Automating the Tolerancing Process 15-11manufactur-Manufacturing process capability data must be organized so that both designers and manufacturingcan readily find the applicable manufac
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manufactur-Manufacturing process capability data must be organized so that both designers and manufacturingcan readily find the applicable manufacturing process information For example, the data could be orga-nized according to machine type, material type, feature type, feature size, and variation type (i.e., length orangular variation) for each manufacturing process Additional organization factors might include vendorname, lead-time required, cost data, and surface finish capability
Finally, the data must be placed in a location that is accessible to the designers The most desirablesetup would allow the designers to access the data from directly inside their tolerance analysis tool Thisrequires either that the tool itself provide an internal mechanism for storing a library of process informa-tion, or both the manufacturing process database and the tolerance analysis tool support a commondatabase format At the same time, the content of the data must be controlled so that it can only beupdated by following a defined procedure
15.5.2 Design Requirements and Assumptions
A second way to automate communication is for the designers to deliver a more complete definition of thedesign to manufacturing Information frequently missing from the design definition is a tolerance modeldescribing what design requirements are most important, and how those design requirements are affected
by manufacturing variation One of the products of the tolerancing process on a design should be a set
of reusable tolerance models The tolerance models and their results can then be delivered along with therest of the design definition to manufacturing
Providing tolerance models to manufacturing can help automate several critical production tasks.First, it helps automate troubleshooting manufacturing problems The tolerance analysis model shouldidentify both the design requirements and the driving dimensions (input variables) Each design require-ment is driven by some critical subset of part dimensions Not all part dimensions are relevant to aparticular design requirement When issues arise in meeting a design requirement, the tolerance model willprovide visibility into what the primary variation contributors to the requirement are This visibility helpsautomate finding the source of manufacturing problems
Second, it helps automate predicting the impact of manufacturing process changes The ing processes used to produce a part may need to be changed in order to reduce costs, free up a specificmachine tool for other production runs, or act as a substitute when the original machine breaks down Ifmanufacturing has access to the original tolerance models, they can pull up the relevant studies andchange the assumptions to reflect the new process, and check conformance to the design requirements.Third, it simplifies communicating design and manufacturing problems back to the designers Byusing the same tolerance models, both design and manufacturing have a common frame of reference andcan speak a common language when problems arise The process of identifying the problem and finding
manufactur-a solution cmanufactur-an be much quicker
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Fourth, it helps evaluate the usability of parts that are out of specification For example, batches ofparts may come in with mean shifts or excessive dimensional variations With both manufacturing processcapability data and a tolerance model accessible, the tolerance model can be updated to test the effect onthe design requirements and see if the parts can be accepted
15.6 CAT Automation Tools
Sections 15.2 through 15.5 discussed principles of automating the tolerancing process in terms of thecreation, analysis, and optimization of tolerance analysis models, as well as methods of automating thetransfer of information between design and manufacturing The practical way these principles can berealized is by implementing them in a tolerance analysis tool
There are a growing number of tolerance analysis tools marketed commercially, and even more thathave been developed internally by various companies Whether or not a specific tolerance analysis tool
is suitable for a company’s efforts to automate their tolerancing process is determined by the capabilityand usability of the tool
15.6.1 Tool Capability
When selecting CAT tools, it’s important to distinguish between specialized tools and general-purposetools Specialized tools are optimized for a specific type of tolerance analysis, such as optical lenses orelectrical connector interfaces General-purpose tools are generic enough to adapt to many commonanalysis situations — mechanisms, fixturing, assembly process variations, and others
Defining the capability requirements of a tool requires understanding the common tolerance analysissituations seen in the company Answering this requires conscientiously collecting information from avariety of designers and manufacturing personnel, and not simply relying on the judgment of one or two
“experts” in the company Individuals tend to develop tunnel vision about what types of toleranceanalysis are important It is important that a CAT tool comprehends the majority of the analysis situationsand simplifies the current analysis methods
While tool capability is very important, it is not the only criteria to consider when shopping for CATtools Several usability issues must be considered In many ways, the usability issues eclipse the impor-tance of tool capability Sections 15.6.2 through 15.6.8 will discuss issues related to the usability of CATtools
15.6.2 Ease of Use
Ease of use is the single most important factor in determining the success of a CAT tool’s deployment Ifthe tool is not easy to use, acceptance among designers and manufacturing personnel is unlikely Defin-ing what is easy to use is highly subjective, but several general characteristics should be considered
• The user interface should have an intuitive layout The information should be well organized with themost important data readily accessible
