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Precision vs Accuracy in Machining: Key Differences

October 23, 2024 · About 3 minutes
precision and accuracy in cnc machining
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How do you know whether the machined parts are precise and accurate or not? Different inspections and measurements are used to ensure this. However, it is essential to understand the concept of accuracy and precision first.

They mainly deal with the degree of dimensioning correctness and repeatability, which are critical for the overall quality of machining CNC parts and products. Although they are independent of each other, their collective measure decides the part’s precision or functionality.  This article will discuss differences in accuracy and precision, elaborating on how to implement these to achieve the desired results.

Let’s start it!

What’s Precision in Machining?

precision cnc machining

The term “precision” refers to how close the machining measurements are to each other. It means the repeatability of values across multiple trials. For example, if you read the diameter of shafts 6mm, 6.01mm, 6mm, and 6.02 mm in three consequent measurements, you can say it is highly precise. However, precision has two important aspects in machining: Repeatability and Reproducibility.

Repeatability means the consistency in parts from a specific machining process or operation. For instance, a milling machine setup creates multiple batches with the same features, dimensions, and surface finish.

Reproducibility is the closeness of parts converted from the same design but different machines, setups, and tooling. It means the parts do not deviate from each other even if the tools or other setups are changed. For example, a custom CAD model is convertible into physical parts using both milling and turning machining with closeness in dimensions.

So, Precision Machining = Repeatability +Reproducibility

What’s Accuracy in Machining?

accuracy inspection

It is the degree of closeness to the actual value in measurement. If the measurement is the same as the theoretical(true) value, the measurement is perfectly accurate.

Next, the machining accuracy refers to how close the dimension or other properties are to the mentioned value in the initial CNC machining design. It has three critical components; Bias, Stability, and Linearity.

AspectDefinitionImportance in Machining Accuracy
BiasDifference between measured and true/reference value (systematic error).It ensures accurate measurements by minimizing fixed deviations.
StabilityConsistency of measurements over time.It prevents variations due to environmental changes, tool wear, etc.
LinearityConsistency of measurement error across the range of operations.It ensures accurate results for different part sizes and dimensions.

Therefore, accuracy in machining is not just about hitting the target value, but ensuring that the process remains unbiased, stable, and linear across all operating conditions.

Key Differences Between Precision and Accuracy

Next, let’s objectively compare precision vs accuracy in three different aspects; their definitions, error type, and measurement.

Different Definition

The precision is the degree of closeness between a set of measured data. So,  as the values are near to each other precision increases. Meanwhile, accuracy is the closeness of the measured value with the true or reference value.

Error Type Classification

Systematic and random are the two error types, random error is associated with precision, and systematic with accuracy.

Random Errors

These are errors due to unpredictable causes, the measurements only show the fluctuations. It is the limitation of measuring instruments or technology. For example, the capability of CNC technology itself has some limitations in the exactness of the mentioned dimensions in the drawing.

Systematic Errors

Systematic errors are the result of fluctuations in temperature, tool wear, misalignment, G and M-code errors, fault setups, and measuring instrument defects. As the systematic error increases, the machining delivers poor accuracy.

Measurement Quantity

The high precision means a precise data set from measurement (values close to each other). So, standard deviation or variance quantifies the precision. On the other hand, accuracy concerns a single measurement or the mean of multiple measurements in comparison to a reference value.

Examples of Typical Precision and Accuracy in Machining

Although accuracy and precision both are related to measurement, they have different characteristics. A measurement can have any combination; precise and accurate, accurate but not precise, etc. There is a popular example of throwing darts at a target to illustrate the difference between precision and accuracy.

examples of throwing darts

Next, let’s take an example of a machined steel shaft with a 10 mm diameter mentioned in the design to discuss typical scenarios of accuracy and precision combination;

Precise and Accurate

The measurement device shows 10.00, 10.02, 10.01, and 10.00 mm respectively. The diameters are close to the true value (10 mm) and also close to each other with a maximum variance of 0.02 mm.

Not Precise but accurate

The measured values are 10.01, 10.02, 9.98, 10.03, 9.99, and 10.01mm respectively. Here values are close to reference(10mm), but they are not close to each other. The maximum difference is 0.06mm.

Precise but not accurate

The measured values are 9.90, 9.91, 9.90, 9.91, and 9.92 mm. They are precise, the reason data are close to each other but not close to the true value of diameter.

Not Precise and not accurate

The measurement shows 9.90, 9.86, 9.95, 9.79, and 9.80 mm. Here, they are neither close to each other nor close to the true value(10mm). It means the values are unprecise and inaccurate.

