Tolerances in CNC Machining

Basic Guide of Tolerances in CNC Machining

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In CNC machining, high-precision CNC equipment can achieve very high levels of accuracy. Still, the CNC machined part’s dimensions cannot fully meet the theoretical values because of the different materials, designs, and machining processes. So the dimensions that meet the manufacturing standards should have a reasonable range of variation. That’s the tolerances in CNC machining. 

What is Machining Tolerance?

Tolerances represent the level of precision required to manufacture a part. Machining tolerances describe the acceptable variation in a part’s final dimensions or measured value. Part designers develop tolerance criteria based on the part’s function, fit, and form. Machining tolerances are essential for the parts assembly. Usually, a corresponding tolerance callout appears next to the applicable dimension of the part.

Machining tolerances usually start with a ± symbol. For example, suppose a 1.5″ height part requires a tolerance range of ±0.005″. The final part should have a variable height measuring between 1.495″ and 1.505″ to pass quality inspection. 

The smaller tolerance represents a tight tolerance, which means more precision the part needs. Conversely, the loose tolerance range means less accuracy the part requires. Smaller tolerances lead to higher costs because you will need more setups, longer cycle times, and additional specialized tools.

What are the Common Terms in CNC Machining Tolerances?

Setting appropriate tolerances ensures that manufacturers create parts within the required specifications. The following terms are widely used and very important to understand tolerance in mechanical engineering. Let’s take a closer look at them:

Terms of CNC Machining Tolerance

Basic size – the exact theoretical size to which limits of deviation are assigned.

Actual size – the actual measured size of the machined final part.

Deviation – the difference between the part’s size and the basic size.

Limits of size – the minimum and maximum sizes between which the actual size should fall. The upper limit is the bigger value, while the lower limit is the smaller value.

Lower deviation – the difference between the basic size and the minimum limit of size.

Upper deviation – the difference between the basic size and the maximum limit of size.

Fundamental deviation – the deviation that is closest to the basic size. It is represented with a letter.

Tolerance – the difference between a part’s minimum and maximum size limits.

Tolerance zone – a spherical zone limited by the part’s upper and lower limit dimensions. It is determined by the tolerance and its position related to the basic size.

International tolerance grade (IT) – indicating tolerance groups that vary based on the basic size but have the same level of accuracy with a specific grade. It is shown by the combinations of IT0, IT1, and IT01 to IT16. There are 18 IT grades in total.

tolerance grade by different machining process

Application of Different Tolerance Grades

Why Tolerances are So Important?

All manufactured parts have some degree of intrinsic variance. Tolerances control these variations, assuring better consistency and optimal parts performance. Here are five reasons why tolerances are important.

#1 Tolerance Clarify the Specifications of your Parts

When you’re outsourcing CNC machining service, If you don’t communicate your part’s specification needs clearly with the manufacturer, you may end up with an unusable part. However, suppose you explicitly tell the manufacturer that you want 3″ parts with a 0.2″ tolerance. In that case, they know you can accept 2.9″ parts. Tolerances tell manufacturers the part’s precision and what they must do to achieve those tolerances.

#2 Tolerance Improve the Fit and Function of Parts 

Tolerances are crucial when one part must interact with other parts. To guarantee that parts are compatible with other components, you must precisely describe your tolerances.

Additionally, some features of a part are critical to its functionality. For fixtures with high demands on location and size, any variation outside of tolerance can render them defective and unusable.

#3 Tolerance Provides a Margin of Error 

Any manufacturing process has a certain amount of variation.  Tolerances account for this variation by defining the margins within which a part can operate. If tolerances are defined from the start, there is less chance that a part will fail or need to be remanufactured.

#4 Tolerance Improves the Final Look of the Product

Tolerances are also important to improve the final look of the product. For example, suppose two parts must be flush with each other without significant gaps. In that case, tight tolerances must be defined for both parts in advance.

#5 Tolerance is Critical to the Cost Control

In general, the tighter the tolerance, the higher the cost. Parts with a tighter tolerance need additional processes such as grinding or superfinishing. A part can be completed using only fundamental machining processes for a looser tolerance.

Therefore, defining tolerances becomes especially important in manufacturing. On the one hand, for parts that require tight tolerances, you can ensure that tolerances are met the first time, avoiding overshooting the costs. On the other hand, for parts where looser tolerances are acceptable, defining tolerances allows you to not pay for extreme precision.