• Model creation should follow a logical process that uses a clearly defined set of operations The modelcreation process should be designed around a systematic approach that can be generically applied to
a wide range of problem types
• Model creation should be quick Time is a scarce resource to designers Few industries have the luxury
of long tolerance analysis cycles If the designers cannot quickly create a model, run the analysis, andget on to their next task, they are likely to use another means to analyze the tolerances or skip italtogether
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• The tool should have useful documentation The tool’s documentation is often the last place searchedfor answers to questions However, when it is finally referred to, the user should find that the docu-mentation is well organized and contains useful examples The documentation should be availableboth on-line and as hard copy
The importance of a CAT tool’s ease of use cannot be overemphasized
15.6.3 Training
The nature of tolerance analysis requires training Tolerance analysis covers a wide range of specializedconcepts: dimensioning, tolerancing, GD&T standards, optimization, statistics, mechanisms, kinematics,manufacturing, inspection, SPC, and others The amount of training required is determined by the back-ground of the trainee, the difficulty of the tool, the quality of the training program, and the complexity ofthe analyses to be performed Purchased tools should provide training classes and materials Companiesthat develop CAT tools in-house bear the burden of developing classes and materials to train its users
15.6.4 Technical Support
The complexity of tolerance analysis guarantees that questions will arise about the use or behavior of aCAT tool Extra assistance may be needed to understand problems in specific application situations.Software bugs will also occur There must be resources available to answer the users’ questions andassist in workarounds until fixes are available
Commercially purchased tools should have a help line and a mechanism for distributing technicalinformation (such as known bugs and workarounds) Help-line access usually requires a company topurchase a software maintenance package in addition to the tolerance analysis tool itself
If tools are developed in-house, help-line resources must be budgeted yearly and skilled help-linepersonnel developed internally to support the users
15.6.5 Data Management and CAD Integration
Computer-based tolerance analysis tools generate data files that must be maintained Tolerance modelfiles developed for a specific CAD model need to be stored with that CAD model This may also be true ofthe analysis output files To this end, the tolerance analysis files should integrate smoothly with thecompany’s CM/PDM (Configuration Management/Product Data Management) system
To help the designers achieve concurrent engineering, the CAT tool should work natively with theCAD system The easier it is to keep the CAD model and the tolerance model in sync, the better Havingthe CAT tool integrated with the CAD system also helps the manufacturing and quality control personnelfind and use the tolerance models when they need them
15.6.6 Reports and Records
Documenting a tolerance study and distributing the results should be quick and easy The reportsthemselves should have a format that covers the important information At a minimum, the reports shouldinclude:
• Output statistical/worst case variation plots
• Sensitivity/Percent contribution pareto of each performance or fit requirement to the part dimensions
• Part dimensions, manufacturing variations, and process capability metrics
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Reports need to be modifiable by the user They should be output as straight text or another commonformat that can be easily read and edited by a word processor Any graphic should also be output in astandard format that can be easily imported into a word processor
15.6.7 Tool Enhancement and Development
It is unlikely that any existing tool on the market will meet all the requirements of a company The CAT toolindustry is still relatively immature and is changing rapidly Therefore it’s important to understand a CATtool’s future development path Issues to understand include:
• What future enhancements are planned for the tool?
• Do future enhancements address all the outstanding issues (e.g., missing functionality) that thecompany has with the tool?
• Is there an effective mechanism for entering enhancement requests and bug reports?
• How rapidly is the tool being improved?
• If it is a commercial product, is the tool provider stable? If it is a tool developed in-house, does it have
a stable funding source?
It is vital that the selected CAT tool is growing and the tool provider is reliable If it is, the investment
in a CAT tool has a far greater chance of delivering real returns to the company in terms of improvedquality and reduced cost
15.6.8 Deployment
The issue of deploying a CAT tool in a company is too large to address within the scope of this chapter.However, some questions that must be answered relative to deployment include:
• Who has responsibility for implementing the tool in the company?
• How much effort will be required internally to install and maintain the tool?
• Does the tool work on company-supported hardware and operating system versions?
In short, a deployment plan must comprehend all the infrastructure required to install and maintainthe CAT tool
Automation can provide great benefits to the tolerancing process Through automation, tolerance modelcreation and analysis can be simplified and accuracy improved The time it takes to develop an optimaldimension scheme for a design can be greatly reduced Automation can also improve the communicationbetween design and manufacturing and help develop a more concurrent engineering environment Finally,careful consideration of the important capability and usability issues will enable the successful selectionand deployment of tolerance automation tools
15.8 References
1 Bralla, James G.1996 Design For Excellence New York: McGraw-Hill, Inc.
2 Bralla, James G 1986 Handbook of Product Design for Manufacturing: A Practical Guide to Low-Cost Production New York: McGraw-Hill, Inc.
