Since 2017, the shortage of linear guide rail pairs has attracted a large amount of funds to enter this capital and technology intensive small industry. It can be foreseen that there will be a large surplus of low-end linear guide rail pairs around 2019. However, due to the lack of core technology, the demand for mid to high end linear guide rail pairs will remain in short supply for a long time. To achieve the mid to high end level of domestic linear guide rail pairs, the vast majority of enterprises still have a lot of technical problems to solve. Even with the simple process of drilling installation holes for linear guide rails, the qualification rate is not high. The reason behind this is that some producers lack mastery of product technical requirements and excessively pursue efficiency or cost savings. When selecting equipment, it is already determined that the product quality qualification rate is low.
The unqualified content of the installation hole of the linear guide rail not only includes inconsistent depth of the sinking hole, oversized diameter of the sinking hole, and uneven chamfering of the sinking hole mouth, but also more serious cumulative error of the installation hole distance of the linear guide rail, which directly affects the installation accuracy of the linear guide rail. The oversized sinking hole size also affects the sealing effect of the nut.
The cumulative error of the installation hole spacing of a linear guide refers to the fact that one linear guide has multiple holes (such as 67 holes in a 4mm 25mm specification linear guide). Generally, machine tools complete drilling by moving the drilling head multiple times relative to the linear guide, resulting in the accumulation of errors in the front and back hole spacing, commonly known as "cumulative error" in the industry. The reasons for accumulated errors include uneven guide rail material leading to irregular elongation or shortening during drilling, insufficient positioning accuracy of the machine tool itself, drilling speed exceeding the allowable range of spindle accuracy or rigidity, and systematic errors in the unreasonable design scheme of the machine tool.
Processing requirements for installation holes of linear guide rails
As shown in Figure 1, there are certain accuracy requirements for the installation holes of the linear guide rail, the chamfering of the hole mouth, the size and center distance of the bottom hole, and the chamfering accuracy of the bottom hole is low. Among them, the accuracy of center distance Z is crucial, and the accuracy of adjacent hole spacing does not need to be as high as the ± 0.05mm required by some enterprises, but the cumulative error must be well controlled. Taking the 25mm specification guide rail as an example, the tolerance drawing for the center distance of the installation hole of a certain German brand linear guide rail only requires ± 0.3mm, but it includes cumulative errors. This article suggests that the center distance tolerance standard for any two holes in a linear guide rail should be controlled within ± 0.2mm.
Figure 1 Requirements for machining installation holes of linear guide rails
Analysis of general linear guide drilling machine schemes
(1) A linear guide drilling machine with guide rail movement and fixed drilling head. This type of drilling machine occupies a large area and generally uses four power heads to drill holes from both sides of the guide rail. The guide rail is horizontal and moves one hole by one to complete the drilling of all holes.
This drilling machine has two driving schemes for the movement of the guide rail. One is the continuous movement of the entire length range of the linear guide rail driven by the servo mechanism as a whole (hereinafter referred to as the "four-axis machining scheme with full stroke servo drive of the guide rail"). This scheme can achieve the required drilling accuracy, but it is not convenient to change the model; The second is to install movable hydraulic vises on a servo slide with a short stroke, exchange clamping with fixed vises, and release the linear guide rail. Each time, move one hole position to complete the machining of all holes in the linear guide rail (hereinafter referred to as the "guide rail hydraulic clamping exchange drive four spindle machining" scheme). This scheme is difficult to achieve qualified cumulative error in hole spacing, for example, even if the adjacent hole spacing deviation is only 0.01mm, there are more than 60 holes, The cumulative deviation exceeds 0.6mm. Due to poor repeatability in elongation or shortening during drilling of the guide rail, even if the sliding table error is compensated by the CNC system, the cumulative error is difficult to solve stably.
(2) Single spindle movement and division of labor step machining scheme. Using a traditional single head moving column drilling machine, all bottom holes are drilled first, and then the sinking holes are drilled one by one from each hole, and the sinking hole mouth is chamfered. Each work step is drilled in stages. Domestic machine tools using this scheme have lower costs but also lower efficiency.
(3) Single spindle moving and forming composite cutting scheme. Using a composite drill bit to drill bottom holes, counterbore holes, and chamfer holes in one go results in high tool costs and high requirements for the rigidity of the machine tool.
