CNC machining tolerance is very important in CNC machining because it decides the cost, time for your product. This article will help you better understand the details and how to choose CNC machining tolerance for different parts and applications, especially in precision manufacturing and functional design.
What is CNC Machining Tolerance?
CNC machining tolerance is the allowable dimensional deviation of your parts, which is specified in the drawing. Due to the inevitable manufacturing errors caused by different machines, tools, materials, and processing environments, it is impossible to achieve an absolutely perfect theoretical size.
Setting reasonable tolerance can not only ensure the assembly and usage functions of your parts, but also balance the processing difficulty and production cost.
For example, ±0.01mm tolerance is a high-precision narrow tolerance and is used for precise mating parts, while ±0.05 mm tolerance is the industry-standard general tolerance and is applicable to the vast majority of non-critical structural dimensions.
Why is CNC Machining Tolerance So Important?
Assembly Accuracy
If your product involves the assembly of multiple parts, then tolerances are the basis enabling these parts to be assembled smoothly. If the tolerances of each part deviate from specifications, your entire product or equipment may not be able to be installed.
Product Functions
If the product you are currently designing is a rotating component such as a bearing or a shaft, excessive dimensional variation will cause instability during operation and wear easily, while a too small size will prevent it from functioning at all.
Product Appearance
For enclosures or cases which involve top and bottom cases, excessive tolerances will result in large gaps, making the appearance extremely ugly. If your product aims for long-term stability, then the consistency of the chamfers, tolerances, and overall shape of the same batch and different batches becomes extremely important.
Cost & Lead Time
In general, tighter tolerance increases CNC machining cost and lead time, because it requires slower cutting speed, more inspection steps, and higher process control. The stricter the tolerance requirements are, the longer the processing time for your parts will be. Moreover, the labor costs for part processing and quality inspection will also increase accordingly.
This is why understanding CNC machining cost vs tolerance relationship is critical during the early design stage.
Key Factors Which Influences CNC Machining Tolerance
CNC Machine Centers
As a professional designer or engineer, you will find that machine condition determines the accuracy. Different machine capabilities directly affect CNC machining tolerance capability and accuracy range. The accuracy of the spindle, the transmission between the guide rail and the ball screw, and the rigidity of the machine tool all have a significant impact on the accuracy.
Spindle Accuracy
If the radial runout and axial runout of the spindle are too large, it directly causes deviations in roundness, hole diameter, and end face runout. When you run high-speed processing, the thermal expansion of the spindle will change the Z-axis dimensions.
Guide Rail And Ball Screw Transmission
The clearance of the ball screw, the wear of the guide rail, and the inconsistency of the tightness of the insert will cause reverse clearance. All these can easily let you find unstable step dimensions, contour tolerances and the thermal deformation of the ball screw causes positioning drift.
Repeated Positioning Accuracy
If you find large dimensional dispersion in different batches, insufficient accuracy of the optical encoder is the main reason and you will see the position of each cutting stroke is inconsistent.
Machine Tool Rigidity
If the rigidity of the machines, column, and spindle box is poor, you will obviously find cutting force will cause micro tremors, resulting in vibration patterns, inconsistent dimensions, and fluctuating sizes. And heavy cutting deformation is more obvious.
Materials
The machines determine the theoretical upper limit of the tolerance, while the material determines whether this tolerance can be consistently and stably maintained in mass production. For example, aluminum, stainless steel, and plastics all have different CNC machining tolerance capabilities and stability levels.
The thermal expansion, internal stress, cutting deformation, and tool wear of different materials are all different. Therefore, the processing tolerances for different materials are also different.
When you are machining soft, highly expandable, and highly stressed materials, the tolerance cannot be solely controlled by the machine tool hardware. Process optimization must be coordinated as well if you want stable tolerance for different batches.
Structures
If you are designing a simple 2D simple structure (such as simple blocks, planes, shallow holes, etc., which are solid and thick structures), the required tolerance can be easily achieved. And the machining part does not shake, so the size and flatness can be well controlled and the tolerance is the most stable.
But for structures with arcs and complex 3D inclined surfaces, two axes of the machine tool need to work synchronously. Even a slight gap in the machine’s ball screw can cause the arc to be not smooth. For three-dimensional inclined surfaces, you will find the three axes need to be linked together. Minor errors will accumulate, and the surface height is prone to fluctuation.
When you want to manufacture deep cavities and closed groove structures, the internal deep grooves and closed space have a narrow processing space. Only slender cutting tools can be used. The tool shank is soft, and during cutting, it is prone to deflection. The cutting fluid in the groove cannot enter and the heat cannot be dissipated. Therefore, the depth and internal shape tolerances fluctuate significantly.
For thin-walled structures, the rigidity of the parts is poor. During cutting, the tool deforms when squeezed. After processing, the internal stress gradually releases, and the size will continue to change after several hours. The wall thickness and flatness are difficult to stabilize, and the tolerance fluctuates greatly.
This is especially important for you to consider thin wall CNC machining tolerance, deep cavity machining, and complex geometry parts.
