Four-Axis Machining Techniques for Guaranteeing Complex Planar Angularity
This paper outlines a method for machining high-precision stop keys that are uniformly distributed on the outer mating surface of landing gear components. The method encompasses the entire machining process, including the rotation angle and control techniques for each individual point. Additionally, it details the calculations for determining the angles and positions of points during the machining of stop keys at various angles. The paper also presents reliable inspection methods to evaluate the completed stop keys.
01 Introduction
The design of piston rods for aircraft landing gear is increasingly incorporating the axle into the piston rod body for main cushioning functions. This integration leads to more stringent requirements regarding the number of stop keys on the axle, including geometric tolerances, key width tolerances, and angular tolerances between the keys. Traditionally, these components typically have only two keys, but recent developments in piston rods now often feature three to four keys (see Figure 1). This complicates the machining quality due to the component’s overall structure. The presence of four stop keys, evenly distributed around the circumference and varying in thickness, poses significant machining challenges. As the demand for larger aircraft grows, it is likely that future designs will include even more stop keys on axles.
To achieve the necessary tolerances for the four stop keys, it is essential to maintain the correct angular relationship between the keys and to ensure that the key mating plane is symmetrical relative to the wheel axle during the machining process. A reliable and efficient machining method, along with suitable tooling, has been developed to ensure high machining quality and to enhance production efficiency.
02 Technical Requirements for Piston Rod Stop Keys
The technical requirements for the stop keys are detailed in Figure 2. These four stop keys are critical structural components that secure the axle to the aircraft wheel. They must adhere to a dimensional tolerance of just 0.02 mm, an axial symmetry tolerance of 0.05 mm, and a uniform angular tolerance of ±10°. Multiple workstations are necessary to complete the machining of all the stop keys.
3.1 Processing using a five-axis machining center
When finishing this type of stop key after heat treatment, a five-axis machining center or a four-axis horizontal CNC boring machine is primarily used. The advantages of a five-axis machining center significantly reduce the need for strict clamping postures and positional adjustments of the parts during processing. To begin, the wheel shaft needs to be aligned with the plane of the main shaft axis of the machine. After this alignment, the coordinate conversion can be achieved by rotating the main shaft, allowing for the completion of the machining process.
3.2 Processing process using a four-axis horizontal CNC boring machine
(1) Processing technology for stop keys of this type of parts
The processing steps are as follows:
- Material collection
- Turning: Machining the outer base circle of the part
- CNC milling: Rough machining the outer contour of the stop key at four locations
- CNC milling: Finishing the outer contour of the stop key at four locations, leaving a 1mm finishing allowance for the mating surface
- Heat treatment: Altering the material properties to meet the technical requirements specified in the design drawing
- Turning: Re-finishing the outer circle of the part’s positioning base to correct any deformation caused by heat treatment
- CNC boring: Finishing the four stop keys; this step is crucial for meeting the technical specifications of the stop key
- Inspection: Conducting a comprehensive check of the size and geometric tolerance requirements of the stop key
- Protective storage
This sequence ensures that all technical specifications and quality requirements are met throughout the processing of the part.
(2) The processing process of the CNC boring process after heat treatment is as follows.
1) Clamping and alignment of parts. The four-axis machining center utilizes the A-axis and a central stand positioned at the same height as its center. The parts are secured and aligned using a “one clamp and one hug” method. The radial runout of the reference outer circle at both ends must be ≤ 0.03 mm. Additionally, the outer circle axis where the stop key is located, as well as the part positioning outer circle axis, must lie in the same horizontal plane. In this configuration, the settings are A-axis = 0° and B-axis = 0°. A schematic diagram illustrating the part clamping and rotation is shown in Figure 3.
2) Actual Stop Key Machining Steps.
After clamping and aligning the workpiece, the machining surface of the stop key must be rotated to a position that is perpendicular to the machine tool’s spindle. This requires a coordinated rotation of both the A-axis and the B-axis (worktable). Once the angular rotation is complete, a dedicated T-shaped cutter (see Figure 4) is used to machine both the front and back sides of the same stop key.
