CNC Machining Tolerances: Common Types and Factors to Consider

CNC machining tolerance
Jack Lie CNC machining expert

Specialize in CNC Milling, CNC Turning, 3D Printing, Urethane Casting, and Sheet Metal Fabrication Services.

The degrees of accuracy that may be achieved with CNC machines are exceedingly high. Certain machines have the capability of achieving accuracies of up to +/-0.0025mm. Milling a component on a CNC machine at the highest accuracy requires a significant investment of money and time. Still, it also produces the finest quality results. This factor determines tolerances in machining. Because various components need varying levels of accuracy, it is in the best interest of an organization for the designer to identify the particular tolerance required for each component.

What Are Tolerances?

To put it more simply, tolerances are measures that indicate the amount of accuracy that is required for an item that you want to fabricate. To be more specific, machining tolerances denote the amount of allowable fluctuation in an item’s final dimensions or predicted values.

Machinists measure the tolerances using numerical numbers, frequently accompanied by a sign that looks like ±. For a component whose length is 2.550 inches, for instance, you may decide to apply a tolerance of ±0.001 inches to it. This would imply that the finished product might have a different length ranging between 2.549 inches and 2.551 inches. Suppose a component with a height of 1.5 inches has a tolerance of ±0.005 inches. In that case, the finished component must fall between 1.495 inches to 1.505 inches to qualify for quality inspection.

When creating a product, manufacturers and CNC machining services may determine the appropriate level of accuracy to utilize based on the specified machining tolerances. More accuracy is needed if there is less of a margin for error, which is referred to as a tighter tolerance in the world of manufacturing. The greater the tolerance, which may also be referred to as looser, the less accuracy is required.

Common Types of Tolerances in CNC Machining

When it comes to determining product measures, engineering tolerances are a necessity that absolutely must be met. The components will usually be manufactured following a general tolerance grade unless the client provides specific tolerance requirements.

Before turning it over to CNC machining, the designer has to first establish the component tolerances for the necessary characteristics. This assures customer satisfaction and lowers the expenses associated with retooling at the CNC machine shop.

When we talk about appropriate features, we are referring to the fact that setting tolerance for each dimension is unnecessary. This has the contrary impact that was intended since adding specifications to each metric would result in a very expensive component.

Typically, we only specify it for those elements of a component that are necessary for it to pair with other parts. When we accomplish this, we confirm that the components will perform as intended and fit together in the assembly. The following is an overview of the numerous sorts of tolerances that may be applied to machined parts:

General Tolerances

Linear dimension range (mm)f (fine)m (medium)C (coarse)V (very coarse )
0.5 up to 3+0.05+0.1+0.2
over 3 up to 6+0.05+0.1+O.3+0.5
over 6 up to30+0.1+0.2+0.5+1.0
over 30 up to 120+0.15+0.3+0.8+1.5
over 120 up to 400+0.2+0.5+1.2+2.5
over 400 up to 1000+0.3+0.8+2.0+4.0
over 1000 up to 2000+0.5+1.2+3.0+6.0
over 2000 up to 4000+2.0+4.0+8.0

Linear and angular metrics, chamfers, and other rounded features may all be described using general tolerances. These tolerances define the standard tolerances for four distinct classes, each based on the range that a component dimension may take.

These classifications are laid out in a chart that divides the various tolerance levels into very coarse, coarse, medium, and fine.

The International Standards provide the range for each specified tolerance for a given dimension bracket. Those International Standards are EN 20286, ISO 286, JIS B 0401, ISO 1829, ANSI B4.1, ISO 2768, and ANSI B4.2.

Limit Tolerances

We represent the limit tolerance as the smallest and largest allowable values for a given dimension. Regarding the assembly, the appropriate dimension of the created components should fall between these two positions.

For instance, if the limit tolerance for a measurement is specified as 12…12.5 millimeters, then the final result must fall somewhere in the middle of these two limitations.

Unilateral Tolerances

In the practice of unilateral tolerance, the range of acceptable values is defined in just one direction. In another sense, we only permit variation on one side of the value that is considered nominal.

Take, for example, a shaft with a diameter of 70 millimeters that has to be inserted into a hole of identical size. Suppose the diameter of the shaft is even slightly larger than 70 millimeters. In that case, it will not be possible to insert it into the hole.

As a result, we cannot use the shafts that were made with a diameter of more than 70 millimeters. This would result in greater waste and lengthen the turnaround time.

Unilateral tolerances are applied to these components to avoid this problem. A tolerance range of 70 +0.00/- 0.05 mm has been agreed upon for this shaft. The largest and smallest diameters of the allowed shafts fall within this range at 70.00 and 69.95 millimeters, respectively.

One of the many benefits of using unilateral tolerance is the simplicity with which we can check it. We can standardize a go gauge since the dimensions change only on one side, with the top value remaining the same. This will allow us to examine more quickly and with less effort.

Bilateral Tolerances

Tolerance on both the left and right of the nominal value or actual profile is known as “bilateral.” If the nominal value is 30 millimeters, a tolerance of +/- 0.05 millimeters on both sides would indicate a range of 29.95 millimeters to 30.05 millimeters.

