What Is CNC Machining Tolerance?

Introduction

As long as you are in the CNC manufacturing industry, you will often hear a word, that is - CNC machining tolerance. This also reflects another aspect of the importance of CNC tolerance in the CNC industry.

If you have just entered the CNC manufacturing industry, then this article about CNC tolerance is something you must not miss. This article will introduce you to information about CNC tolerance from the following content.

What Is CNC Machining Tolerance?

Perhaps you have seen the definition of CNC tolerance, such as some people will tell you: "Machining tolerance is the limit of dimensional variation allowed relative to the ideal blueprint value.

Machining tolerance describes the accuracy of any manufacturing process." Or "Machining tolerance refers to the allowable deviation between the size, shape, and position of the machined part and the expected design specifications." Both descriptions are correct, but not concise enough.

The so-called CNC machining tolerance is simply understood as the actual manufactured CNC part data has a certain error range with the CNC machining drawing. This range is called CNC machining tolerance.

What Are the Common Types of Tolerances in CNC Machining?

In CNC machining, the most common types of tolerances we have mainly include dimensional tolerances and form and position tolerances.

Dimensional tolerance

Dimensional tolerance focuses on the dimensional accuracy of parts, such as the deviation of length, width, diameter, etc. from the nominal size.

For example, the standard prototype and production processing tolerance of some websites is plus or minus 0.005 inches (0.13 mm), which means that the position, width, length, thickness, or diameter of any part feature will not deviate from the nominal size by more than this value. This is called dimensional tolerance.

Form and position tolerance

Form and position tolerance involves two concepts: the geometric shape and position accuracy of parts, including form tolerance and position tolerance.

Form tolerance refers to the deviation between the geometric shape of the actual CNC parts produced and the ideal shape (design drawing). Such as straightness, flatness, roundness, cylindricity, line profile, surface profile, etc. These form tolerances are used to ensure that the geometric shape of the parts meets the design requirements.

The focus of position tolerance is the position accuracy of CNC parts. These position accuracies include orientation tolerance, positioning tolerance, and runout tolerance, such as parallelism, verticality, inclination, coaxiality, symmetry, position, etc. These position tolerances are used to ensure the correct position and direction of parts in space.

The Most Frequently Used Tolerance Terms in CNC Projects

In the CNC manufacturing industry, due to the differences between manufacturers, there will be certain deviations in the names of different tolerances, but the following tolerances are the default unified names in the CNC manufacturing industry:

Standard tolerance

Standard tolerance means that when the customer does not specify the specific CNC part tolerance standard, the machining manufacturer will manufacture parts for the customer according to the default tolerance standard. The typical tolerance used by this part is called standard tolerance.

So far, the manufacturing standards for machined parts are mainly divided (defined) by two American associations (the American Society of Mechanical Engineers (ASME), and the American National Standards Institute (ANSI)) and an organization (Organization for Standardization (ISO)) to divide (define) this manufacturing range for this machining standard. At RT manufacturing, the CNC manufacturing standard we use is the ISO2768MH tolerance standard.

Restricted tolerance

Restricted tolerance means limiting the manufacturing standard of machined parts to a predetermined range. For example: the marking of a part position is 10 (±0.02) mm. That means that the upper limit tolerance of this position of the part cannot exceed 10.02mm, and the lower limit tolerance cannot be lower than 9.98mm. When the size of the part is exactly 10mm, we call it the limit tolerance, also known as 0 CNC tolerance.

Unilateral Tolerance

‌Unilateral tolerance refers to the tolerance that allows the dimension to deviate in only one direction based on the basic dimension, but not in the opposite direction.

Unilateral tolerance often appears in solving problems such as product parts assembly interference (conflict). Therefore, the most common scenario for unilateral tolerance is the new car development stage (prototype stage).

Especially when dealing with parts such as body length, width, height, or appearance quality, reasonable unilateral tolerance will play a role in reducing development costs, shortening development cycles, and improving product quality.

