The difference between precision and accuracy in CNC machining , their importance, and methods for improvement

In the field of CNC machining, “accuracy” and “precision” represent two different meanings. A batch of parts can be accurate but not precise, and vice versa. They mainly involve the correctness and repeatability of dimensional annotations, which are crucial to the overall quality of CNC parts and product machining. Although they are independent of each other, their combined measurement determines the precision or functionality of the parts.

Understanding the difference between accuracy and precision is highly beneficial, as it helps in making informed decisions about custom CNC parts. It also enables improvements to current manufacturing processes and the implementation of effective quality control measures. This article will guide you through understanding all the relationships between precision and accuracy.

What is precision in machining and how is it defined?

“Accuracy” refers to the degree of closeness between machining measurement results, that is, the repeatability of values after multiple trials. Accuracy in machining has two important aspects: repeatability and reproducibility. Repeatability refers to the consistency of parts produced by a specific machining process or operation. Repeatability refers to the closeness of parts derived from the same design but using different machines, settings, and tools. This means that even if tools or other settings change, the parts will not deviate from each other. Therefore, precision machining = repeatability + reproducibility . In general , CNC machining accuracy refers to the degree of conformity between the actual dimensions, shape, position, etc., of the machined part and the design requirements. It typically includes the following types of accuracy:

• Dimensional accuracy: Dimensional accuracy refers to the deviation between the actual dimensions of a machined part and its design dimensions. It is commonly expressed in terms of tolerance zone; the smaller the tolerance zone, the higher the dimensional accuracy.

• Shape accuracy: Shape accuracy refers to the deviation between the actual shape of a machined part and its designed shape, including roundness, flatness, and straightness. The smaller the shape error, the higher the shape accuracy.

• Positional accuracy refers to the deviation between the actual position of a machined part and its designed position, including coaxiality, symmetry, and parallelism. The smaller the positional error, the higher the positional accuracy.

What is accuracy in machining ?

Accuracy is another measure of observation error. It refers to how close a measured value is to a specified value. If the measurement result is the same as the theoretical value (true value), the measurement result is perfectly accurate. In the field of CNC machining, to determine the accuracy of a part, the difference between the calculated value and the value provided in the blueprint must be compared. The smaller the value, the higher the accuracy, and vice versa.

Machining accuracy refers to how close the dimensions or other properties are to the values stated in the initial CNC machining design. It comprises three key components: deviation, stability, and linearity. Therefore, manufacturing accuracy is not only about achieving target values, but also about ensuring that the process remains unbiased, stable, and linear under all operating conditions. Determining the measurement accuracy of a part typically involves comparing the actual measurements of the part with the expected design specifications. Elimold typically uses the following tools and techniques to ensure part accuracy.

• Coordinated Measuring Machine (CMM): CMMs provide precise measurements of parts to verify that they meet required dimensions. These machines can detect deviations as small as a few micrometers .

• Statistical Process Control (SPC): SPC charts track the process over time. If data points consistently cluster around the target value, the process is accurate .

• Measurement repeatability and repeatability (Gage r&R): While primarily used for measuring accuracy, Gage r&R can also help identify systematic errors that affect accuracy .

What are CNC machining tolerances?

Machining tolerance refers to the allowable range of deviation from the expected size. It is usually expressed as “±”. For example, if the drawing tolerance for a 10 mm shaft is ±1 mm, it means that the length of the shaft after machining should be between 9 and 11 mm. Therefore, tolerance determines the precision of part machining. Maintaining this allowable deviation range across multiple production batches is known as precision machining.

In addition to linear tolerances (linear, angular, and radial), there are other types of tolerances; roundness, perpendicularity, flatness, etc.Tolerances play a crucial role in the assembly of parts. Furthermore, tolerances promote consistency in mass production.Furthermore, design tolerances are also related to manufacturing costs. Designs with strict tolerances require precision CNC machine tools for machining.

