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How To Control CNC Milling Accuracy

Nowadays, more and more machine have realized intelligence and automation, and the precision and stability of CNC milling have made great progress. However, sometimes it is inevitable that machining errors will occur, resulting in economic losses. So what are the reasons for cnc milling errors?

The Causes Of CNC Machining Errors

1.CNC Milling Machining Error

Machining errors include spindle rotation errors, guide rail errors and transmission chain errors.

Spindle rotation error refers to the change of the actual rotation axis of the spindle relative to the average rotation axis at each moment, which directly affects the accuracy of the workpiece. The main causes of spindle rotation error are spindle coaxiality error, bearing error, coaxiality error between bearings, and spindle winding.

The guide rail is the benchmark for determining the relative position of various machine tool parts on the machine tool, and it is also the benchmark for the movement of the machine tool. The manufacturing error of the guide rail itself, the uneven wear of the guide rail, and the installation quality are important factors causing the error of the guide rail.

The transmission chain error refers to the relative motion error between the transmission elements at the end of the transmission chain. This is due to the manufacturing and assembly errors of the various components of the transmission chain and the wear and tear during use.

2.Tool Error

The tool inevitably wears out during the cutting process, changing the size and shape of the workpiece. The impact of tool geometric errors on cnc milling errors depends on the type of tool. When using fixed-size tools, tool manufacturing tolerances directly affect the machining accuracy of the workpiece. In addition, for general tools (such as turning tools), manufacturing errors do not directly affect machining errors.

3.Fixture Error

The function of the fixture is to make the workpiece and the tool equivalent, so that the machine tool is in the correct position, so the geometric error of the fixture has a great influence on the machining error (especially the position error for cnc automotive parts process).

4.Positioning Error

Positioning errors mainly include datum misalignment errors and positioning sub-manufacturing errors. When machining a workpiece with a machine tool, it is necessary to select some geometric elements on the workpiece as positioning data during precision cnc milling. If a positioning datum and a design datum (the datum used to determine the size and location of a specific surface on a single-part drawing) are selected, a mismatch will result in a misalignment error in the datum.

The workpiece positioning surface and the fixture positioning element together form a positioning pair. The maximum position change of the workpiece caused by the manufacturing inaccuracy of the positioning pair and the fit gap between the positioning pairs is called the manufacturing inaccuracy error of the positioning pair. The inaccurate manufacturing error of the positioning pair only appears when using the adjustment method, not in the trial cutting method.

5.The Error Caused By The Force Deformation Of The Process System

  • Workpiece stiffness: In the process system, if the workpiece stiffness is relatively low compared to machine, tools and fixtures, under the action of cutting force, the deformation of the workpiece due to insufficient stiffness will have a greater impact on the deformation of the workpiece. Machining errors. Give.
  • Tool rigidity: The rigidity in the normal (y) direction of the machining surface of the outer circle turning tool is very high, and the deformation is negligible. When drilling a small-diameter inner hole, the rigidity of the tool bar is very poor, and the force and deformation of the tool bar have a great influence on the machining accuracy of the hole.
  • Rigidity of machine tool parts: Machine tool parts are composed of many parts. So far, there is no suitable simple calculation method for the stiffness of machine tool parts. Currently, test methods are mainly used to determine the stiffness of machine tool parts. Factors affecting the rigidity of machine tool parts include the influence of contact deformation of the joint surface, the influence of friction, the influence of low rigidity parts, the influence of clearance, etc.

6.Errors Caused By Thermal Deformation Of The Process System

The thermal deformation of the process system has a great influence on the machining error, especially in precision machining and large-scale parts machining. Machining errors due to thermal deformation can account for 50% of the total error of the workpiece.

7.Adjustment Error

Every machining process requires tuning the process system in some way. Alignment errors occur because the alignment is not completely accurate. The process system ensures the mutual position accuracy of the workpiece and the tool on the machine tool by adjusting the machine tool, tool, fixture or workpiece. When the original accuracy of the machine tool, tool, fixture and workpiece blank all meet the process requirements without considering the dynamic factors, the adjustment error plays a decisive role in the machining error.

8.Measurement Error

When measuring parts during or after machining, the measurement accuracy is directly affected by the measurement method, measurement tools, workpiece accuracy, and subjective and objective factors.

9.The Error Caused By The Internal Stress Of The Workpiece

Stresses that exist inside a part in the absence of external forces are called internal stresses. When internal stress occurs in the workpiece, the metal of the workpiece will be in a high-energy unstable state, and will instinctively transform into a low-energy stable state, accompanied by deformation, so that the workpiece loses its original machining accuracy.

The Element Of Controlling CNC Milling Accuracy

The accuracy of a CNC machined part depends on many factors, including part design, tool selection, toolpath programming, how fixtures are held, and more.

Different industries have different precision requirements for parts, such as medical, aerospace, automobile, chemical and other industries have relatively high precision requirements for parts. Of course, there are many other industries that also have high requirements for the accuracy of parts, and some industries do not have such high requirements for the accuracy of parts, and pay more attention to the stability of parts accuracy, that is, the size difference between parts should be as small as possible.

So how does the CNC machining factory ensure the accuracy of the parts?

CNC milling machine accuracy: 3 measurements

The overall accuracy of a milling machine is determined by three distinct measurements: positional accuracy, repeatability, and tolerance.

1.Position Accuracy

Positional accuracy is equal to the difference between the distances between specified points on the part and the actual measured distances of those points after milling. With a CNC mill, accuracy depends on the ability to follow a programmed path. The machining shop determines the positional accuracy of the milling machine by taking multiple measurements and calculating the statistical average of the deviations.


Define the precision of the milling machine by repeating exactly the same commands to get the same result one after the other. Similar to accuracy, the accuracy of a CNC mill is calculated by taking multiple measurements when setting the cutting path. The measure of accuracy lies in the repeatability of cutting the same exact path, resulting in repeated exact reproductions of the part.


The goal of CNC milling machines is to keep them within specified tolerances by repeatedly maintaining high positioning accuracy. For example, imagine shooting an arrow at a target. The distance between the arrow and the bullseye measures accuracy.

How To Judge CNC Milling Precision

To maintain the required precision of parts, a good machining shop prioritizes the following in its processes and procedures:

1.Working Environment:

Accuracy depends on the operating environment providing stable conditions such as temperature and humidity.

2.Device Status

The precision of a CNC milling machine requires ongoing preventive maintenance to keep all equipment in perfect working order.

3.Machine Calibration

All precision milling machines are periodically recalibrated to maintain factory settings.

4.Tool Monitoring

Cutting tools are monitored, and maintenance and replacement are performed according to the status.


In order to minimize the effects of vibration, it is necessary to use suitable clamps to fix the workpiece.

6.Mechanic Experience:

An experienced machinist is more likely to understand the nuances that lead to the manufacture of precise parts.


Experienced manufacturing engineers understand how to use different tools in different methods and speeds to efficiently manufacture precise CNC milled parts.

Working With An Experienced CNC Milling Partner

Be-Cu prototype is a China-based CNC machine shop, but we have been known nationally since 1995 for being on time, within budget, and within tolerance. We understand that each project is unique, which is why we offer custom cnc milling and 5 axis cnc machining service tailored to meet specific customer requirements. Our expertise in providing high-quality custom copper parts manufacturing and fabrication services is unmatched. Be-Cu prototype’ commitment to quality assurance and inspection ensures that every precision cnc milling service we provide is of the highest standard.