A Complete List of Errors in CNC Machining – Causes and Solutions

This paper makes a more detailed analysis of CNC machining errors and their causes, summarizes the compensation methods for system errors, and elaborates on various types of error compensation methods on this basis.

What is machining accuracy?

Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of a part after machining conform to the ideal geometric parameters. The difference between them is called machining error. The size of the machining error reflects the level of machining accuracy. The larger the error, the lower the machining accuracy, the smaller the error, the higher the machining accuracy.
Machining accuracy includes three aspects.

• Dimensional accuracy: refers to the actual size of the machined part and the center of the tolerance zone of the size of the part in line with the degree;
• Shape accuracy: refers to the actual geometry of the machined part surface and the ideal geometry of the degree of conformity;
• Position accuracy: refers to the actual position between the surface of the machined part and the ideal position of the degree of conformity.

What are the main errors of machining?

In the same processing benchmark and meet the production conditions of the product processing method processed out of a number of parts, due to the impact of processing factors, its size, shape and surface mutual position will not be absolutely consistent, there is always a certain amount of processing errors. At the same time, from the premise of meeting the required tolerance range, to take reasonable, economic methods and measures to improve the productivity and economy of machining. The main reasons for machining errors are some of the following.

The geometric error of the machine tool

The original manufacturing error of the machine
The original manufacturing error of the machine tool refers to the geometry of the working surface of the components of the machine tool, the surface quality, the mutual position error caused by the machine tool motion error, is the main reason for the geometric error of CNC machine tools.
The so-called manufacturing error refers to the error generated in the entire machine tool mechanical production process. And manufacturing errors are mainly divided into three aspects.

• First, the rotary error. For the rotary error, this error, generally will appear in the position near the spindle. Generate rotary error, usually due to the machine tool machinery in the manufacturing process of bearing work problems caused by. Due to the bearing problem, the spindle will not work properly, so the error will be generated in a moment. In the process of generating rotary error, bearings, spindle winding, coaxiality, etc. are all possible sources of error. Therefore, for the rotary error, it is a mobile error. Once the rotary error will be generated in the process of machine tool machinery production, reduce the machine tool for the control of the accuracy of the parts.
• Second, the guide error. Guide rail errors are likely to arise and machine tool machinery manufacturing in the position of the relationship. When the machine tool in the process of machinery manufacturing production, once the position of the relationship deviation will produce rail error. If the machine produces the error, it will produce uneven wear on the parts. Thus increasing the probability of machine tool error.
• Third, the error of the transmission chain. When the machine tool work process, the transmission chain began to drive continuously. Once the machine tool drive chain in the process of transmission, especially at the end of the first and last, for the transmission of the parts occurred in the process of movement of the error, it will lead to the entire machine tool drive chain error, affecting the normal work of the machine tool.

Machine control system errors
Including the machine axis system servo error (contour following error), CNC interpolation algorithm error.
Thermal deformation error
Due to the internal heat source of the machine tool and environmental thermal disturbance caused by the thermal deformation of the structure of the machine tool and the resulting error.
Error caused by the deformation of the process system due to cutting load
Including machine tools, tools, workpieces and fixtures deformation caused by the error. This error is also known as “let the tool”, which causes distortion of the shape of the machined parts, especially when processing thin-walled workpiece or the use of slender tools, this error is more serious.
Vibration error of the machine
In the cutting process, CNC machine tools due to the flexibility of the process and process variability, its operating conditions have a greater possibility of falling into the unstable region, thus provoking strong chatter. This leads to deterioration of the surface quality and geometric shape errors of the machined workpiece.
Testing errors of the inspection system
Includes the following aspects.

• (1) Due to the manufacturing error of the measurement sensor and its installation error on the machine tool caused by the error of the measurement sensor feedback system itself;
• (2) The error of the measuring sensor due to the error of the machine parts and mechanism and the deformation in use.

External interference error
Random errors caused by changes in the environment and operating conditions.
Other errors
Such as errors caused by programming and operation errors.
The above errors can be in accordance with the characteristics and nature of the error, into two categories: namely, the system error and random error. The system error of CNC machine tools is the inherent error of the machine itself, with repeatability. The geometric error of the CNC machine tool is its main component, which also has repeatability. The use of the characteristics of the “off-line measurement”, can be used to “off-line detection – open-loop compensation” technology to correct and compensate, so that it is reduced to achieve machine accuracy to strengthen The purpose.