(4) Dual spindle single drive machining scheme. This plan involves fixing and installing two main shafts on the Z-axis slide of a traditional column drilling machine, and cooperating to complete the bottom hole, countersunk hole, and hole chamfering. Compared with the single spindle moving and forming composite cutting scheme, the efficiency of the scheme is equivalent, but the tool cost has been reduced. The sinking hole still needs to be completely countersunk with the forming tool, and the tool consumption cost is still relatively high.
(5) Three spindle single drive machining scheme. This plan involves fixing and installing three main shafts on the Z-axis sliding plate of a traditional column drilling machine, and completing the bottom hole, countersunk hole, and hole chamfering in conjunction. Compared with the "dual spindle movement and single spindle drive machining scheme", the efficiency of the scheme has been improved, and the tool cost has been reduced. However, the three heads are fixed, and it is difficult to adjust the drill bit when replacing. The machining efficiency, sinking depth, and chamfer uniformity quality are difficult to balance, and workers often sacrifice the machining quality.
(6) Three spindle and three drive machining plan. This plan involves installing three sliding tables on the column of the moving column drilling machine, with one drilling head installed on each sliding table. Each drilling head independently drives the drilling through servo drive. Three main axes can independently control the depth and cooperate to complete the bottom hole, counterbore, and hole chamfering. Compared with the "three spindle movement and single spindle drive machining scheme", the efficiency of the scheme is significantly improved, and the tool cost is reduced. The advantages of this machine are high efficiency and high quality qualification rate. The disadvantage is that the cost of machine tools is relatively high.
(7) Three spindle and three drive machining scheme with chamfering on the bottom surface. This plan is based on the "three spindle movement and three spindle drive machining plan", with one chamfering power head added to the bottom, and all holes in the guide rail are machined in one clamping. The advantages of this machine are high efficiency and high quality qualification rate. The disadvantage is that the cost of machine tools is high.
The linear guide drilling machine developed in this article
The scheme is shown in Figure 2, and the scheme description is as follows:
Figure 2: Scheme of linear guide rail drilling machine in this article
1. Saddle; 2. Column; 3. Three sets of dual milling spindles; 4. Workbench and fixtures; 5. Bed body
(1) On the basis of the spindle three drive machining scheme, this article proposes to change a single row of three spindles into three sets of double linked milling spindles, and develop and design a new linear guide drilling machine with a "double row three spindle three drive machining scheme". At the same time, two guide rails are drilled, which greatly improves machining efficiency.
(2) This article presents three sets of dual milling spindles for a linear guide drilling machine. As shown in Figure 3, the dual precision milling spindle independently developed and designed in this article adopts a combination of high rigidity precision spindle bearings to control the axial and radial circular runout accuracy of the spindle within 0.01mm. This spindle can be used for self milling linear guide rail installation positioning surface, ensuring the synchronous parallelism between the two guide rail positioning reference surfaces and the two rows of spindles. The positioning accuracy is high during drilling, No need for central drilling positioning to meet the accuracy requirements of drilling positioning.
Figure 3 Installation of Three Groups of Double Joint Milling Spindles
(3) The principle of high-quality and high-efficiency operation in the dual row, three spindle, and three drive machining scheme. As shown in Figure 4, each row of three main axes corresponds to one linear guide rail, and the bottom holes, counterbore holes, and hole chamfers of two guide rails are processed in one molding process. Install standard drill bits slightly smaller than the countersunk holes for positioning and drilling countersunk holes on each row of spindles, leaving a small margin; The second spindle automatically centers and drills the remaining thickness through the drilling of the first spindle, with good concentricity control; The third spindle is equipped with a forming countersink cutter with a guide pillar, which has a small amount of allowance and chamfering in the countersink hole, a small cutting amount, and good surface quality. Due to the fact that most of the cutting workload for countersunk holes is replaced by standard drill bits with lower costs and higher drilling efficiency, the consumption of expensive forming spot facers is significantly reduced.
Figure 4 Schematic diagram of the principle of high-quality and high-efficiency operation of the dual row, three spindle, and three drive machining scheme
This plan can quickly optimize the coordination of three sets of drill bits to share the drilling amount, making the working time of the three sets of drill bits more average and saving drilling time.
In addition, the bottom hole chamfering can be manually completed by workers using ordinary equipment within the automatic drilling time range of the machine tool, or by using a separately equipped, low-cost fully automatic pneumatic chamfering machine, without affecting the overall drilling efficiency and quality level.
