CNC machined processing is an indispensable part of the machinery manufacturing industry, occupies a crucial position, including the instrument, machine tool, prevent machinery, chemical machinery, metallurgical mining machinery, farm machinery, lifting and transporting machinery! Power machinery and other mechanical parts processing. Science and technology are the primary productive forces. With the improvement of science and technology, more and more advanced technologies and processes emerge and are applied in actual production. Numerical control technology (CNC) and the birth of numerical control machine tools, while improving the efficiency and quality of mechanical processing, has brought a broader development space for the field of mechanical production. In this article, the influencing factors of CNC machining accuracy are explored, and reasonable and effective measures are put forward to practice, in order to promote the development of machinery field.

2 What Is CNC Machine Accuracy?

What does accuracy mean in terms of machining? To explain this concept in easy terms, accuracy refers to the difference between the actual measurement and measurement from your system. Let’s consider an example to make it a bit easier to understand, an accurate machine will execute an operation (cut, polishing, bore, etc.) the way that the operator intends. If the CNC tells the machine to drill a hole at 6 from the outside edge, the machine will perform this step at exactly 6, without errors.

The quality of CNC-machined parts depends on many factors, including part design, tool selection, toolpath programming, mechanics skills, workpiece clamping strategies, etc. Parts in critical-path functions, such as robotic arms used for laser surgery or landing gear components in high-performance aircraft, must operate reliably. Reliability requires precision, precision, and the ability to maintain tight tolerances when manufacturing parts in the CNC machine shop.

Machining accuracy is the main factor affecting the quality of mechanical manufacturing, the level of machining accuracy directly affects the life and performance of the machine, including the relative position Geometry. Therefore, in order to improve the machining accuracy, it is necessary to the relative position Geometry! Dimensions and other parameters are strictly controlled to ensure that the parameters are accurate.

3 Types of Accuracy of CNC Machine Tools

Under the term machine tool accuracy, you can imagine several features of the machine. From the perspective of designers and metrologists, the understanding of accuracy is different. From a metrological perspective, accuracy describes how close the measurement result is to the actual quantity. In the field of machine tools, we can talk about several types of accuracy, and the accuracy of the determination is only qualitative (small, medium, high). These are geometric accuracy, working accuracy and production accuracy. Each accuracy has its own reasons.

These three basic accuracy of CNC machine tools are supplemented by other types of accuracy, namely positioning accuracy, interpolation accuracy, volumetric accuracy, and thermal expansion.

3.1 Geometric Accuracy

Geometric accuracy describes the geometric structure of the machine tool, from which the characteristics of the functional components that affect its work accuracy can be evaluated. It also describes the production quality of the machine and its assembly in an unloaded state. These tests are carried out on machines working under no-load or finishing conditions of machining.

Geometric accuracy of axes, their measurement and evaluation are given by the standard ČSN ISO 230-1. This section only applies to accuracy testing. It does not deal with the functional tests of the machine (vibrations, jerky movements of parts, etc.) or the determination of characteristic parameters (rotation numbers, feeds), as these tests are to be performed prior to the accuracy tests. Geometric tests consist of verifying the dimensions, shapes, and positions of components and their relative alignment. They include all operations that affect a part of the machine, such as flatness, centering, axis intersection, parallelism, straight or flat squares.

They relate only to dimensions, shapes, positions, and relative movements that may affect the accuracy of the machine operation. According to the standard, there are six kinds of geometric errors in linear (according to ČSN ISO 230 – 1) and rotary (according to ČSN ISO 230 – 7) axes, namely three translational errors—positioning error, horizontal and vertical straightness error and three angular errors. A typical three-axis CNC machine tool contains 21 geometric errors—3 × 3 translation errors, 3 × 3 angular errors. To these errors, the errors of the relative squareness of the linear axes are added. All these errors will adversely affect the overall positioning accuracy of the machine, thereby affecting the accuracy of the processed parts. When the actual position does not match the position displayed on the machine control unit, an error usually occurs. The error increases with the dynamic effects of axis interpolation.