Random error has a random nature, you must use the “online detection – closed-loop compensation” method to eliminate the impact of random error on machine tool accuracy, the method of measuring instruments, measurement environment requirements are strict, difficult to promote.

Geometric error of the tool

In the whole process of direct production of machine tools, the tool is very easy to wear, therefore, due to tool wear and error problems are not uncommon. Once the tool has worn, the actual process of machining the parts will change the size and shape of the parts, thus making the production of the parts produce errors. For example, Figure.1 is a common tool in the machine tool machinery production process.

Figure.1 Tool in the production process
However, the error caused by the wear of different tools in this machine production process is also different. For example, for common tools, the errors that can be caused by common tools have a relatively small impact on the parts. Therefore, it can be said that under normal circumstances, the error caused by the tool is generally acceptable. But in the process of machine tool machining, many parts have very strict requirements for accuracy. Even in the process of processing will also use special tools. At this time, if the tool wear, then it will directly affect the service life of the parts.
Any tool in the cutting process will inevitably produce wear and tear, and this will cause changes in the size and shape of the workpiece. The impact of tool geometric error on machining error varies with the type of tool: when using fixed-size tools, the manufacturing error of the tool will directly affect the machining accuracy of the workpiece; while for general tools (such as turning tools, etc.), the manufacturing error has no direct impact on machining error.

Geometric error of the fixture

In general, in the process of machine tool machinery production, fixtures are in production to play a fixed role. Their main role is to be in the process of production of machine tools, clearly fixed equipment should be in the position. Therefore, once he produced an error, the impact on the parts will be more serious than other parts of the error brought about by the impact. It also destroys a fixed position of all the parts and causes errors in the machine and the tool.
The role of the fixture is to make the workpiece equivalent to the tool and the machine tool has the correct position, so the geometric error of the fixture on the machining error (especially the position error) has a great impact.

The process of error

For the machining process in the machine tool, the error generated by the process is also very common. In general, due to process problems and errors arising from the following main reasons.
First, the stiffness of the components. When the process of machine tool machining, many parts of the stiffness are low. Therefore, the machine tool machinery in the process of processing, the process of cutting tools will allow the parts to accept a certain force, and if the entire part of the stiffness is not enough, it will make the parts in the process of processing deformation, thus causing errors.
Second, the stiffness of the tool itself. For the tool, generally can be applied in the machine tool mechanical processing of the tool itself is very rigid. Therefore, in the process of machining, the impact on the tool itself, generally negligible. However, in the process of generating different parts, the hardness of the parts varies due to their hardness. Therefore, sometimes if the cutting hardness of the part is too large. This may cause damage to the tool itself, which may result in errors in the production of the part later.

Positioning error

Positioning errors mainly include the benchmark does not overlap error and positioning sub manufacturing inaccuracy error. When processing the workpiece on the machine tool, a number of geometric elements on the workpiece must be selected as the positioning datum for processing, if the positioning datum selected and the design datum (used to determine the size and position of a surface on the part drawing based on the datum) does not overlap, it will produce the datum non-coincidence error.

Workpiece positioning surface and fixture positioning elements together constitute the positioning vice, due to the inaccuracy of the positioning vice manufacturing and positioning vice of the maximum position of the workpiece caused by the fit gap, known as the positioning vice manufacturing inaccuracy error. The inaccuracy of positioning sub manufacturing error will only be generated when the adjustment method is used, and will not be generated in the trial cutting method.

Process system deformation of the error generated by the force

Workpiece stiffness: process system if the workpiece stiffness is relatively low compared to the machine tool, tool, fixture, under the action of cutting force, the workpiece due to insufficient stiffness and deformation caused by the impact of machining errors is relatively large.
Tool stiffness: external turning tool stiffness in the direction of the normal (y) of the machined surface is large, and its deformation can be negligible. Boring smaller diameter bore, tool bar stiffness is very poor, tool bar deformation of the hole processing accuracy will have a great impact.
Machine tool components stiffness: machine tool components consist of many parts, machine tool components stiffness so far there is no suitable simple calculation method, at present the main experimental method to determine the stiffness of machine tool components. Factors affecting the stiffness of machine tool components are combined with the impact of contact deformation, the impact of friction, the impact of low stiffness parts, the impact of clearance.
Process system thermal deformation on the impact of processing errors is relatively large, especially in precision machining and large parts processing, caused by thermal deformation of the processing error can sometimes account for 50% of the total error of the workpiece.

