Our Location
Mastering CNC Machining Tolerances: A Complete Guide
If you are operating a CNC machine, then it is important to consider part tolerance. This guide will cover all the essential information you need to know about CNC machining tolerances. So, please continue reading to enhance the accuracy and consistency of your parts during CNC machining.
What Are Tolerances?
In machine design and manufacturing, tolerance refers to the allowable variation in the actual parameter values of machinery or machine parts. For instance, if a certain product specification’s upper and lower limits are 100 and 60, respectively, its tolerance is 40. If the upper and lower limits are +100 and -100, respectively, then its tolerance is 200.
“parameter values” encompasses geometric parameters in mechanical processing and parameters from other disciplines such as physics, chemistry, and electronics. Hence, tolerance is a widely used concept. For mechanical manufacturing, the purpose of setting tolerances is to determine the geometric parameters of the product so that its variation is within a certain range to fulfill the interchangeability or fit requirements.
The tolerances of geometric parameters include dimensional tolerances, shape tolerances, positional tolerances, and so on.
What Are Tolerances In CNC Machining
Machining tolerances are the acceptable range of variation in a dimension relative to its ideal blueprint values. It is used to measure the accuracy of any manufacturing process.
The machining tolerance value should be as low as possible for high accuracy and precision. In other words, the accuracy of a process is inversely proportional to its machining tolerance.
Even though a perfect process does not exist, modern manufacturing techniques such as CNC machining have lowered the machining tolerance value to a minimum.
In CNC machining, tolerances are usually measured using the format ±0.x”.
Why Are Machining Tolerances Critical?
All machined parts have inherent variation. Tolerances are used to manage this variation, ensuring consistent and optimal performance of CNC machined components.
Enhance the Fit and Function of the Part
Precision is of utmost importance when it comes to CNC parts. Tolerances are crucial in ensuring the parts fit perfectly in an assembly. It is essential to define the tolerances clearly to ensure compatibility with other components. Additionally, certain features of the part contribute to its proper functioning in the intended application. Any deviation from the permissible limits can render the parts defective and unusable. Therefore, it is crucial to maintain the necessary tolerances to ensure that CNC parts are high quality and meet the required standards.
Rule a Margin of Error
Every manufacturing process has some degree of variation. But machining tolerances determine the limits within which a part can function despite this deviation. It reduces the risk of a part failing or needing to be reproduced if tolerances are defined from the outset.
Control The CNC Machining Cost
Tighter tolerances tend to increase the cost of CNC machining as they require additional processing, such as grinding and superfinishing. However, if fundamental machining processes are sufficient for looser tolerances, a product can be completed without incurring significant expenses. It is important to define tolerances to prevent unnecessary cost increases when handling parts that require tighter tolerances. Similarly, defining tolerances in parts that require looser tolerances helps avoid paying for extreme precision.
Ensure the Part’s Final Appearance
High precision tolerance enhances the final appearance of machined parts. For instance, tight tolerances must be defined to ensure perfect assembly if two parts flush together without obvious gaps.
Different Types Of CNC Machining Tolerances
1. Limit Tolerances
Regarding dimensions, the maximum and minimum values define the limit tolerance. It means that when a part is connected to the final product, its values should perfectly fit into the designated contact points. For instance, a dimension with a limit tolerance of 10 to 10.5mm indicates that the final values will fall between these boundaries. Generally, these boundaries are the limits within which the values will lie.
2. Unilateral Tolerances
Unilateral tolerances refer to situations where only one part of a direction can be tolerant. The variation can either be more or less than the actual part size. Let’s consider an example of a metal rod that needs to be machined to fit a product. If not machined properly, the metal rod may be out of dimensional range by even a fraction of a millimeter. However, such errors can be avoided by ensuring proper unilateral tolerances. By allowing a 0.05mm maximum deviation, we can have a 45+0.00/-0.05mm tolerance range, which means that the maximum and minimum diameter values permissible for the shaft are 45mm and 44.95mm, respectively. The benefit of unilateral tolerances is the ease of inspecting the part. Standardizing a go gauge can increase inspection speed. The key here is having a fixed upper value with varying dimensions on one side.
3. General Tolerances
Machinists generally follow standard CNC tolerances for commonly manufactured parts such as threads, pins, and pipes. If customers do not specify their tolerance levels, many milling solutions provide international standards of around +/-0.1mm. You can refer to the detailed CNC machining chart for a comprehensive understanding of these standards. These standards are defined by various international organizations, including the American National Standards Institute (ANSI), the International Organization for Standardization (ISO), and the American Society of Mechanical Engineers (ASME).
