Basic Guide To CNC Machining Tolerance

CNC machining refers to the use of computer programming and electromechanical equipment to automatically process metal parts (and non-metallic) as required. CNC machine tools perform all operations on the workpiece according to the program and provide us with the final product.

Although CNC machining service is very accurate in product size, it is not perfect. From the part material to the machining process used, various factors may lead to differences. Therefore, engineers assign machining tolerances to parts during the design process.

What is Machining Tolerance?

Machining tolerance, also known as dimensional accuracy, is the acceptable deviation of part size.

This is expressed as the maximum and minimum size limit of the part. If the size of the part falls between these limits, the part is considered to be within the tolerance range. However, if the size of parts exceeds these limits, these parts exceed the acceptable tolerance range and are considered unusable.

The machining tolerance usually starts with a ± symbol. For example, suppose a 2.0 “(50.8mm) high part requires a tolerance range of ± 0.005” (0.127mm). The variable height of the final part should be between 2.005 “(50.927mm) and 1.905” (48.387mm) to pass the quality inspection.

The tolerance can also be expressed in any decimal place. The more decimal places included, the tighter the tolerance. These different types of tolerances are expressed as follows:

Retain one decimal place, expressed as (. x), (for example, ± 0.2 ″)

Two decimal places, expressed as (. 0x), (for example, ± 0.01 ″)

Three decimal places, expressed as (. 00x), (for example, ± 0.005 ″)

Four decimal places, expressed as (. 000x), (for example, ± 0.0005 ″)

aluminum machining

What Are the Standard Machining Tolerances?

There is no true standard machining tolerance, mainly because different applications require different tolerances. However, some manufacturers and industry organizations have established standard tolerances they use or recommend for certain parts and materials. This is particularly true in the military and aerospace manufacturing industries.

Generally, the customer will provide the tolerance of their project to the mechanical workshop. Some mechanical workshops require customers to provide tolerances, but if not, other mechanical workshops operate according to the common tolerance list. For example, in SANS, our standard manufacturing tolerance is ± 0.004 “(0.1mm).

What are Considerations When Selecting Tolerances?

Many factors need to be considered when determining the tolerance. These include:


Materials behave differently under pressure. Some materials are easier to use than others. These material characteristics must be considered when determining tolerances. For example, the following common features

Abrasion resistance:

Some very wear-resistant materials may be very hard during processing. Due to the abrasiveness of materials, these materials will wear the cutting machine, thus affecting the design tolerance.

Hardness and stiffness:

soft and flexible materials are usually more difficult to process to the specified tolerance because they can change the size. Therefore, additional measures may be required to cut the material to fit the tolerance.

Thermal stability:

Some non-metallic materials, especially plastics, are easy to deform when heated. This limits the acceptable machining process types and affects the tolerance of parts.

Processing Type:

The processing method used will significantly affect the possible tolerance of finished parts, because some processes are more accurate than others. For example, drilling may be more accurate than turning or milling. In addition, CNC machine tools with different number of axes may also have different basic tolerances.

The tolerance of CNC machine tool determines the type of parts it can process. In some cases, it is necessary to carry out further operations on the parts to achieve strict tolerances. Different machining processes will also produce different surface roughness or features. If you want the part to have small or ultra-fine machining features, the part may need to go through several different machining processes.

Electroplating And Finishing:

When determining the size and tolerance of parts, any electroplating or finishing process shall be considered. Although electroplating and finishing will add a small amount of materials to the surface of the part, these small amounts of materials will still change the size of the final product, so they should be considered before production.

Tolerance Level Affects Finished Product Inspection

The tighter the tolerance, the more difficult and time-consuming it is to check them. Parts with strict tolerances require special measuring tools and testing methods, which increases production costs


The tolerance should be accurate, but it must not be more stringent than necessary, because the cost of achieving a more stringent tolerance is higher. If your part will use a three-digit decimal tolerance, do not set it to a four-digit decimal tolerance.

A smaller tolerance means a strict tolerance, which means that the part needs higher accuracy. On the contrary, a loose tolerance range means that the required accuracy of the part is low. Smaller tolerances will lead to higher costs because you will need more settings, longer cycle times and additional special tools.

It is also important to remember to check the tolerance carefully. The old part specification that you want to reuse may use unnecessary strict tolerances, or the tolerances may be incorrectly transcribed. Even new part specifications may contain errors. Taking a few more minutes to carefully examine the existing tolerances of new and old projects can help avoid future restructuring costs.

Taking these factors into account and using tolerances correctly, engineers can rest assured that their parts will be installed correctly after the manufacturing process is completed.

Why Are Machining Tolerances Important?

All manufactured parts have certain internal differences. Tolerance controls these changes to ensure better consistency and optimum part performance. The following are the reasons why tolerances are important.

1. Tolerance specifies the specification of the part

When you outsource CNC processing services, if you do not clearly communicate the specification requirements of parts with the manufacturer, you may eventually get unusable parts. However, suppose you explicitly tell the manufacturer that you want a 3-inch part with a tolerance of 0.2 inch. In that case, they know you can accept 2.9 inch parts. Tolerances tell manufacturers the accuracy of parts and what they must do to achieve these tolerances.

2. Tolerance improves the fit and function of parts

When a part must interact with other parts, tolerance is critical. To ensure that parts are compatible with other components, you must accurately describe your tolerances.

In addition, some characteristics of the component are critical to its function. For fixtures with high requirements for position and size, any change beyond the tolerance range may cause them to be defective and unusable.

3. Tolerance improves the final appearance of the product

Tolerance is also important for improving the final appearance of the product. For example, suppose that two parts must be flush with each other without obvious gaps. In this case, strict tolerances must be defined for the two parts in advance.

Many manufacturers refuse to start manufacturing parts until engineers define all features with tolerances. The reason for this is that the manufacturer does not have a reference frame to understand how the part will interact with other parts. Due to the lack of information, the manufacturer does not know the importance of specific dimensions for the final design.

4. Tolerance is critical to cost control

Generally speaking, the tighter the tolerance, the higher the cost. Parts with more stringent tolerances require additional processes, such as grinding or superfinishing. A part can only be completed with basic processing technology to obtain a looser tolerance.

Therefore, defining tolerances becomes particularly important in manufacturing. On the one hand, for parts requiring strict tolerance, you can ensure that the tolerance is met for the first time and avoid excessive cost. On the other hand, for parts that can accept looser tolerances, defining tolerances allows you to avoid paying for extremely high accuracy.


In design and production, tolerance refers to the allowable variation range of part size. Tolerance is very important in CNC machining. On the one hand, tolerance defines the specification of parts, thus minimizing the cost and shortening the turnaround time. On the other hand, tolerance can ensure higher consistency and proper performance of parts.

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