How to do CNC milling thread machining for hard aluminum alloys
Duraluminum alloys have a lower density than steel or copper and a higher specific strength. Under the same load conditions, duraluminum alloy parts can effectively reduce the structure’s weight. In the aerospace industry, power components and automotive machinery manufacturing, duraluminum alloy parts have been widely used. With the development of technology and the deepening of industrial transformation, the application scenarios of duraluminum alloy structural parts are increasing, accelerating the research development and promotion of duraluminum alloy processing technology.
With the development of intelligent manufacturing technology, CNC industry has occupied the mainstream position, in some large parts or high-precision thread processing, the traditional tap, plate tooth processing or thread turning has been difficult to meet the needs of intelligent manufacturing. Adopting an advanced control system of a CNC machining center for thread milling not only improves the efficiency of the thread machining process but also obtains good economic benefits.
The large-diameter bore threads in hard aluminum alloy structural components have the characteristics of high machining difficulty and high demand. The optimization under parametric calculation is realized by taking the milling thread machining process, tool path, and machining technology parameters as the controlling factors to provide a new technical way for high-precision CNC milling of hard aluminum alloy threads. Optimize and adjust the parametric calculation model to solve the technical problems of high-precision milling of large aperture threads in batch, improve the automation level of thread processing, and help the development of new quality productivity.
Establishment of the parameterized calculation model
The basic size of the thread can be calculated according to the pitch P of the thread cutter and the thread diameter D marked on the drawing. The pitch P of the thread cutter is proportional to the thread’s theoretical triangular tooth height H. Therefore, the theoretical triangular tooth height H of the thread can be obtained by solving equation (1):
The theoretical internal thread diameter D1 can be obtained by solving equation (2):
The theoretical triangular height of the thread, H, influences the accuracy of the screwed thread after machining. When machining hard aluminum alloys, the internal thread diameter D1 often becomes smaller due to the ductile properties of the material, and attention should be paid to this index to improve machining accuracy.
The milling of cylindrical internal threads is a bottom-up movement of the thread milling cutter’s toolpath along a set standard helix. In contrast, the thread milling cutter’s rotational movement cuts triangular tooth-shaped excess material.
When point A on the tip of the thread milling cutter is projected to the XY plane, point A on the cutter’s tip can be regarded as a rotary motion with fixed angular velocity centered on the Z-axis. Point A on the cutter’s tip is projected to the Z-axis in a uniform axial motion. The parametric equation can solve these two motions, as shown in equation (3).
Where: α is the helical radius from the tip point A to the Z-axis, the center of the rotation circle; θ is the rotation angle, which is the rotation angle of the tip point A around the Z-axis; h is the ascending distance of the same helical angle on the machining trajectory line.
Threads of different standard diameters can be machined when milling threads, using the same thread milling cutter and adjusting the helix radius in the toolpath. And when the helix radius α is changed, there will be a change in the machining trajectory, to adjust the thread curves of different diameters.
In the machining process, when the helix radius is set as a variable that conforms to the rule of change of conical thread, the machining tool trajectory obtained is a conical helical curve, and the conical thread will be obtained in the end.
In the machining process, when the helix radius is set to a fixed value, the machining tool trajectory obtained is a cylindrical helical curve, and a cylindrical thread will be obtained in the end.
In the production process, it is necessary to change the production process according to the workpiece material, the difference of standard pitch, the depth of tooth shape processing, the cooling condition in the processing and so on.
For example, if the workpiece material is elastic or difficult to cut, it needs to be milled many times, and the threaded holes with large depth-diameter ratios need to be milled many times in small quantities; the threads with large pitches have a large machining allowance and must be roughly milled and semi-finish milled before finishing milling. Otherwise, the phenomenon of letting the knife appear in the process of machining, which will make the thread pass the specification but not pass the standard, or the tool will vibrate the tool because of the large amount of allowance, which will make the processed threaded tooth surface ripples, destroying the threads. Precision.
In the machining process, the hard aluminum alloy will have obvious differences between the machined thread and the theoretical thread because of the material extension, so the machined parts can not meet the requirements of the drawings.