Geometric Dimensioning & Tolerancing (GD&T)

Geometric Dimensioning & Tolerancing (GD&T)

The standard of dimension tolerances may be improved upon by using geometric dimensioning and tolerancing (GD&T). In addition to guaranteeing that the final value stays within specified limits, it also specifies the dimensional properties of concentricity, smoothness, and true position.

The final measurement must conform to these standards to be accepted. Although GD&T is not as common as other tolerances, it is crucial in CNC manufacturing. GD&T provides options for ensuring dimensional accuracy for several characteristics, which is especially useful for these parts due to their typically stringent requirements.

Factors That Impact Machining Tolerances

When establishing tolerances, there are a lot of different elements that need to be taken into account. The following are some examples of these:


When determining the machining tolerances for a project, it is necessary to consider the materials used. The amount of tolerance that may be achieved with various materials is contingent upon various factors, including the properties of the materials themselves. Some examples of these aspects are as follows:

  • Hardness: Machining softer materials to tighter tolerances might be challenging due to the increased difficulty in doing so. This is mostly because their dimensions may alter when the cutting tool comes into contact with them. You will need to exercise more patience when working with materials of this softer consistency.
  • Abrasiveness: Materials that are rough and gritty are often difficult on the cutting tools, and they might even cause the equipment to wear out more quickly. Because of these materials’ nature, obtaining a certain tolerance is challenging since any modifications to the cutting tool may result in decreased precision. When working with abrasive materials, it is common practice for the technician to switch out the tool many times during the course of the machining operation.
  • Heat Stability: Most of the materials that are not metals are impacted by this problem. During the process of machining, as the temperature rises, these materials begin to get deformed and lose their original shape. Because of this, the techniques that may be used on that specific material are limited.

Machining Type

Because certain machining methods are more exact than others, the possible tolerances for the completed product will be considerably impacted by the method used for the machining. 

Plating and Finishes: When calculating the dimensions and tolerances of a component, it is important to consider any plating or finishing procedures that will be performed on the item. Plating and finishing both add very tiny amounts of material to the exterior of a component; nonetheless, these additions still cause a change in the overall dimensions of the product. Thus they need to be taken into consideration before manufacturing begins.


Tolerances should be exact, but they should never be more stringent than is absolutely required. This is because achieving tighter tolerances comes at a higher financial cost. Suppose your component can function properly with a tolerance of three decimal places. In that case, there is no need to increase it to four decimal places.


The inspection process becomes more challenging and time-consuming as the tolerances get narrower. They need more accurate measuring instruments and more thorough inspection procedures. This causes a rise in the component’s total production costs.

When these considerations are taken into account, together with the appropriate application of tolerances, engineers can have confidence that their designs will result in finished products that are of the appropriate size and shape.

How Can You Find the Right Tolerance?

Inspection with a CMM

There are situations when choosing the tolerance level is not the challenging part. It’s about being sure that you’ve chosen the best option. It is essential for the people who design products or parts to calculate the amount of wiggle room that is allowed for the tolerances of a certain item. It may have a significant bearing on determining the appropriate tolerance, the amount of time needed to complete the project, and the associated cost.

When requesting the “best quality possible,” a client dealing with a CNC machining service runs the risk of unknowingly spending twice as much as necessary. Because CNC milling is inherently a process that requires a high level of accuracy, even with looser tolerances, the end result is often still very near to what was initially specified. Some tips are provided below to help you make a decision:

Consider the purpose of your part

Not all parts are required to be manufactured with tight tolerances. The specific purpose of your part often dictates the level of precision required for machining. For example, parts that do not combine with others don’t need high accuracy. Therefore, it’s unnecessary to spend much expense on tight tolerance when your parts actually don’t need it.

Find a reliable CNC machining partner

A reliable and dependable CNC machining manufacturer means almost everything for tight tolerance. Whether you are an engineer, a product developer, or a part designer, you are supposed to specify your desired tolerances, goals, and specifications with the experts before you send your manufacturing request online to a CNC machining or rapid prototyping company. This will help you save costs and time.

Also, It’s of great significance to remember that if you don’t ask for or specify any tolerances when submitting your part for production, most CNC machining services will automatically use their standard tolerances, usually around ±0.005 inches (±0.127mm). This is a very small deviation imperceptible to the naked eye, but it can affect how your final part fits in the assembly.


No business can afford to take dimensional tolerances for granted. If we have a good set of machining tolerances, it will help boost process efficiency and reduce expenses while leading to the success of the overall project. Runsom Precision can assist you if you are searching for a company with experience working within tolerances and can assist you in completing your project successfully.

An operator at Runsom with inspection equipment
An operator at Runsom with inspection equipment

We host full sets of advanced CNC machining equipment including 3-, 4-, and 5-axis CNC machining machines. Besides comprehensive CNC machining services, such as CNC milling, CNC turning, and Swiss CNC machining services, we also provide all kinds of surface treatments to enhance the properties of CNC machined parts. And we have a strict manufacturing quality control system in all manufacturing processes to ensure all these measures can be implemented in place.

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