3 Cox, N.D 1979 Tolerance Analysis by Computer Journal of Quality Technology 11(2):80-87.
4 Creveling, C.M 1997 Tolerance Design Reading, Massachusetts: Addison Wesley Longman, Inc.
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5 Gao, Jinsong 1993 “Nonlinear Tolerance Analysis of Mechanical Assemblies.” Dissertation, Mechanical neering Department, Brigham Young University
Engi-6 Glancy, Charles 1994 A Second-Order Method for Assembly Tolerance Analysis Master’s thesis MechanicalEngineering Department, Brigham Young University
7 Harry, Mikel, and J.R Lawson 1992 Six Sigma Producibility Analysis and Process Characterization Reading,
Massachusetts: Addison Wesley Longman, Inc
8 Johnson, N.L 1965 Tables to facilitate fitting SU frequency curves Biometrika 52(3 and 4):547-558.
9 Ramberg, J.S., P.R Tadikamalla, E.J Dudewicz, E.F Mykytha 1979 A Probability Distribution and Its Uses
in Fitting Data Technometrics 21(2):201-214.
10 Stoddard, James 1995 Characterizing Kinematic Variation in Assemblies from Geometric Constraints Master’sthesis Mechanical Engineering Department Brigham Young University
Trang 7Paul Matthews has been practicing mechanical design engineering for the past 12 years In his 10 years
of experience with Texas Instruments, he was part of the design team for the F-117 Stealth Fighter infrared night sight and a major author of the Mechanical Product Development Process for the Defense System and Electronics Group At TI, he gained a high proficiency at 3-D solid modeling using ProENGINEER and developed several standard best practices for modeling and data management For the past two years he has been employed as a design mechanical engineer and division director at Ultrak, specializing in the design of larger volume commercial and professional security-related CCTV products.
16.1 Introduction
One question I’ve dealt with as a mechanical engineer is: “Why generate so many paper drawings anddocuments to get a product built?” A simple answer to this question is to provide a manufacturer informa-tion on how to make the product parts and assemblies However, a more important and often forgotten
reason is to make a profit for the company that pays me.
I get paid to design and build a product to sell In today’s environment, if I can’t accomplish this fasterthan my competition, I might as well not do it at all If I’m really paid to produce a product faster and betterthan my competition, will I have the time to generate 2-dimensional (2-D) paper documentation to capturethe 3-dimensional (3-D) design information and notes referred to in the previous chapters? Will I everconsistently generate a drawing that everyone in the product life cycle interprets the same way? And willthis drawing provide the information necessary to build the component? Even if I did, does a manufactureruse this information in a way that helps an improved product move faster to market?
Chapter
16
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The main reason for writing this chapter is to give you ideas for capturing and sharing design
information to manufacture products with minimal paper movement The ideas presented here are not
limited to drawing dimensions and tolerances, but include all information associated with the productdevelopment process and the data formats used to better support today’s rapid product development andproduction
16.2 Paperless/Electronic Environment
16.2.1 Definition
I’ve been in several situations where design programs advertise hours saved by going to a paperlessdesign and manufacturing environment When asked how they do it, the responses usually indicate thatdrawings are transferred to the manufacturing facility by modem, e-mail, or LAN-based communications.After the drawings are downloaded, the manufacturing engineers print the files and pass the paper to thenext person in the process This saves numerous hours compared with the hand delivery of the samepaper drawing Yet this does not reflect the true meaning of “Electronic/Paperless Environment” that Iwant to discuss here There’s more to this environment than the speed in which electronic data can betransferred from point to point
An electronic environment process has two distinct functions:
• To capture the design and manufacture information in a data format best suited to the person makingthe decisions for the particular process step
• To share and reuse the captured information in concurrent engineering for later steps in the process
For many of the designs done in industry today, this data format is a computer-aided engineering(CAE) database; a 3-D computer aided design (CAD) database, and various other formats for supportingnotes By putting less emphasis on paper documentation and more emphasis on a well-documentedconcurrent design/manufacture data capture and share process, the cycle time, cost, and quality of newdesigns is improved
Figure 16-1 Information flow in the
product development process
Specification Definition
Conceptual Design
Detail Design Prototype
Document and Qualify Production
Customer Service
Time
Quantity of Information1
2
4 3
5 6 7
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A typical product development process is shown in Fig 16-1 During the product developmentprocess, the quantity of information increases rapidly and each prior process block’s information sup-ports the process block above it The majority of this information is in several types of computer formatsand each separate block in the process represents not only a process step, but possibly a different person,department and even company completing the task It is critical to the process that this information iscaptured and seamlessly shared from block to block As seen in the figure, the bigger the informationoverlap on the blocks, the shorter the time and inherently the increased strength of the product designprocess
16.3 Development Information Tools
What we all want to do is make the product development process better To make the process better, weneed to capture and share design and manufacturing information in the most efficient way possible Themost efficient way, for some companies, is to use paper and pencil and many manila folders to navigateinformation through the development process For the majority of the competing companies in the market-place, the computer is used to help guide the information flow
This section describes several techniques to help the product team with design and manufacturinginformation in electronic forms
16.3.1 Product Development Automation Strategy
Electronic automation is a simple concept for most companies today The best automation is generatedfrom a simple idea put together with other ideas to form a completed tool It starts with something knownand builds on solutions until the requirements are met
What generates a good automation solution?