Due to the symmetry of the part, one angle can be utilized to machine two stop keys simultaneously. However, if the part lacks symmetry, as seen with three stop keys, each must be rotated and machined individually. This approach ensures that the dimensions of the two stop keys in that direction conform to the geometric tolerance requirements relative to the center datum. After machining these, the A-axis and B-axis are then rotated again to machine the remaining two stop keys.
3) Other matters.
During the machining process, the rotation of the B-axis will result in the special “T-type tool,” used for machining the stop key, having a long overhang. Additionally, when machining the “T-type tool,” factors such as the root R angle, the rigidity of the machining system, and the rigidity of the tool itself can lead to significant vibration marks on the surface of the stop key. This can negatively impact product quality. Therefore, it is essential to monitor the tool condition and machining process parameters throughout the machining procedure.
(3) Key points of the stop key machining process
In summary, the following key aspects should be emphasized during the machining process of the stop key:
1. The angle of the key should be adjusted based on the actual angle of the wheel shaft, utilizing three-coordinate measurements to achieve an angle value closest to the theoretical position.
2. For each stop key being machined, it is essential to compensate for the size of every machining surface to ensure the dimensions are as close as possible to the theoretical measurements.
3. It is recommended to use circular blades, particularly the special “T-type” knives, and limit each set of blades to processing no more than two parts. Circular blades provide better dimensional stability for machining these keys.
Before starting the machining process, ensure that the tool is sharp and carefully control the equipment processing parameters to minimize chatter marks and maintain high processing quality.
(4) Principle of calculation of the processing size of the stop key The principle is as follows.
1) Principle of calculation of processing points.
When compiling the CNC program, the actual angle of the outer circle where the stop key is located is combined with the rotation of the machine tool station. This process determines the angle and processing point that can be directly used on the production floor, resulting in a finalized CNC program. By leveraging mathematical formulas, a macro program is employed to calculate the starting and ending points of key machining based on actual positions and angle transformations, effectively achieving B-axis rotation origin tracking.
Once the part is installed on the machine tool and the actual position is determined, the processed part is initially rotated around the Y-axis to a predetermined angle, which serves as the positive direction for angle transformation. Subsequently, the axle part is rotated to comply with the right-hand screw rule: the thumb points to the end face, with the other fingers indicating the negative direction. This methodology yields the final position of the key.
With the dimensions of the key, including its width, length, and the starting point’s distance from the origin, the theoretical tool cutting point for machining the key can be calculated. Utilizing coordinate system projection, this point is rotated and projected onto a new coordinate system. Finally, through macro program calculations, the transformed point coordinate is determined. The calculation procedure is outlined as follows.
#101=54;(INI. COS NO.); Initial coordinate system
#102=-999.95;(XPVOT AT B=0); X-axis offset between the worktable rotation center and the spindle center in the initial coordinate system
#103=450;(ZPVOT AT B=0); Z-axis offset between the workpiece rotation center and the worktable rotation center in the initial coordinate system
#104=0.0;(BROT FROM B=0); Rotation angle relative to the B-axis datum = 0 for aligning feature locations on the machined part
#105=59;(FINAL COS NO.); Final coordinate system output
#106=0.0;(XOFS AT B=0); X-axis offset from the tool setting point in the initial coordinate system
#107=0.0;(YOFS AT A=0); Y-axis coordinate offset from the tool setting point in the initial coordinate system
#108=0.0;(BROT FROM B=0); B-axis rotation angle (rotation around the Y axis) after the coordinate offset
#110=0.0;(AROT FROM A=0); A-axis rotation angle (rotation around the X axis) after the coordinate offset
G#101; Activate the machining coordinate system
#111=#[[#4014-53]*20+5201]-#102; X-axis coordinate increment when the initial coordinate system coincides with the worktable center and the spindle center. #112=#103; Z-axis coordinate increment of the tool setting point in the initial coordinate system relative to the worktable center
#113=COS[#104]; Is the inverse angle ±90° or ±270°?
IF [#113 EQ 0.0] GOTO666 #114=[#111+#112*SIN[#104]]/COS[#104]; Calculation formula used when the inverse angle is ±90° or ±270°.
#116=#103 GOTO888 N666 #114=#111*COS[#104]+#112*SIN[#104]; X-axis rotation increment calculation when the inverse angle is not ±90° or ±270°. #116=#112*COS[#104]-#111*SIN[#104]; Z-axis rotation increment calculation when the inverse angle is not ±90° or ±270°.