In the case of three-axis kinematics, we can find 21 error parameters, 18 translational errors and 3 parameters of squareness of individual machine axes. These errors, including spindle errors, are shown for the three-axis vertical milling machine in Figure 1. The kinematic chain of the three-axis machine tool presented below corresponds to W (Workpiece) -X-Y-Z-T (Tool).

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Figure 1.

Scheme of deviations of three-axis kinematics at the machine MCV 754 QUICK, KOVOSVIT-MAS.

The error description for one linear X-axis and one rotary C-axis is given in Table 1.

Linear axis X Rotary axis C
EXX – positioning error EXC – radial motion in X direction
EYX – straightness error in Y direction EYC – radial motion in Y direction
EZX – straightness error in Z direction EZC – axial motion of C axis
EAX – angular roll error EAC – tilt error motion around the X of the C axis
EBX – angular pitch error EBC – tilt error motion around the Y of the C axis
ECX – angular yaw error ECC – angular positioning error

Table 1.(The error description of a linear axis)

As early as in 1932, German professor Georg Schlesinger published a book “Inspection Test on Machine Tools,” which became the basis for a unified system for assessing the accuracy of machine tools. In this book, he introduced guidelines for the use of devices and equipment for machine tool inspections. Measurement procedures and tolerances for permitted deviations are also given. The name of prof. Schlesinger is used to informally call the geometric accuracy tests of machine tools.

The devices and aids most commonly used to measure geometric errors in machine tools are, for example, granite rulers and cubes, dial gauges, digital inclinometers, autocollimators or laser interferometers, which are increasingly used for measurement. The principle of light interference as a measurement tool can be traced back to 1880, when Albert Michelson invented the interferometry. The Michelson interferometer consists of a wavelength (monochromatic light) light source, a silver-plated mirror and two other mirrors. Although modern interferometers are more precise, with a measurement accuracy of one part per million or higher, they still use the basic principles of Michelson interferometers.

The straightness measurement shows deflection (bent component) or misalignment in the machine guides. This may be due to wear, an accident that could damaged them, or poor machine foundations that cause the axis or the entire machine to fall.

Squareness is measured by comparing the straightness of two nominally orthogonal axes. The measurement can be arranged in different ways using different fixtures and devices. Measuring prisms, mandrels, or granite cubes may be included in the fixture, while the equipment is between the dial indicator and the laser.

Planeness measurement is performed to check the planeness of CMM tables and machine tools, plate fields and surfaces. It determines whether there are any significant peaks or valleys and quantifies them. If these errors are serious, corrective operations are required. A certain number of measuring lines are required to measure the planeness of the surface.

3.2 Positioning Accuracy

This parameter describes the accuracy and repeatability of positioning in linear and rotary numerically controlled axes. “Determination of accuracy and repeatability of positioning in numerically controlled axes” is described in the standard ISO 230-2/6 (ISO 230-2 Test code for machine tools—Determination of accuracy and repeatability of positioning numerically controlled axes. ISO 230-6 test code for machine tools—Determination of positioning accuracy on body and face diagonals), but very often the directive VDI/DGQ 3441is also used.

Positioning accuracy is the most common form of measurement for laser interferometer (Figure 2). The laser system measures linear positioning accuracy and repeatability by comparing the position displayed on the machine with the actual position measured by the laser system.

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Figure 2.(Setting of measuring system for measurement of positioning accuracy)

A more advanced device for positioning accuracy measurement is the Laser Tracker, which allows for immediate evaluation of the x, y, and z deviations. The geometric accuracy of the machine and the accuracy of positioning can be evaluated simultaneously (Figure 3) for an already assembled and activated machine. For this reason, the aforementioned accuracies are usually considered simultaneously.

