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cnc machining tolerances

CNC Machining Tolerances: Overview, Benefits, and Tips

CNC machining has significantly improved manufacturing results as part of the emergence of Industry 4.0. It utilizes various fabrication methods and advanced machines to create superior quality and high-precision components. These CNC machines help achieve the highest levels of accuracy and tight CNC machining tolerances.

However, CNC turning and milling parts with the tightest tolerances are often expensive and time-consuming. Thus, it is essential to specify the appropriate tolerances that a part requires when machining.

This article discusses every essential aspect of CNC machining tolerances, including the types, benefits, and how to determine the ideal tolerance for your parts. Let's get to it!

What Does Tolerance Mean in CNC Machining Process?

what does tolerance mean in cnc machining process
What Does Tolerance Mean In Cnc Machining Process

CNC machining tolerances are measurements indicating the acceptable level of variance required in the dimension of machined parts. It is sometimes called dimensional accuracy. Machining tolerances involve marking the minimum and maximum acceptable degree of variation in the product’s measured value or final dimensions.

Part tolerances are a crucial variable in the machining process of components for assembly. As a result, product designers often set dimensional limits based on the component’s form, fit, and function.

Generally, machinists calculate dimensional accuracy using digits, and a ± symbol often precedes it. For instance, a product with a height of 1.5 inches needs a tolerance within ±0.005 inches. To fulfill all requirements, the final parts should have a varying height measurement between 1.495 and 1.505 inches.

Also, you may consider setting a tolerance of ±0.001 inches for a part with a length measurement of 2.550 inches. Ultimately, the manufactured component would possess variable measures between 2.549 and 2.551 inches in length.

Specified tolerances in CNC machining are used to guide manufacturers on what degree of precision to employ in machining parts. Looser tolerance often requires less precision, while a tighter tolerance requires more precision. However, these tighter tolerances incur higher costs due to the need for additional setups, extended cycle times, and specialized toolings.

Types of CNC Machining Tolerances

types of cnc machining tolerances

There are various types of tolerances in CNC machining. Here are the most common tolerances used in CNC machine shops:

Standard Tolerances

Most product designers and engineers utilize standard machining tolerances for most commonly machined parts. Machined parts, such as pins, threads, pipes, etc., often use standard machining tolerances. Machinists often use normal tolerances when customers fail to specify the required dimensional limit. Many CNC milling services often provide standard tolerances of ±0.1mm.

You can easily locate these typical tolerances on a CNC machining tolerance chart. The standard machining tolerance range is often fixed by different international standards bodies. These bodies include the American National Standards Institute (ANSI), the International Organization for Standardization (ISO), and the American Society of Mechanical Engineers (ASME).

In addition, there are four separate tolerance categories based on their range for a workpiece’s dimension. These include fine (f), medium (m), coarse (c), and very coarse (v).

Unilateral Tolerances

at machining unilateral tolerance
Unilateral Tolerance

Unilateral tolerances have permitted variance in only one direction. It can occur either in the negative or positive direction only. An excellent example of a unilateral tolerance would be +0.00/-0.07mm. It means a finished workpiece can be below the size of 0.07mm but not more than the defined measurement.

You can utilize unilateral tolerance in designing a component that needs to fit with others. As a result, the components shouldn’t exceed the defined measurement because that can make them unsuitable for mating.

Bilateral Tolerances

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Bilateral Tolerance

Bilateral tolerances permit the variation in the final measurement on either side of the dimension. That is, the deviation from the nominal value can either be positive or negative; the deviation can be bigger or smaller.

An excellent example of bilateral tolerance is +/-0.05mm. It shows that the machined workpiece can be shorter or longer by 0.05mm than the preferred measurement. Machinists often use the bilateral tolerance for exterior dimensions when machining parts.

Geometric Tolerances

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Geometric dimensioning and tolerancing are more complicated and advanced than typical machining tolerance systems. It typically involves highlighting measurements and permissible deviations.

