CNC Machining Tolerances: Everything You Need to Know

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:

Introduction to CNC Machining Tolerances

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:

• Bilateral tolerance

  : Permits variation above and below basic size and has equal or unequal amounts of variance. The upper variance is expressed with a + symbol, lower variance is expressed with a - symbol.

• Unilateral tolerance

  : Permits variation above or below basic size and does not permit variation in both (the size may only deviate in one direction). The upper variance is expressed with a + symbol, lower variance is expressed with a - symbol.

• Limit tolerance

  : Does not use a + or - symbol, shows upper and lower limits of dimension. Anything between these values is acceptable.
  

They can also be expressed by a number of decimal places. The more decimal places, the tighter the tolerance.

• One decimal place, written as .x (example:  ±0.1")
• Two decimal places, written as .0x (example: ±0.02")
• Three decimal places, written as .00x (example: ±0.006")
• Four decimal places, written as .000x (example: ±0.0004")
  

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:

• Basic size

  : The diameter of the bolt, or shaft, and the hole

• Upper deviation

  : The difference between the part’s maximum possible size and basic size

• Lower deviation

  : The difference between the part’s minimum possible size and basic size

• Total tolerance

  : The value that describes the maximum amount of variation

• International tolerance grade

  : The maximum size difference between the component and the basic size

• Fundamental deviation

  : The minimum size difference between the component and the basic size

• Maximum material condition (MMC)

  : Contains the most material within tolerance, part is heaviest at MMC

• Least material condition (LMC)

  : Contains the least material within tolerance, part is lightest at LMC

• Allowance

  : The allowance between mating parts is the minimum amount of clearance and maximum amount of interference

• Datum

  : Some tolerances reference a specific datum or datums, or an exact plane, line, axis or point location that GD&T or dimensional tolerances are referenced to
  

What Are Standard Machining 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:

• Material

  : No two materials are exactly alike, and some are easier to work with than others. It’s important to consider the heat stability, hardness and rigidity and abrasiveness of the material in order to determine tolerances.

• Method of machining

  : The type of machining used can greatly impact the end product, as some processes are more exact than others.

• Plating and finishes

  : Plating and finishing add small amounts of material to the surface of a part, which can alter the dimensions of the part just enough to require a different tolerance.

• Cost

  : The tighter the tolerance, the more costly the process. In order to remain cost-efficient, it’s important to ensure your tolerance is precise, but not tighter than necessary. 

Limitations of Tolerancing Before GD&T

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.

A Closer Look at Typical Machining Tolerances

In general, there are five types of tolerances specified in GD&T:

• Form tolerances

  : A basic geometric tolerance that determines the form of the part

• Profile tolerances

  : Sets a boundary around a surface within which the elements of the surface must lie

• Orientation tolerances

  : Determines the orientation for the form in relation to a reference

• Location tolerances

  : Indicates the location of the feature in relation to a reference

• Runout

  : Specifies the run-out fluctuation of a target’s feature when the part is rotated on an axis
  

CNC Machining Tolerance Chart

The following symbols are used for specifying geometrical characteristics on engineering drawings. This geometric tolerancing chart is based on the ASME Y14.5:

Why It’s Important to Understand Machining Tolerances

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.1

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.10

Train to Become a CNC Machinist

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.

NASCAR Technical Institute Campuses That Offer CNC Training