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Figure 3.(Synergy when using laser trackers to evaluate geometry and positioning accuracy)

3.3 Interpolation Accuracy

In theory, if the CNC machines were perfectly accurate, then the circular path of the machine would exactly match the programmed circular path. In practice, however, any kind of the errors (measuring error, straightness, clearance, reverse error, etc.) will cause the radius of the circle to deviate from the programmed circle. If we are able to accurately measure the actual circular path and compare it with the programmed (nominal) path, we would get a ratio of the machine tool accuracy.

The measurement and evaluation of circular interpolation accuracy are the subject of the ČSN ISO 230-4 standard . The purpose of the tests is to provide a method for the evaluation of capabilities of the contour forming of numerically controlled machine tools. These errors are affected by geometric errors and the dynamic behavior of the machine during the feed used. Results are visible on machined parts under ideal processing conditions if the diameter and feed are the same for both machining and interpolation testing.

3.4 Volumetric Accuracy

Advanced and highly progressive methods include the assessment of volumetric accuracy and its subsequent compensation. The purpose of these advanced compensations is to minimize the tool center point (TCP) deviation at any point in the machine measured workspace. TCP volumetric deviation is defined as the sum of partial deviations in the individual axes.

Volumetric accuracy of machine tools is represented by the vector diagram of the error deviation in the working space. In the standard ISO 230-1, the concept of volumetric accuracy for a three-axis center is defined as the maximum range of relative deviations between the actual and ideal position in the X, Y, Z directions and the maximum range of deviations orientation for directions of A, B, C axes for motions in X, Y, Z axes in the specified volume, where the deviations are the relative deviations between the tool and the workpiece on the machine tool for specified alignment of the primary and secondary axes.

The Laser Tracer measuring device (Figure 4) is mainly used for measuring of volumetric accuracy and subsequent volumetric compensation. The principle of the Laser Tracer measurement is based on measurement of beam lengths (HeNe laser wavelengths, 632.8 nm) and calculation of the measured point in the workspace by the method of sequential multilateration.


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Figure 4. (Principle of measurement with Laser Tracer)

With this method, it is necessary to measure gradually from multiple locations on the machine (it is recommended to measure from at least four Laser Tracer positions). The method is presented as an analogy to the GPS system.

3.5 Working Accuracy

This is a property of a machine tool that expresses the quality and productivity of a potential workpiece production. Working accuracy is expressed by the production of a test workpiece or a series of test workpieces. The working accuracy of the machine is affected by the accuracy of the relative tool path.

Geometric accuracy of the machine.

Tool positioning accuracy relative to the workpiece (positioning accuracy).

Resistance of the machine to elastic deformations (caused by cutting forces, workpiece weight, etc.)

Resistance of the machine to thermal expansion (“thermal stability”).

Selection of cutting conditions, etc.

An overall summary of factors affecting the accuracy of the machine tool is shown in Figure 5. The resulting error in the Cartesian coordinate system is shown by Eq. as a spatial error between the programmed and the actual TCP position.

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Figure 5. (Overview of the error budget in a machine tool and its influencing factors).

Test workpieces to be tested for working accuracy are given, for example, by ISO 10791–7. Here, a test workpiece for 3-axis machining is designed. Furthermore, test workpieces are aimed at continuous 5-axis machining. An example is the test workpiece defined by the directive VDI NCG 5211-1.

3.6 Production Accuracy

Production accuracy describes the production process accuracy evaluated on the workpiece. Production accuracy is influenced by geometrical accuracy, positioning accuracy, working accuracy, and also by the errors of machine operator (incorrectly adjusted tool, poorly clamped workpiece) and by changes of ambient conditions. Variations in the dimensions of the test workpieces during the production process provide direct information on production accuracy.

Production accuracy is usually monitored by SPC (statistical process control). This method has already been overcome in some production processes with 100% product control. Due to the spectrum of workpieces of medium-sized and large CNC machine tools, the SPC method can still be considered valid.

