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CNC machining requires great precision. In this industry, being off by just millimeters can lead to critical errors. Unfortunately, however, no machine is 100% accurate, 100% of the time.
From the material of the part to the machining process used, there are many different factors that can cause variances. This is why machining tolerances are assigned to parts in the design process—an amount of acceptable variance in the dimension of a part.
So what exactly are machining tolerances, and why are they important? Keep reading to learn all about how this concept applies to the career of a CNC machinist:
Machining tolerance, which is commonly referred to as dimensional accuracy, is the amount of permitted variance in the dimension of a part. This involves setting a maximum and minimum dimensional limit for the part.
Essentially, this process defines how wide the tolerance can be while staying within the necessary range to create a part that meets the required specifications. If a part is manufactured with dimensions that are out of tolerance, it is considered unusable for its desired purpose.
The range a dimension can vary is referred to as the ‘tolerance band.’ The larger the allowed difference between the upper and lower limits, the looser the tolerance band. The smaller the difference is, the tighter the tolerance band.
Tolerances are expressed a few different ways, including the upper and lower limits, the permitted amount below and above a certain dimension, and the allowable variance by itself. Three basic tolerances that commonly occur on working drawings include:
They can also be expressed by a number of decimal places. The more decimal places, the tighter the tolerance.
When preparing a design, setting the appropriate tolerances is essential, as this ensures the part will be created within the required specifications. However, this process can be difficult and requires an in-depth understanding of machining tolerances and how they apply to different materials and types of machinery.
The following terms are often used when applying tolerances:
As mentioned, different materials and machining processes require different tolerances. This means there aren’t exactly ‘standard’ machining tolerances. However, some manufacturers have set guidelines they follow for particular applications.
Some machine shops will require customers to provide tolerances, and if they are not provided, they will either refuse to work on the part or will apply a standard tolerance of, for example, ±0.005". This indicates that the diameter of the part may be 0.005" smaller or 0.005" bigger than the specified diameter.
When determining tolerances, there are several factors that are important to consider:
Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances. Essentially, it tells the manufacturing staff and machines what degree of accuracy and precision is needed on each controlled feature of the part.
GD&T uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describe nominal geometry and its allowable variation. Before GD&T, X-Y areas were used to specify manufacturing features. For instance, if you were drilling a mounting hole, you would need to ensure the hole was within a specified X-Y area.
However, an accurate tolerancing specification would define the position of the hole and how it relates to the intended position—the accepted area being a circle. X-Y tolerancing leaves a zone where inspection would produce a false negative. While the hole is not within the X-Y square, it would still fall within the circumscribed circle.
Stanley Parker, an engineer who was developing naval weapons during World War II, noticed this failure in 1940. Driven by the need for cost-effective manufacturing and meeting deadlines, he worked out a new system through several publications. Once proven as a better operational method, the new system became a military standard in the 1950s.
Currently, the GD&T standard is defined by the American Society of Mechanical Engineers (ASME Y14.5-2018) for the USA and ISO 1101-2017 for the rest of the world.
In general, there are five types of tolerances specified in GD&T:
The following symbols are used for specifying geometrical characteristics on engineering drawings. This geometric tolerancing chart is based on the ASME Y14.5:
Proper application of GD&T will ensure that the part defined on the drawing has the desired form, fit (within limits) and function with the largest possible tolerances. GD&T can add quality and reduce cost at the same time through producibility.
The correct interpretation of machining tolerances takes practice. It often requires the completion of a CNC training program, such as the one offered at NASCAR Technical Institute, to learn.
Created in conjunction with Roush Yates, a leading brand in the performance industry, this 36-week program covers everything from reading blueprints to interpreting geometric dimensioning and tolerancing. Your training will combine online and classroom instruction with hands-on application in order to prepare you for a career in the field.
UTI’s CNC program begins every six weeks, giving you an opportunity to get going and start training for your career sooner. Additionally, UTI offers housing assistance for students who need to relocate to complete their program, and scholarships and grants to help lower education costs.
Does a career in the CNC industry sound like the right fit for you? With UTI’s CNC Machining Technology program, you can train to become a skilled machinist in less than a year. To learn more, visit our CNC program page and request information to get in touch with an Admissions Representative today.
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