Therefore, there are two special forms of unilateral tolerance in unilateral tolerance, namely single upper limit and single lower limit. A single upper limit means the smaller the better, and a single lower limit means the larger the better.

Bilateral Tolerance

‌Bilateral tolerance‌ refers to a type of tolerance that has both upper and lower deviations. Bilateral tolerance exists to ensure that the dimensions of manufactured products are within the specified range, neither larger nor smaller than the dimensions required by the design, thereby ensuring the quality and performance of the product‌.

Bilateral tolerance is the opposite of unilateral tolerance, which has only one of the upper and lower deviations, while bilateral tolerance considers both the upper and lower limits of the size at the same time, providing stricter dimensional control.

This type of tolerance is particularly important in mechanical engineering because it is directly related to the interchangeability and functionality of parts. By setting appropriate bilateral tolerances, it is possible to ensure the precise fit of parts during assembly and avoid functional failures or safety hazards caused by excessive dimensional deviations‌.

Geometric Dimensioning and Tolerancing (GD&T)

Geometric tolerance is a part of form and position tolerance. GD&T (Geometric Dimensioning and Tolerancing) is a standard method for describing the geometric features and tolerances of parts, aiming to ensure the accuracy and interchangeability of parts during manufacturing and assembly.

The application of GD&T can bring many benefits, including improving the accuracy of parts, reducing scrap rate, optimizing the assembly process, etc. In addition, the GD&T annotation method does not exist independently of the traditional linear dimension tolerance annotation method but is a member of the tolerance annotation family. Due to its unique advantages, it is gradually accepted by more mechanical practitioners.

In the CNC manufacturing industry, geometric tolerance appears most frequently, mainly including dimension annotation methods and specific geometric pattern annotations. From the aspects of design ideas, detection process, and function realization, it sets benchmarks, allocates tolerances, and reflects the requirements for parts, which reduces the manufacturing cost and measurement difficulty to a certain extent.

To help everyone understand the tolerances in these CNC manufacturing industries, the author of this article has compiled several common CNC machining symbols in the CNC industry below. The details are as follows:

Do CNC Tolerances Have to Be Strictly Enforced?

People in the CNC industry are very familiar with the implementation standards of CNC tolerances. It should be determined according to the actual use scenarios of customers. In general, CNC machined parts only need to meet the general machining manufacturing standards. Only industries with relatively high requirements such as automobiles and medical care will strictly limit the CNC machining tolerances.

Therefore, if you are a customer who needs CNC machining services, you will often hear your business connection ask you, "Can you please introduce the application areas of your CNC parts?" Because the CNC manufacturing standards of different industries are different, asking this question is to make the best-machined parts for your industry, which are closest to the standards of your industry.

Therefore, when a business connection asks you this question, you can tell them the use areas or application scenarios of the parts, so that the parts produced can better meet your expected results, or even exceed your expectations.

What do CNC Manufacturing Tolerances Stand for?

Part manufacturing cost. The higher the tolerance requirements of CNC parts, the more stringent the manufacturing conditions, and the more precise the size of the manufactured parts.

But in this process, it is easy to damage parts and scrap them, and the manufacturing cost is very high. It not only tests the technology of the CNC manufacturing team but also tests the manufacturing time of parts compared to other workpieces. This means that strict part-tolerance requirements will take longer.

What Is the Normal CNC Tolerance Range?

In the modern CNC manufacturing industry, the CNC manufacturing standard that is generally followed is the "ISO2768MH tolerance standard". The ISO2768MH tolerance standard applies to various types of mechanical parts, including but not limited to bearings, gears, connecting rods, nuts, bolts, etc.

The standard covers parts of different precision levels, from roughness grade A to fine grade H, suitable for different use environments and processing methods. The following is a table about the ISO02768mh tolerance standard:

What Factors Affecting CNC Machining Tolerances?