What are Common Types of Tolerances in CNC Machining?

Tolerances are expressed in manufacturing designs based on the feature under control and the engineer’s intentions. The following are the most typical types of CNC machining tolerances in the manufacturing industry:

General/ Standard Tolerances

We can define standard tolerances for linear or angular measurements and chamfers or other rounded parts. For example, standard machining tolerances exist for parts such as pipes, threads, pins, etc. Some milling services can offer typical tolerances of +/-0.1mm.

The range of standard tolerances is usually regulated by various international standards governing associations like the American Society of Mechanical Engineers (ASME), the International Organization for Standardization (ISO), and the American National Standards Institute (ANSI).

There are four different tolerance classes based on their range for a part dimension, including very coarse (v), coarse (c), medium (m), and fine (f). The details are shown in the figure below.

General Tolerance Chart

To learn more about the general tolerances, please look through ISO 2768-mk.

Bilateral Tolerances

Bilateral tolerance allows the final measurement of the part to vary on either side of the nominal value or true contour. An example of bilateral tolerances is 20 +0.06/-0.06mm. This shows that the final measurement of the CNC machined part has a maximum of 20.06mm and a minimum of 19.94mm. The nominal value is 20mm. Bilateral tolerances are mainly used for external dimensions.

Bilateral Tolerances

Unilateral Tolerances

Unilateral tolerances allow deviation only on one side of the nominal value. An example of unilateral tolerances is 70 +0.00/-0.05mm. This means the finished part measurement has a maximum and minimum allowable value of 70.00mm and 69.95mm, respectively. In the example above, when applying unilateral tolerances, we only allow a negative change from the nominal value. Generally speaking, we use unilateral tolerances on mating parts.

Unilateral Tolerances

Limit Tolerances

Limits tolerances are a sort of CNC machining tolerance expressed as a range of values in which the part is considered acceptable as long as the measured value falls within the range. An example of limit tolerances is 14 – 14.5mm. This indicates that the part’s measured value must lie between the upper and lower limit. The upper limit is 14mm, while the lower limit is 14.5mm. 

Limit Tolerances

Geometric Dimensioning and Tolerancing (GD&T)

Geometric dimensioning and tolerancing (GD&T) is more advanced and complex than regular machining tolerance systems. This sort of tolerancing uses feature control frames to showcase specific forms and dimensional tolerances. 

GD&T ensures that the final measurements of the part remain within defined boundaries. It also outlines the geometric characteristics of the parts, such as their concentricity, flatness, and true position. 

Some parts usually have higher processing requirements, and GD&T’s tolerance control methods help ensure the dimensional accuracy of part features.

Geometric Dimensioning and Tolerancing

What are Considerations When Selecting Tolerances?

The proper tolerance limit guarantees flawless part performance without incurring unnecessary costs. Let’s look at what should be considered while selecting the tolerances.

#1 Tighter Tolerance Level Means Higher Costs

Different tolerances have a significant impact on machining costs and turnaround time. Tighter tolerances usually result in higher costs because it requires more time and labor to manufacture the part slowly.

In addition, applying tight tolerances also increases the risk of a part falling outside its tolerance range and being scrapped. Therefore, the machine will require special jigs and fixtures to achieve the expected tolerances.

Finally, highly tight tolerances necessitate specialized measuring tools for quality control. Overall, high-tolerance machining increases quality inspection costs and production costs.

#2 Tolerance Level Depends on the Material

When determining machining tolerances, you must consider the selection of materials. Different materials have unique properties. These properties affect the tolerance levels that a material can achieve. Here are a few things to consider when choosing your materials.

  1. When starting a cutting operation, frictional heat can cause the deformation of some non-metallic materials like plastics. Therefore, there may be cases where the material type is incompatible with the manufacturing process.
  2. Soft materials are difficult to hold in place because they constantly bend and change size when in contact with the cutting tool.
  3. Abrasive materials that are too hard can wear down the cutting tool, making it difficult to achieve specific tolerances. Examples of some common abrasive materials include phenolics such as GP 11, GP 03, or any glass laminate.

#3 Tolerance Level Depends on the Machining Methods

Tolerances might depend on the choice of the manufacturing method. For example, drilling may be more precise than turning or milling. Furthermore, CNC machines with different numbers of axes may also have different basic tolerances. 