The three main influences that affect the machine tool and the production process and cause workpiece dimensional variations can be more closely assigned to:

  • Production Technology 15%.
  • Working Accuracy of The Machine 25%.
  • Measurement 15%.
  • Ambient conditions 20%.
  • Machined part 5%.
  • Machine operator 20%.

The above-mentioned partial accuracies of the machine tool can be divided into individual parts of the life cycle (Figure 6). Production accuracy can, therefore, be monitored at the phase of customer’s machine use and is influenced by both the working accuracy of the machine and long-term stability of geometric accuracy.

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Figure 6. (Relationships between individual accuracies of a CNC machine tool throughout its life cycle).

4 The Main Cause of Errors In Machining Accuracy

Machining accuracy refers to the degree of conformity between the actual geometric parameters (size, shape, position) of the processed parts and the ideal geometric parameters. In processing, the error is inevitable, but the error must be within the allowable range. Through error analysis, master the basic law of its change, so as to take corresponding measures to reduce processing errors and improve processing accuracy.

Then there will be errors and there will be reasons, roughly as follows:

4.1 Spindle Rotation Error

The spindle rotation error refers to the variation of the actual rotation axis of the spindle relative to its average rotation axis at each moment. The main reasons for the radial rotation error of the main shaft are: the coaxiality error of several sections of the main shaft journal, various errors of the bearing itself, the coaxiality error between the bearings, and the deflection of the main shaft.

4.Guide Rail Error

The guide rail is the benchmark for determining the relative positional relationship of various machine tool components on the machine tool, and also the benchmark for machine tool movement. The uneven wear and installation quality of the guide rail are also important factors that cause the guide rail error.

4.Transmission Chain Error

The transmission error of the transmission chain refers to the error of the relative movement between the transmission elements at the first and the end of the transmission chain. The transmission error is caused by the manufacturing and assembly errors of each component of the transmission chain and the wear during use.

4.4 The Geometric Error othe Tool

In the cutting process of any tool, it is inevitable to produce wear, and thus cause the change of the size and shape of the workpiece. In the machining of CNC machine tools, the geometric accuracy of the machine tool is affected by the external factors such as the external force and the heat generated by the machining process. The geometrical distortion of the machined parts on the machine tool causes geometric errors. According to research, the main reasons for the geometric error of CNC machine tools are nothing more than the following two factors: internal factors and external factors. The internal factors that cause geometric errors of the machine tool refer to the errors themselves caused by the geometric factors of the machine, such as the level of the working surface of the machine tool, the level and straightness of the machine tool guide, and the geometric accuracy of the machine tool and fixture. External factors mainly refer to the geometric errors caused by the external environment and thermal deformation during processing. For example, when the tool or part is in the cutting process, due to thermal expansion and deformation, geometric errors will occur, which will affect the machining of the machine tool. The precision and machining accuracy of the parts.

4.Positioning Error

One is the error of benchmark non-coincidence. The datum used to determine the size and position of a certain surface on the part drawing is called the design datum. The reference used to determine the size and position of the processed surface of this process on the process drawing is called process reference. When machining a workpiece on a machine tool, several geometric elements on the workpiece must be selected as the positioning datum during processing. If the selected positioning datum does not coincide with the design datum, a datum misalignment error will occur. The second is the inaccurate error of the positioning pair manufacturing.Position error refers to the deviation or degree of deviation of the actual surface, axis or symmetry plane of the processed part from its ideal position, such as perpendicularity, positioning, symmetry, etc. The position error in CNC machining usually refers to the dead zone error. The cause of the position error is mainly due to the machining error caused by the gap and elastic deformation generated during the machining of the machine parts, and the machining head needs to overcome the friction during machining. Other factors lead to positional errors. In the open loop system, the position accuracy is greatly affected. In the closed loop servo system, it mainly depends on the accuracy of the displacement detecting device and the speed amplification factor of the system, and the general influence is generally small.

