Precision vs Accuracy in Machining: Key Differences

Getting CNC machining right means more than hitting a dimension once. It means knowing how close you are to the target. It also means knowing how consistently you can stay there. This is what precision vs accuracy in machining describes.

A process can be precise but not accurate. It can be accurate but not precise. It can be both, or it can be neither. If you do not understand the difference, you might make bad parts. These parts could look fine on paper but fail later. They might fail in assembly, testing, or long-term use. This article explains precision and accuracy for a machinist. It covers what they mean, how they relate to tolerances, and how to measure and improve them.

What Are Precision and Accuracy in Machining?

In machining, precision describes how consistent repeated parts are. In contrast, accuracy describes how close measurements are to the true value on the drawing.

Accuracy in machining tells you if a feature is “on target.” Imagine a shaft needs to be 10.00 mm. If you measure it and get 10.00 mm or very close, the process is accurate. In formal terms (ISO 5725-1), accuracy combines trueness and precision. But in shop talk, accuracy usually means “closeness to the drawing.”

Precision in machining is about repeatability. Imagine you machine many shafts. If they all measure within a few microns of each other, the process is precise. This is true even if they are all slightly off the target size. Precision shows the random changes in a process. This includes small shifts in tool engagement, chip load, or vibration.

Here is a simple way to remember:

Precision vs Accuracy

Precision and accuracy have different focuses. Precision looks at the spread of results, while accuracy looks at the distance to the target. Mixing them up causes poor process decisions.

From an error standpoint, precision is affected by random errors. These are small, unpredictable changes in cutting forces, tool wear, or temperature. They vary from one cycle to the next. Accuracy is affected by systematic errors. This could be a wrong tool offset or a worn reference surface. These errors shift all results by a similar amount.

For measurement, you need a set of measurements to check precision. You need many parts to see how results cluster. You can check accuracy with a single measurement by comparing it to the target. In practice, we often use the average of several readings.

A common mistake is saying “high precision machining” when you mean “high accuracy within a tight tolerance.” A process can be very repeatable (high precision). But it could be 0.05 mm too small every time (poor accuracy). This is a problem if your tolerance is only ±0.02 mm.

How Precision, Accuracy, and Tolerance Work Together in CNC Machining

Precision, accuracy, and tolerance are linked. They describe what deviation is allowed, how close you are to the target, and how consistently you stay there.

• CNC Tolerance is what the drawing allows. It is the accepted range around the target value.
• Accuracy describes where your process sits inside that tolerance band.
• Precision describes how wide your process spread is. It shows how much parts vary around their average.

For any feature, good machining means two things:

• The average measured value is very close to the target (high accuracy).
• The spread of measurements is small compared to the tolerance (high precision).

In process terms, precision is about σ (sigma). Accuracy is about the mean shift from the target. You need to control both. This keeps the process within tolerance without constant sorting and rework.

Typical Tolerance Examples in Machined Features

A simple shaft example shows the relationship:

• Nominal diameter: 10.00 mm
• Tolerance: ±0.02 mm → acceptable range is 9.98–10.02 mm

Now, think about three different results from a CNC turning process:

• The average diameter is 10.00 mm. Almost all parts are between 9.995–10.005 mm. The process is accurate and precise. There is plenty of room within the tolerance limits.
• The average diameter is 9.97 mm. Parts are between 9.965–9.975 mm. The process is precise but not accurate. All parts are too small, even though they are very consistent.
• The average diameter is 10.00 mm. But parts range from 9.97–10.03 mm. The process is accurate on average but not precise. Many parts will be out of tolerance on both sides.

When you ask for tighter tolerances, you are asking for better precision and accuracy. This usually costs more.

Precise vs Accurate Machined Parts

The best way to learn about precision and accuracy is to see the four classic combinations. We can relate them to real shop situations.

Precise and Accurate

A process is precise and accurate when measurements are tight and centered on the target value.

Imagine a CNC mill cutting a 20.00 mm pocket. The tolerance is ±0.01 mm. Measurements from parts read 20.00, 20.01, 19.99, and 20.00 mm. All values are very close to 20.00 mm and to each other. Tooling, fixtures, offsets, and temperature are all under control. This is the ideal state. Parts fit, assembly is easy, and scrap is low.

Precise but Not Accurate

A process is precise but not accurate when results are consistent but shifted away from the target.

For example, a shaft should be 15.00 ±0.02 mm. But measurements show 14.94, 14.95, 14.94, and 14.95 mm. The spread is very small, so precision is high. But all parts are below the lower tolerance limit. This is often caused by wrong tool offsets or a bad work coordinate setting. Fixing accuracy here means removing a bias, not fighting random changes.

Accurate but Not Precise

A process is accurate but not precise when the average result is near the target, but parts vary widely.

Suppose a drilled hole should be 8.00 ±0.05 mm. Measurements read 7.95, 8.03, 7.98, 8.05, and 7.99 mm. The average is close to 8.00 mm, so the process is accurate on average. But the wide spread shows poor precision. This can be caused by unstable clamping, an inconsistent feed rate, or vibration. You might pass an initial check, but your long-term results will be poor.

Neither Precise nor Accurate

A process is neither precise nor accurate when results are scattered and centered away from the target.

A dimension should be 50.00 ±0.05 mm. But your readings are 49.80, 49.92, 50.10, 49.85, and 50.05 mm. The process has both a mean error and a large spread. This usually points to bigger problems. You might have worn machine parts, poor fixtures, or a serious setup error. At this point, you are not fine-tuning. You are troubleshooting a process that is out of control.

