In certain industries like the medical or aerospace industry, even a few microns of deviation can cause catastrophic assembly failures. The CNC machine tolerance directly impacts the quality, reliability, compliance and profitability of a manufacturing process.
More importantly, the knowledge of tolerance helps manufacturers to choose the right production process and equipment. Therefore, before starting the next machining project, read this guide on how to optimize the production process.
What is CNC Machine Tolerance?
The acceptable range of dimensional variation that a CNC machine can achieve during the production process is the machine’s tolerance. Simply put, the machine’s tolerance determines how accurate the machine is in terms of the expected dimension of a manufactured part.
With this knowledge, it is easier to optimize a specific manufacturing process to meet the specific project requirements. For instance, the Swiss CNC machining process is mostly common in medical or dental devices, the aerospace and semiconductor industries.
Why CNC Machine Accuracy Matters
· Product Quality
The machine’s accuracy plays an integral role in product quality, as it determines the accuracy of part dimensions. Dimensions are especially critical as it influences how effectively you can use the manufactured product as well as the parts’ durability. Elevated precision also minimizes the possible defects that occur in a part and eliminates the need to adjust the parts, saving on time and materials.
· Part Interchangeability
Tight tolerance machining also ensures that the quality of the parts produced remains consistent throughout the manufacturing process. That is to say, the dimensions of the first piece you produced should be the same as the thousandth part produced, and can easily be interchanged without any customization. This is especially key for sectors that require industry-compliant parts such as medical components, aerospace, and automotive parts.
· Assembly Accuracy
Assembly is typically the last test carried out after the production process, and it is done by joining the parts together as intended by design. Assembly accuracy is therefore how perfectly these pieces can fit to create the final product, and it tests the accuracy of the minor pieces involved. Failure to fit, if the pieces fail to properly align or there are small gaps between the pieces, is viewed as failure.
Failing the assembly accuracy often leads to rejection of these parts, causing you both time and materials. It also causes a delay in production as the parts need to be remade.
· Industrial Compliance
There are several industries in which all parts are regulated using industry standards and often require certification. This is done to ensure that all your components meet the set standards and are therefore safe to use, and it also ensures traceability, which fosters accountability. Industries such as aerospace, medical, and automotive are regulated for safety reasons and therefore can only utilize industrial-compliant parts.
· Customer Satisfaction
Customer satisfaction is one of the key metrics that you need to consider, and it is ensured by providing high-quality parts. If you are able to offer standard products within the set timeframe and offer competitive prices, then you can ensure customer satisfaction. Offering after-sales services also gives you important feedback, allowing you to improve the quality of your products and increase repeat customers.
Common CNC Tolerance Requirements by Industry
| Industry | Recommended Tolerance for Each Part in inches |
| Aerospace | · ±0.0002″ · +0.0000/-0.0005″ for critical components |
| Medical | · ±0.001″ – ±0.003″ · ±0.001″ for Small parts · ±0.002″ – ±0.003″ for Larger parts |
| Automotive | · ±0.001″ – ±0.005″ · ±0.005″ for Standard parts |
| Electronics | · ±0.002″ – ±0.005″ · ±0.003″ for PCB drill diameter and advanced manufacturing |
| Oil and gas | · ±0.002″ – ±0.005″ |
| General manufacturing | · ±0.002″ – ±0.005″ · ±0.005″ for Standard CNC |
Types of CNC Tolerances
- Dimensional Tolerances
The acceptable variance in physical measurements of the parts produced by your CNC machine is referred to as dimensional tolerance. The physical dimensions in question are the angle, diameter, length, width, and height of the manufactured parts. There are three main types of dimensional tolerance, which are:
Limit Tolerances: This dictates the range that is acceptable for the parts you produce. Assuming the nominal size is 0.60mm, then the upper limit will be 0.65mm while the lower limit is 0.55mm.
Unilateral Tolerance: This refers to the permissible positive or negative variation the parts you machine can have in one direction. Acceptable range falls between1.500 mm and +0.000/-0.005 mm.
Bilateral Tolerance: Acceptable variation when it comes to the symmetry of these parts is the bilateral tolerance. This value represents the acceptable positive and negative variation and should vary between ±0.0005 mm.
- Geometric Dimensioning and Tolerancing (GD&T)
This refers to select symbols used in the design and engineering drawings to accurately highlight the acceptable variations for the various parts, as well as the intention of the design. These symbols guide you in the manufacturing of the parts and are typically standardized to allow for uniform meaning.
Using these symbols, you can determine the shape, size, acceptable tolerance, and the geometrical properties of the parts you are producing.
For manufacturers using CNC machines to fabricate parts in North America, the GD&T practices are governed by the ASME Y14.5. It stipulates the uniform rules, symbols and definitions that help in interpreting engineering designs.
- Surface Finish Tolerances
It is the texture of the surface that determines the surface finish tolerance. This tolerance helps you ascertain how much texture is acceptable for various components. Various measurements are factored into this type of tolerance, such as:
- Arithmetic Average that denotes the acceptable Average roughness height
- Root-Mean-Square is used to determine the acceptable Roughness
- Maximum Height, which measures the peak-to-valley difference
Standard Tolerance Values
There is no fixed standard tolerance value; it varies depending on various factors such as the dimensions, geometry, shape, and size of the parts. However, there are standard CNC machining tolerances, which are as follows.
| Tolerance Type | Standard Value |
| Basic Linear Dimensions | ±0.005″ |
| Hole Locations | ±0.005″ |
| Standard | ±0.005″ |
| High Precision | ±0.001″ – ±0.002″ |
Factors Affecting CNC Machine Tolerance
Several factors affect CNC machine tolerance, including:
- Machine-Related Factors
The type of machine you have influences the range of tolerance you can achieve. General machines typically have a tolerance of ±0.005″ whereas high precision machines give you ±0.001″ – ±0.002″. Machine features such as the quality of the spindle and the machine’s frame also influence the machine’s tolerance, as they determine the machine’s interaction with the part to be machined
- Tooling Factors
For you to achieve tight tolerance, you first need to utilize high-quality specialized tools designed specifically for precision machining. The tools also need to be regularly maintained and replaced if necessary, as precision machines cause tools to wear at a faster rate.
