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:

One key part of effective tolerancing is accounting for tolerance stack-up. By performing tolerance stack analysis, you can make sure a component’s tolerances are mathematically correct, physically possible, and truly beneficial to part production and performance. This article will give a broad overview of what tolerance stacking is, why it’s important, and how to utilize it in your part designs.

What is tolerance stacking or stack-up?

Tolerance stack-up is the process of adding tolerances together before manufacturing in order to understand their cumulative effect on part production. Final results from a tolerance stack are compared to tolerancing standards, regulations, and other limits in order to ensure the part design will produce high-quality components. This tells you the total amount a part can differ from specified dimensions.

Proper tolerance stack analysis also enables you to predict how your final component will look, function, and interact with other components — this is particularly important when it comes to manufacturing mating parts.

Accounting for tolerance stack-up helps ensure tolerances can be manufactured before manufacturing even begins. This prevents you from having to go back to the design process after moving to the prototyping or production stages, which can save time, money, and resources. Calculating tolerance stack-up can also save you money by helping you understand your tolerances in context, so you can optimize for cost and manufacturability.

There are two main kinds of tolerance stack analysis:

Worst-case tolerance analysis

This involves adding up all of the individual tolerances of a part or assembly to find the total sum. When performing worst-case tolerance analysis, you should set each tolerance to either the largest or smallest value in its range. Both the upper and lower limits should be evaluated to provide a full picture of the allowable tolerance range. Then, compare the total tolerance to the part’s performance limits in order to ensure proper design. Worst-case tolerance analysis should be used when mating parts are absolutely critical and there is little chance for rework or design modification once production has started.

Statistical tolerance analysis

This combines all the probabilities of the different dimensions — meaning the likelihood of each dimension being above or below its ideal value by a given amount— to determine the part’s chances of failure or success. There are a number of different statistical tolerance analysis methods, such as the Monte Carlo method and root sum square (RSS). Statistical tolerance analysis is useful for high-volume production where a small percentage of scrap is acceptable, as long as the majority of parts fall within the allowable tolerance range. This allows for larger tolerances upfront that make manufacturing easier and lower costs.

Regardless of which method you use to find your part’s tolerance stack-up, having an accurate understanding of your tolerance stack-up will help you create manufacturable products.

Tolerance stacking best practices

Here are some best practices for product teams to keep in mind during the design process, which can help ensure they’re properly accounting for tolerance stack-up.

Avoid over-dimensioning your part

When each part feature is labeled with upper and lower tolerances, a design drawing can become overcrowded and unclear. Not only does this cause confusion and make your part design harder to understand, but conflicting dimensions can also bring errors into your tolerance stack analysis. One way to counteract over-dimensioning is to only explicitly define tolerances for part aspects that truly need them. Undimensioned features would then be controlled by a general tolerance that is applied to the entire part unless otherwise specified.

Evaluate your tolerance stack’s sensitivity

Make sure you understand the consequences of tolerance stacking before you calculate your stack-up. Will it be a total disaster if tolerance design conditions aren’t met or will the part still be able to function properly? By contextualizing your final tolerance stack-up with manufacturing and performance, you can understand how large or small your tolerance stack can be without compromising success. Keep in mind that tighter tolerances will require more expensive manufacturing methods, so it’s important to balance cost considerations with tolerance sensitivity.

Consider post-manufacturing changes

Post-assembly changes, such as deflection and normal wear-and-tear, can affect the precision of a part after it’s been produced. Since this information impacts the creation of tolerances, it’s important to keep any post-production changes in mind as you determine your tolerance stack.

Follow general tolerance best practices

When it comes to tolerance stack-up, all the usual GD&T standards still apply. This includes being conscious about how an individual part feature will interact with other component elements, making sure a part is machinable and within reasonable limits of manufacturing capabilities, and keeping track of important part characteristics like material selection. Most importantly, you should make sure your tolerance stack-up is allowable and within any relevant GD&T requirements for a specific component.