Interaction between machining tolerances, precision, and accuracy

Precision and accuracy are interrelated and help determine measurement quality. Tolerance accuracy refers to the allowable variation of the final part dimension relative to the ideal value. CNC machining processes are never perfect, and tolerance accuracy specifies the permissible deviation of the result from the standard. When a system is accurate and yields a value close to the true value, it is likely within acceptable tolerance limits. However, a system can give precise results, but still exceed the permissible tolerance range.

When part tolerance limits are not explicitly specified, most machinists will define tolerances for metal and plastic parts as +/- 0.005 and +/- 0.01 inches, respectively. If the dimension is 20,000 inches and the tolerance is +/- 0.005, then the tolerances for all metal parts must be between 19.995 and 20.005 inches. Note that machinists do not always strive to achieve the most stringent tolerances possible. More lenient tolerances can be used when the features on a part are not critical to safety.

Precision and accuracy are so important in CNC machining projects

Precision in the manufacturing process has three important aspects: functionality, performance, and fit during assembly. Furthermore, precision is crucial for the consistency of parts across multiple batches.

Precision machining	It refers to the production of mechanical parts with high precision and repeatability. Therefore, precision CNC machining uses various instruments to measure accuracy and precision to ensure that the required standards are met.
Calibration of manufacturing equipment	Any systematic error in the manufacturing process indicates a defect in the machinery, tools, or instruments used to check precision. This acts as a notification, reminding you to recalibrate the manufacturing and measuring equipment.
Reduce material waste and improve reliability	Measuring the precision of individual machined parts helps identify defective parts and allow for immediate adjustments to the production process. Early identification of defective parts can reduce waste.
Parts assembly	Precision components are crucial for a smoother, faster assembly process. Precise components work together effectively, reducing downtime and labor costs. Ultimately, this improves the overall efficiency of the production line.

The main difference between precision and accuracy

We can objectively compare precision and accuracy from three different aspects: their definitions, error types, and measurement.

Different definitions	Precision refers to the degree of closeness between a set of measurement data. Therefore, the closer the values are, the higher the precision. Accuracy refers to the degree of closeness between a measured value and the true or reference value.
Error type classification	Systematic error and random error are two types of error. Random error is related to precision, while systematic error is related to accuracy.
random error	These errors are caused by unpredictable factors, and the measurement results only show fluctuations. This is due to limitations in the measuring instruments or technology. For example, the performance of CNC technology itself can limit the accuracy of dimensions marked on drawings.
Systematic error	Systematic errors are caused by temperature fluctuations, tool wear, misalignment, G and M code errors, setting errors, and defects in measuring instruments. As systematic errors increase, machining accuracy decreases.
Quantity of measurement	High precision means the accuracy of the measurement dataset (numerical values are close to each other). Therefore, standard deviation or variance quantifies precision. On the other hand, accuracy involves comparing the average of a single measurement or multiple measurements with a reference value.

Factors affecting the manufacturing precision and accuracy of CNC parts

The accuracy and efficiency of CNC machining are influenced by a variety of factors, especially when manufacturing large batches of parts. These factors include the physical properties of the materials, the structure and precision of the machine tools, the material and shape of the cutting tools, and the settings of the machining parameters. A proper combination and optimization of these factors can improve the accuracy and efficiency of CNC machining, thereby achieving higher quality and more efficient processing.

Machine tool manufacturing errors

Errors include spindle rotation error, guide rail error, and transmission chain error, which may be caused by uneven wear, installation quality, manufacturing and assembly errors of components, etc., and directly affect the accuracy of the workpiece being processed.

External force on machine tools

When force is applied to an object, a force that resists deformation is generated; this is called rigidity. For high-precision machining, the rigidity of a machine tool is crucial; the greater the rigidity, the higher the dynamic accuracy. Machine tool rigidity includes the rigidity of the machine tool itself and the contact rigidity between components. The former mainly depends on the material, cross-sectional shape, and size of the machine tool, while the latter is influenced by the contact material, geometry, hardness, machining method, and surface roughness. Dynamic and static rigidity are also contributing factors.