Adjustment error

In each process of machining, it is always necessary to adjust the process system in one way or another. Since the adjustment cannot be absolutely accurate, thus generating adjustment errors. In the process system, the workpiece, tool in the machine tool mutual position accuracy, is through the adjustment of the machine tool, tool, fixture or workpiece to ensure. When the machine tool, tool, fixture and workpiece blanks, such as the original accuracy of the process requirements and do not take into account the dynamic factors, the adjustment error plays a decisive role in machining errors.

Measurement error

Parts in processing or measurement after processing, due to the measurement method, gauge accuracy and workpiece and subjective and objective factors are directly affected by the measurement accuracy.

Internal stress

There is no external force and the existence of internal stress in the parts, known as internal stress. Once the internal stress on the workpiece, the workpiece metal will be in a high-energy unstable state, it instinctively to the low-energy stable state, and accompanied by deformation, so that the workpiece lost the original processing accuracy.

Processing errors caused by unreasonable manufacturing process

In the real production, the errors caused by unreasonable process design generally have the following forms.
The error caused by unreasonable machining route
Since the hole position accuracy is required to be high, the arrangement of the boring route becomes more important, and the improper arrangement may bring in the backlash of the coordinate axis, which directly affects the hole position accuracy.
Errors caused by improper arrangement of tool cut-in and cut-out
When milling a full circle, arrange the tool to enter the circumference from the tangential direction for milling, and when the full circle is finished, do not cancel the cutter complement or retreat at the cutting point, but arrange a distance to continue the movement along the tangential direction, so as to avoid the workpiece and tool scrap caused by the collision between the tool and the workpiece when the cutter complement is canceled. When milling the inner circle should also follow this cut-in and cut-out method, it is best to arrange the transition from the arc to the arc of the processing route, cut out should also arrange a section of the transition arc before retreating, so as to reduce the joint marks at the joint, which can reduce the roughness of hole processing and improve the accuracy of hole processing.
Error caused by insufficient process analysis
Universal part structure processability is not fully applicable in CNC machining, but the following points of special attention.
The use of a unified positioning reference, CNC machining without a unified positioning reference, will be caused by the re-installation of the part and the processing of the contour position and size of the two surfaces are not coordinated, resulting in a large error.
Avoid causing undercutting or overcutting phenomenon, when machining the inner and outer contour of the arc and straight line or arc and arc connection on the CNC lathe, the size of its transition arc radius should be fully considered, because the size of the tool tip radius may cause undercutting or overcutting phenomenon.
Errors caused by improper process handling
Determine a reasonable process can improve the accuracy of the machined parts. For example, when processing a plane with a cutting allowance of 1.5-2.0mm, the processing error of the plane is 0.3-0.5mm, if two tool walks are used to remove the allowance, the last tool walk 0.2-0.5mm, the error can be stabilized at about 0.1mm. So in the processing according to the accuracy requirements can be carried out several times to complete the processing.

The error caused by unscientific programming

Errors caused by improper selection of the programming point of origin
The programming origin is usually used as the starting and ending point of the programming coordinates, and its correct selection will directly affect the machining accuracy of the part and the difficulty of calculating the coordinate size. The following principles should be noted when selecting the programming origin.
Programming origin as far as possible with the drawing on the size of the reference (design benchmark and process benchmark) overlap, such as the positioning of the parts should be the center of the hole as the programming origin, for some irregularly shaped parts, can be selected in its reference surface (or line) programming origin, when the processing route is closed form, should be in the higher accuracy requirements of the surface to select the programming origin (or processing starting point).
The choice of the programming origin should be conducive to programming and numerical calculation.
The processing error caused by the programming origin should be minimal.
The programming origin should be easy to find, and the measurement position is more convenient.
Errors caused by improper handling of dimensional tolerances in the part drawing
Such a problem is mainly manifested when the dimensional tolerance is not symmetrical. If the machining size is 300+0.2, this data should be processed to 30.1+0.1 during programming, so as to ensure that the machined part better meets the requirements of the drawing. Programming according to the basic size is a common mistake of programmers, should follow the principle of converting the size into the average difference before programming.