4. Bilateral Tolerances
Bilateral tolerances in CNC machining are symmetrically distributed around the base dimension. It implies that the upper and lower limits deviate from the base dimension by the same value. Unlike unilateral tolerances, bilateral tolerances differ in positive and negative directions.
5. Geometric Dimensioning and Tolerancing
Geometric Dimensioning and Tolerancing (GD&T) is a communication system between engineers and manufacturers to specify acceptable deviation levels from a part’s design dimensions.GD&T tolerances are often used for components that have extremely accurate sizes.
Common Standards for CNC Machining Tolerance
CNC machining includes various procedures with different tolerances due to the various cutting tool types. Below are standard tolerances for specific CNC processes:
| Processes Types | Tolerance Standards（mm） | Tolerance Standards（In.） |
| Lathe | ± 0.13 mm | ±0.005” |
| 3-axis Milling | ± 0.13 mm | ±0.005” |
| 5-axis Milling | ± 0.13 mm | ±0.005” |
| Engraving | ± 0.13 mm | ±0.005” |
| Screw Machining | ± 0.13 mm | ±0.005” |
| Router | ± 0.13 mm | ±0.005” |
| Steel Rule Die Cutting | ± 0.381 mm | ±0.015” |
| Router (Gasket Cutting Tools) | ± 0.762 mm | ± 0.030” |
| Rail Cutting Tolerances | ± 0.762 mm | ± 0.030” |
| Surface Finishes | 125RA | |
You will notice tighter tolerances when comparing CNC machining processes to alternative machining technologies.
Different Terms For Measuring CNC Tolerances
It is important to understand the different terms associated with CNC tolerances to get a better idea of how to measure tolerances in machining. Below are some typical terms that you should know.
Basic Size
A part’s basic size is the size specified in the engineering drawing. Product engineers understand that manufacturing techniques often require some level of tolerance. Hence, product designers use the basic size as a reference point and remain mindful of possible deviation during machining.
Actual Size
The actual size of a part refers to its dimension after completing the machining process. The actual size represents the practical realization of the final product, while the basic size represents its theoretical values. However, it is almost impossible to make the actual size the same as the basic size. Hence, product manufacturers work towards bringing these two values within the same range.
Limits
Limits are the minimum and maximum allowed dimensions of a part. The minimum permitted dimension is the “lower limit,” while the maximum allowed dimension is the “upper limit.” However, a part will be considered unusable if it falls outside these limits.
Deviation
Deviations mean the variances of the maximum permissible size from the basic size. As there are two types of allowable limits, there are two types of resultant deviations – lower and upper deviation. However, it is easier to calculate these deviations:
Upper Deviation is the difference between the Upper Limit and the Basic Size.
Lower Deviation is the difference between the Lower Limit and the Basic Size.
Datum
Datum is a physics term that means an imaginary line or plane picked arbitrarily as a reference point for measurement tools. It is also common in various kinds of Geometric Dimensioning and Tolerancing areas.
Considerations When Deciding on CNC Machining Tolerances
Having prior knowledge about tolerances can lead to better planning and resource utilization.
Choice Of Tight Tolerance
Tolerance is always better since it directly reflects the accuracy of a part. However, tight tolerances can increase production costs and lead to a time-consuming process for CNC machined parts. Therefore, it is important to incorporate tight tolerances only when necessary.
In cases where a part will be used in secondary assembly processes, tight tolerances are generally needed to prevent any failure in acceptable assembly. That is why there is a high focus on the tolerance band.
Another scenario where tight tolerance machining is needed is designing innovative parts prototypes. Engineers expect the prototype to function exactly like the finished products, so they use as tight requirements as possible.
High Cost Of Tight Requirements
For several reasons, aiming for tighter tolerances can significantly impact machining costs and turnaround time. Tighter limits require more time and labor, and increase the probability of a part falling outside the tolerance range and being discarded. Additionally, machines will need special jigs and fixtures to achieve these limits, and very tight tolerances will require special measurement tools for quality control.
The Tolerance Levels Vary Depending on the Type of Material
Different materials have unique characteristics that can impact their CNC machining tolerance.
- Heat stability: This issue mainly affects non-metallic materials. During the machining process, heat buildup causes them to lose shape, limiting the available processes for that material.
- Hardness: Soft materials are difficult to machine with high precision due to dimensional cutting changes.
- Abrasiveness: When working with rough and coarse materials, the cutting tools tend to wear out faster and can cause difficulty in achieving strict dimensional accuracy. These materials also make it harder to maintain accuracy because changes in the cutting tool can result in less precision. Moreover, abrasive materials often require the technician to change the tool multiple times during machining.