Through long-term data analysis and practical operation of parts processing, machine positioning has a great influence on the machining accuracy of CNC machine tools. The machining error of CNC machine tools is caused by the positioning accuracy from the structural point of view. The feed system of the machine tool is the main link that affects the positioning accuracy. The feed system of CNC machine tools is usually composed of two parts: mechanical transmission system and electrical control system. The positioning accuracy is related to the mechanical transmission system in the structural design. In a closed-loop system, CNC machine tools can often prevent positional deviations in the main components of the feed system, such as ball screws, by means of position detection devices. For the open-loop system, due to the many influencing factors and the complicated situation, positioning and monitoring cannot be carried out, which greatly affects the machining accuracy of the CNC machine tool.

4.6 Errors Caused by Force And Deformation othe Process System

One is the stiffness of the workpiece. In the process system, if the rigidity of the workpiece is relatively low compared to the machine tool, cutting tool, and fixture, the deformation of the workpiece due to insufficient rigidity will have a greater impact on the machining accuracy under the action of cutting force. The second is tool stiffness. The rigidity of the external turning tool in the normal direction of the machined surface is very large, and its deformation can be ignored. Boring an inner hole with a small diameter, 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. The third is the stiffness of machine tool components. Machine tool components are composed of many parts. So far there is no suitable simple calculation method for the stiffness of machine tool components. At present, experimental methods are mainly used to determine the stiffness of machine tool components.

4.7 Errors Caused by Thermal Deformation othe Process System

The thermal deformation of the process system has a relatively large impact on the machining accuracy, especially in the precision machining and large parts processing, the machining error caused by the thermal deformation can sometimes account for 50% of the total error of the workpiece.

5 Causes and Analysis of Machining Accuracy and Machining Error

Machining accuracy and machining error are used to evaluate the surface parameters of machining parts. Usually machining accuracy is the application of mathematical tolerance grade to express, the smaller the numerical value of the grade, the higher the precision requirements of machining parts, in machining need to have a smaller processing error! And the machining error is mainly to reaction with concrete numerical value, the greater the numerical also suggests that the machining error, the greater the processing and the lower the accuracy, in the process of mechanical parts processing, regardless of any machining manner is likely to be one hundred percent accurate, there will always be some machining error, only machining error is smaller, the higher the quality of the processing, Machining parts to meet the needs of the user.

But in the actual machining process, machining precision of the confirmation need in relation to the actual need, only from the point of processing quality, natural machining accuracy is higher, the machining quality is better, but the machining precision of the ascension, often require a greater economic input, increase the cost of machining. Therefore, at the time of confirmation for machining precision, it also needs to be determined according to the actual needs, so that the machining can not only meet the needs of use, but also maintain the economic cost of processing.

5.1 Process System Error Analysis

Among the factors affecting precision machining, process system error is a very important point, which can be analyzed, mainly divided into two parts: Mechanical parts in the process of processing mainly need through machine tools and tools to achieve, and then the machine itself is also a mechanical product, there is a certain error, this error is bound to affect the accuracy of mechanical parts processing! At the same time in the process of machining, machine tool will produce vibration, the vibration will affect the precision of processing and the use of the machine tool bearings in a long time, there will be a certain degree of wear and tear, it is inevitable there is a phenomenon, but this kind of phenomenon will influence on the precision machinery processing, reduce machining accuracy in addition to this, Tool is an essential part of mechanical processing, there will be differences between different tools, which will affect the final machining accuracy, but this factor on the accuracy of machining has a small impact, is usually ignored.

5.2 Position Error Analysis

In the machining process, you first need to fixed parts, then according to the related parameters for processing in the process of all mechanical parts the location are relatively fixed, but the location of the setting need to be the initial positioning, there may be some errors in positioning, and ideally the location produce certain deviation, the processing process behind nature will also be affected, thus affecting the quality of precision machining. When the positioning error is greater, the greater the impact on mechanical precision machining, making the final machining parts of the effect of the worse therefore, in mechanical processing should strengthen the control of positioning error, as far as possible to reduce the error in this respect.