Importance of Precision and Accuracy in Part Production

There are three important aspects of accuracy in manufacturing; functionality, performance, and fitting during assembly. Meanwhile, precision is essential for the consistency of parts across multiple batches.

Precision Machining

It refers to the production of mechanical parts with high accuracy and repeatability. So, precision CNC machining uses various instruments and measures the accuracy and precision to ensure desired standards.

Calibration of Manufacturing equipment

Any systematic error in the manufacturing process signals defects in machinery, tooling, or instruments to inspect precision. This is like a notification to re-calibrate the manufacturing and measurement equipment.

Reduced Material Waste and Reliability 

Measuring the accuracy of individual machined parts helps to identify the defective parts and immediately adjust the manufacturing process. This early identification of defective parts individual reduced the waste.

Parts Assembly

Accurate parts are essential for smoother and faster assembly processes. Precise and accurate components that fit correctly reduce downtime and labor costs. As a result, the overall efficiency of the production line increases.

Measuring and Inspecting Precision and Accuracy in Machined Parts

Measurement System

Typically, two types of measurement systems are employed for the machining parts; dimensional and non-dimensional measurements. Coordinate measuring machines, gauge blocks, calipers, and 3D laser scanning are popular for the measurement of dimensional features. On the other hand, roughness testing, Rockwell hardness testing, and non-destructive testing fall under non-dimensional measurements.

Regarding the inspection, First Article Inspection (FAI) is the most important factor for measurements. It typically involves compliance with standards and regulations. For example, ISO 9001 for general mechanical components and AS9100 for aerospace components.

Measurement System Analysis

The purpose of measurement system analysis is to determine how much variation in the measurement data comes from the system itself and how much is due to the part or process.

Some analysis approaches are;

  • Stability: Does data from multiple trials indicate the right calibration?
  • Bias: What is the difference between the average measured value and the true value?
  • Linearity: How does measurement accuracy change over the range of precise measurements?

Precision Measurement

It includes checking repeated measurements of the same part under the same conditions to produce similar results through repeatability and machining reproducibility.

  • Repeatability Study: How consistently the same operator can measure the same part using the same tool, which can be a micrometer or caliper. Minimal variation indicates high repeatability.
  • Reproducibility Study: It tests whether different CNC machinery setups or operators using the same instrument produce similar results or not.

Accuracy Measurement

Accuracy is about how close the measurement is to the true value, whereas bias, stability, and linearity are the main influencing factors. Precision measuring instruments like CMM machines and calipers take the data, which can be used for calculations. Next, it is often expressed in the %’ of accuracy;

Accuracy = (Measured Value -True value/ True value) x 100%

How to Improve the Precision and Accuracy of Your Machining Process?

cnc machined steel shaft

The design phase, machine capability, machining variables, setup and tooling, and every other phase of CNC machining collectively decide the overall precision and accuracy. Therefore, many considerations are essential for improving them.

Reduce human input, CAM, and consistent accurate parts. Automating the operations, and the reliability of measurement instruments.

Incorporate Design for Manufacturability (DFM) Principles

Design for manufacturability refers to creating a 3D model that is well aligned with the capability of high-precision CNC machinery, choosing the optimal material, design simplicity, and following the standard compliances. For example, maintain a consistent GD & T and reduce the intricacies that are challenging for tool access. Only a design compatible with the available machines and tooling can fabricate precise and accurate parts.

  • Choose a material balancing performance and cost
  • Create a design compatible with the manufacturing process
  • Consider the factors that ease the assembly process.
  • Standardization

Ensure Design Adheres to Tight Tolerance Limits

While labeling the dimensions and tolerances, check the range of a particular CNC machine and set your tolerances accordingly in the machining drawing. Here, ensure that labeled tolerances are very close to the achievable range. For example, if a CNC mill can maintain tolerances as low as ± 0. 13mm, you can set tolerances in your design closer to this value, i.e. ± 0. 125mm, ± 0. 120mm, etc. Meanwhile, also be uniform with tolerances throughout the drawing.

Optimize Machining Cutting Parameters

The machining cutting parameters should be chosen based on the work material, geometry complexity, and type of the tool. Different parameters like feed rate, spindle speed, depth of cut, and coolant flow affect the precision vs accuracy differently. For instance, the feed rate is more impactful toward accuracy, whereas spindle speed highly influences the precision.

