Methods for Judging the Accuracy of Vertical Machining Centers
In the field of mechanical processing, the accuracy of vertical machining centers is of crucial importance to the processing quality. As an operator, accurately judging its accuracy is a key step in ensuring the processing effect. The following will elaborate on the methods for judging the accuracy of vertical machining centers.
Determination of Related Elements of the Test Piece
Materials, Tools and Cutting Parameters of the Test Piece
The selection of test piece materials, tools and cutting parameters has a direct impact on the judgment of accuracy. These elements are usually determined according to the agreement between the manufacturing factory and the user and need to be properly recorded.
In terms of cutting speed, it is approximately 50 m/min for cast iron parts; while for aluminum parts, it is approximately 300 m/min. The appropriate feed rate is roughly within (0.05 – 0.10) mm/tooth. In terms of cutting depth, the radial cutting depth for all milling operations should be 0.2 mm. The reasonable selection of these parameters is the basis for accurately judging the accuracy subsequently. For example, too high a cutting speed may lead to increased tool wear and affect the processing accuracy; improper feed rate may cause the surface roughness of the processed part to fail to meet the requirements.
The selection of test piece materials, tools and cutting parameters has a direct impact on the judgment of accuracy. These elements are usually determined according to the agreement between the manufacturing factory and the user and need to be properly recorded.
In terms of cutting speed, it is approximately 50 m/min for cast iron parts; while for aluminum parts, it is approximately 300 m/min. The appropriate feed rate is roughly within (0.05 – 0.10) mm/tooth. In terms of cutting depth, the radial cutting depth for all milling operations should be 0.2 mm. The reasonable selection of these parameters is the basis for accurately judging the accuracy subsequently. For example, too high a cutting speed may lead to increased tool wear and affect the processing accuracy; improper feed rate may cause the surface roughness of the processed part to fail to meet the requirements.
Fixing of the Test Piece
The fixing method of the test piece is directly related to the stability during the processing. The test piece needs to be conveniently installed on a special fixture to ensure the maximum stability of the tool and the fixture. The installation surfaces of the fixture and the test piece must be flat, which is a prerequisite for ensuring the processing accuracy. At the same time, the parallelism between the installation surface of the test piece and the clamping surface of the fixture should be inspected.
In terms of the clamping method, a suitable way should be used to enable the tool to penetrate and process the full length of the center hole. For example, it is recommended to use countersunk screws to fix the test piece, which can effectively avoid interference between the tool and the screws. Of course, other equivalent methods can also be selected. The total height of the test piece depends on the selected fixing method. A suitable height can ensure the stability of the position of the test piece during the processing process and reduce the accuracy deviation caused by factors such as vibration.
The fixing method of the test piece is directly related to the stability during the processing. The test piece needs to be conveniently installed on a special fixture to ensure the maximum stability of the tool and the fixture. The installation surfaces of the fixture and the test piece must be flat, which is a prerequisite for ensuring the processing accuracy. At the same time, the parallelism between the installation surface of the test piece and the clamping surface of the fixture should be inspected.
In terms of the clamping method, a suitable way should be used to enable the tool to penetrate and process the full length of the center hole. For example, it is recommended to use countersunk screws to fix the test piece, which can effectively avoid interference between the tool and the screws. Of course, other equivalent methods can also be selected. The total height of the test piece depends on the selected fixing method. A suitable height can ensure the stability of the position of the test piece during the processing process and reduce the accuracy deviation caused by factors such as vibration.
Dimensions of the Test Piece
After multiple cutting operations, the external dimensions of the test piece will decrease and the hole diameter will increase. When used for acceptance inspection, in order to accurately reflect the cutting accuracy of the machining center, it is recommended to select the final contour machining test piece dimensions to be consistent with those specified in the standard. The test piece can be repeatedly used in cutting tests, but its specifications should be kept within ±10% of the characteristic dimensions given by the standard. When the test piece is used again, a thin-layer cutting should be carried out to clean all the surfaces before conducting a new precision cutting test. This can eliminate the influence of the residue from the previous processing and make each test result more accurately reflect the current accuracy status of the machining center.
After multiple cutting operations, the external dimensions of the test piece will decrease and the hole diameter will increase. When used for acceptance inspection, in order to accurately reflect the cutting accuracy of the machining center, it is recommended to select the final contour machining test piece dimensions to be consistent with those specified in the standard. The test piece can be repeatedly used in cutting tests, but its specifications should be kept within ±10% of the characteristic dimensions given by the standard. When the test piece is used again, a thin-layer cutting should be carried out to clean all the surfaces before conducting a new precision cutting test. This can eliminate the influence of the residue from the previous processing and make each test result more accurately reflect the current accuracy status of the machining center.
