How do you balance the trade-offs between GD&T accuracy and cost?

Perfect GD&T only exists in theory...we all spend a lot of time and friction interpreting customer drawings only to hear that the designer wasn't really all that experienced with GD&T, or the drawing was from the 1990's and no one will change it for fear of making a mistake. CAD Technologies such as MBD/PMI will help go a long way to reducing interpretation, but the design engineers also need to step up their game when it comes to adding more compliant GD&T to their drawings. Lack of 'holistic' process knowledge when applying tolerances is increasing the costs of parts unnecessarily. I would estimate the cost of for example a car could be reduced easily 10-20% with a sobering review of tolerances vs the GD&T being applied to drawings today.

Yes, you want parts accurate. But, not so accurate that the parts cost too much. Why buy a diamond, when all you really need is glass? And, you want it to be accurate "enough" to be functional and appropriately priced. You want parts to fit each and every time. Not most of the time. Not some of the time. All of the time.

Consider if/how the rest of the manufacturing cycle will use GD&T before implementing it. I've had experiences where GD&T was used at the design stage, while neither machinists (who are making the parts) and inspectors (who are approving the parts) had the inspection tools or the understanding to use GD&T effectively.

Balancing GD&T accuracy with costs is a crucial part of the tolerance design and optimization process. This involves making considered decisions to achieve a high-quality product without incurring excessive expenses. Here are some strategies to strike this balance: Understand the Functional Requirements; Evaluate the Impact on Quality; Analyse Manufacturing Costs; Consider Statistical Tolerances; Involve a Multidisciplinary Team; Perform Sensitivity Analyses; Iterate and Learn. This approach helps ensure the product meets desired quality standards without exceeding available budget resources.

I agree that perfect GD&T does not exist. The released GD&T…about 80% of releases have major errors… and…even if the release is good…somewhere between 15-20 re-interpretations by non-experts of the GD&T occur as the release works its way downstream.

First… Let’s get the correct terms… Accuracy describes how centered the mean is relative to the target. Precision describes the spread of the distribution. So…if there is no mean shift….+/- 0.1 and +/- 10 have the same accuracy…even though the first tolerance is much more precise. In terms of GD&T, Precision is what increases costs, not accuracy.

Different GD&T symbols can affect the part or feature requirements, but measurement method should be agreed to in a Dimensional Measurement Plan. As for manufacturing GD&T symbols do not drive manufacturing processes, tolerance values and the interrelationship between features will. Do not mistake design requirements for GD&T constraints. Some designs are simply more complex than others.

A melhora no custo da produção de produtos cotados em GD&T começa com o devido treinamento dos projetistas na execução do projeto. Se bem definidos o projeto vai agradar na qualidade, porém se a definição for muito fechada e sem necessidade vai onerar e muito. Necessário sempre em um projeto novo envolver o metrologista da empresa que por circunstâncias deve ser bem treinado nesse quesito.

This is a good point, the problem that I'm facing all the time is basically to spread the knowledge for all involved on it to get the same visualization about the tolerance specified. Other issue is when we are comparing ISO with ASME. It's another hot issue, inside the brand we have manufacturing sites in EUROPE and USA.

With the symbology it is important to understand their native "hierarchy" and use only those that obtain the intended and desired effect. It is also understand how, if applicable, the datum scheme constrains the part for ease of end measurement.

I use characters that need cheapest inspection. for instance, runout tolerances instead of circularity. the other importantly is to use a bigger feature as being a datum. good luck

In what world is even circular runout (combination of coaxial, parallelism, and circularity) less costly than just circularity?!? In addition to MMC, LMC, and RFS for tolerances the MMB, LMB, and RMB effects on darum feature.eeference frames can be powerful tools.

With tolerances for feature size, it is best to apply the math before signoff of the design. MMC is typical for most parts, sometimes in rare needs is LMC and in between the two is RFS. With RFS, though, one is stating that no matter the size of the feature, the result is between the value cited and zero. LMC & MMC award bonus tolerance, hence do the math! On a side note: boundary shifts LMB & MMB apply to how the datum of the boundary is to be treated and in some designer's view may award further bonus tolerance.

