A Complete Guide to CNC Machining Tolerances
By Lucas Lo | Updated: Mar. 18, 2025
Table of Contents
As we all know, though precision is important in machining process and decides the availability of features, it is impossible to make the features manufactured as same as they are designed. But designers can confine the permissible deviation scale between actually manufactured features and ideally designed one. That is where CNC machining tolerances come into play.
CNC(Computer Numerical Control) machining is one of the most popular manufacturing techniques in various industries. It relies on the use of computer programming and electro-mechanical devices to automate kinds of features as demanded.
What’s more, extreme high level of precision can be achievable through CNC machines if needed. However, CNC close tolerance machining services are generally expensive and are not necessary in many cases.
Therefore, designers and manufacturers are supposed to comprehensively know the concepts of machining tolerances, correct usage of them and considerations for machining tolerances choosing. You can get a complete guide on CNC machining tolerance in this blog.
What Are Machining Tolerances?
Machining tolerance is the acceptable value scope of deviation from the original designed feature dimensions indicated on drawings. Machining tolerances stand for the precision of manufacturing processes and products. The lower the values of machining tolerances are, the higher the accuracy level would be.
However, since there is no perfect manufacturing, the value of machining tolerances could never be zero in actual practice. What we could do is make the value be low as soon as possible as needed. In modern industries, CNC machining is such a beneficial manufacturing technique to low down the values.
In general, the CNC machining tolerances would be expressed in the form of “±0.x”.
How to Interpret and Calculate CNC Machining Tolerances?
If you want to apply machining tolerances correctly in your manufacturing, you should know how to interpret and calculate them first. The essential terms of machining tolerances are as follows:
1. Basic Size
The basic size of a feature is the dimension designed originally by engineers, standing for the perfect theoretical size of features. Generally, it would be indicated with tolerances on engineering drawings.
2. Actual Size
The actual size is the dimensions of the final manufactured features after machining process. Because of the limitation of material and technical machines, it is impossible to make the final features completely as same as the ideal designed one. That’s to say, there is always deviation between basic size and actual size.
3. Limits
Limits refer to range of the maximum and minimum permissible dimensions of features. The upper limit stand for the maximum permissible dimension while the lower limit stand for the minimum permissible dimension. Pay attention to that the actual size of the final manufactured features must be controlled within these two limits, or they would be list into rejects.
4. Deviation
Deviations refer to the variations of the maximum permissible size from the basic size. Because there are two types of limits, naturally, there are two types of deviations: the upper deviation and the lower deviation. They are calculated as follows:
Upper Deviation=Upper Limit-Basic Size
Lower Deviation=Lower Limit-Basic Size
5. Datum
Datum is a term in physics, referring to a hypothetical point, line or plane picked arbitrarily as a reference for measurement tools. More importantly, it is also popularly used in various kinds of geometrical dimensioning and tolerancing fields.
6. Maximum Material and Least Material Conditions
Maximum Material Condition(MMC) and Least Material Condition(LMC) are two key concepts in Geometrical Dimensioning and Tolerancing(GD&T).
MMC stands for the condition where a component of a part contains the maximum account of material within its specified tolerance limits. The largest shaft and the smallest hole within allowable size are typical examples of MMC.
And when a feature is at MMC, it allows additional geometrical tolerance( bonus tolerance) as the feature deviate from MMC.
Bonus tolerance=MMC-Actual Size
While LMC stands for the condition where a component of a part contains the minimum account of material within its specified tolerance limits. The smallest shaft and the largest hole are typical examples of LMC.
Since MMC is the worst condition scenario where the components would still fit, it generally used when the assembly and fit between mating parts matter most.
While LMC mostly used when the primary concern is material strength, clearance and walk thickness to ensure a feature has enough material to withstand stress.
7. Tolerance Calculating
After learning about those basic terms of CNC machining tolerances, we can enter into calculating tolerance. To calculate machining tolerances, what we should know basically is the upper limits and the lower limits required for features.
The formula is as follows:
Tolerance=Upper Limit-Lower Limit
For example, let us consider a shaft of its basic size diameter 8mm, and its permissible variances lie between:
Upper Limit: 10mm
Lower Limit: 6mm
Tolerance=10-6=4mm
Pay attention to that there is also situation where the upper limits and lower limits are not indicated directly, but present in the form of variances, like 8±2mm. In such a case, the upper limit and lower limit can be figured out by adding and subtracting the variance respectively.
Types of CNC Machining Tolerances
Tolerances of CNC machining are various for different geometrical features and machining processes. The most common CNC machining tolerances are as follows:
1. General Tolerances
General tolerances are popularly applied in many machining processes for common manufacturing features. In such cases, designers often do not specify their own tolerances of features on engineering drawings, but indicate the general tolerances.
In general, the scope is authoritatively standardized by various international organizations, such as the International Organization for Standardization(ISO).
For CNC machining, ISO 2768 MK is considered to be as the standard CNC machining tolerance. To learn more about ISO2768 tolerance, please refer to our blog What is ISO 2768.
2. Unilateral Tolerances
Just as the name imply, unilateral tolerances allow deviations occurring only in one direction. That is to say, either the upper limit or the lower limit is the same as the basic size.
Unilateral tolerances are crucial in the fit between two mating parts, such as a shaft with a hole. For instant, consider a shaft of basic size diameter 10mm that needs to fit into a hole of the same size.
Even an extreme little dimension would not be allowed to exceed, or it could not fit into the hole. But deviation can not be avoided. Therefore, unilateral tolerances come into effect.
Though the above shaft can not be larger, it is allowed to be smaller to some extent. If the maximum deviation of 0.05mm is accepted, the tolerance range of this shaft is set as 10+0.00/-0.05mm.