Positioning of the Test Piece
The test piece should be placed in the middle position of the X stroke of the vertical machining center and at an appropriate position along the Y and Z axes suitable for the positioning of the test piece and the fixture as well as the length of the tool. However, when there are special requirements for the positioning position of the test piece, they should be clearly specified in the agreement between the manufacturing factory and the user. Correct positioning can ensure the accurate relative position between the tool and the test piece during the processing process, thereby effectively ensuring the processing accuracy. If the test piece is inaccurately positioned, it may lead to problems such as processing dimension deviation and shape error. For example, deviation from the central position in the X direction may cause dimension errors in the length direction of the processed workpiece; improper positioning along the Y and Z axes may affect the accuracy of the workpiece in the height and width directions.
The test piece should be placed in the middle position of the X stroke of the vertical machining center and at an appropriate position along the Y and Z axes suitable for the positioning of the test piece and the fixture as well as the length of the tool. However, when there are special requirements for the positioning position of the test piece, they should be clearly specified in the agreement between the manufacturing factory and the user. Correct positioning can ensure the accurate relative position between the tool and the test piece during the processing process, thereby effectively ensuring the processing accuracy. If the test piece is inaccurately positioned, it may lead to problems such as processing dimension deviation and shape error. For example, deviation from the central position in the X direction may cause dimension errors in the length direction of the processed workpiece; improper positioning along the Y and Z axes may affect the accuracy of the workpiece in the height and width directions.
Specific Detection Items and Methods of Processing Accuracy
Detection of Dimensional Accuracy
Accuracy of Linear Dimensions
Use measuring tools (such as calipers, micrometers, etc.) to measure the linear dimensions of the processed test piece. For example, measure the length, width, height and other dimensions of the workpiece and compare them with the designed dimensions. For machining centers with high accuracy requirements, the dimension deviation should be controlled within a very small range, generally at the micron level. By measuring the linear dimensions in multiple directions, the positioning accuracy of the machining center in the X, Y, Z axes can be comprehensively evaluated.
Accuracy of Linear Dimensions
Use measuring tools (such as calipers, micrometers, etc.) to measure the linear dimensions of the processed test piece. For example, measure the length, width, height and other dimensions of the workpiece and compare them with the designed dimensions. For machining centers with high accuracy requirements, the dimension deviation should be controlled within a very small range, generally at the micron level. By measuring the linear dimensions in multiple directions, the positioning accuracy of the machining center in the X, Y, Z axes can be comprehensively evaluated.
Accuracy of Hole Diameter
For the holes processed, tools such as internal diameter gauges and coordinate measuring machines can be used to detect the hole diameter. The accuracy of the hole diameter includes not only the requirement that the diameter size meets the requirements, but also indicators such as cylindricity. If the hole diameter deviation is too large, it may be caused by factors such as tool wear and spindle radial runout.
For the holes processed, tools such as internal diameter gauges and coordinate measuring machines can be used to detect the hole diameter. The accuracy of the hole diameter includes not only the requirement that the diameter size meets the requirements, but also indicators such as cylindricity. If the hole diameter deviation is too large, it may be caused by factors such as tool wear and spindle radial runout.
Detection of Shape Accuracy
Detection of Flatness
Use instruments such as levels and optical flats to detect the flatness of the processed plane. Place the level on the processed plane and determine the flatness error by observing the change in the position of the bubble. For high-precision processing, the flatness error should be extremely small, otherwise it will affect subsequent assembly and other processes. For example, when processing the guide rails of machine tools and other planes, the flatness requirement is extremely high. If it exceeds the allowable error, it will cause the moving parts on the guide rails to run unsteadily.
Detection of Flatness
Use instruments such as levels and optical flats to detect the flatness of the processed plane. Place the level on the processed plane and determine the flatness error by observing the change in the position of the bubble. For high-precision processing, the flatness error should be extremely small, otherwise it will affect subsequent assembly and other processes. For example, when processing the guide rails of machine tools and other planes, the flatness requirement is extremely high. If it exceeds the allowable error, it will cause the moving parts on the guide rails to run unsteadily.
Detection of Roundness
For the circular contours (such as cylinders, cones, etc.) processed, a roundness tester can be used to detect. The roundness error reflects the accuracy situation of the machining center during the rotation movement. Factors such as the rotation accuracy of the spindle and the radial runout of the tool will affect the roundness. If the roundness error is too large, it may lead to imbalance during the rotation of mechanical parts and affect the normal operation of the equipment.
For the circular contours (such as cylinders, cones, etc.) processed, a roundness tester can be used to detect. The roundness error reflects the accuracy situation of the machining center during the rotation movement. Factors such as the rotation accuracy of the spindle and the radial runout of the tool will affect the roundness. If the roundness error is too large, it may lead to imbalance during the rotation of mechanical parts and affect the normal operation of the equipment.