Also, it indicates specific geometric characteristics of the machined components, such as their flatness levels, centricity, and true position. Some machined parts often require a specified diameter, which only geometric dimensioning and tolerancing can offer.

Limit Tolerances

at machining limit tolerances
Limit Tolerances

A limit tolerance is commonly indicated as a range of values where the CNC machined parts are considered tolerable, provided their measured value is within the allowable variance. For example, 19.996- 20.005 is a limit tolerance showing that the workpiece must possess a measurement within the upper and lower limits. In this case, 20.005 mm is the upper limit, while 19.996 is the lower limit.

Importance of Tolerances in CNC Machining

importance of tolerances in cnc machining
importance of tolerances in cnc machining

Tolerance in custom machining helps to manage variations in machining operations, ensuring great consistency and efficiency in component performance. This section discusses some of the importance of tolerance in machining parts:

Clarity in Part’s Requirements

Whenever you outsource CNC machining service, you must turn in your part’s requirements to your manufacturing partner. Failure to do so will most likely result in the production of unusable parts.

For instance, inform your manufacturing partner that you need 6 inches parts with a 0.4-inch tolerance. Such a manufacturer would know that you can take 5.8 inches of machined parts. Tolerances give manufacturers an idea of the part’s precision and what needs to be done to realize these tolerances.

Cost Control

Generally, tighter tolerances mean high costs because the parts that require a tighter tolerance undergo post-process machining operations like grinding. Meanwhile, a looser-tolerance part can be finished using only basic machining processes.

It would help to note that determining tolerances is a crucial aspect of manufacturing. As a result, you must ensure you achieve tight tolerances the first time when machining parts that require tight machining tolerances to prevent higher costs. Likewise, determining tolerance for parts compatible with looser tolerances helps you save costs and unnecessarily tight tolerances.

Excellent Surface Finish for Products

excellent surface finish for products
Excellent Surface Finish for Products

One of the many benefits of tolerances in CNC machining is that it helps to enhance the final surface aesthetic of products. For instance, suppose there are two machined parts to be connected with each other without any significant loopholes or gaps. Both parts would require defined tight machining tolerances to fit and function perfectly.

Creates a Margin of Error

Each manufacturing technique has a particular degree of variation. Tolerance estimates this variation by defining the margins within which the machined part can function. If you provide tolerances for a part from the beginning, the chances that the machined parts will fail or will have to be recreated are low.

Enhances the Fit and Function of Machined Parts

Machining tolerance is a crucial variable to consider in cases where one component would have to interact with others. So, to ensure the compatibility of these parts with other parts, you must be precise in defining your tolerances.

Furthermore, certain features of a machined part determine its functionality. Hence, any variation that exceeds the tolerance band of fixtures with high specification needs on location and size may render them faulty or unusable.

Factors to Consider to Determine the Right CNC Machining Tolerances

factors to consider to determine the right cnc machining tolerances
factors to consider to determine the right cnc machining tolerances

Most complex machined parts, like engines, require high precision. Here are factors to keep in mind when determining the tolerance band for your parts:

When Your Parts Require Tight Tolerances

Machining tolerances within a single component often vary. Although the entire structure of a part may not require the tightest tolerances, certain aspects of the machined part may require tight tolerances. The structural points of a part where it connects with other components may require the highest precision.

So, a part must possess tight machining tolerances if it needs to fit perfectly with another. Therefore, you must communicate the preferred tolerances to the manufacturing partner. In addition, it would help to note that very tight tolerances incur more costs due to wear caused by CNC machine tools.

Although a new machine tool might easily attain tolerances of ±0.001 inches, the precision becomes less constant as it creates multiple copies of the same part. Suppose your order is above 1000 units using  ±0.001 tolerances. In that case, your manufacturing partner may need to adjust the machining speed or change the machining tool leading to high retooling costs and cycle time.