The tolerance of a CNC machine dictates the types of parts it can process. Further operations on the part will be required in certain cases before it can achieve tight tolerance. Different machining processes also create different surface roughness or features, if you want small or super finishing features on your parts, the parts may need to go through several different machining processes.

Different machining processes also create different surface roughness

#4 Tolerance Level Affects the Finished Product Inspection

The tighter the tolerances, the more difficult and time-consuming it is to inspect them. The parts with tight tolerances need specialized measuring tools and inspection methods, raising production costs.

Inspection of parts tolerances

When do You Need Tight Tolerances?

The tight tolerances may not be that essential for a part’s entire structure. Tight tolerance should be considered if the part needs special features. For example, engineers generally apply tight tolerances at structural points where parts must fit or join with other components.

Tight tolerances are essential when manufacturing complex parts, especially in the aerospace & defense, medical & life sciences, and diversified industrial field. A few thousandths of an inch can be the difference between a component that fits and one that does not. If tight tolerances are not achieved properly, the final products may not that good as expected.

What are Tips for Achieving Optimal Tolerances?

Effective tolerances are crucial in CNC machining, as tighter tolerances incur additional expenses. For instance, creating a part with a specific shape and tight tolerance can involve several cutting operations with different tools, resulting in more machine usage time and higher costs. But if you approach the production process correctly, you can frequently account for or offset these costs. Here are some tips for achieving optimal tolerances.

#1 Consider the Application of Your Parts

Not all parts require a tight tolerance design. The tolerance level needed for the machining part is frequently determined by its intended application. For example, creating parts that do not combine with others requires less milling precision. Given how much more costly it is to achieve tight tolerances, it is generally not used if it is not required.

#2 Consider the Material Used

Standard machining tolerances are usually +/- 0.005” for metal parts and +/- 0.010” for plastic parts. However, some parts may require even tighter tolerances to guarantee an appropriate fit.

Dimension precision can be challenging for some materials and easy for others. Remember that some raw materials expand and contract when exposed to different temperatures or moisture levels. Therefore, you should define a new tolerance based on this factor. 

#3 Find the Right CNC Machining Service 

Customers may achieve the appropriate tolerance by locating a reputable CNC machining service. Then, you can discuss your requirements with a manufacturing specialist and determine the optimal tolerances for the project. Generally, part designers specify tolerances before submitting manufacturing requests to CNC machining services, saving production costs and time.

#4 Use High-performance Cutting Tools

The dimension variations in a work part might result from improper cutting tool usage, tool deflection, or a dull cutting edge. Tool deflection often occurs on long-ended features such as long shafts and deep holes. Furthermore, dull cutting tools place your parts in an unfavorable position and affect the precision of the spindles.

#5 Don’t Ignore Parallelism and Perpendicularity

Don’t ignore parallelism and perpendicularity when considering tolerances, especially when working with multiple parts. Parallelism and perpendicularity are especially important for assembly because even slight misalignments can lead to larger misalignments over time, ultimately affecting the overall quality of the part.

#6 Maintain the Accuracy of Work Holders

Work holders are important in ensuring that CNC machining tolerances are as required. They help keep the part in place as it is machined and serve as reference points for locations.

Conclusion

In design and production, tolerance refers to the permissible range of variation in a part’s dimensions. Tolerances are critical in CNC machining. On the one hand, tolerances clarify the specification of the part, thereby minimizing costs and reducing turnaround time. On other other hand, tolerances ensure greater consistency and proper performance from the parts.

In short, to ensure your parts meet the required tolerances, you should consider material selection, part design, and the machining process. As a trusted CNC machining company, LEADRP has been producing precision-tolerance parts and prototypes for customers around the world in almost every industry. If you’re unsure about what tolerances are right for your application, don’t worry. We are here to help identify the mission-critical tolerances for your parts.

References

Principles of Tolerancing – From McGill

Why Are Tolerances Important in Manufacturing? – From READING PLASTIC MACHINING AND FABRICATION

FAQ

ISO 2768 is an International Standard published by the International Organization for Standardization (ISO) to simplify mechanical tolerance drawing standards. It makes design and manufacturing more convenient and facilitates cooperation between different companies.

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