Typical CNC Machining Tolerances
Different processes have different CNC machining tolerance capability ranges, depending on machine type and cutting method.
CNC Milling Tolerances
For CNC milling machining parts, the general tolerance is ±0.10 mm and standard CNC machining tolerance can be controlled within ±0.05 mm. The key dimensions ( such as holes ) can reach ±0.01 mm.
CNC Turning Tolerances
Due to the machining in a rotating state, CNC turning part usually has higher dimensional consistency, with the standard tolerance controlled within ±0.02 mm, and the key dimensions such as precision shafts and hole diameters can reach ±0.01 mm. These ranges represent typical CNC machining accuracy and tolerance capability in production environments, not theoretical limits.
Standard Tolerance System
For parts whose tolerances are not specified in the drawings, the industry usually follows the ISO 2768-m general tolerance standard. ISO 2768 is the most commonly used general tolerance standard for CNC machining drawings and engineering parts.
Please check the following chart for your reference.
ISO 2768-M General Tolerances Chart
| Feature Type | Nominal Dimension | Tolerance |
| Linear Dimensions | 0.5 – 3 mm | ±0.10 mm |
| > 3 – 6 mm | ±0.10 mm | |
| > 6 – 30 mm | ±0.20 mm | |
| > 30 – 120 mm | ±0.30 mm | |
| > 120 – 400 mm | ±0.50 mm | |
| > 400 – 1000 mm | ±0.80 mm | |
| > 1000 – 2000 mm | ±1.20 mm | |
| Angular Dimensions | Up to 10 mm (shorter side) | ±1° |
| > 10 – 50 mm | ±0°30′ | |
| > 50 – 120 mm | ±0°20′ | |
| > 120 – 400 mm | ±0°10′ | |
| Radii & Chamfers | 0.5 – 3 mm | ±0.20 mm |
| > 3 – 6 mm | ±0.50 mm | |
| > 6 mm | ±1.00 mm |
How to achieve 0.01mm CNC Machining tolerance?
Achieving tight tolerance CNC machining such as ±0.01mm requires control of machine, tooling, material, and process stability.
Step 1 Control Workshop Environment and Machine Tool Accuracy
The workshop should maintain a constant temperature and calibrate the gap of the machine tools regularly. When choosing the machining centers, you should consider that the repeat positioning accuracy of the machine tools for the manufacturer should be at least within ±0.003mm.
Step 2 Optimize Tooling Configuration and Tool Management
You must ensure they use good tool holders and new tools. Please do try to keep the tool rods as short as possible. and tell them regularly to measure the dimensions. Replace the tools in time when they are worn out to reduce the single processing error.
Step 3 Standardize Clamping Operation and Eliminate Material Internal Stress
Do not clamp too forcefully and provide proper support for thin-walled parts. You can suggest to them the internal stress of the material before performing fine processing to prevent deformation after the processing is completed.
Step 4 Optimize Cutting Process to Reduce Processing Deformation
Step 5 Standardize Measuring Process and Make Program Compensation
Let the workpiece cool down completely before measuring. If the dimensions deviate, you can immediately ask your manufacturer to make fine adjustments to the program for compensation.
This level of precision is usually required in high precision CNC machining parts such as aerospace, automation, and precision assemblies.
How to mark the tolerance in the drawings?
Properly defining tolerances on drawings is a key part of CNC machining design guidelines and DFM.
Key Mating Dimensions
They directly determine the assembly tightness and motion clearance. If high precision is required in your design, you can mark ±0.005 to ±0.01mm because these are typically the most important functional tolerance dimensions.
If it is an aluminum thin-walled structure, the tolerance can be moderately relaxed to ±0.01 to ±0.015mm to avoid batch scrapping.
General Installation Dimensions
They are usually only for fixation, without precise mating. The tolerance can be ±0.02 to ±0.05mm, and there is no need to pursue high precision.
These are usually used for standard CNC machining tolerance settings in production drawings.
Free Non-functional Dimensions
They will not affect assembly and use. They are uniformly marked with free tolerance, and there is no need for you to give strict numerical values separately, which can reduce processing costs.
FAQ
What is the difference between ISO 2768-f and ISO 2768-m in CNC machining?
Both are common free tolerance standards for drawings.
ISO2768-M represents medium accuracy, and this is the default for most ordinary mechanical parts while ISO 2768-F represents high precision level and small tolerance ranges.
What is standard vs tight CNC machining tolerance?
For the assembly tolerances with strict requirements, you need to consider more stringent tolerance specifications. For parts without specific requirements, you can use standard tolerances.
What tolerance should I specify on a drawing?
You have to mark the strict/tight tolerance requirement on the drawings.
Does tighter CNC machining tolerance increase cost?
Yes, if the tolerance is tight. We have to consider different machining processes to realize it and spend more time machining, processing and measurement quality control.
Final Thoughts
Yecheng is a professional high-precision CNC machining and tight tolerance manufacturing supplier. If you need custom CNC machining parts with tight tolerance requirements such as ±0.01mm, please send us your drawings for quotation.
