ASM Metals HandBook Vol. 14 - Forming and Forging
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Formability Testing of Sheet Metals
Brian Taylor, General Motors Corporation
Selected References
•
A.K. Ghosh, S.S. Hecker, and S.P. Keeler, Sheet Metal Forming and Testing, in Work
ability Testing
Techniques, American Society for Metals, 1984, p 135-195
•
S.S. Hecker and A.K. Ghosh, The Forming of Sheet Metal, Sci. Am., Vol 235, Nov 1976, p 100-108
•
R.M. Hobbs and J.L. Duncan, Material Selection--Tests for Formability, Metals Engi
neering Institute,
American Society for Metals, April 1979
•
S.P. Keeler, Understanding Sheet Metal Formability, Vol 1, Machinery, Feb-July 1968
•
S.P. Keeler, Sheet Metal Forming in the 80's, Met. Prog., July 1980, p 25-29
•
J.R. Newby, Formability Testing and Deformation Analysis of Sheet Steel, in Formability Topics--
Metallic
Materials, STP 647, American Society for Testing and Materials, Philadelphia, 1978, p 4-38
•
R. Pearce, Sheet Metal Testing--From the 19th Century Until Now, in Sheet Metal Forming and Formability,
Proceedings of the 10th Biennial Congress of the International Deep Drawing Research Group, University of
Warwick, Warwick, England, April 1978, Portcullis Press Ltd., 1978, p 355-362
•
"Sheet Steel Formability," AISI Committee of She
et Steel Producers, American Iron and Steel Institute, Aug
1984
CAD/CAM Applications in Sheet Forming
Introduction
Virginia Mouch, Electronic Data Systems Corporation
COMPUTER-AIDED DESIGN AND MANUFACTURE (CAD/CAM) typically involves the use of graphic displays,
which allow the interactive creation and modification of geometric shapes, but CAD/CAM is not limited to this technique.
In its broadest definition, CAD/CAM can be the application of any software program--batch or interactive--that facilitates
product design and manufacture.
There are two principal advantages of applying CAD to the design of dies for sheet metal stamping. First is the generation
of computer surface data for downstream CAM applications to generate numerically controlled (NC) cutter paths and to
eliminate the need for die models. Second is the reduction of downstream die tryout time and the reduction of die
construction aids by producing more geometrically accurate data. These advantages can result in both time and cost
savings. This article will discuss the application of CAD/CAM to the diemaking process, first in general terms and then in
a specific case study.
CAD/CAM Applications in Sheet Forming
CAD/CAM as Applied to Diemaking
Diemaking, whether manual or computerized, is currently an art, not a science. The methods of approaching the problem
may be as numerous as the number of people doing the job. Once these methods are known, it is easier to select the best
system package. Most currently available CAD/CAM systems are generic; few systems have specific software tools
tailored to the designing of dies. However, with some attention to several key issues, these generic systems can be
successfully applied to the design and manufacture of dies. Some of the key issues involved in moving from a manual
process to a computerized one are identified below. Flow charts for manual and computer-aided die processing are shown
in Fig. 1.
*
Fig. 1 Comparison of manual and computer-
aided diemaking operations illustrating the steps eliminated using
CAD/CAM. (a) Traditional manual die processing. Source: Ref 1. (b) Die processing using CAD/CAM.
Establishing the Form of the Master Product Data. It is imperative that the master product data (the computer
model, drawings, and physical model) be established and that the CAD system have the capability of accepting this
information in the form identified. These master product data represent the starting point for development of the die. All
personnel involved in the tooling process must know where to obtain the master product data. Depending on its form, they
may need to perform additional operations to make use of it. For example, prior to computerization, a physical model may
have been the master. When CAD/CAM systems are used to produce the tooling and the physical model is still the
master, it becomes necessary to incorporate scanning devices and data translation software to make use of the master data
in the CAD/CAM system.
Determining the Mathematical Representation Required for the Computer Model of the Die. Computer-
aided design and manufacture systems can present a number of different mathematical methods of representing or storing
data. This can be in the form of traditional two-dimensional drawings, three-dimensional lines, arcs, points, or more
complex three-dimensional surface representations.
If NC cutter path generation is to be done by a CAM system, then a surface representation for most of the product and die
will make the process more efficient. Surface creation and modification will then be a critical feature of the CAD system
selected. Wireframe capabilities will also be needed for designing the mechanical components of the die. Depending on
the CAM system selected, the mechanical components may or may not require surfacing.