How to Measure Precision and Accuracy in Your Machining Process?

To know how precise and accurate your work is, you need to inspect dimensions and check for repeatability. You also must compare results to your target and its tolerance.

Dimensional Inspection and Measurement Systems

Dimensional inspection gives you the data to check precision and accuracy.

Common tools include calipers, micrometers, bore gauges, and CMMs (Coordinate Measuring Machines). For things like surface roughness, you use other tools. The logic is the same. You record measurements and see how they match the goal. Your measurement system must be good. Poor tools or methods will hide the true state of your process.

Repeatability / Reproducibility Checks for Precision

You can check precision with repeatability and reproducibility (R&R) studies.

• Repeatability checks variation when the same person measures the same part many times with the same tool. Small variation means good repeatability.
• Reproducibility checks variation when different people, machines, or setups are used. If results stay similar, the process is reproducible.

You can also calculate the standard deviation of your measurements. Control charts also show how tightly data points cluster. A narrow, stable band means good precision. A wide or drifting band means random variation is a problem.

Comparing to Nominal and Tolerance for Accuracy

You check accuracy by comparing measured values to the target dimension and its tolerance.

For one dimension, you can find the error:

Error = Measured value − Nominal value

Or you can show it as a percentage:

Accuracy (%) = (1 − |Measured − Nominal| / Tolerance range) × 100%

In practice, we check how the average of multiple measurements fits in the tolerance. If the average is close to the target, accuracy is good. If the average is shifted to one side, you have a systematic error. You must correct it through calibration or offset changes.

How to Improve Precision and Accuracy in CNC Machining

Improving precision and accuracy requires different actions. You reduce variation for better precision. You remove bias for better accuracy. These steps are even more important in CNC machining 5 axis settings, where tight specs are often a must.

Controlling Machine, Tooling, and Fixturing for Better Precision

To improve precision, you focus on making the process stable and repeatable.

Key actions include:

• Maintaining the machine to avoid play and backlash.
• Using good fixturing so the part is held the same way every time. This reduces movement and vibration.
• Standardizing cutting parameters like feeds and speeds. This keeps cutting loads similar for each cycle.
• Managing tool wear with a clear plan. Change tools before they fail.

The goal is to make each cycle look like the last one. This minimizes random changes in the final part.

Calibration, Compensation, and Environment Control for Better Accuracy

To improve accuracy, you focus on how close your process runs to the target value.

Practical steps include:

• Regular calibration of CNC machines and measuring tools. This removes systematic errors.
• Correctly setting and checking tool and work offsets. Do this after tool changes or fixture adjustments.
• Using compensation functions on the machine control. Examples are tool wear and thermal compensation.
• Controlling factors like temperature. Let the machine and workpiece stabilize before final cuts on critical jobs.

These actions reduce systematic errors. They move the average value of your process back to the target.

Balancing Required Quality vs Cost and Cycle Time

Engineers and planners must ask an important question. It’s not just “how good can we be?” but “how good do we need to be?”

Tighter tolerances require better machines, tooling, and inspection. They often mean lower output. A simple bracket might work fine with a tolerance of ±0.1 mm. Specifying ±0.01 mm only adds cost, not value. The best approach is to:

• Specify tolerances that are tight enough for function, but no tighter.
• Align drawing tolerances with realistic CNC capabilities—including those of CNC machining 5 axis.
• Save ultra-tight tolerances for critical features, not whole parts.

Conclusion

In CNC machining, precision and accuracy are different tools for controlling quality. Precision tells you if your process is consistent. Accuracy tells you if it is correct. Tolerances define how much error is acceptable.

If you only watch accuracy, you might hit the target sometimes but struggle with bad parts. If you only chase precision, you might make very consistent scrap. By measuring both and fixing their root causes, you can build stable, capable, and cost-effective machining processes.

FAQ

Can a machining process be precise but inaccurate?

Yes. A process is precise but inaccurate if it produces consistent results that are all off-target. This happens when tool offsets or work coordinates have a systematic error. For example, every part is 0.03 mm too small but they are all very similar. Fixing this means adjusting an offset, not fighting random changes.

Which is more important in CNC machining, precision or accuracy?

Neither is always more important. You need enough of both to stay within tolerance. For key parts, accuracy is critical so they match the design. Precision is vital for consistency across batches. A capable process first gets accuracy right, then improves precision to reduce scrap.

How can I tell if I have a precision problem or an accuracy problem?

You have an accuracy problem if your data cluster tightly but are off-center from the target. You have a precision problem if the average is near the target but the data are widely spread. Plotting measurements on a chart makes this clear. A narrow but shifted band shows an accuracy issue. A wide band centered on the target shows a precision issue.

How do precision, accuracy, and tolerance influence part cost?

Tighter tolerances need higher precision and accuracy, which almost always raises the cost. Reaching them may demand better machines, tooling, and more inspection time. If you specify tolerances that are too tight, you make the job more expensive without improving its function. Tolerances should be based on real needs.

Does better measurement automatically improve precision and accuracy?

No, better measurement does not fix the machining process itself. But it makes problems visible sooner. Good, calibrated tools help you see the difference between precision and accuracy issues. They help you measure change and check that your fixes are working. However, you still need to adjust the machine, tools, and process to actually improve your results.