Tooling errors

Errors in tooling include errors in tools, gauges and fixtures.
In CNC machining to try to use CNC tools, because the tool needs to be sharpened after a period of time, the tip position of the tool will change when re-installed after sharpening the ordinary tool, and need to re-align the tool. The CNC tool is characterized by high accuracy of tool manufacturing and repeat positioning accuracy of about 0.02mm after insert indexing, which greatly reduces the tool setting time, and at the same time, the insert surface is coated with wear-resistant layer, which can well guarantee the machining accuracy.
Selecting suitable gauges can check the real size of the formed parts from time to time to ensure the machining of products that meet the requirements of the drawings.
Fixture can realize the role of rapid installation of parts, but at the same time requires the fixture to ensure that the amount of deformation to meet the requirements of the clamping, or excessive deformation will affect the machining accuracy of the parts.

CNC machine tool error itself

CNC machine tool error is mainly divided into two parts: the error generated by the CNC operating system and the error generated by the machine part.
CNC system error
CNC control system in accordance with the control mode of the servo system, can be divided into open-loop control system, closed-loop control system and semi-closed-loop control system. The open-loop control system does not carry a detection device and there is no feedback circuit. Since there is no feedback detection device in the feed system, the error generated in its forward route cannot be compensated by feedback information, which leads to the output position error. The closed-loop control system can detect the actual position at all times, and feedback to the CNC system, and the command value in the system for comparison, until the elimination of the difference in value to stop moving, and thus can achieve a high degree of control accuracy. The semi-closed-loop control system still has the part of no feedback, so the error not included in the feedback information will still affect the position accuracy of the moving parts, and thus affect the accuracy of the machining.
Errors generated by mechanical parts
The drive axis of any machine tool has to go through a speed-up and speed-down process when moving and stopping. In high-grade CNC machine tools, due to the hydraulic drive and locking system, the rate of ascending and descending can be completed in a very short period of time. And economic CNC machine tools due to the absence of hydraulic drive and locking system, it is up and down rate in a certain period of time to complete. This drive system hysteresis (i.e. inertia) and stagnation in the processing of internal grooves and contours such as surface, the tool to go through the corner when easy to produce “overtravel” and lead to processing errors. Programming should be close to the corner before the appropriate reduction in feed rate, after the corner and then gradually increase the speed.
CNC machine tool drive vice contains gears, shafts, drive screws and other components of the reversal gap, when the table reverses the movement, this gap will cause the motor to go empty and the table does not move the phenomenon, resulting in transmission errors, transmission parts of the force deformation and thermal deformation caused by the deformation error, the error of the table guide. For the above several error causes can be used as follows.

• (1) For transmission backlash, the gap value can be measured in open-loop and semi-closed-loop systems and input to the control system as parameters, as the actual compensation value of backlash to be compensated in reverse motion.
• (2) For the error caused by force deformation and thermal deformation of the components in the mechanical drive chain, it can be solved by increasing the rigidity of the drive chain, reducing friction and cooling.