N888 #117=#114+#106 #118=#107*COS[#110]+#108*SIN[#110] #119=#116+#108*COS[#110]-#107*SIN[#110] # 1 2 4 = # 1 1 7 *C O S [ # 1 0 9 ]- # 1 1 9 * S I N [ # 1 0 9 ] + # 1 0 2 #125=#[[#4014-53]*20+5202]+#118#126=#[[#4014-53]*20+5203]+#119*COS[#109]+#117*S IN[#109]-#103 G10 L2 P[#105-53] X[#121+#124] Y[#122+#125] Z[#123+#126]; Coordinates written
2) Machining Angle Calculation Principle.
Machining the wheel axle stop key involves working with angles, so angle analysis and calculations are necessary before starting the CNC machining process. This angle calculation is similar to calculating the position of a point, as it also utilizes projections within a coordinate system. To perform the angle calculation, you simply need to transform the perpendicular vector of the machining plane of the part so that it aligns parallel to the spindle axis of the machine tool. The rotational position of the part during the machining process is illustrated in Figure 5.
In actual machining, the program must take into account the machine tool’s specific machining range, which varies based on the equipment system, worktable, and machining stroke. Typically, the range for the A-axis during part machining is from -120° to +120°, while the B-axis range is from -90° to +90°.
Table 1 shows an Excel calculator for calculating machining position offsets and angles.
This tool is a macro program developed using FANUC system syntax. It allows field technicians to perform calculations directly on the machine, while process engineers can run the program using VERICUT software to carry out angle calculations. For practical purposes, variable #21 can be configured to adjust the axle orientation during part machining. Once refined to meet actual production needs, this tool can be used to quickly generate finishing programs for all similar machining areas, thus eliminating reliance on software. Below is the local macro program.
#17 = 91.69Q – LZ ANGLE); Angle between the wheel shaft and the piston rod (positive value)
#3 = -45° (C – KJ ANGLE); Enter the wheel shaft rotation angle, following the right-hand screw rule, with the thumb pointing toward the end face and other fingers pointing toward the negative direction
#11 = 25° (H – KJ DEPTH); Key thickness (positive value)
#19 = 150° (S – KJ STAT); Key start and intersection positions (positive value)
#8 = 188° (E – KJ ENDP); Key end and intersection positions (positive value)
#7 = 120° (D – LZ DIAMD); Key root outer diameter (positive value)
#20 = 90° (T – TL DIAMD); Tool diameter (positive value)
#23 = 450° (ZOFF AT B=0); When B=0°, the distance from the piston rod center to the table rotation center
#21=0 (#21=0 A-/A+); The axle can be facing up or down. Use #21 to set it to 0; otherwise, it is not 0. N005 G0 G91 G28 Z0.0; Before the table rotates, move the table away from the tool.
N010 G90 N015 G54 N020 IF [#3 GE 0.0] THEN #4=1.0 N025 IF [#3 LT 0.0] THEN #4=-1.0 N030 #3=180.0-#3 N035 #5=-COS[#3] N040 #6=SIN[#3] N045 #12=#6*SIN[#17] N050 #13=#5 N055 #14=#6*COS[#17] N060 #1=ATAN[#14]/[#13] N065 #2=ASIN[#12] N070 IF [#21 NE 0.0] GOTO666; Calculate whether the journal is facing up or down.
N075 IF [#2 LT 0.0] THEN #1=#1+90.0; Calculate the required rotation angle for the A-axis.
N080 IF [#2 GE 0.0] THEN #1=#1-90.0; Calculate the required rotation angle for the A-axis.
N085 IF [#2 LT 0.0] THEN #2=-#2; Calculate the required rotation angle for the B-axis.
N090 IF [#21 EQ 0.0] GOTO888 N666; Calculation for a journal that always faces upward
N095 IF [#2 LT 0.0] THEN #1=#1-90.0; Calculate the required rotation angle for the A-axis
N100 IF [#2 GE 0.0] THEN #1=180.0+#1; Calculate the required rotation angle for the A-axis
N105 IF [#2 LT 0.0] THEN #2=180.0+#2; Calculate the required rotation angle for the B-axis
3) The calculation principle for integrating machining points and angles is essential for completing the machining of the axle stop key. This process involves combining the calculations of each stop key point and angle to create a comprehensive program for this type of key. When the part is rotated to the machining position, its actual position shifts relative to the original position, as illustrated in Figure 6. Therefore, it is crucial to account for this change in the origin in the final program.