Determining the Form and Use of the CAD Die Data. In a typical manufacturing operation, several different
departments may need these data in order to generate NC cutter paths, to develop castings, to produce prototype models,
and to schedule construction. The CAD-developed die data may need to be transmitted through drawings or mathematical
models to a CAM system.
In a noncomputerized environment, a drawing is the standard output for those sectors of an organization concerned with
die construction. Depending on the extent of computerization, this may be sufficient. However, if NC machines are to be
programmed offline using an automatic part-programming system, then the most efficient form of the output is an
electronic mathematical representation of the tool. If these needs are many and varied, they must be identified, and the
data translation tools included in the CAD/CAM package(s) must be selected.
Determining How CAD and CAM Will be Linked. In the previous paragraphs, an assumption was made that the
same system need not be used to design the product, to design the tool, and to manufacture the tool. However, if CAD and
CAM systems will be used, the method for establishing the electronic mathematical model between the two systems must
be analyzed.
Again, in a noncomputerized environment, this link may consist of passing a drawing from one person to the next.
Ideally, it is most efficient for moving data between processes if the systems are the same. However, there may be trade-
offs in accomplishing this total integration. It may be difficult to find one CAD/CAM system that meets all of the
functional requirements. Before branching off into different systems, the cost of data movement and translation should be
analyzed.
Determining the Type of CAM System Required. This issue gets closer to the shop floor. The sophistication
required of the NC cutter path generation software will depend on the NC machines to be programmed (that is, three axis
or five axis). In addition, postprocessors must be acquired for each NC machine. Numerically controlled cutter paths can
be postprocessed directly on the CAM system or downloaded to another system.
Determining the Method of Downloading Posted Machine Data to the NC Machines. A number of
methods are available, ranging from paper tape to floppy disk to direct numerical control (DNC) (direct communication
link from a host computer to the NC machine controller). In determining which method is appropriate for a given
installation, the anticipated volume of NC data to be downloaded versus the cost of the link should be considered.
Reference cited in this section
1.
Tool and Manufacturing Engineers Handbook, Volume 2, Forming, Society of Manufacturing Engineers, 1984, p 6-16
Note cited in this section
*
The flow chart shown in Fig. 1
(b) represents one point of view of the application of CAD/CAM to the product life
cycle. It is intended to be a generic description of the cycle. The diagram may vary f
rom organization to organization
based on the amount of time computer assists have been used and the degree of success achieved in phasing out manual
methods. The figure attempts to show that computer assists are being added to the process but some manual
assists, in
reality, have not been phased out. Several processes in the cycle can be phased out. These include wood/plaster master
models, checking fixtures and spotting aids. There are also several processes that can be added to the cycle.
These
include formability analysis, die inspection and part inspection.
CAD/CAM Applications in Sheet Forming
CAD/CAM System Selection
The processes and data flow should be analyzed before any system is purchased. In addition, an analysis of future
processes and data flow is needed to assess what, if any, organizational or procedural changes will be required. This
future analysis also facilitates the selection of the CAD/CAM systems and the peripheral equipment, such as printers and
plotters. These analyses can be done by in-house personnel, outside independent consultants, or companies that provide
such services along with their systems.
The application of CAD/CAM technology to the diemaking process is ongoing. Key to its success are careful analysis and
planning, quality training, and commitment from all levels of the organization. The process must be considered in terms
of the application of CAD/CAM as well as the interface with the shop floor. The following sections provide more specific
details on some of the issues discussed in this introduction.
CAD/CAM Applications in Sheet Forming
Equipment, Advantages, and Applications of CAD/CAM for Diemaking
Lee Spruit, Autodie Corporation
The advent of CAD/CAM has transformed the manufacture of dies for automotive sheet forming from what was once
essentially a manual process that involved making expensive models and trial and error for arriving at a die design into a
highly accurate and automated process that is augmented by the use of computers. Many of the steps taken in traditional
die design can be eliminated (see Fig. 1), resulting in much shorter lead times. Die quality is also improved through the
use of computer-aided manufacture, and as a data base of standard die designs is developed, a cost savings can be
realized.
CAD/CAM Applications in Sheet Forming
Computer-Aided Design
Computer-aided design has been defined as the technique by which geometrical descriptions of three-dimensional objects
can be created and stored in the form of mathematical models in a computer system; once created, the models can then be
displayed, manipulated, and analyzed in a number of ways on a CRT (Ref 2).
