Wheel frame parts usually have high technical requirements such as dimensions and geometric tolerances. The traditional two-pin positioning system on one side uses clearance fit, which leads to large positioning errors and unstable part processing accuracy. Over-positioning has two sides. On the one hand, it violates the six-point positioning principle and affects clamping and positioning. On the other hand, if handled properly, it can improve the rigidity and processing accuracy of the part. Correctly analyzing and processing over-positioning can improve positioning accuracy without affecting the loading and unloading of workpieces. This is the key to the rational design of over-positioning fixtures. With the assembly and motion simulation functions of UG NX software, the impact of fit clearance on the positioning error of round holes at different positions can be intuitively displayed. The positioning accuracy of the double-expansion two-pin structure with improved positioning error has been improved, but it still has its limitations. For porous wheel frame workpieces, a reasonable three-pin positioning method on one side can achieve higher and more stable positioning accuracy than the two-pin positioning method on one side.
1 Preface
Over-positioning means that a certain degree of freedom of the workpiece is restricted twice or more. The over-positioning phenomenon can easily lead to the failure of the rigid workpiece to be installed correctly and should be avoided as much as possible [1]. The positioning pins used in the two-pin-on-a-side clamping and positioning process are roughly divided into two categories: rigid pins and flexible pins. Both rigid and flexible pins have their limitations. The gap-type fit of the rigid two-pin on one side structure limits the machining accuracy. The flexible two-pin on one side is troublesome and costly to produce. Moreover, the two-pin on one side has a limited scope of application and cannot meet the requirements for processing porous parts such as wheel frames. How to ensure the positioning accuracy of porous parts on vertical machining centers is an issue worth studying.
2 Limitations of two pins on one side
2.1 Gap type with two pins on one side
The traditional gap-type two-pin structure on one side uses rigid positioning pins. In order to avoid over-positioning, a cylindrical pin and a cutting edge pin are used. Its positioning principle is cylindrical pin positioning and diamond pin orientation. The cylindrical positioning pin limits the freedom of movement of the workpiece in the X and Y directions and plays the main positioning role; the diamond positioning pin (the purpose of edge cutting is to increase the pin hole gap and compensate for the hole spacing error of the workpiece and the pin spacing error of the fixture. When installing, it should be ensured that it is a non-edged cylinder in the direction of the vertical line connecting the centers of the two holes) only limits the freedom of rotation of the workpiece around the Z-axis, and usually plays the role of angular positioning. The datum displacement error of process dimensions in the horizontal direction is usually determined by the cylindrical pin hole positioning pair, which is mainly due to the random wandering and floating of the main positioning hole on the workpiece relative to the cylindrical positioning pin. The datum displacement error in the vertical direction is related to the center of the two holes. The connection line is related to the X-axis angle, which is determined by the angle error of the workpiece caused by the gap between the fixture positioning pin and the workpiece positioning hole.
Although the traditional gap-type two-pin structure on one side avoids over-positioning, it increases the positioning error at the positioning hole of the edge-cutting pin. As shown in Figure 1, when the reference hole of the maximum limit size meets the positioning pin of the minimum limit size, the pin hole contact lines are located on both sides of the line connecting the two holes, and when the limit angle deflection occurs between the line connecting the two holes and the line connecting the two pins, The most unfavorable positioning conditions will occur, which can easily cause the hole position to be out of tolerance [2].
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Figure 1: Rotation error of two pins on one side
To reduce the reference displacement error and rotation angle error caused by random floating, the matching gap of the pin holes must be eliminated, that is, the size deviation of the positioning holes and pins must be reduced. However, the extent to which the accuracy of workpieces and tooling can be improved is limited by the machining accuracy of machine tools. The smaller the hole pitch tolerance and hole diameter tolerance, the more difficult and higher the cost will be in processing, and if the fit gap is too small, it will cause great trouble in the loading and unloading of workpieces. It can be seen from Figure 1 that under the condition of a certain hole-pin clearance, the longer the distance L between the two holes, the smaller the rotation angle error Δφ, and the positioning error caused by the rotation angle is relatively reduced.
2.2 Expandable type with two pins on one side
In actual production, in order to improve the positioning accuracy and facilitate the loading and unloading of workpieces, the expandable two-pin structure on one side is often used. The expandable two-pin structure on one side first uses the pin hole gap for flexible clamping, and then uses the pin's expansion mechanism to expand the positioning pin to eliminate the pin hole matching gap and reduce the corner error. At the same time, due to the difference between the spacing between the positioning holes and the spacing between the positioning pins, the workpiece will move slightly due to the expansion of the positioning holes, and the spacing difference is effectively evened out, thereby improving the positional accuracy of the processed holes. The application of an expandable two-pin structure on one side can also reduce the machining accuracy of the workpiece positioning hole while meeting the design requirements, thereby saving production costs [3].
The expansion structure of the positioning pin is divided into two types: full circle expansion and several-point expansion, which respectively correspond to the cylindrical positioning pin that plays the main positioning role and the edge-cutting pin that limits the workpiece angle error. The expandable two-pin structure on one side can be divided into single expansion type and double expansion type.