5.3 Force Deformation Error Analysis

Gravity is generated during machining and cutting force! Transmission force and other forces, when these forces act on machined parts and machine tools, will make machined parts and machine tool parts appear a certain degree of deformation, and then affect the mechanical precision processing, the following launch a specific analysis.

5.3.1 Influence of Machined Parts Stiffness

Machined parts due to the influence of the material, the stiffness between different parts is also different, when the cutting tool for its cutting, will produce cutting force on the parts, this cutting force in different stiffness of the parts of the deformation effect is not the same, thus affecting the mechanical precision processing. This is also the precision processing should focus on the analysis and consideration of the place.

5.3.2 Impact of Machine Tool Component Stiffness

Machining on machine tool, and machine work in the long run, the machine tool components may appear to some extent, the deformation of the machine parts and the difference between the ideal and affect the machining precision of the deformation of machine tool parts and there is a direct link between the stiffness of machine components and machine parts stiffness has become one of the influence factors of mechanical precision machining.

Essentially causes the mechanical processing force deformation error has two main reasons: first, the cutting force in the process of cutting caused by changes in the whole machining system, resulting in the emergence of machining error, for this situation is usually because of the processing allowance change. Unequal processing workpiece material caused by; Second, the need to be in the process of machining parts clamping, fixture can produce strong force for processing parts, can cause deformation of machining parts, cause the occurrence of error, for this kind of situation lies in the process of the workpiece clamping position is inaccurate and cause the deformation of parts produced bigger, makes the deformation error increases, affect the machining accuracy.

5.4 Thermal Deformation Causes Machining Errors

Machining error caused by thermal deformation is one of the most important factors in machining accuracy, and its influence may be neglected in ordinary machining. But in the processing of precision workpiece and large workpiece, its influence is very obvious, and sometimes the processing error caused by thermal deformation will account for more than half of the total error of the processed workpiece. And this thermal deformation error is unavoidable, machining parts! Machine tool! Cutting tools in the process of processing will generate greater heat, which may cause thermal change shape, affect the machining accuracy of should be fully considered in the process of production and processing, therefore, that the impact of actively do a good job in heat makes the unit time equals the heat coming from the heat and maintain, maintain between state of heat balance, to reduce the ill effects of thermal deformation of machining parts.

5.5 Error Caused by Internal Stress

For every mechanical parts, the stress is independent of external stress, belongs to the inner power of machined parts, but this kind of inner strength can also cause deformation of mechanical parts, and increase the machining error when machining parts itself contains high internal stress, high internal could in an unstable state. They will move to the low energy, and in the process of this transfer will produce a certain degree of deformation, so that the machining accuracy is greatly affected.

6 Ways to Improve the Machining Accuracy of CNC Machine Tools

In the machining process of CNC machine tools, the accuracy of the parts processed directly affects the quality of the products. Some mechanical parts and precision equipment parts require high machining accuracy. Improving the machining accuracy of CNC machine tools is the key to solve this problem. The key is. Through extensive research and analysis, it is concluded that there are several ways to improve the machining accuracy of CNC machine tools:

6.1 Improve Machining Accuracy by Controlling the Original Error of CNC Machine Tools

In the machining process of CNC machine tools, the error itself is inevitable, and there is an inevitable error between the machined parts and the CNC machine tools. This certain error is called the original error.

Therefore, to improve the machining accuracy of CNC machine tools, controlling the original error of CNC machine tools is one of the important countermeasures. Analyze the possibility of the original error of the system, and take corresponding improvement measures according to the cause of the error and the type of error. In the machining process, the position accuracy and geometric accuracy of the CNC machine tool have an important influence on the machining accuracy of the parts. The position control and geometric precision control are used to reduce the influence of position error and geometric error on the parts. At the same time, for the deformation error generated during the processing, air-cooling, water-cooling and other methods should be used to control the thermal deformation during the machining process, and the machining accuracy caused by the thermal deformation error is reduced. At the same time, for the position error, it is necessary to choose a tool suitable for the material of the part reasonably to avoid the deformation of the tool, and choose a reasonable fixture according to the shape of the part to be processed. If necessary, specially design fixtures for the shape and size of the parts to avoid them. Position error.