Detection of Position Accuracy
Detection of Parallelism
Detect the parallelism between processed surfaces or between holes and surfaces. For example, to measure the parallelism between two planes, a dial indicator can be used. Fix the dial indicator on the spindle, make the indicator head contact the measured plane, move the workbench, and observe the change in the dial indicator reading. Excessive parallelism error may be caused by factors such as the straightness error of the guide rail and the inclination of the workbench.
Detection of Parallelism
Detect the parallelism between processed surfaces or between holes and surfaces. For example, to measure the parallelism between two planes, a dial indicator can be used. Fix the dial indicator on the spindle, make the indicator head contact the measured plane, move the workbench, and observe the change in the dial indicator reading. Excessive parallelism error may be caused by factors such as the straightness error of the guide rail and the inclination of the workbench.
Detection of Perpendicularity
Detect the perpendicularity between processed surfaces or between holes and surface by using tools such as try squares and perpendicularity measuring instruments. For example, when processing box-type parts, the perpendicularity between the various surfaces of the box has an important impact on the assembly and use performance of the parts. The perpendicularity error may be caused by the perpendicularity deviation between the coordinate axes of the machine tool.
Detect the perpendicularity between processed surfaces or between holes and surface by using tools such as try squares and perpendicularity measuring instruments. For example, when processing box-type parts, the perpendicularity between the various surfaces of the box has an important impact on the assembly and use performance of the parts. The perpendicularity error may be caused by the perpendicularity deviation between the coordinate axes of the machine tool.
Evaluation of Dynamic Accuracy
Detection of Vibration
During the processing process, use vibration sensors to detect the vibration situation of the machining center. Vibration may lead to problems such as increased surface roughness of the processed part and accelerated tool wear. By analyzing the frequency and amplitude of the vibration, it is possible to determine whether there are abnormal vibration sources, such as unbalanced rotating parts and loose components. For high-precision machining centers, the vibration amplitude should be controlled at a very low level to ensure the stability of the processing accuracy.
During the processing process, use vibration sensors to detect the vibration situation of the machining center. Vibration may lead to problems such as increased surface roughness of the processed part and accelerated tool wear. By analyzing the frequency and amplitude of the vibration, it is possible to determine whether there are abnormal vibration sources, such as unbalanced rotating parts and loose components. For high-precision machining centers, the vibration amplitude should be controlled at a very low level to ensure the stability of the processing accuracy.
Detection of Thermal Deformation
The machining center will generate heat during long-term operation, thereby causing thermal deformation. Use temperature sensors to measure the key components (such as the spindle and the guide rail) temperature changes and combine with measuring instruments to detect the change in the processing accuracy. Thermal deformation may lead to gradual changes in the processing dimensions. For example, the elongation of the spindle under high temperature may cause dimension deviations in the axial direction of the processed workpiece. To reduce the impact of thermal deformation on the accuracy, some advanced machining centers are equipped with cooling systems to control the temperature.
The machining center will generate heat during long-term operation, thereby causing thermal deformation. Use temperature sensors to measure the key components (such as the spindle and the guide rail) temperature changes and combine with measuring instruments to detect the change in the processing accuracy. Thermal deformation may lead to gradual changes in the processing dimensions. For example, the elongation of the spindle under high temperature may cause dimension deviations in the axial direction of the processed workpiece. To reduce the impact of thermal deformation on the accuracy, some advanced machining centers are equipped with cooling systems to control the temperature.
Consideration of Repositioning Accuracy
Comparison of the Accuracy of Multiple Processing of the Same Test Piece
By repeatedly processing the same test piece and using the above detection methods to measure the accuracy of each processed test piece. Observe the repeatability of indicators such as dimensional accuracy, shape accuracy and position accuracy. If the repositioning accuracy is poor, it may lead to unstable quality of batch-processed workpieces. For example, in mold processing, if the repositioning accuracy is low, it may cause the cavity dimensions of the mold to be inconsistent, affecting the use performance of the mold.
By repeatedly processing the same test piece and using the above detection methods to measure the accuracy of each processed test piece. Observe the repeatability of indicators such as dimensional accuracy, shape accuracy and position accuracy. If the repositioning accuracy is poor, it may lead to unstable quality of batch-processed workpieces. For example, in mold processing, if the repositioning accuracy is low, it may cause the cavity dimensions of the mold to be inconsistent, affecting the use performance of the mold.
In conclusion, as an operator, to comprehensively and accurately judge the accuracy of vertical machining centers, it is necessary to start from multiple aspects such as the preparation of test pieces (including materials, tools, cutting parameters, fixing and dimensions), the positioning of test pieces, the detection of various items of processing accuracy (dimensional accuracy, shape accuracy, position accuracy), the evaluation of dynamic accuracy, and the consideration of repositioning accuracy. Only in this way can the machining center meet the processing accuracy requirements during the production process and produce high-quality mechanical parts.