Discussion on CNC machining accuracy issues

CNC, as a cutting-edge technology, can be applied in machinery production to effectively improve the level of automation, combine with preset machining programs to achieve automated processing of parts, higher production efficiency and machining accuracy than traditional processes, and obtain stable quality products. In the production process of machining, the change of machining procedures will lead to different shapes, but due to the influence of objective factors and subjective factors of the operator, the production process fails to reasonably control the machining accuracy, resulting in the final production of product quality does not meet the requirements. Therefore, by exploring and analyzing the factors affecting CNC machining accuracy, we can provide support for the follow-up of effective improvement measures and lay the foundation for higher level development in the field of machinery production.

1. Analysis of the technical advantages of CNC

The technical advantages of CNC are more distinctive than the traditional technology, mainly in the following aspects.

(1) high productivity, CNC technology in actual production applications, compared to traditional machine tools spindle speed is greater, can reduce the processing time of parts. At the same time, the speed of the moving parts of CNC machine tools is faster, while improving production efficiency, reducing the time of manual assistance.

(2) High processing accuracy. CNC machine tools are a cutting-edge technology equipment, compared to the previous machine tools for higher processing accuracy, to achieve automatic compensation, to provide a solid guarantee for the quality of product processing. CNC machine tools can achieve high-precision parts processing, machining accuracy in the range of 0.004mm ~ 0.100mm, through the preset program in the command code, automatic control of the tool and speed. CNC machine tools can achieve automated processing, in reducing human operation and subjective awareness of the shortage at the same time, the maximum to avoid human error caused by safety accidents or quality problems.

(3) higher economic efficiency. CNC in practice, you can achieve precision machining, reduce the human auxiliary time at the same time, accelerate production efficiency, reduce time costs and production costs. Even if the cost of CNC machine tools is higher, but compared to traditional machine tools to create higher economic benefits, worthy of widespread promotion and application in the field of mechanical production.

2. Factors affecting the accuracy of CNC technology processing

I. Workpiece coordinate system factor

Workpiece coordinate system has a greater impact on CNC machining accuracy, mainly refers to the establishment of the corresponding coordinate system of the workpiece during the programmer becomes, following the corresponding principles. The workpiece origin is determined on the process positioning datum, which can improve machining accuracy; the workpiece origin fits with the dimensional datum of the part as much as possible to reduce the calculation error as much as possible; the workpiece surface with high accuracy is selected as much as possible to improve machining accuracy; the horizontal machining center sets up the workpiece origin on the rotary center to ensure that the rotary center and the Z-axis connecting position fit together. If it is a general part, following the above principles can effectively ensure the machining accuracy, but if it is a

But if it is a special structure of parts, it is necessary to consider more other factors.

(1) Machining of box parts on horizontal machining centers. This type of parts, due to its structural characteristics, requires the construction of four positioning coordinate systems, or the establishment of multiple coordinate systems on each side, which has a greater impact on the machining accuracy. Therefore, when establishing the coordinate system, it is necessary to take into account that the accuracy of the machine tool rotation center coordinates affects the coaxiality of the relative side holes, and in establishing multiple coordinate systems, whether the machining process is reasonable will directly affect the machining accuracy. The accuracy of the reference ground will affect the hole center high precision.

(2) Vertical machining center on the deformed workpiece processing. This type of workpiece has distinctive structural characteristics, and usually requires the establishment of two or more coordinate systems on the part, combined with the structural characteristics of the part and the clamping method to consider the adverse effects on machining accuracy. This includes how many coordinate systems are established and which coordinate system can be machined first to ensure the accuracy of the part; the accuracy of the positioning dimensions between coordinate systems also affects the machining accuracy of the part.

II. The machine itself structural factors

(1) programming error. Programming error is the machine itself affects the machining accuracy of one of the factors, for the economic CNC machine tools, in the workpiece processing process, the tool along the plane of the two coordinate axes to go in the direction of the folding line for operation, which may lead to the workpiece surface jagged interpolation error. Causes such errors appear more factors, including the degree of uniformity of the pulse, machine resolution and control system dynamic characteristics and calculation methods, etc., the calculation process will also appear cumulative error, after reaching a certain value to drive the machine movement, affecting the machining accuracy of the workpiece. For such errors, in programming as far as possible to choose the absolute way programming, insert back to the reference point instructions, coordinate clearing to eliminate cumulative errors.

(2) Approximation error. Approximation error will also affect the machining accuracy, mainly is the approximation algorithm approximation part contour error. If the non-circular curve is machined, the non-circular curve is approximated and the linear profile replaces the curve profile, which will lead to inaccurate profile and affect the overall machining accuracy. Approximation error value dk, the formula is as follows: Rmin is the theoretical minimum radius of curvature of the contour, if the workpiece contour is unknown, the list curve approximation is used in programming, resulting in the approximation error value error is determined.

(2) rounding error. This error refers to the CNC machine tool machining process, the pulse equivalent for the minimum value of linear displacement plays a decisive role. This requires the operator to strictly follow the requirements of the size of the part, into the control of the number of pulses for programming. Pulse equivalent value for machine tool processing accuracy boundaries have a greater impact, if the mathematical processing of rounding to a pulse equivalent value, inevitably resulting in rounding error, which in turn affects the final workpiece processing accuracy.