An important aspect to consider besides the definition of the tolerances on the design stage is that the manufacturing process it will be subjected to statistical evaluation, therefore the tolerances must go hand in hand with the capacity of the control means to ensure that the process is not subjected to over-demand and/or the appropriate control means is chosen. For example, a CMM would not be feasible to control a perpendicularity with a tolerance of 0.3 μm constructed from a median plane of two contact zones with a length of just 5 mm (since the aspect of stability for an R&R). In this case, an option would be to invest in a precision fixture to subject the part to assembly purposes and validate its magnitude of perpendicularity.

Evite adicionar modificadores aos elementos de rosca, a menos que você disponha de métodos de medição altamente eficientes e precisos para medir isso.

Sometimes tight tolerances can be avoided by making parts adjustable. Slotted holes, eccentric bushings, and other design features that allow in field alignment can be cost effective alternatives to tight tolerances.

GD&T does not mean tight tolerance. The CP for a process or equipment is the only control available. When using any tolerance CP will drive tolerancing and design for manufacturing. Improper use of clearance (slots) is your enemy. GD&T is used to remove ambiguity from verification. Measurements are not controls.

Proper and appropriate datums are super important. We see a lot of constructions on drawings that don't make sense and then it takes a lot of time and effort to go back and forward with the client until we come to a proper datum system that is stable and linked to the function of the part. We also see more and more very tight tolerances of 0,02mm and less on plastic parts for example. With the right mould iterations and inspection process this can be achieved, but those small numbers are indeed costly and might not always be needed.

Best thing to do when reviewing for tolerance versus cost to adjust the design is to consult with those individuals such as the CMM programmer, the inspection technicians, and machine setup technicians who eventually have to prove said feature on design is in specification in their hourly, initial, or shipment ready reports. For accuracy of GD&T tolerance, it is advisable to have references to either existing parts / actual data do verify limits of design acceptability and redo the math on features changed before signoff. One doesn't want to allow for a position tolerance on a washer-like part to be offset by the entire wall thickness.

@Kurt Hallquist - I understand your perspective. The statement about 'tighter tolerances' can be interpreted in different ways, and the choice between tighter or looser tolerances is indeed a complex consideration. I fully agree that the application of GD&T aims to obtain tolerances that align with the desired quality. It is crucial to adopt tolerances that meet the functional and quality requirements of the product. However, this is where the balance between the desired accuracy and associated costs comes into play. Tighter tolerances often result in greater precision and quality but can increase production and inspection costs and, consequently, the final product price.

"You need to explain how your GD&T meets the design requirements, the performance criteria, and the customer expectations." Are we still adressing working with the ignorant and GD&T illiterate? Shouldn't this step have already been accomplished via the tolerance analysis? I would be interested in the authors idea of this 'explanation'.

Absolutely, I completely agree. In a project environment where GD&T plays a crucial role, it is essential that everyone involved has at least a minimum knowledge of GD&T. And do we have this in companies? This is rhetoric.

This step is one the engineers lacks the most. For me it seems that most of the drawings applies a standard procedure to apply DATUM features position and tolerance in spite of specific study of part function and environment assembly... and maybe this way of doing things has something to be with cost-effectiveness of design process. Particularly, I don't like some traditions that tend to forget that manufactured parts are never perfect as theory and practice are different worlds. E.G.: Situations where different flat surfaces of a part share the same A Datum (with an offset) that needs a perfect manufacture of all the surfaces in between to ensure a good assembly, clearly needing a thightener tolerance than the drawing shows.

It is important after the industrialization project for the part, don't be closed to improvements suggestions, because is different define the control of the part on a simulation (commonly named desk job), unlike to submit these control to the real environment of manufacturing.

GD&T is a language. If authored properly it can be an encoded language enabling unambiguous decoding. GD&T is most valuable when properly applied to an entire product. It doesn't have to be peppered everywhere. Critical features need to be mapped through coordinated Datum schemes a from part through to product level. Some call this the optimized Datum Flow Chain. When that is done, and when proper analysis is performed, a product is robust for variation control. 2 major benefits: 1. Common cause variation won't affect product assembly or quality, and 2. When special cause variation does occur, the root cause may be quickly found using the well mapped Datum Scheme and a good 3D Variation Simulation software.

GD&T should describe the function and largest possible variance still providing full function. Unfortunately it’s usually rather „if I use super tiny tolerances at least it won’t be my fault if it breaks…“