Deviations and Non-Conformance Reports (NCRs) are wasteful and expensive.
You lose whether the part you made is red-tagged and rejected for being out of spec, or you sourced parts with values that aren’t within specified tolerances. Even if you can get an ECR to permanently change a print, held shipments still cost money and lost productivity.
We’ve had clients receive non-conformance reports for a variety of reasons. Maybe the tolerance is too tight, or inspection is measuring things improperly. Perhaps manufacturing just cannot create a stable part with the existing design.
These issues create friction internally and externally -- whether your suppliers don’t deliver the quality you need, your internal departments don’t get the root cause of a problem, or they simply don’t understand the goal of the drawings.
Those deviations can be catastrophic. Imagine you have an overseas supplier that isn’t inspecting its parts correctly, so the assembly bolts don’t fit, and the entire production line has to shut down. Best case scenario? It only costs you time and money. Worst case? Your lowered performance costs you your reputation, your customers, and eventually, your business.
Better communication and clearer drawings between you and your customers and suppliers are how you get it right the first time and avoid deviations entirely. Implementing best GD&T practices lets you spend your time refining processes and products instead of fighting fires.
In this article, we’re going to take a closer look at the elements that contribute to the creation of deviations and how you can fix (or better yet, prevent) the problems that flow from OT parts.
Communication between Design, Production and Inspection
When design communication is good, you get a clear, functional design that makes it easy to produce a part that makes your customers happy. More specifically, good design communication ensures that production at the limits of the tolerances on the print still results in parts that fit and function as they should.
When design communication is bad, you wind up with unclear drawings and parts not designed for manufacturability, inspection, or assembly. Then deviations occur because the tolerances are too tight to manufacture, or the import of certain tolerances wasn’t made clear to inspectors, or the stacked tolerances of multiple parts weren’t accounted for, and they don’t fit together.
When manufacturing is good, you’re producing parts efficiently and meeting drawing requirements -- and your machinists know how to do sanity checks to ensure that the production process is working as it should.
When manufacturing goes wrong, you get deviations because dimensions in the drawing are out of tolerance or the production team misunderstood the requirements. These misunderstandings are correctable with the use of Geometric Dimensioning and Tolerancing, which is a language that simplifies the manufacturing process by facilitating accurate and efficient communication.
When inspections are good, they’re reliable, repeatable, and mindful of the proper functional intent of the design -- they reject poorly functioning parts and accept parts that meet requirements. Not only that, the inspectors are able to defend their decisions because no room is left for interpretation, and they’re operating efficiently, inspecting only what’s important and needed to maintain standards. Generally, good inspections result from utilizing good design communication and functional inspection planning to create effective processes to catch problematic issues.
When inspections go wrong, there’s a misunderstanding regarding the design’s functional intent, or parts are measured incorrectly. The former flows from poor design communication, and the latter results from inspection not following a clear, repeatable method for inspection. And the absence of functional inspection planning results in either over-inspection (which slows down production and costs money) or under inspection, which results in the production of bad parts.
Ideal Situation - Manufacturing and Shipping Good Parts
What does it look like when you’re firing on all cylinders and design, manufacturing and inspection are all on the same page? It means your design is clear and functional, manufacturing can hold the tolerances set forth in the drawings, and inspection is accurate and efficient.
You’re happy. Your customers are happy. Your costs, time for production, and quality of products are all acceptable and approach optimal levels.
That’s the ideal. But we don’t live in a utopia, do we? The real world is complex and ever-changing. Even if you’ve hit manufacturing nirvana at some point, it’s impossible to stay there forever. So, let’s take a closer look at the problems that come with operating in the real world.
Production Issue 1: Part is OT and Won’t Function
This is a situation where bad parts are being rejected. It’s among the most common issues our clients face and is a natural part of a functional manufacturing process. It’s bad news for the manufacturing team, who either cannot meet the capability needed to hold the tolerance or misinterpreted the drawing. But it’s good news for your other departments -- it means that the design was clear and functional, and inspection is accurate and reliable.
It’s also bad for the bottom line, as the resulting parts are scrap, which means a financial loss for you.