CNC programming is another factor. The tool path and G & M-codes determine the final dimensions of the part to be machined. So, simulate the tool path in computer software and optimize it for high precision and accurate capabilities. 

Furthermore, the precision of machining tools also impacts the results. Any tool wear, slight deviation, or excessive heat can cause many defects or even complete failure.

Implement Effective Fixturing Techniques

The effective fixture is not only about securing the workpiece and cutting tool but also ensuring the stability and rigidity of the machine base and frames.  The precision fixtures tightly hold the workpiece and eliminate tool movement, vibration, and misalignment errors. Additionally, the experience of the operator who set the fixtures also influences the precision. So, fixtures should be set by an expert machinist.

Here are some tips for implementation;

  • Use custom and hard fixtures for mass production
  • Apply the 3-2-1 workpiece locating principle
  • Modular fixtures are suitable for prototyping and small volumes

Utilize Kinematic Coupling for Consistent Alignment

The main benefit of kinematic coupling is that it provides excellent repeatability and reduces the interchangeability error. Additionally, the minimal contact points provide a higher degree of movement freedom for dynamic machining processes. Another characteristic of kinematic coupling is the alignment is independent of clamping force. The workpiece remains aligned with the tool even with varying forces. Furthermore, kinematic coupling also helps with precise control of the tool path over the workpiece.

Fabricating Precise and Accurate Parts for Different Industries

accurate and precise custom parts

In many industries, highly precise and accurate parts are the first requirement for overall functionality and performance. For example, automotive engine components, medical implants, military defense system parts, aircraft fuel injection systems, etc. In such sensitive applications, CAM-integrated CNC manufacturing can deliver the optimal machining outcomes.

The table below shows the precise machined parts examples from different industries;

IndustryWhy Precision and Accuracy?Parts Example
AerospaceHigh safety and performance, standard compliance, and weight reductionTurbine blades, engine mounts, fuselage panels, and landing gear
AutomotiveVehicle reliability, safety, and fuel efficiencyExhaust systems, chassis components, suspension arms, gears
Medical DevicesCritical for patient safety and functionalitySurgical tools, implants, prosthetic limbs, diagnostic equipment parts
ElectronicsPerformance and miniaturizationHeat sinks, circuit board frames, battery casings
Energy SectorEfficiency and longevity of energy systemsWind turbine blades, solar panel frames, inverter housings

What’s Machining Tolerances?

Machining tolerances refer to the permissible range of deviation from intended dimensions. It is typically expressed in terms of “±”. For example, if a drawing of a 10 mm shaft has ± 1 mm tolerance, it means the length of the shaft after machining should fall under the range of 9 to 11 mm. So, tolerances define how accurately you want to machine your parts. Subsequently, maintaining this allowable variation range over multiple production batches refers to precision machining.

Besides linear tolerances (liner, angular, and radial), there are other types are tolerances also; roundness, perpendicularity, flatness, etc.

The tolerances play an essential role in parts fitting and assembly. Additionally, tolerances also facilitate the measures for uniform production in large volumes.

Furthermore, design tolerances are also associated with machining costs. The designs with tight tolerances require precision CNC machines to convert them,

Do You Need High Precision and Accurate Machined Parts?

Choosing the right manufacturer for your high precision and accurate machined parts can be confusing, you need to consider their technical capabilities, human resource expertise, and cost-effectiveness.

RapidDirect is a leading manufacturing company that offers precision machining services for diverse industries. Our CNC machining facilities are equipped with 3, 4, and 5-axis machining centers, lathes, milling machines, and laser cutters. They can produce complex machined components fulfilling strict requirements. Meanwhile, gauges, calipers, CMM machines, and other various instruments are used to inspect the precision and exactness.

Furthermore, the experienced product designers and machinists at our factory can handle the designs with a tolerance down to mm or even lower. So, request a machining service quote from our engineers by uploading your drawing.

Try RapidDirect Now!

Conclusion

In the context of machining or manufacturing, the difference between precision and accuracy is repeatability and conformity. The precision ensures the parts are closely identical (repeatable), whereas the accuracy confirms the machined parts are close to the true dimensions. Different calibration and measurement systems work together to confirm the closeness and repeatability.

These measurements are crucial for more than maintaining the functionality and performance of manufactured components, they also help in continuous improvement, waste minimization, and cost reduction.

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    Written By
    Noah Harrison is the Director of Content Marketing at RapidDirect with over a decade of experience in content writing. He specializes in creating custom content for research projects, thesis, and reports, and is skilled in 3D modeling, rendering, and designing innovative products for engineering assignments and advertising.

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