- Material Factors
Hard materials can maintain dimensional accuracy, which makes them suitable for critical applications. However, to achieve tight tolerances during the machining process, they require specialized tools and higher cutting forces.
. On the other hand, if you are using softer materials such as aluminum or plastic, given their flexibility, they are more likely to shift, leading to looser tolerance.
- Process Factors
Faster cutting speed and feed rates can result in some errors, whereas slower cutting speed and feed rates mean more time spent on individual parts, minimizing any error. Temperature control is key, as high temperatures during the production process may result in thermal expansion, leading to error.
How CNC Tolerance Affects Manufacturing Costs
Tolerance in CNC machines directly impacts the manufacturing cost in various ways, including:
Time
Producing components with tight tolerances often requires more time. In order to increase precision, the production process is slowed to eliminate any errors that may occur as a result of rushing. More time translates to fewer products produced while expending more energy, which in turn drives up the production cost.
Special Tools and Machines
Specialized high-quality tools and precision cost more to purchase, which directly influences the production cost. Maintaining these machines also costs more as they are more intricately designed in order to achieve the results, which further increases the operational cost.
Higher Waste
Given the small margin of error for tight tolerance parts, produced components that fail to meet the standard are often scrapped, leading to a lot of wasted materials. Therefore, more material is utilized to design these components.
Tool Wear
Precision machining is a demanding production process that takes its toll on the cutting tools, leading to the tools wearing out at a faster rate. This means that these expensive tools need to be regularly replaced.
Costlier Materials
Materials compatible with tight tolerance machines typically are harder with a higher quality, thereby costing more.
How to Measure Tolerance in CNC Machines
Measuring a CNC machine involves the following steps:
- The first step is to inspect the geometry and the position of the part, which is then followed by assessing the machine’s dynamic behavior.
- You can then use a dial indicator or a laser interferometer to ensure that the axis is accurately placed.
- Then execute a command to move the machine before returning to the initial position, mimicking a continuous production to test repeatability.
- Using this command also accesses the machine’s ability to accurately return to its original position
- Verify the accuracy of its axis and perpendicular alignments
- Test the spindle runout by accurately measuring several points along the rotation axis of the spindle.
- Conduct a test run on a material to determine the accuracy of the cutting tools
Best Practices for Achieving Tight CNC Tolerances
There are several practices that you can implement to obtain tight tolerance; they include
Design: When generating the initial design, you need to ensure that it factors in the tolerance. Achieving tight tolerance tends to cost more; you should therefore include this feature for integral parts only. Sections that require tight tolerance should be rigid to ensure the desired dimensions are achieved.
Machine Selection: When selecting the machine to use, ensure that you take the desired tolerance in mind, as not all machines are designed to achieve the same result. For tight tolerances, high-precision machines are more suitable.
Maintenance: You should also factor in the required maintenance practices, as they are essential in maintaining the machine’s tight tolerance.
Tooling Quality and Strategy: Ensure that you utilize high-quality tools, as they are more precise. During the production process start with the rough product before finishing it smoothly.
Workholding: Ensure that your workpiece is properly fixed onto the worktable to prevent it from shifting during the cutting process, leading to wrong dimensions.
Process Optimization: Ensure that you factor in all the required details before you initiate the production process, and keep a keen eye on the entire production process
Environmental Control: Ensure that all the environmental conditions in your manufacturing area are stable and conducive to the material and production process.
Material Selection: Tight tolerances are easier to achieve when using hard materials as opposed to soft materials, which may flex during the production process.
CNC Accuracy Testing and Calibration: Conduct regular tests using the recommended equipment and calibrate the machine as required to maintain precision
FAQs
What is the standard CNC Machine Tolerance?
The standard tolerance your CNC machining should have is 0.005″.
How does tolerance vary in various CNC machining operations?
Tolerances in these machines vary depending on the material, type of production, and the machine precision.
Can the manufacturer maintain consistent CNC machine tolerances in high-volume production?
It is possible for you to maintain consistent CNC machine tolerance, but it is a challenge for high-volume production.
What is the difference between CNC machine accuracy and repeatability?
The machine’s accuracy is determined by how precise it is in following the original design. Whereas repeatability is the machine’s ability to continuously maintain that precision for each part.
Can all CNC machines achieve tight tolerances?
Not all CNC machines are able to have tight tolerances; this ability is largely impacted by the type of machine you have.
Are there ISO standards for CNC machining tolerances?
Yes, there is the ISO 2768 for general tolerance, ISO 2768-2 for geometric tolerance, ISO 286 for systems of limits and fits.
How does material selection affect tolerance in CNC machines?
Soft materials are more flexible and are therefore less likely to have tight tolerances, whereas rigid hard materials tend to have higher tolerances.
Looking for Right CNC Machine for Tight Tolerance Manufacturing?
TSINFA is your trusted CNC machine manufacturer in China. From standard to high-precision CNC machining, the TSINFA team can help your factory achieve high dimensional accuracy, consistent machining and to achieve high quality part finish. Let us know your industry, and we will recommend the right CNC machine.