This Section will describe typical CAD equipment and processes used for sheet forming die design. Figure 2 shows a
flow chart of the operations described in this Section.
Fig. 2 Flow chart of die design and processing. See also Fig. 1. Courtesy of Autodie Corporation.
Equipment
Hardware. Central to a CAD system is a series of individual workstations, each of which may be equipped with its own
processing unit or may operate through a central computer on a time-sharing basis. Even when each workstation has its
own processing unit, they are usually linked to all of the other workstations at the facility. This avoids the system
slowdown that may be encountered during peak computing periods when using a central computer and prevents the idling
of an entire department in the event that the main computer "crashes" (fails or "goes down").
Each workstation must be equipped with memory capacity to handle the size of the data files used. A typical workstation
may have 8 megabytes of random-access memory. Additional memory capacity is supplied by a disk drive. Peripheral
equipment may include magnetic-tape readers, telephone modems for communication with other computers, and a
printing device for output.
Software used for CAD/CAM applications is often available from workstation suppliers. One function of the software is
to maintain, update, and store a constantly expanding parts library. This data base will continue to provide the key to the
increasing cost-effectiveness of computer-aided design for diemaking. As tooling for new parts is required, it can be
patterned after designs for existing tooling rather than created from the beginning, as is usually done in manual
operations. However, to realize this economy, standard details for tooling components must be adopted and shared. When
this is accomplished, there will ideally be almost no original design work undertaken; all new tooling will come from an
assembly of standard die components.
Costs and Advantages Associated With CAD
Costs. Unquestionably, the cost of a tool designed with CAD will be higher than one designed by the traditional method.
Some shops have estimated this increase in cost to be approximately 20% or more. In addition, although die design has
traditionally represented about 10% of total die cost, with CAD, the current percentage is closer to 15%. However, these
figures are transitory and deceptive.
Computer-aided design requires a significant financial commitment to the purchase of computers and associated
equipment and software. Modifications to the plant are also often necessary to accommodate all of the sensitive
equipment. Even when the commitment has been made to spend hundreds of thousands of dollars to purchase the system,
skilled operators will then be needed to handle it. Aside from the training costs for these operators, efficiency will suffer
while the designers become familiar with the power of the new techniques. Beyond all of this, there are the financial and
time costs associated with building a library of parts and standard die components. In all, any firm deciding to enter this
technology should allot approximately 1 year of relative inefficiency to setting up the entire system.
As discussed later in this Section, the primary advantages of CAD are its tie-in to CAM and the smooth, efficient
machining of dies that results. Therefore, the cost of machining the die is reduced, and this in turn raises the percentage of
total cost represented by the design function, even if absolute design costs were to remain constant. Nevertheless, die
design costs may eventually fall below the levels associated with traditional design.
Advantages. Although the costs of implementing computer-aided design are readily apparent, the advantages are far
less so and are often unquantifiable. In fact, most of the advantages accrue downstream in the manufacturing process,
rather than directly in the design function. These advantages include DNC part programming, better scheduling, reduced
number of setups, and faster machining times. These will be covered more fully in the discussion "Computer-Aided
Manufacture" in this section.
Within the design phase itself, most advantages derive from enhanced productivity in terms of speed and accuracy.
Depending on the specific task, productivity can typically increase from three to ten times for particular functions. As
mentioned above, standardized components help tremendously in raising this figure; other factors include the complexity
of the part, the degree of part symmetry, and its similarity to previously designed components. Another element of
productivity gain is found in the reaction time to any changes in input, which may range from days to literally moments.
As important as productivity increases may be, perhaps the greatest contribution of CAD is in design accuracy. On one
hand, there is dimensional control that is often well above that attainable by manual drafting. A designer can precisely
scale and view the work from any perspective, rotating the design on the screen at will. Further, with this three-
dimensional capability, when an alteration is made to a design in one dimension, the computer automatically makes the
equivalent change in the remaining views, thus preserving continuity and minimizing the possibility of errors. These
features lead to vastly improved comprehension by the designers, because they are not required to assemble a series of
two-dimensional drawings into a mental three-dimensional image. On the other hand, CAD systems provide the
opportunity to avoid errors in drafting and documentation because the computer maintains records and continuity. In
addition, computer tolerances are much closer than those of a drafter; inconsistencies that may be difficult to detect on
paper are immediately obvious to the system.