Inspection
Why is it necessary to inspect a part after machining it using CNC? Simply put, it is the foundation of our Quality Assurance Policies. The main drawback of unnecessary tolerances is the required resources for inspection. The worst-case scenario is when manufacturers need the appropriate inspection machine grade.
The Right Manufacturing Method Choice
The tolerances the designer specifies determine which manufacturing processes are best suited to achieve them. As tolerances become tighter, fewer CNC machines will be capable of completing the job satisfactorily because their precision capabilities vary. In some cases, the part may require further operations, such as lapping and grinding, to achieve the desired tolerances. However, Swiss machining can produce turned parts with extremely tight tolerances without any secondary finishing processes. This is economically feasible for higher volumes of production.
Suggestions To Achieve Tighter Tolerances In CNC Machining
When machining parts, avoiding some degree of variation is almost impossible. Precision is crucial to create a good quality part. Tighter tolerances are now being demanded in CNC machining, but this comes with the downside of additional costs. However, various ways exist to cost-effectively offset and account for these additional costs. Some two wins strategies include:
Piece Tolerance Optimization For Typical Applications
We can ensure a proper fit by minimizing deviations in CNC Machining. We allow tolerances of +/- 0.05 mm for metal and +/- 0.010 mm for plastics during production. Tighter tolerances can increase production costs, so we only apply them to necessary component surfaces that meet design requirements. By doing this, we can avoid additional expenses. However, we adhere to standard tolerances if tight tolerance is not necessary.
Consider Performing a Final Inspection During the Design Phase.
Measuring tolerances on complicated parts is challenging and requires specialized equipment for accuracy. This task becomes even more daunting when the components contain smaller features that are difficult to manufacture and inspect. However, with the right tools, ensuring tight tolerances through precise inspection and measuring of the dimensions is possible.
Align the Machinability Of Material With Tolerance Expectations
Normally, parts are designed with typical properties in mind based on the material used. These properties are essential for the end-use application. However, it’s important to consider how a material’s properties can affect its machinability. Softer materials may struggle to hold tighter tolerances due to their low durometer, while it’s tougher to mill hardened steel than plastic.
Consider Parallelism and Perpendicularity
It is essential to prioritize these two aspects, especially when dealing with multiple components. The main issue is that even a slight misalignment can cause problems over time. This can negatively affect the part’s credibility and viability, leading to substandard quality.
Consider Choosing a Reliable CNC Machining Company
Outsourcing your project to a reliable CNC machining company can help you avoid the annoyance of dealing with technical aspects such as tolerances and many other things. Enze is considered one of the top CNC service providers from China.
Enze CNC machining services are provided by skilled and the most advanced equipment. It means that you will be provided with professional guidance on the best tolerances to use, and you can also expect the best tools to achieve those tolerances.
Which Industries Require Accurate CNC Tolerances?
Precision CNC machining tolerances are necessary for industries that require strict design and functional standards. Some of these industries include:
- New Energy
- Aircraft & Aerospace
- Automotive
- Agriculture
- Electronics
- High-Tech Industry
- Elevator
- Medical
FAQS
How Can Tighter Tolerances Increase Production Cost In CNC Machining?
Tighter tolerances are desirable for parts, but they come at a cost. High-tolerance machining can be expensive because it requires expensive tools and the process runs slower. Part inspection also requires a precise CNC procedure with very small margins of error. However, even with all the precautions in place, the failure rate of these parts can still be high, leading to increased production and inspection costs.
What is The Highest Tolerance Achievable With CNC?
CNC machining can achieve tolerances of +/-0.005 inches (0.127 mm) or better. In some cases, achieving even tighter tolerances of +/-0.001 inches (0.0254 mm) or +/-0.0005 inches (0.0127 mm) is possible. Machining tolerances smaller than ±0.001 inches (0.0254 mm) are extremely challenging.
What Are the Standard CNC Machining Tolerances for ISO 2768?
We use a standard for tolerances in angular and linear dimensions, which minimizes inconsistencies and determines machining tolerances. Drawings have two categories – Angular and Linear General Tolerances, with four precision levels – Very coarse (v), coarse (c), medium (m), and fine (f). These levels are also called the four tolerance classes in DIN ISO 2768. Geometrical tolerance defines precision-level features for tolerance classes H, K, and L. For instance, a part drawing labeled ISO 2768-mK shows a medium tolerance range and tolerance class K.
Conclusion
Enze guarantees precise CNC machining according to your specifications. This guide can help you achieve the necessary tolerance. Contact us if you have more questions.