(4) Analysis of the layout of the machine tool spindle in this article. The drilling scope of this article includes all specifications of ball guide and roller guide from 15mm to 65mm, with adjacent hole spacing of 30mm, 40mm, 52.5mm, 60mm, 80mm, 105mm, 120mm, etc. The Z-small common multiple scheme covering the above hole spacing is adopted, and factors such as adjustment efficiency, machine tool cost, and processing efficiency are considered. The center distance between the three groups of main axes is divided into two types: 240mm (refer to Figure 4) and 315mm, Only one adjustment is needed to meet all the machining requirements for the above hole spacing. This article designs a fast positioning adjustment device, which is very convenient and fast to adjust.
(5) Analysis of X-axis dynamic column driving scheme. This article adopts a gear rack drive scheme that has already been maturely applied to profile machining centers, and uses high-precision magnetic grating rulers for closed-loop control to ensure the stability of accuracy. This article also reserves a dynamic nut ball screw drive scheme in the bed design, for users who trust the ball screw drive scheme more to choose.
(6) Fixture design and clamping scheme analysis. A centrally controlled clamping mechanism ensures reliable positioning and clamping. Adjustable clamping head, able to adapt to different types of guide rails for quick change and adjustment. Due to the significant differences in the shape of the guide rails, some have equal width on the upper and lower sides, while others have uneven width. Therefore, except for the 15mm model which uses clamping on both sides, all other models only press the lower side of the guide rails tightly, and a limit cover is set on the other side of the top surface to prevent the guide rails from flipping (see Figure 5).
Figure 5 Fixture diagram
Convenient chip removal and drainage: The machine tool is designed with multiple models sharing one set of fixtures. The fixtures are designed with guide rail drilling holes to avoid empty spaces, which have drainage functions. At the same time, within the allowable range of fixture support strength, chip removal grooves are designed between every two clamping heads, and the drilling chips can directly fall into the automatic chip rolling groove below, making cleaning very convenient.
Analysis of drilling capacity of the linear guide rail drilling machine designed in this article
This article designs a spindle motor, which drives a set of two spindles to rotate through a 3:1 reduction ratio reduction mechanism. A forced cooling system for the spindle is designed, allowing the spindle to work continuously for a long time.
(1) Assessment of Z small drilling capacity. The constraint on the ability of Z small drilling is the spindle speed. If a hard alloy drill bit is used, a high Z-speed of about 3000r/min is required. In this article, a servo spindle motor with a speed of 10000-12000r/min is used, which can meet user requirements.
(2) Z drilling capacity assessment. The main spindle output torque is the limiting factor for the drilling capacity of Z, with a diameter of 20mm for the 45mm specification linear guide and 26mm for the 65mm specification linear guide. A 7.5kW/47.7N · m main spindle motor is selected, and the output torque of Z exceeds 170N · m, which can meet the machining needs of all holes. If the user only processes guide rails below 35mm, they can choose a 5.5kW/34.7N · m spindle motor.
(3) Other factors that affect the drilling ability of machine tools. The driving force of each axis of the machine tool, the rigidity of the guide rails of each axis of the machine tool, and the bearing capacity of the ball screws and screw support bearings of each axis will all affect the drilling capacity of the machine tool. In this article, the design is uniformly matched with the requirement of drilling a hole with a diameter of 26mm to meet the drilling requirements of various models of linear guides.
(4) Comparison and analysis of various CNC linear guide rail drilling machine schemes (including A drilling time for one 4m25mm specification guide rail) are shown in the attached table.
Conclusion
The new type of linear guide drilling machine developed in this article is designed with three sets of double reduction precision spindles driven by three independent servo axes. The prototype that has been produced has processed 100 linear guide rail samples with a specification of 4mm and 25mm. After testing, any two hole spacing errors are all within ± 0.2mm, and the vast majority of hole spacing errors are measured within ± 0.1mm. The aperture is within the design tolerance range, the chamfer is uniform, and the processing quality is qualified. The processing time of each two guide rails can be controlled within 10 minutes (average 5 minutes per guide rail), and the processing efficiency is 2-12 times that of other linear guide rail drilling machines, achieving the research and development design goal of high quality and efficiency.
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2024-01-31