Solutions for machining errors

Compensation of errors

Static compensation methods
There are various methods of compensation for the errors generated in the machining process of machine tools. One of the most common is static compensation. The so-called static compensation is a very basic compensation measures in the process of machine tool machining manufacturing, this compensation measures are generally in the process of increasing the hardware structure of the parts. The goal of compensation is generally due to the role of external forces and directional error, through static compensation can reduce the probability of error occurrence. For example, screws are very common among the types of machined parts in machine tools. If an error occurs during the machining of a screw part, this static compensation method can be used to reduce the probability of error. At the same time, the static compensation method can also be applied to other error compensation in the machine building process.
Dynamic compensation method
In the machine tool manufacturing process, if you want to use the dynamic compensation method. Generally more common, is the combination of dynamic compensation and static compensation. Because these two methods in combination, you can successfully ensure that the machine tool machining manufacturing process error control. For example, in the process of using the cutting process of machine tools, the machine tool is prone to spatial position changes, and this spatial position change, but also in the subsequent production process on the production of parts error. Therefore, after such changes occur, it can be timely feedback compensation, control the heat and temperature in the entire cutting process, etc. Dynamic compensation is completed.
Coordinate compensation method
In the process of machine tool machining production, there is a very common compensation method is the coordinate compensation method. This compensation method, is generally the use of the entire machine tool CNC system to the machine design against the reference position.
position. This can be based on the origin of the coordinates to feedback the real-time error generated by the machine tool. In the process of machine tool production, this method is generally applied to the axis type of CNC machine tools. In the process of control, the machine’s mechanical processing data is first determined, and then the compensation results are given. Then you can effectively control the probability of error generated by the machine tool.
To carry out the error compensation of CNC machine tools, error measurement is the key, the error model is the basis. Through the error compensation, you can effectively improve the accuracy of the machine tool and contribute to the improvement of China’s manufacturing industry.

Process measures to improve machining accuracy

Reduce the original error. Improve the geometric accuracy of the machine tool used for machining parts, improve the accuracy of fixtures, gauges and tools themselves, control the error generated by the force and heat deformation of the process system, reduce tool wear, deformation errors caused by internal stress, minimize measurement errors, etc. are all directly reduce the original error. In order to improve the machining accuracy, the original error of the machining error needs to be analyzed, according to the different circumstances of the main original error caused by the machining error to take the appropriate measures to solve. For the processing of precision parts should be used to improve the geometric accuracy of precision machine tools, rigidity and control of processing heat deformation as much as possible; for parts with forming surface processing, it is mainly how to reduce the forming tool shape error and tool installation error.
Error compensation method
Some of the original error of the process system, can take the error compensation method to control its impact on the processing error of the parts.
Error compensation method: the method is to artificially create a new original error, so as to compensate or offset the original process system inherent in the original error, to reduce the processing error and improve the processing accuracy of the purpose.
Error offset method: the use of the original one original error to partially or fully offset the original original error or another original error.
Dividing or homogenizing the original error
In order to improve the machining accuracy of a batch of parts, can take the method of dividing certain original errors. The processing accuracy requirements of the parts surface, but also can be taken in the process of continuous trial cutting processing, and gradually homogenize the original error method.
Dividing the original error (grouping) method: according to the law of error reflection, the size of the workpiece of the blank or process is measured and divided into n groups, the size range of each group of workpiece is reduced to the original 1n. Then adjust the accurate position of the tool relative to the workpiece according to the error range of each group, so that the size of each group of workpiece dispersion range center is basically the same, in order to make the size of the whole batch of workpiece dispersion range is greatly reduced.
Homogenization of the original error: the process of this method is to make the original error of the machined surface continuously reduced and averaged through processing. The principle of homogenization is to find out the difference between workpieces or tool surfaces that are closely related by comparing and checking them with each other, and then carry out mutual correction processing or benchmark processing.
Transferring the original error
The essence of this method is to transfer the original error from the error-sensitive direction to the error-non-sensitive direction. Transfer the original error to the non-sensitive direction. A variety of original error reflected in the degree of the part processing error and whether it is in the error-sensitive direction is directly related. If in the process of machining to try to make it transfer to the non-sensitive direction of the machining error, it can greatly improve machining accuracy. Transfer the original error to other aspects of the machining accuracy does not affect.

Conclusion

The use of CNC machine tools when processing parts error sources are very complex, this paper analyzes the common sources of error, it can be seen that the process analysis is the CNC machining of the preparatory work, and directly affects the machining accuracy. And CNC machine tools own structure and working mechanism of the changes in the processing of parts produced a great deal of new operating habits error, so CNC machine tool operators to strengthen the process knowledge and learning of CNC machine tool structure and working principles, digest the new knowledge, the formation of good habits, in practice, continue to explore and find targeted solutions.
In short, in the process of machining, the generation of errors is inevitable. Only a detailed analysis of the causes of errors can take the appropriate preventive measures to minimize machining errors, so as to effectively improve the accuracy of machining.
Source: China CNC Machining solutions provider – Yaang Pipe Industry (www.machinedsgn.com)