GD&T in Metal Fabrication:  Why Geometric Dimensioning & Tolerancing Comes at a Cost

Geometric Dimensioning & Tolerancing

Could engineers unknowingly drive up metal fabrication project costs by 50% or more simply by overutilization of geometric dimensioning and tolerancing (GD&T)? It’s entirely plausible.

Engineers in the precision metal fabrication industry are well versed in GD&T. It is a complex system that allows them to define geometric specifications for parts with the intent to control variances and achieve extreme precision.

GD&T standards work in parallel with the American Society of Mechanical Engineers (ASME Y14.5-2018) and ISO 1101-2017, using the same library of symbols and values. As such, it can be a helpful tool, ensuring that everyone uses the same standardized language and definitions. GD&T helps bring clarity surrounding complex design specifications.

Technically, GD&T helps to determine how much a feature on a part can deviate from the listed geometries, potentially opening up tolerances. However, it is increasingly used to restrict tolerances when not necessary to sometimes unreasonable degrees, significantly driving up costs and making some parts more complex to manufacture than they really need to be.

There is definitely a place for GD&T in certain high tolerance manufacturing applications, but it’s important to consider the potential risks and significant cost implications associated with overengineering mistakes and applying GD&T to every component of an assembly whether it needs it or not.

Appropriate Use of GD&T in Metal Fabrication

To some degree, the extent to which GD&T is required often depends on the critical nature of a component and its relation to other parts in an assembly. 

One of the most beneficial uses of GD&T standards is when there are interchangeable parts that require a very precise assembly process. For example, a part may have an eight-hole pattern that’s designed to have another component — also with eight holes — bolted to it. Each piece needs to have the holes be the same dimension. Each piece must also have the holes located in the proper true position to ensure they fit together correctly. The relationship between each component’s hole dimensions and position calls for GD&T.

Another example might be a cylinder that needs to fit inside a chamber. It might require GD&T tolerancing in the form of straightness control to ensure that it isn’t warped. Any variance along the length of the cylinder might cause it to pinch, bind, or leave too large of a gap. 

Unnecessary Use of GD&T in Metal Fabrication

The aforementioned eight-hole pattern calls for GD&T. However, consider a 2-inch access hole for wiring or cables that has no relationship to another component. It likely will not need a strict GD&T tolerance to signify precisely 2.000 inches with a 0.014-inch tolerance.

Profiles are another area where GD&T tolerancing is typically not required, such as the side of a cabinet or enclosure. Does the profile need to be precisely 0.500 inches thick across its entire span or will a slight variance that is undetectable to the eye be acceptable

Why does it matter? Additional steps are required to ensure those tolerances. A hole that must be precisely 2.000 inches will require machining vs. lasering. Likewise, a plate of stainless steel or aluminum that needs to be precisely 0.500 inches thick across its entire span will require that a 0.625-inch thick plate of material be machined down to that tolerance.

Cost Implications of Unnecessary GD&T

When used for its intended purpose, GD&T can save money because it reduces errors and rework for parts with complex configurations and connections. On the other hand, when GD&T tolerances are specified for parts that don’t require such strict tolerances, it can cost much more

In our experience at Fox Valley Metal-Tech, we see the overutilization of GD&T in elements where it is not necessary more often than not, especially in defense projects. Tight tolerances might be included “just to be sure,” or it may simply be easier to apply GD&T to an entire project rather than pick and choose components. Sometimes engineers are trained to apply GD&T to everything. However, it’s important to know that unnecessary use of GD&T in some instances can increase manufacturing costs by 50% or more.  

That figure sounds startling, but take into consideration one of the examples mentioned above. Machining a thicker metal plate down to exactly a half inch requires significantly more labor and time. A laser system can cut at 140-inches per minute whereas a machining center operates at only 20-inches per minute to achieve extremely tight tolerances. You’ll not only pay more for labor, you’ll also end up paying for more material than necessary, only to machine it down and have the rest go to scrap. With the costs of steel and aluminum lately, that can add up quickly.

It’s not just production that’s impacted. More time is required ahead of time in the form of operations and setup, plus the inspection process becomes more lengthy on the back end.

How to Address Excessive Use of GD&T

Once an engineering team’s drawings and supporting documentation are created, they’re often sent on to a procurement team to acquire bids. Not all purchasing agents are aware when excessive GD&T tolerances are included in a package nor have an appreciation for the additional costs they’ll incur.

In an ideal scenario, the metal fabrication company’s engineering and estimating team will identify potential GD&T issues during the quoting process and request a technical discussion with the buyer and their engineering team where they’ll help identify potential cost savings. A 3D model file of the fabrication is helpful to determine what will populate the inside of a component and what is mounted to surfaces, showing where GD&T tolerances are absolutely necessary.

Once opportunities are identified, the metal fabrication team can work with the customer’s engineers to modify datums where possible and provide an accurate quote, all while ensuring the customer still gets what they need.

When considering which metal fabrication company to use for your next project, also consider their value-added services. It’s these types of conversations and insights that make for a true partnership. Contact the experts at Fox Valley Metal-Tech today to see whether an engineering and/or DfM review is in order, and download our complimentary RFQ guide below with additional considerations.

Metal Fabrication RFQ Guide