3. Bilateral Tolerances
Different from unilateral tolerances, bilateral tolerances allow deviations in both directions. The deviation can be either positive or negative, allowing the features to be little larger or smaller than required. Generally, the variations in both directions are equal, usually expressed in the form of ±0.xmm.
Below is a drawing showing the unilateral tolerances and bilateral tolerances.
4. Limit Tolerances
As another common expression of CNC machining tolerances, limit tolerances are not described by any symbolic language like + or -, instead, it is a range of values within which the any sizes of manufacturing features are acceptable.
The two extremes of the range are respectively upper limit and lower limit. The basic size does not matter that much. The only requirement is to make the actual size fit within the limits indicated.
For example, 9.5-10.5mm is a limit tolerance of CNC machining, indicating that the upper limit is 10.5mm and the lower limit is 9.5mm, and the any feature whose actual dimension is within 9.5-10.5mm is acceptable.
Pay attention to that though limit tolerances are expressed different from bilateral tolerances, their outcome is going to be the same. The key is that limit tolerances are easier to interpret and cut out needs for calculation.
5. Geometric Dimensioning and Tolerancing(GD&T)
Geometric Dimensioning and Tolerancing is a standardized system for machining. It defines the permissible variances in the form, size, orientation, and location of features. It helps to ensure the parts function correctly and be manufactured consistently.
Typical CNC Machining Tolerances
CNC machining covers various processes with kinds of tolerances due to different CNC machines or cutting tools types. Below is a common standard CNC machining tolerance chart with different CNC machines.
Processes | Tolerance Standards |
Milling (3-axis) | ± 0.13 mm or 0.005” |
Milling (5-axis) | ± 0.13 mm or 0.005” |
Lathe | ± 0.13 mm or 0.005” |
Router | ± 0.13 mm or 0.005” |
Router (Gasket Cutting Tools) | ± 0.762 mm or 0.030” |
Engraving | ± 0.13 mm or 0.005” |
Screw Machining | ± 0.13 mm or 0.005” |
Steel Rule Die Cutting | ± 0.381 mm or 0.015” |
Rail Cutting Tolerances | ± 0.762 mm or 0.030” |
Surface Finish | 125RA |
After reading the above chart, you may find that tolerance of CNC machines is much tighter compared with other manufacturing technology.
Considerations for Machining Tolerances Choosing
Since the tolerances used in machining processed have decisive effect on the final products, it is important to elect suitable tolerances for manufacturing features. Here are some crucial factors needing to consider.
Material
The material properties make huge impact on the achievable tolerances in CNC machining. These properties commonly include abrasiveness, hardness, and heat stability.
Abrasiveness
CNC machines would be hugely affected by extremely rough or coarse materials, and the wearing speed would be faster because of those material. Therefore, it is challenging to make features used such materials achieve high precision level as cutting tools change during the machining processes.
Hardness
It is difficult to manufacture soft material to a high precision level. Once the cutting machines touch them, their dimension would change a lot to some extent. That is to say, much more patience and time is needed during machining soft material.
After heat treatment, CNC machined steel parts have a higher hardness and are difficult to process thus obtaining close tolerances requires more techniques and time.
Heat Stability
It is a key factor commonly considered when machining non-metals. The shape of materials with low heat stability would gradually change as the heat increases during the machining process. For such material, to achieve high precision is also difficult and costly.
For CNC machining plastics, it is very difficult to obtain 0.001mm close machining tolerance, because the plastics are easily deformed by heat during CNC machining process.
Take the CNC machined Telflon parts for exmaple, the dimensions of the finished Telflon parts are differenct at 20℃ and 30℃. CNC plastic parts with tight tolerance up to 0.005 can be challenge for many CNC machining shops.
Machining Techniques
The machining techniques also would influence the tolerances of features as different surface features and roughness would be carried out by various machining techniques. Machining techniques like turning, milling and grinding have their own capabilities and limitations respectively.
In the chart above, we show the accuracy that can usually be achieved by different CNC processing machines.
Surface Finishes
Pay attention to that finishing operations such as painting, anodizing, and plating can also influence the final precision level. These surface finishing techniques can change the dimensions of features. Though the change may be very small, it still needs to be considered carefully.
Cost
For any manufacturing, cost is one of the most important factors to be considered. Manufacturers often choose the least tolerances within budget, which would balance cost and precision well.
When to Use CNC Close Tolerance Machining
Generally, using CNC close tolerance machining means getting high precision level. But it is not necessary to choose CNC close tolerance machining services for any products. You should comprehensively consider the above factors, and the most crucial one is where would the features be applied in. You should know that CNC machining with tight tolerances is very costly and time-consuming.
We usually recommend using ISO2768MK as the standard tolerance, while some key dimensions in 2D can be marked with closer or tighter tolerances.
Conclusion
Machining tolerances really have decisive effect on the features function. If you can apply machining tolerances correctly and smoothly, you will get lots of benefits like good precision, lower cost and shorter machining time etc. Therefore, read this guide carefully and apply suitable tolerances in CNC machines and other manufacturing. ECOREPRAP is always here to help you.
As we have talked about CNC machining tolerances, we will introduce engineering fits,GD&T symbols and the QC equipment to check the tolerances in the coming blogs.
Lucas is a technical writer at ECOREPRAP. He has eight years of CNC programming and operating experience, including five-axis programming. He also spent three years in CNC engineering, quoting, design, and project management. Lucas holds an associate degree in mold design and has self-taught knowledge in materials science. He’s a lifelong learner who loves sharing his expertise.