6.2 Reasonable Design of the Core Components of the Machine Tool to Avoid Positioning Errors

The positioning accuracy of the machine tool has an important influence on the accuracy of the components, and the core components that affect the positioning accuracy of the machine such as the straightness and level of the feed system, guide rails, and work surfaces. When designing a CNC machine tool, it is necessary to select the core components reasonably. For example, when selecting the ball screw that is widely used in machine tools, the accuracy of the ball screw should be fully considered. The ball screw technology that is more mature should be selected and installed. The support of the ball screw can not be ignored. The support of the ball screw is closely related to the transmission accuracy of the system. The support of the ball screw is mainly determined by the axial load and the rotation speed, which has an important influence on the machining accuracy of the CNC lathe. Accuracy fixing and support. In the design process, the bearing capacity of the ball screw should be strictly checked.

The drag chain, as part of the machine’s outer protection, is now indispensable for machine tools. Due to its chain structure, certain vibrations will also occur when following the movement of the tool holder. This vibration will be transmitted directly to the tool, which will have a certain impact on the machining accuracy.

6.3 Improve the Machining Accuracy of CNC Machine Tools Through Real-time Monitoring Technology

With the continuous improvement of numerical control technology, the CNC machine tool is processed in real time in real time, the error link in the machining process is adjusted in time, and the error data of each link in the machining process is collected and fed back to the control terminal. The error data adopts the corresponding error compensation mechanism to perform timely error compensation, which can effectively improve the machining accuracy of the parts.

7 Measurements to Calculate CNC Millings Accuracy

Three different measurement methods determine the overall accuracy of a milled part: positioning accuracy, precision or repeatability and tolerance.

7.1 Positioning Accuracy of the Mill

The positional accuracy is equal to the difference between the specified distance between points on a part and the actual measured distance of those points once they have been milled. For a CNC milling machine, the accuracy depends on how well it can follow its programmed paths. The machining workshop determines the accuracy of the rolling mill through multiple measurements and calculating the statistical average of the deviations.

7.2 Repeatable Precision 

The machining shop defines the precision of the rolling mill by precisely repeating the same commands, or G-code, to deliver the same results one after another. Similar to accuracy, when a CNC milling machine is required to cut a path, the factory can calculate the accuracy of the CNC milling machine through multiple measurements. The measurement of accuracy lies in the repeatability of cutting the same precise path, and finally producing accurate repeating parts.

7.3 Tolerance of the Finished Part

Machinists and engineers use the term tolerance when referring to the allowable deviation of a CNC milling machine from a measured set value. CNC programmers and machinists program the rolling mill to manufacture parts within the tolerances required by the customer. When cutting materials and producing parts, the rolling mill should stay within these set tolerances.

The goal of a CNC milling machine is to keep it within a specified tolerance range by repeatedly maintaining a high positioning accuracy. For example, imagine shooting an arrow at a target. The distance between the arrow and the bull’s eye is the key to measuring accuracy. The distance between the arrows is equal to the repeatable accuracy. When processing a batch of parts, the repeatability is not accurate and will not produce any usable results because the parts will all be out of tolerance equally. Or, accuracy without repeatability will result in only one or two parts available. The combination of precision and repeatability will produce a whole batch of qualified products. The machine shop uses this precision to ensure the quality of the parts.

8 Conclusion:

To sum up, the machining process is more complicated, which need to take many factors into account, in the mechanical processing, need to various aspects considering the influencing factors, and then make a scientific and reasonable processing scheme or processing steps, such ability can effectively reduce the error that exist in the process, thus effectively enhance machining accuracy. The quality of machining becomes higher, in line with the needs of more machined workpiece.


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