• Product Process Requirements Knowledge
The product process must be defined Often companies build automation and then figure out how theprocess needs to flow to use the automation that was constructed Inherently, this forces the automationand process to iterate until a common compromise on both automation and process is met Clearly,successful companies know what information is needed during the product life cycle and what the pro-cess needs to be to support the capture and flow of the information The automation of the informationflow becomes very well defined and simple to implement
• Automation Experience
Solid experience is critical To know when something worked before (or didn’t work!) enablesautomation designers to think ahead and not waste time pursuing paths that will dead end later A newtechnology is always alluring to automation designers, but may not be the best solution to the problem.Experience, with not only the latest and greatest technologies, but also the tried and true technologies,will usually generate the best solutions
• Process Tool Proficiency
Tools are meant to help someone complete a task When a person who generates automation isproficient in the process tool that the automation is designed for, the automation is stronger The profi-cient tool user enhances the features in the process tool and does not construct the automation to forcethe desired outcome A simple example is a person writing a Visual Basic script to add up a column ofnumbers in a spreadsheet program Obviously, the spreadsheet program has built-in functions to do thistask and a script would be foolish
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• Imagination
Without the ability to solve a problem in many different ways, automation designers can get easilystuck There is always a way to complete the desired task If you don’t think of the best way to do it, yourcompetitor will Don’t underestimate the importance of this point Most often, the simple obvious choice
is the right choice In those situations, when the obvious choice does not produce the desired outcome,the automation designer needs to think outside the confines of previous solutions Here is an example of
a problem and a solution
Process step: During this particular product development process step, a design team member isresponsible for providing a marketing team member with a photorender of the new product for marketingliterature, such as an advertisement for new company products
Problem: The new product’s 3-D solid model is so complex and has so many features, the photorendersoftware used to automate this process step will not run to completion on the current computer system.Solution: The automation designer develops the parameters associated with this size of the solidmodel and flags solid models this size or larger as candidates for Stereolithography and paint After thescaled model is built and painted, a real picture can be taken
In this example, the automation designer has the ability to think outside his expertise for a solution tothe problem A more powerful computer helps (by the way, you can never have enough!), but for thisparticular company, it was not cost justified for the number of products that fell into this category
• Automation Flexibility
No product development process will remain fixed long enough to develop a full set of automationsupport Automation that is built to endure modification in the process is very costly and almost impos-sible The process must be able to change with the company’s growth and expectations When theprocess changes, the automation must be updated to support the change without major rework
• Support
Like any tool, automation requires maintenance and repair Support personnel are required to keep thetool current with the process and also with changing technologies Automation that is left alone willslowly wilt like a plant without water The difference is that the plant will show signs of fatigue, where thetool will just stop growing with the process The first sign of trouble is when the product competitors beatyou to market with better designs
• Luck
Luck is a relative word Anyone who claims they can control product development team expectations,keep key employees from leaving the company, and prevent lightning strikes to the main computer, hashad incredible luck in their career I prefer to anticipate bad luck (even expect it) and always be ready to re-group and attack
The above concepts together create good process automation Keep in mind, automation is not themost important point here The main effort with any automation is to support the process that needs theautomation A tool never dictates what a process should be
16.3.2 Master Model Theory
As computer software becomes more advanced, it enables the design team to capture more informationinto a single database This single database is referred to as the master model The information captured
in this database appears in many forms Some are listed in Table 16-1
The master model is the controlling design database, capturing all relevant design data in one centrallocation The key to the master model concept is to generate the design and manufacturing process basedaround a focused design data set and use this master set to generate all supporting documents Oncecaptured, other engineering and manufacturing disciplines reference this information in formats best
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Graphical Data The nominal geometrical representation of the design