4) Recommended cutting parameters during machining. The machining parameters used during the process are closely linked to the rigidity of the machining system, the stiffness of the machine tool spindle, the cutting performance of the selected circular insert, and the material being machined. For load-bearing structural components of aircraft landing gear, ultra-high-strength steel is commonly employed. A recommended cutting speed is between 30 to 40 m/min. To achieve the desired surface roughness, a feed rate of 0.05 to 0.08 mm/s is generally utilized. In practice, cutting parameters can be adjusted within a specific range based on the machining conditions.
04 Stop Key Inspection Method
The inspection of stop keys generally focuses on two main aspects: the thickness of the stop key itself and its geometric tolerance. The thickness is a general dimension that can be measured at multiple points using a conventional micrometer to ensure it falls within the required tolerance range while maintaining stability. However, the geometric tolerance of the stop key is of primary importance. This part’s stop key is mainly tested for its uniformly distributed angle of 90° ± 10′ and the symmetry requirement of 0.05 mm relative to the center of the wheel axle.
There are various methods to test these requirements. The most commonly used method involves employing a three-coordinate measuring machine. This machine constructs a measuring coordinate system based on the main axis of the wheel shaft and the end face of the stop key. Using a probe and a “four-point sampling” technique, the position and geometric tolerance of the stop keys can be accurately measured. This measurement method demands high accuracy from the three-coordinate measuring machine and comes with a certain tolerance. Additionally, the overall measurement process is influenced by the quality of the outer circle processing of the wheel shaft. It is also important to note that it is impossible to achieve 100% detection of the positional relationships between all matching surfaces of the wheel shaft and each stop key.
An alternative method involves designing and manufacturing a copy gauge through comprehensive positional gauge detection. In this case, the outer circle of the wheel shaft, where the stop key is located, is used as the reference point to create a comprehensive gauge. The “trial assembly” method allows for direct measurement with this gauge. While this method requires high accuracy from the comprehensive gauge used for detection, its sheet metal manufacturing process is relatively simple, making it easier to meet the necessary accuracy requirements. The gauge inspection can effectively eliminate the “high points” of each matching surface, achieving 100% inspection of both the final processing size and position accuracy of the part, thereby ensuring full quality control. The design schematic diagram and detection principle of the comprehensive gauge are illustrated in Figure 7.
05 Comparison of Processing Results Using Different Equipment
5.1 Processing Quality
As of now, more than 40 products have been machined using the four-axis machining center, resulting in a qualified rate of 95.34% after thorough metrology and comprehensive gaging inspections. In addition, over 70 products have been machined with the five-axis machining center, achieving a qualified rate of 95.89%. Both machining methods effectively ensure quality, and we have not received any customer feedback concerning quality issues for the products delivered.
5.2 Processing Efficiency
When machining on a five-axis machining center, once the part is clamped and aligned, there is no need for multiple angular rotations. The spindle of the five-axis machining center can directly achieve the necessary angular positioning, allowing for complete machining. As a result, the machining time is approximately 2.2 hours per part.
In contrast, a four-axis machining center requires multiple rotations and post-rotation verification, leading to a longer setup time and a machining time of about 2.6 hours per part. This means that the efficiency of a four-axis machining center for this type of stop key is approximately 18.1% lower than that of a five-axis machining center. Additionally, the machining process on a four-axis center demands more attention and effort.
5.3 Other Aspects
While four-axis machining centers may have slightly lower machining efficiency compared to five-axis machining centers, they are significantly more affordable. In situations where common equipment in a machining shop faces bottlenecks, or if a five-axis machining center malfunctions, a four-axis machining center can effectively redirect resources. This ensures consistent quality while maintaining smooth production.
06 Conclusion
This article outlines a method for machining stop keys using a horizontal machining center. It also summarizes two tools that aid in production, enabling high-precision machining. This method can be widely applied to the machining and manufacturing of stop key components for integrated landing gear designs.
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