Ⅲ. Feed structure

The feed structure clearance affects the CNC machining accuracy mainly because the feed structure transmission structure consists of several parts including coupling joints, reduction gears, support bearings and ball screw sub-constituents. The clearance between any one of these parts will affect the overall machining progress, among which the clearance generated by the motion reversal has the greatest impact on the machining accuracy. Therefore, the clearance of each part can be reduced appropriately to provide guarantee for machining accuracy.

Bearing axial clearance on the table feed accuracy brings a greater impact, bearing axial clearance SZ, the reverse will appear table feed error, the formula is d2 = SZ, through the positioning preload can eliminate the axial clearance, to provide security for the overall stability of the structure. However, the processing error brought by the thermal deformation of the bar should be taken into account in this process. The clearance between the ball screw and the nut is SF, and the table feed error caused by the reversal is d1=SF. The clearance of the screw-nut pair will adversely affect the overall stiffness, and the machining error caused by the clearance can be effectively solved by test driving the preload between the ball screw and the nut. It should be noted that when the applied preload force is the same as the maximum axial force 1/3, the machining error can be basically eliminated and the overall machining accuracy can be guaranteed.

Ⅳ. Temperature and process system thermal deformation factors

In the production process of CNC machine tool processing, the temperature factor has a greater impact on the overall machining accuracy, mainly because of the metal

Metal heat will appear thermal expansion and contraction changes, resulting in changes in the size of the part. If the high precision requirements, in the summer processing production

If the accuracy requirements are high, in the summer processing production need to reasonably control the temperature conditions, as far as possible to reduce the deviation; winter processing, it is necessary to close to the lower deviation. And the process

The impact of thermal deformation of the system on machining accuracy is shown in the frictional and cutting heat factors during the machining process, and the process system temperature rises to different degrees.

The temperature of the process system increases to different degrees, or the part is deformed, and the position between the tool and the workpiece changes, which affects the final

The final machining accuracy is affected. Therefore, it is necessary to strictly control the machine structure and the thermal stability of the drive system, with the help of automatic compensation of errors

and clearance, and to measure and adjust the tool compensation size according to the actual situation.

3. Improvement measures of CNC machining accuracy

Ⅰ. Optimize the design of CNC machine tools

CNC technology in machinery manufacturing has been commonly used to reduce production costs while accelerating production efficiency and quality, and has accumulated a wealth of experience, but there are still some of the factors that affect the high precision given to you. Based on this, in order to solve this problem need to raise control CNC machine tools applicable range, optimize the design of machine structure, in the actual production experience to summarize and improve, so that the CNC machine design more scientific and reasonable.

First, the external purchase of higher precision parts, improve the overall processing performance of CNC machine tools, reduce processing costs at the same time, the production of higher quality products.

Secondly, to ensure that the structure has sufficient rigidity, the structure of the force is uniform, so that even with large external forces will not lead to mechanical geometry deformation problems, to enhance the workpiece will be high precision. At the same time, the machine tool processing accuracy for a comprehensive analysis, to understand the performance requirements based on the adjustment of parameters to ensure that the accuracy of the parts in line with technical standards. And the choice of tools and fixtures, should be based on the actual production needs of a comprehensive consideration, choose the right parts processing.

Finally, optimize the structure of CNC machine tools center of gravity, adjust the parameters to reduce the adverse effects of mechanical form. You can appropriately lower the mechanical center of gravity, enhance the frequency of the machine tool swing mode, in reducing the machine tool load at the same time, effectively improve the CNC machine tool deformation resistance performance. The structure is concentrated in the lower part, through the spindle system optimization design, to improve the thermal state performance of CNC airports, even at higher temperatures will not deformation, effectively reduce the adverse effects of thermal effects on CNC machine tool production, and improve processing accuracy.

Ⅱ. Do a good job of equipment inspection

In order to ensure processing accuracy, you can combine the actual situation to prepare a reasonable plan, a comprehensive inspection of CNC machine tools before production, to understand the operation of various parts of the equipment and the overall performance, and check the operating environment of the equipment, the preparation of reasonable preventive and control measures to eliminate factors that potentially affect the processing accuracy. Through such methods, machining errors can be effectively reduced and machining accuracy can be improved.

Ⅲ. Improving the geometric accuracy of machine guide

Combined with the machining accuracy requirements, the geometric accuracy of the existing CNC machine tool guideway is adjusted and improved, requiring the guideway to have sufficient stiffness and stability to avoid strong vibration during machining and to meet the machining needs while improving the machining accuracy.

4. Conclusion

In summary, in the face of rapid development in the field of machining, CNC technology has been widely used with its unique advantages, compared with traditional machine tools can effectively reduce production costs and time costs, improve product processing efficiency and quality. To achieve this goal, it is necessary to strictly control the machining accuracy and prepare a plan to control the possible influencing factors to promote the higher level of development in the field of machinery production.