Abstract : Based on the analysis of a CNC gantry deep hole drilling machine, this paper proposes mechanical structure design and CNC system modification design. The improved design utilizes mechatronics knowledge, and proposes a method that integrates dual-grating detection, servo hydraulic cylinder and hydraulic valve control, wire rope adjustment of the crossbeam and power head balance, and differential comparison within the CNC system to achieve micro-compensation.
Keywords : Mechanical structure design, dual-grating inspection, CNC system and balance compensation
The CNC gantry deep hole drilling machine is an independently developed special-purpose machine tool that fills a gap in the domestic market. The structure, similar to that of a gantry planer, will not be elaborated here; the focus is on the innovative structural design and CNC system analysis of the machine tool. This special-purpose machine is suitable for machining large disc-shaped parts. Its main technical specifications and parameters are as follows: maximum drilling diameter 30mm, maximum drilling depth 500mm; worktable size 2.5m × 1.6m, worktable travel 2.5m, sliding plastic-coated guide rails with a load capacity of up to 10t, and ball screw drive; crossbeam travel 0.6m, column with plastic-coated sliding guide rails, maximum column movement 2m, vertical movement of the crossbeam using ball screws; left and right movement of the drilling power head on the crossbeam using ball screw drive.
1. General requirements for CNC gantry deep hole drilling machines
(1) The test piece is a machined part from Dalian Fushun Refinery: deep hole drilling of heat exchanger tube sheet holes.
Each piece has 160 holes measuring 20mm x 350mm, as shown in Figure 1.
(2) A CNC machine tool with a moving beam gantry type and a double deep hole drilling power unit. The overall structure of the main machine is similar to that of a gantry planer, as shown in Figure 2.
(3) The worktable moves horizontally (X direction), the crossbeam moves vertically up and down (Z direction), the deep hole drilling power head moves laterally (Y direction), and the drill rod box moves vertically to process the coordinate holes of the tube sheet.
(4) The vertical movement guide rail of the crossbeam adopts a plastic-coated sliding guide rail.
(5) The worktable moves, the two deep hole drilling power head devices move laterally, the crossbeam moves vertically up and down, and the drill rod box moves vertically, all driven by AC servo motors.
(6) Chip collection box, for collecting chips after drilling.
(7) Numerical control system, which integrates electromechanical components, improves the internal quality of machine tools.
2. Measures to improve borehole positioning accuracy
The overall assembly drawing of the machine tool is shown in Figure 2. Deep hole drilling has the following characteristics: the dimensional tolerances and surface roughness of the holes are guaranteed by the deep hole drilling tool itself; the positional accuracy of the holes is related to the guide frame and the cutting tool; and the distance between the holes is related to the position control of the machine tool.
Simply piecing together a CNC system, linear guides, and optical gratings will not improve the working accuracy of a CNC gantry deep hole drilling machine. Improving the machine tool's internal quality is the primary issue that needs to be addressed.
As can be seen from the partial view in Figure 3, although the deep hole drilling power head and crossbeam adopt a balancing device, the distance L between the drilling centerline and the guide rail surface between the gantry column is too large. The weight of the deep hole drilling power head is about 2t, and the crossbeam is about 10t. Their center of gravity forms an overturning moment with the vertical guide rail of the column. Long-term use will cause the lower part of the crossbeam side of the joint surface between the column and the guide rail to wear quickly, resulting in the centerline of the drill rod of the power head, that is, the centerline of the guide sleeve on the guide frame, tilting and deviating from the vertical position in the front and back direction, thus affecting the position accuracy of the drilling.
This is the first problem that needs to be solved. As can be seen from the partial view in Figure 4, the left and right movement of the deep hole drilling head in the Y-axis direction causes the center of gravity of the crossbeam to shift, which in turn causes the axis of the guide sleeve on the guide frame to deviate from the vertical position in the left and right direction, thus affecting the position accuracy of the drilling.
This is the second problem that needs to be solved. The above two problems are key to the success or failure of the "CNC gantry deep hole drilling machine" design. The first problem is that the overturning moment causes a deviation in the position of the guide sleeve in the front and rear directions, which can be compensated for by using a reverse pre-tilting method. During final assembly, the tilting amount caused by the overturning moment is measured, and the crossbeam slide between the column and the crossbeam is removed again. The corresponding part is scraped off, with more scraping on the upper part than the lower part (about 0.03~0.05mm), so that the axis of the power head drill rod is slightly tilted up relative to the column guide rail surface.