Educating your manufacturing team on GD&T so that they can read drawings correctly and understand parts’ functional requirements is how you ensure they produce parts to spec.
Ignorance of GD&T not only results in OT parts, but it can also cost you business opportunities. Some machinists see the position symbol and incorrectly assume that tolerances are too tight for them to work with. Other shops will mark up the price for GD&T prints because they assume the parts will be more expensive to produce.
Machinists who truly understand GD&T know that the number after the symbol is what matters, not the symbol itself. That ensures that they can communicate back to design about problems and manufacturing limitations before production starts, saving time and money for everyone involved. Remember, a good machinist can look at a well-designed print and understand the design intent to anticipate (and prevent) issues.
Production Issue 2: Part is OT, but Functions / Part is Falsely Assumed OT and Functions
These two situations have different causes, but the same result: inspection rejects parts that should be accepted. In a reversal of the previous production issue, this situation results from a problem in design and/or inspection, while manufacturing is doing its job well.
When an OT part functions, the tolerances are too tight and the part isn’t designed for manufacturing. This occurs because design is making tight tolerances everywhere, instead of just in the places where they’re functionally needed. The bad design wastes time and money on needless deviations.
When a part is falsely flagged as OT, it’s because inspection has misinterpreted a measurement, isn’t measuring accurately, or lacks understanding of the design intent. This situation sends manufacturing on a wild goose chase trying to fix something that’s not broken, which again wastes time and money.
Designers must ensure that the part’s function is communicated properly to manufacturing and inspection, and open up tolerances to their functional limits. The design department must also communicate with manufacturing to understand and integrate existing production capability limits, and understand the tools available to make cheaper parts easier to manufacture. Finally, designers should understand the standards and ways that the part will be inspected to keep from misleading inspectors.
Inspection must understand how to read design drawings, measure accurately, and use the proper inspection methods for the callouts. Additionally, they must implement correct inspection plans and procedures that are aligned with design to ensure they’re measuring according to design intent.
Production Issue 3: Part Meets Design Tolerances, but Will Fail in the Field / Inspection Misses OT Parts
These last two problems are the worst-case scenario that you want to avoid at all costs: accepting bad parts that should have been rejected. When it happens, It’s like a time bomb set to eventually explode on your customers -- souring your relationship with them at best, or shutting down your company due to lawsuits and recalls at worst. The cause can be design, manufacturing or inspection, or any combination of the three.
When a part meets the design requirements, but fails in the field, it means that design has incorrectly designed the part. This may be a failure of R&D to perform a correct finite element analysis on the part, or it may be that the part wasn’t designed for assembly and the tolerance calculations were inaccurate. In either case, this results in manufacturing unwittingly making bad parts, and it can’t correct the issues until there’s a dispute (which may not ever happen).
When a part is OT, but passes inspection, it means that inspection has misinterpreted its results, is not obtaining precise measurements, is using inaccurate methods, or doesn’t have a clear understanding the design intent. As with a bad design, inspection failures result in manufacturing producing bad parts. Either way, the end result is not only wasted time and money creating scrap, but also the creation of massive legal liability.
R&D and design must be functioning properly and making accurate calculations. Additionally, designers must understand assembly tolerances and the standards and ways that parts will be inspected. For its part, inspection must understand how to read drawings and align with design when communicating with drawings. Additionally, inspection must create quality inspection plans and procedures and ensure accurate measurements are taken to catch problems.
The most powerful and effective way to deal with the problems posed by the realities of production is accurate communication throughout the production process. GD&T is the tool to ensure that your communication is as good as it can be. The language of GD&T provides the specificity needed to ensure that design, manufacturing and inspection are in alignment. When everyone -- from design engineers to machinists on the floor -- can communicate using GD&T, it becomes much easier to not only identify and solve problems when they occur, but to prevent them from ever happening in the first place.
GD&T Basics training courses will teach you the vocabulary of GD&T and how to use it in the real world to make your team better. Contact us today if you want to discuss how GD&T can help you or if you’re ready to move forward with training!
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