Graphical Data Geometry attributes such as line colors, widths, and visibility
Attributes
Dimensional Dimension and tolerance attributes associated with the geometry
Attributes Dimensional attributes provide the scale of the geometry
Design Notes Notes and design calculations used in the product process that may be
needed for future revisions of the product
Parameter Data Information such as cost, part name, designer name, part number, material,
and design revision are a few examples The number of fields of parameterdata can be quite large and provide excellent process automation
opportunities
Software-Generated Calculations done by the software using designer parameters and
Parameters attributes as inputs: mass properties, number of parts in an assembly,
and measurement calculations are several possibilities
Manufacturing Manufacturing specifications needed to complete the fabrication of theProcess Data design Material finish, packaging/shipping requirements, surface
roughness, special tool requirements, and regulatory conformancerequirements are examples
Table 16-1 Information captured in a database
Figure 16-2 Master model process
information
suited for what they need during any particular process step When the master model is updated, ing information is updated concurrently, with little interpretation This update process can be very effi-cient if automated
support-Fig 16-2 shows a simple example of when the engineer decides to add a screw to an assembly Themost logical place for this to take place is in the CAD model, where he parametrically adds the screw modelinto the CAD database The database is considered the master model in this case Other documents arelinked to this master model, and because of this, are directly updated with the new information Theprincipal point here is that all the other product design disciplines know to look at the master model for
A screw is added
to an assembly
Master Model is updated
Bill of materialupdates
Assemblyinstruction updatesAssembly drawing
updates
Mass propertiesupdates
Structuralanalysis updates
Cost updatesService manual
updates
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changing information Once again, if this process is automated, very little effort is needed for this change
to be cleanly incorporated across the product design group
There are many examples of how the master model can be used in the product design process
• Computer Aided Process Planning (CAPP) software for the manufacturing process uses the mastermodel as the seed for generating detailed work-flow estimates and numerical-controlled (NC) code formachining
• Purchasing may use the master model source as a guide for ordering purchased hardware for theassembly
• The structural analysis of a part may automatically be recalculated for updated geometry A documentmay be autogenerated showing inspection dimensions that fall below a certain process capability of amachining center
• The tolerance analysis may be directly linked to the solid model CAD database, so that when thetolerance is changed in the model, the analysis is automatically updated
Theoretically, information is captured one time in a single database file by one software program used
by all disciplines of the product development process In reality, this is unfortunately not the case Aprinted circuit board assembly (PCBA) design is a good example A PCBA will have a mechanical database
to specify packaging constraints constructed in one CAD software, electrical schematic data to define thecircuit in another CAD software, a circuit board layout for the etch runs, bill of materials in a third software,and possibly simulation data in a fourth There are also numerous soldering specifications, materialspecifications, component data sheets and any other referenced document All of these together capturethe design intent for the product One of the most important pieces to the success of the product process
is to know the master model or master data set, and let this single data set control the design automationand reference
The following is an example of a very common occurrence that illustrates the importance of the mastermodel:
I used ProENGINEER™ solid modeling software to create the design database It was commonpractice to take the 3-D solid ProENGINEER™ files and convert them (using a DXF conversion standard)
to 2-D AutoCAD® files to generate the drawings These drawings were taken to the shop where 3-DComputer Vision (CADDS4X) databases were generated to create the NC program Remember the designdatabase (master model) was ProENGINEER™
Here are the problems:
• The design was interpreted five times, with each conversion moving farther away from the designer’sthought
Designer thought à 3-D CADà2-D Drawingà3-D CAMà NC ProgramàInspection
• When making changes, the change was updated and interpreted in at least four different databases If theparts were measured with a coordinate measuring machine (CMM), this adds another interpretation
• Each step in the process may have a different owner, department, or in some cases company involved
to complete the process step
This simple idea can provide a powerful tool for automation and a strong product process information
set Concentrate on the fundamental purpose behind the master model: Focus all product team members
to a common data set When the product team can quickly and easily find the needed information in a
convenient format, the development process will flow smoothly