Since the column and crossbeam only move relative to each other during workpiece installation and removal, the lower part of the guide rail mating surface between the column and crossbeam wears very slowly, generally within tolerance for about three years, thus improving service life. Wear on the drill rod box guide rail surface on the power head has little impact on accuracy. The second problem is caused by the shift of the center of gravity, which can be eliminated by using "automatic CNC compensation" to improve the positional accuracy of the inner hole. As shown in Figure 5, using mechatronics knowledge, two grating rulers can be installed on both sides of the crossbeam as displacement sensors to measure the displacement on both sides of the crossbeam, and input to the CPU through the I/O interface; take any point in the Z-axis direction as the origin, and use the assembly fixture and gauge bar to adjust the center line of the guide sleeve on the guide frame to a vertical position. At this time, record the values of the left and right grating rulers: Z<sub>left</sub> = a, Z<sub>right</sub> = b. When the crossbeam moves up and down, and the positions of the left and right guide rails are not synchronized (there is a gap in the guide), the center of gravity changes. The relative displacement ΔZleft = Z′left - a, ΔZright = Z′right - b. If ΔZleft > ΔZright, a control signal is generated for compensation. The pressure of the servo cylinder on the right side of the crossbeam increases, and the CNC system CPU controls the horizontal hydraulic cylinder to perform the corresponding action through the I/O interface.
As shown in Figure 4, wire ropes 2 and 3 are counterweights inside the column, balancing the weight of the crossbeam and the power head (approximately 14t). Wire ropes 1 and 4 are connected to the adjusting hydraulic cylinders, causing changes in the force on the left and right sides of the crossbeam, resulting in a shift in the center of gravity and enabling the crossbeam to automatically return to level. When ΔZ left = ΔZ right, the CNC system sends a control signal, and the pressure of the adjusting hydraulic cylinder system at both ends no longer changes, achieving full closed-loop digital control; the reverse is also true.
After balancing, the hydraulic cylinder at the rear of the crossbeam locks the crossbeam, ensuring that the centerline of the guide sleeve on the guide frame no longer deviates from the vertical position in the left and right directions, thus guaranteeing the correct position of the drill hole.
The vertical feed motion of the crossbeam is controlled by a grating closed-loop system, achieving three decimal places. Balanced control ensures accurate drilling position. In practice, a hole spacing accuracy of 0.02mm is sufficient for machining parts. The horizontal movement of the power head on the crossbeam and the feed motion of the drill rod box on the power head are controlled by an encoder on an AC servo motor, which does not affect the machining accuracy of the holes (dimensional tolerances, surface roughness, straightness).
3. Hydraulic balance of the drill pipe box
There are three methods to achieve automatic balancing for vertically moving components: one method is to use a brake on the electric motor for balancing when the component is relatively light; another method is to use a mechanical method with counterweights; and the last method is to use a hydraulic automatic balancing method.
As can be seen from the above introduction, the crossbeam has obviously been balanced by a counterweight, which has already occupied the space. The drill rod box has a vertical feed motion, so the only way to eliminate the downward sliding due to gravity is to use a hydraulic cylinder to balance it. The difficulty lies in the swing of the Y and Z axes of the pulley. It cannot be disengaged, nor can it affect the balance. This requires creative thinking and a novel pulley structure.
As shown in Figure 6, the rope can be made of steel wire rope or sprocket rope. Since the sprocket in the sprocket rope is difficult to reverse, steel wire rope is used here.
The wire rope is inserted deeply into the pulley to ensure it doesn't slip off. Pulley 1 can rotate along the bearing and is located at the rear; pulley 2 can rotate along the bearing and move along the linear guide rail. The combination of the two pulleys ensures that the wire rope neither slips off nor affects the balance, satisfying the oscillation of the Y and Z axes. Clearly, the tension of the wire rope on the fixed pulley is constant, as shown in Figure 7. T1=T2=T3=T4, the tension is constant, and the weight of the drill rod box on the deep hole drilling power head is balanced by a hydraulic cylinder. This pulley structure can meet the left and right movement requirements of the deep hole drilling power head, achieving normal operation.
4. Conclusion
Through the mechanical structure design and CNC system modification of a CNC gantry deep hole drilling machine, a method to ensure drilling accuracy is proposed. The overall design incorporates dual-grating detection, servo hydraulic cylinder and hydraulic valve control, wire rope adjustment of the crossbeam and power head for balance, and differential comparison by the CNC system to achieve micro-compensation. The machining accuracy is the same as that of the horizontal deep hole drilling machine ZK2102. The successful design of this product opens up a new layout for deep hole drilling machines, provides a theoretical basis for practical applications, and has practical significance.