Heat distortion

Objects expand with temperature changes, and different materials are heated to varying degrees. Therefore, it is crucial to pay attention to temperature changes during processing, especially as the operating time increases. The deformation caused by temperature cannot be ignored. Understanding the relationship between machine tool operating time and temperature is very important for accuracy.

Use of cutting fluid

Cutting fluid is very important for machine tools, as it can provide effective protection for workpieces and cutting tools, reduce tool wear, and improve the machining accuracy of workpieces.

Effect of rotational speed

Under normal circumstances, the higher the cutting speed, the higher the efficiency. However, it is also constrained by factors such as the hardness, plasticity, carbon content of the workpiece, and the hardness and geometric properties of the cutting tool. Therefore, the speed should be adjusted according to the different materials of the cutting tool, and the resonance zone of the part should be avoided to prevent affecting the machining accuracy.

How to improve the precision and accuracy of the machining process?

The overall precision and accuracy are determined by the design phase, machine capabilities, machining variables, setup and tooling, and every other stage of CNC machining. Therefore, many factors need to be considered to improve precision and accuracy.

Incorporating Design for Manufacturability (DFM) principles

Design for manufacturability (CMAN) refers to creating a 3D model that perfectly matches the performance of a high-precision CNC machine tool, selecting the optimal materials, simplifying the design, and adhering to relevant standards. It aims to maintain consistent geometric dimensions and tolerances (GD&T) and reduce the complexity of tooling operations. Only designs compatible with existing machine tools and tooling can manufacture precise and accurate parts.

Ensure that the part design meets strict tolerance limits.

When dimensioning and specifying tolerances for parts, ensure that the specified tolerances are very close to the achievable range. If the tolerance of a CNC milling machine can be as low as ±0.13 mm, then you can set the tolerances in the design to be closer to that value, such as ±0.125 mm, ±0.120 mm, etc. Furthermore, tolerances should be consistent throughout the entire drawing.

Optimize part machining cutting parameters

The selection of machining cutting parameters should be based on the workpiece material, geometric complexity, and tool type. Different parameters such as feed rate, spindle speed, depth of cut, and coolant flow rate have varying effects on accuracy and precision. Another factor is the toolpath and G&M code, which determine the final dimensions of the part being machined. Therefore, the toolpath needs to be simulated and optimized in computer software to achieve high precision and accuracy. Furthermore, the accuracy of the machining tool also affects the machining results. Any tool wear, slight deviations, or overheating can lead to numerous defects or even complete failure.

Implement effective clamping technology

Effective fixtures not only secure the workpiece and cutting tool, but also ensure the stability and rigidity of the machine tool base and frame. Precision fixtures can firmly hold the workpiece and eliminate tool movement, vibration, and misalignment errors. Furthermore, the experience of the fixture setter also affects accuracy.

Achieving consistent alignment using motion coupling

The main advantages of motion coupling lie in its superior repeatability and reduced interchangeability errors. Furthermore, the minimal contact points provide greater freedom of motion for dynamic machining processes. Another characteristic of motion coupling is that alignment is unaffected by clamping forces. Even if the clamping force changes, the workpiece remains aligned with the tool.

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in conclusion

Precision and accuracy are two different terms with different meanings. Precision concerns consistency and repeatability. It indicates how close two or more parts are to each other. Accuracy, however, refers to how correctly a part’s dimensions match the blueprint. Both accuracy and precision are important in the manufacturing process, ensuring quality and improving customer satisfaction.

Different calibration and measurement systems work together to ensure proximity and repeatability. These measurements are not only crucial for maintaining the functionality and performance of manufactured parts, but also contribute to continuous improvement, waste reduction, and cost reduction. While pursuing high precision and efficiency, Xtproto also closely monitors technological developments and innovations. Artificial intelligence and the Industrial Internet of Things (IIoT) play a significant role in improving efficiency and optimizing operations, effectively utilizing modern technologies to meet ever-changing manufacturing needs.

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