Your choice of material for your project determines the specified machining tolerances. Material properties can influence the tolerance level attained with such material. Some of these features are:

  • Abrasiveness: Rough and coarse materials are typically harsh on machine tools and can make them wear out quickly. These CNC machining materials make it almost impossible to achieve certain tolerance because you have to change the cutting tools regularly, resulting in reduced accuracy.
  • Heat Stability: Heat stability is a common issue that affects non-metals. These non-metallic materials lose shape as heat builds up during CNC machining. More so, this factor limits the compatible processes with the particular material.
  • Hardness: Harder materials are easier to machine to tight tolerances than softer materials. This is because softer materials change dimensions as the machine tool touches the material. Hence, you must be more patient when machining more delicate fabrics.


Tolerances directly impact the quality of a machined part and its costs. Higher tolerances cost more since you must machine the parts more slowly and consistently with more expensive tools.

Also, parts with tighter operational tolerances need more extensive, thorough quality inspection since the acceptable margin of error is smaller. A high failure rate is another factor that increases the CNC machining cost for parts with tight machining tolerances. High-tolerance machining causes an increase in production costs and quality inspection costs.

Manufacturing Processes

Individual CNC machines have varying capacities. Some machines can achieve tolerances that others cannot. Likewise, a CNC machine’s tolerance decides the type of parts it can work on. However, you must perform secondary operations on machined parts before achieving the desired tolerance.


When machining parts with tight machining tolerances, you will require a longer time to scrutinize the part. Also, you may have to utilize specialized tools and equipment to confirm the tolerance levels of the machined parts.

How Can You Find the Right Tolerance for Your Parts?

how can you find the right tolerance for your parts
CNC Precision Machining

As a product designer, specifying the extent of acceptable variance for your machined part’s tolerance is essential. It significantly affects finding the right tolerance for your parts and the costs and turnaround time.

However, you should note that CNC machining is a typical high-precision process. Therefore, looser tolerances are almost equivalent to the original specifications. Here are tips to help you find the right tolerance for your parts:

Examine the Machined Part’s Use

Although not all machined parts need to be designed with tighter tolerances, the specific use of the part will decide the degree of accuracy required when machining the parts. For instance, producing parts that do not fit with others doesn’t need higher milling accuracy. So, because machining parts with tighter tolerances is more expensive, you shouldn’t go for it if you do not need it.

Partner with a Reliable CNC Machining Company

One of the most effective means of getting the desired tolerances for your parts is to partner with a reliable CNC machining service. You can discuss your project ideas and specifications with a manufacturing expert and determine the appropriate tolerances.

You must identify the tolerances before submitting a manufacturing request to a rapid prototyping company. Providing this information helps a long way in saving costs and time. More importantly, it is crucial to understand that most CNC manufacturers will automatically revert to their standard tolerances if you do not specify a particular tolerance margin.

Although this seems to be an insignificant deviation in tolerance, it can affect the fit and function of your final parts. For example, if you machine a part with a tiny hole even by ±0.127 mm, it won’t be easy to fit its adjoining part within it.

AT-Machining, Your Trusted CNC Machining Service Expert

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AT-Machining cnc service

AT-Machining provides an extensive range of CNC machining service tolerances that perfectly match your design requirements. Our state-of-the-art facilities host the most advanced CNC machines capable of achieving tighter or looser tolerances, just as your projects require. We have expert machinists and engineers with in-depth knowledge of the various machining techniques.

Our customers across the globe benefit from our CNC machining services and other manufacturing capabilities we offer. Our reliable quality inspection system ensures your parts are machined to standard. We guarantee the standard tolerances for CNC machining, which are DIN-2768-1-medium for plastic parts and DIN-2768-1-fine for our metal parts, respectively.

We adhere to ±0.005 inches as a standard tolerance for our CNC machining service. Likewise, our tightest tolerance is approximately ±0.0002 inches (±0.005mm), precisely the width of a human hair. Do not hesitate to contact us for your next project; let professionals handle it correctly!

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From AT-Machining, I’m a CNC Machining Expert in this field for more than 20 years.  We offer cost-effective machining services from China. Ask for a quote for your ongoing or upcoming projects now!

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