Programs like the popular six sigma process and design excellence initiatives have really helped to change culture in lean development and production while putting more focus on voice of the customer. These initiatives have been working and have accomplished some significant improvements in product development. However, mention GD&T and the need for training and the scramble is on for reasons why the training is either unnecessary or otherwise keep the training to a minimum investment. Fundamental GD&T training is hardly enough to perform reliable detailed tolerance stack-ups. But, any discussion on advanced training in GD&T or stack-up analysis is often tabled for another time.
So, it is no wonder that three dimensional tolerance analysis still has not caught on. Organizations typically feel that whatever GD&T practices they have been using for years is working and the same is true with hand calculated 2D tolerance stacks involving GD&T. The questions are; "Is the application of GD&T and tolerance Stack-ups within the organization reducing costs and improving quality the way GD&T is intended to do?" Or, is inappropriate use of GD&T and unreliable tolerance stacks costing time, money, and quality? I believe the later of the two is the reality for many organizations.
The discomfort is obvious in even a fundamental class on GD&T. With a group of experienced engineers who have approved dozens or hundreds of drawings that use GD&T, an instructor can ask a basic question and the class remain silent. Ask for a volunteer to do a relatively simple stack and again no hands. So, no wonder designers and engineers hide behind present day practices. After all, present day practices for tolerance analysis typically do not include a check and approval cycle. The irony is that using a good 3D tolerance analysis tool can actually make designers and engineers comfortable with not only the GD&T but with the results too; Results that are truly value added and help provide more robust design, reduced development costs, reduced part and inspections costs, reduced tooling changes, and reduce time to volume to mention just a few benefits.
I know this is a bold statement. But it is simple to test the perceived importance of tolerance analysis within a company. Ask these questions. Is GD&T used? Do persons defining requirements with GD&T have certifications such as the ASME GD&T Professional technical and/or senior level certifications or some other real world equivalent experience? Do associates typically attend a three day on site course within which they sleep, answer cell phones and e-mail, and stretch ten minute breaks into thirty minutes? Don't get me wrong. I know professionals, very good professionals that offer these courses. The irony is that an on site course is to reduce time and cost of training when it is the knowledge of GD&T that is supposed to reduce time and cost of producing higher quality product delivered to the customer. Who, if anyone, checks the tolerancing for correct application? This is not the same as signing off on a 2D drawing. That is a whole other issue. Who checks the tolerance stack-up and what are his or her credentials to do so? A personal favorite; "Is the tolerance analysis a requirement in any of the stage gates within a design excellence program and is the analysis put into a release cycle for an approval signature and design control?" And my all time favorite; "Are the black belts, in whatever process excellence program your company may have, required to take even one class, much less be certified, in GD&T?"
I know these questions can hit some sore spots; rock the boat. But now go to the people who assemble and test product on production lines. Go to the people who have to inspect components and ask them; "what is the single biggest issue when production assemblies either do not assemble or otherwise do not function correctly?" It almost always comes down to dimensional controls. In particular, production issues come down to not having "knowledge" of specific contributors and therefore impact studies on tolerance deviations are almost useless and home grown tolerance stack-ups are either unavailable or so unclear that the analysis can not be read (sometimes not even by the author of the analysis), or worse the analysis is plain wrong. We know there are huge costs associated with assembly line shut downs, recalls, and customer dissatisfaction, and we know that these costs can be associated in whole or in part to tolerances specifications that are simply not understood.
There are many benefits for implementing a 3D tolerance analysis tool like the UGS Teamcenter Visualization VSA product known as VisVSA. When used early on in the detailed design phase of new product development, it can help reduce the number of physical prototypes just as other CAE processes like FEA. Without valid tolerance analysis prototype parts are received and changes are physically made to features to get prototypes parts to assembly and/or function. Often, the dimensional issues result in prototype tool changes resulting in additional physical prototypes, time, and money just to get a design assembled and functioning for further verification and validation. With VisVSA 3D tolerance analysis, thousands of virtual engineering builds can be performed using Monte Carlo simulations and tolerance iterations can be performed quickly based on each set of results to optimize the design for function with optimal tolerances that allow for easier lower cost manufacturing with better capability to clear, concise, and valid requirements.
Another major efficiency that a 3D tool like VisVSA offers is the ability to virtually change the nominal CAD data, without corrupting or actually changing the CAD model, and then quickly analyze iterations of nominal changes along with tolerance allowances. This is a superior process to going back to the 3D CAD model; Change it; then translate the data over to a supplier to change a prototype tool and get new first shots, in weeks, only to find out the guess was wrong. After a proper VisVSA analysis, or virtual impact study, 3D CAD models can be changed efficiently with more reliability for a successful improvement.
Tolerance libraries that re-use the knowledge of known manufacturing capabilities based on material, process, and supplier can be developed that in time speed the dimensioning and tolerancing, as well as the analysis, resulting in better product definition. Is time efficiency in dimensioning and tolerancing important? It certainly is. Dimensioning and tolerancing for most companies is typically the antiquated process of doing drawings; the "legal" documents so I have been told over and over and over again. Creating drawings is already a waste of time, but when tooling is delayed waiting for a drawing, the dollars add up fast. And if the drawing changes six or seven times, the dollars add up faster and release of product slows down. The root of most drawing issues is the creative re-design of GD&T and guessing tolerance requirements that should already be known and valuable company assets.
Maybe the single biggest advantage to design and process excellence programs is the gain in consistency. When teams work in a similar and consistent process, companies gain huge efficiencies in managing their resources and product development can realize continuous improvements because process errors or inefficiencies will also be consistent, easily identified, and improved upon. Using a consistent method and tool for tolerance analysis will yield similar results where hard dollars can me measured. Gains in consistent use of GD&T can be spectacular. The reduction of team player conflicts that constantly surround the application of GD&T can me reduced not only in time but turned into real value added inputs to product design; not to mention a much improved work environment. Is retention of good talent on any company objectives list? I think so. And, in time, persons wanting to do 3D tolerance analyses will become in high demand. Persons who know 3D tolerance analysis will be in higher demand. This paradigm is true today with 3D solid modeling compared to the manual drawing table. Ultimately culture will change to a point where a company not using 3D tolerance analysis tools will have difficulty in recruitment for persons to do such analysis. And yes, I am saying that the role of tolerance analysis not only will be but is a specialty; no different from FEA or other CAE tools. In the day, we called them checkers.
Many companies eliminated drawing checkers to lean out some of the new product development process. The problem with the drawing checker system was/is that there were huge delays in releasing drawings typically for non-value added reasons like the font of some lettering or re-arrangement of dimensions and or views that would make the drawing look better. But checkers often did some random stack-ups that sometimes had real value. What if such stack-ups were not so random? What if there was a focused requirement for documented, controlled, reliable, and reusable tolerance stack-ups? What companies needed to do, and still need to do, is eliminate the non-value added details of a 2D drawing and develop the checker role into a dimensional management specialist.
The best part about a fundamental, natural, and necessary culture change is that voice of the customer demands it. In world competition to develop world class product, the demand to get there better, faster, and cheaper has not and never will change.
About Norm Crawford
Director of South Region for Geometric Solutions
Dimensional Management / Tolerance Analysis and GD&T Consultant
ASME GD&T Professional Senior Level certification (GDTP S-0386)
Over 25 years in product design with specialty in 3D CAD
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