To explore the relationship between the extension coefficient of the part material and the thread processing result, the set point T is an arbitrary point on the tool trajectory; Rt is the actual helical radius value of the point T in the thread milling process, and the difference between the helical center diameter d2 of the actual thread milling cutter and the theoretical inner thread center diameter value D2 is corrected by the material extension coefficient γ, as shown in Equation (4).
The helical center diameter d2 of the thread milling cutter can be obtained from the tool datasheet.
Experimental study on the machining of large-diameter internal thread helical milling
The spiral angle of rise required on the thread surface and the envelope without the spiral angle of rise formed by the rotation of the thread milling cutter can not coincide completely, which leads to thread machining interference errors during the machining process.
The center axis of the internal thread can only be kept parallel to the center axis of the milling cutter, which leads to a fixed value of the helix angle difference between the helix angle of the thread and the envelope machined by the thread milling cutter all the time. As a result, both thread surfaces will interfere and show center symmetry in actual machining.
Cutting test processing adopts a smooth milling mode, from the bottom of the thread cut into the bottom-up way to program, which is divided into roughing and finishing modes to process. After the thread milling, the workpiece is processed to obtain half of the internal thread profile for observation. The physical drawing of the test piece is shown in Fig. 1.
This test uses different materials to extend the deformation coefficient γ to obtain different machining tool trajectory helical radius Rt, machining M37 × 1.5-6H cylindrical internal threads of hard aluminum alloy, the use of helical cut into the toolpath thread milling process, the obtained threads for cross-section processing, the use of image measuring instrument to measure out the thread contour line and the exact size, through the comparison of detection of changes in machining accuracy.
Measuring the tooth profile of each thread and comparing the standard thread profile, the test results are shown in Table 1, and the machining error after the introduction of the extended deformation coefficient is obtained, which can effectively reduce the tooth shape variation and obtain the high-precision thread.
The milling accuracy of internal threads of hard aluminum alloy changes with the change of feed per tooth, cutting speed, and ductile deformation coefficient, which is calculated by extreme difference analysis after collating the test data. The center diameter deviation and tooth type deviation of the threads obtained after milling decreased with the increase in cutting speed and increased with the increase of feed per tooth.
Therefore, to improve the dimensional accuracy of thread milling, the cutting speed should be increased appropriately, with a reasonable ductile deformation coefficient.
Factory batch processing
Batch machining of hard aluminum alloy threaded parts of large diameters are as follows:
(1) obtain hard aluminum alloy threaded parts drawings, the preparation of a reasonable machining process, and select the applicable tool.
2）Programming software such as power mill and CNC machine tools for the front machining process for cutting trial processing and the milling of large threads to prepare for processing.
(3) Prepare the preliminary machining program of large thread CNC milling and adjust the parameters through a parametric equation.
(4) Carry out the trial machining of hard aluminum alloy large diameter thread parts, and carry out the first piece of workpiece inspection.
(5) According to the test results, adjust the walking tool helix diameter to correct the thread machining section extension deformation coefficient.
(6) Carry out the first sample machining again, check whether the thread accuracy meets the drawing accuracy requirements, and check the tooth profile and pitch data by checking the thread pass/stop gauge.
(7) After the first sample test passes the drawing requirements, mass production is carried out, and the thread parameters are sampled and tested.
(8) In case of fluctuations in the KPI of the measured values, the parameters are adjusted and optimized again.
(9) In the production of several batches of products after the finalization of the relevant technical parameters of the product.
(10) Summarize the shortcomings of the process, comprehensively review the optimization of the production process and production efficiency, and put the product parameters into the database model to accumulate data support for subsequent development.
Milling large threaded holes in engineering structural parts has realized stable 5H-level precision machining. Stable machining of large-size threaded holes in non-through holes has been realized in batch.
Conclusion
Analyzing large-diameter threaded holes in hard aluminum alloy structural components and establishing a parametric calculation model for the technical parameters of thread milling can realize the batch processing of large-diameter high-precision threads. The generality of thread milling is good; it can process large diameter or large pitch threaded workpieces, use the same thread milling cutter to process all kinds of internal and external threads with different nominal diameters but the same pitch, and realize batch stable processing.
We can see the analysis of the corresponding milling thread machining process, tool path and processing technology parameters in the parametric calculation to optimize high-precision CNC milling of hard aluminum alloy threads to provide a new technology path.