by modem or E-mail, they will require less long distance telephone time.

advantages of self-extracting files:

• the company receiving the file can uncompress and separate the CAD files automatically without any experience in archiving or compression.
• the procedure is insensitive to version level of archiving software.

File Transfer

• Disk Transfer. The simplest method of transferring a file is to save it to a floppy disk and send it overnight mail. Unless otherwise arranged, the disk should be a DOS format, 1.44MB 31Ž2" diskette. There are other options available. We recommend the 31Ž2" diskette because it is inexpensive, commonly available, and can be read by the three computer platforms commonly used for CAD -- DOS, Mac®, and UNIX.
• Modem/telephone transfer. Modem transfer of files is a simple and reliable method of file transfer when properly set up. Modem transfer requires a modem at both the customer and supplier businesses, and a good telephone connection between them.

a computer:
(that initiates the file transfer)

• with a modem.
  minimum speed of 9600bps, v.42bis
• set to N,8,1 (no parity, 8 bit, 1 stop bit)
• with communications software that supports file transfer. Also set to 9600,N,8,1 (same as modem). Please note that this is a generally accepted standard, but there are other options.

a host computer:
(other end of the transfer)

• with a modem.
• minimum speed of 9600bps, v.42bis
• set to N,8,1 (no parity, 8 bit, 1 stop bit)- host software that can allow outside access to the computer while unattended.
  also set to 9600,N,8,1 (same as modem).

The computer places a phone call and connects with the host computer, the file transfer can be made in either direction between the two computers. Commands for the file transfer are typically issued by the computer that initiates the phone call. However, most host software programs will allow the transfer to be controlled at either computer. Software at both computers will typically ask what transfer "protocol" you would like to use. Any choice will work as long as you identify the same protocol at both computers. (ZMODEM and YMODEM are recommended. They are well documented, commonly available, and fast. XMODEM is the best choice if they are not available.)

There are many different communications software programs to choose from. Each is well documented and capable of doing the file transfers described here. Microsoft Windows® includes a "Terminal" program with extensive "help" functions and instructions in the user guide. The program lacks a "host" mode that allows other computers to call in, but it is capable of initiating the phone call and connecting with other computers.

9600 bps (bits per second) is a typical modem by today's standards. As CAD files continue to get more complex, file size will continue to grow and speed will become even more important.

With today's technology, 9600 bps is a good tradeoff between speed, cost, and phone line quality. If the quality of the phone line is not sufficient to support 9600 bps, the modem will automatically slow down to a speed where it can reliably communicate. We suggest you test the quality of your local phone service and buy a faster modem if your phone lines are capable of supporting it.

The flowchart summarizes the file preparation and transfer process. Each of the three transfer methods are commonly used today. The one you choose will be a joint decision between customer and supplier, based on file size and urgency.

It is a good practice to include a copy of the Customer/Supplier CAD Agreement form with each CAD file transfer. It will lead the customer to anticipate commonly asked questions that are often overlooked.

This example of an agreement form is not intended to be an all-inclusive one. Companies are encouraged to modify and improve it as they gain experience.

This is not a complete representation of the CAD chapter--illustrations, tables, and other visual examples are missing.

Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association

Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!

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