It is not so easy to get a perfect and precise CNC part. This is a comprehensive consideration. There are many factors to consider. Generally, the factors that affect CNC machining tolerances include but are not limited to the following aspects:

Machine tool factors

Different machine tool types involve different machining processes. Different machining processes will have different machining tolerances. This is inevitable because each machine tool is fixed and its type determines its upper limit for part machining. Therefore, when most people understand the machining process, they should consider the limitations of the machine tool when machining parts.

Material factors

Different material properties will greatly affect CNC machining tolerances. Generally speaking, the CNC machining properties of metals are easier to obtain precise machining tolerances than the CNC machining properties of plastics. It is mainly affected by the following aspects:

Density: The lower the density of the machining material, the more difficult it is to machine to high precision. Generally speaking, materials with low density are lighter.

Lightweight materials are more likely to have problems in the actual machining process and require more patient operation. The more times the operation is performed, the more it will affect the tolerance of CNC machining.

Thermal stability: a common problem in non-metallic materials. During the machining process, heat will be generated, and the heat will become higher and higher. These materials with low thermal stability will deform as the machining heat increases, thus affecting the CNC machining tolerance.

Roughness: The roughness of the material greatly affects the utilization rate of CNC tools. The rougher the material structure, the more serious the impact on the CNC tool, which will increase the frequency of tool change. As the tool changes, the machining accuracy of the parts will naturally decrease.

Surface treatment

Usually, a surface treatment that adds a protective layer to the surface of the part will affect the machining tolerance. For example, powder spraying, painting, sandblasting, electroplating, etc. will affect the machining size of the part to a certain extent.

If you want to minimize the impact of tolerance, choose the finishing treatment (part glossy effect) because this is done on the machine, which can prevent the part from exceeding the dimensional tolerance standard to the greatest extent.

Budget

Among all the factors affecting machining tolerance, the most influential factor is the budget, which determines the quality of the tools used and the better material selection. Generally speaking, more stringent machining requirements mean that more machining time is required and many different special tools are used.

These are often expensive, but sufficient budgets can largely ensure that the parts are within the required machining range and have good surface quality.

Machining Team skills

An excellent machining team is an important factor affecting CNC machining tolerances. It includes two aspects, one is the CNC programmer and the other is the CNC machinist.

An excellent CNC programmer can minimize unnecessary CNC machining paths (this process is also called optimization parameters) to a certain extent and can achieve the tolerance range you want to a certain extent.

An excellent CNC operator is the specific implementer. He must not only ensure the correct use of CNC tools but also ensure that all processes are standardized according to the code path given by the CNC programmer. Only with close cooperation can the smooth manufacturing of parts be achieved and the tolerance of parts be controlled within the range.

Can Parts be Manufactured Smoothly without Clearly Indicating Tolerances In CAD Files?

Usually, if you do not mark the specific part size in the design drawing file, it is difficult to manufacture the parts according to the drawing. Because the machining manufacturer does not know the size of your parts and the specific location markings, the manufactured parts do not conform to the actual use scenario.

As a qualified machined parts manufacturer, when encountering such problems, we will feedback to the customer, and we will only start manufacturing after receiving the confirmed production instructions.

Is There A Zero-Tolerance CNC?

If you want to achieve a perfect fit visually, you think it is a zero tolerance CNC, then this can be achieved, it has high requirements for CNC machines and technology. But in actual CNC manufacturing, zero tolerance CNC is just an ideal conceptual tolerance.

Do You Provide CNC Parts Manufacturing Services with Tolerance Markings?

Yes, as a qualified CNC parts manufacturer, we provide customers with high-quality CNC machining services. The part design drawings with specific tolerance markings are what we like to see most because they allow us to more clearly understand the manufacturing tolerance range you want. So as to provide you with the best parts processing services.

Conclusion

In this article, we focus on the concept of CNC machining tolerances and common machining tolerance types. If you want to engage in the manufacturing industry, such as designers, or CNC quality inspectors, etc., it is necessary to understand this basic knowledge.

If you want to know more about CNC dynamic news, please pay attention to RT blog updates at any time. If you only have a conceptual awareness of CNC, then you can click here to learn what CNC is.