In the single-expansion type two-pin structure on one side, the cylindrical positioning pin that plays the main positioning role is usually designed as an external expansion type, which is used when the diameter of the center positioning hole of the workpiece is larger and the diameter of the angular positioning hole is smaller.
The double-expansion type two-pin structure on one side is mostly used in situations where the diameters of the central positioning hole and the angular positioning hole of the workpiece are both large. The common double-expansion structure with two pins on one side mostly adopts a toothed flap expansion structure, and both positioning pins are made of high-quality spring steel. The new double-expansion type two-pin structure on one side mostly uses thin-walled positioning pins with floating media installed in the inner cavity. Floating media include solid spheres, pastes and liquids. Taking liquid plastic thin-walled positioning pins as an example, when the pressure screw pressurizes the liquid plastic in the thin-walled expansion sleeve through the sliding column, the liquid plastic in the inner cavity of the positioning pin will evenly transmit the pressure it bears, so that the thin-walled The positioning pin undergoes plastic deformation and expands radially, and the axis of the positioning pin and the central hole are coincident, thereby achieving the purpose of reducing positioning error. After the workpiece is processed, the pressure in the thin-walled expansion sleeve is reduced and the positioning pin is separated from the workpiece.
2.3 Limitations of the two-pin structure on one side
The positioning process of two pins on one side can also be regarded as the assembly process of the pin and hole workpiece. Therefore, UG NX software can be used to assemble the pins and holes to simulate the over-positioning method of two pins on one side. Taking a stainless steel rotary disk as an example, N (odd number) coaxial holes of φD1 are evenly distributed on both end surfaces, and the center is a large through hole of φD2. UG NX software is used for pin and hole assembly. There are three contact constraints between the tooling and the workpiece, namely the end surface contact between the base plate and the workpiece, and the contact between the two sets of pin holes. In order to more intuitively present the positioning error amplification phenomenon of a two-pin positioning structure in a porous workpiece, the matching gap between the two pairs of cylindrical pins and holes is set to 3 mm.
As shown in Figure 2, if the central large hole Q1 and a small hole Q2 on the distribution circle are used as the benchmark, because there is a matching gap, even if it is over-positioned, when the pin and the hole cylinder are in partial contact, the workpiece can still be in a small range. internal float. In addition to the two positioning holes, the positioning errors of the remaining two holes K3 and K4 on the distribution circle of the rotary disk vary in size due to their relative positions to the two positioning pin holes Q1 and Q2. From Figure 2, it can be seen intuitively that the positioning error of the small holes K3 and K4 on the distribution circle far exceeds the mating gap of the pin hole by 3 mm, that is, the positioning error is amplified relative to the mating gap. Using the center hole and the small holes on the distribution circle The two-pin positioning method on one side of the hole cannot meet the processing requirements.
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Figure 2: Error amplification phenomenon in the positioning of central holes and circumferential holes
As shown in Figure 3, if the two small holes Q2 and K4 on the distribution circle of the rotary disk are used as the benchmark, it is obvious that the pin spacing of this method is larger than that of the previous method. Although the pin spacing is increased, resulting in a relative reduction of the rotation angle error, the positioning error of the remaining two holes Q1 and K3 still exceeds the matching gap by 3 mm, and there is also a phenomenon of different hole positions and different positioning errors. This kind of two-pin positioning on one side still cannot meet the technical requirements.
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Figure 3: Error amplification phenomenon in double circumferential hole positioning
Even if a double-expansion structure with two pins on one side is used, systematic errors such as measurement, manufacturing and assembly are inevitably introduced during the production process of the fixture positioning components. Due to the manufacturing error of the fixture itself, the axes of the pin and the shaft cannot completely coincide. At the same time, although in the vertical direction of the connection between the two pins, the angle error is reduced due to the elimination of the fit gap; in the direction of the connection of the two pins, the pin, The difference in the hole spacing reference will be homogenized due to the slight displacement of the workpiece, but the positioning error is only reduced relative to the rigid cylindrical pin and cannot be eliminated. Its size depends on the shape, position and dimensional accuracy of the fixture itself when it is manufactured. , and except for the two positioning holes, the positioning errors of the other holes will still vary due to their relative positions to the positioning pin holes. There is still a tendency for the positioning error to be amplified relative to the two pins on one side, and there are out-of-tolerance Phenomenon.
3 Dual nature analysis of over-positioning
The phenomenon of over-positioning can easily lead to the failure of rigid workpieces to be installed normally. However, under certain conditions, reasonable use of over-positioning can achieve good results and obvious benefits.
For workpieces with weak rigidity and high precision requirements, such as thin-walled workpieces, slender rods or workpieces with a large flat surface as the positioning reference, large parts, etc., over-positioning clamping is more beneficial. For workpieces with poor rigidity, any places that are easily deformed should be restrained as much as possible. The purpose is to prevent deformation caused by cutting forces during the processing, increase the rigidity of positioning and clamping, ensure the stability of the processing process, and improve processing accuracy.
When turning a long-axis workpiece, one end of the workpiece is clamped with three claws and the other end is supported by a tail tip. The freedom of movement of the workpiece in the Y and Z directions is limited twice, resulting in over-positioning. Compared with tipless support, the contact area and clamping reliability are increased, the rigidity of the workpiece is strengthened, the processing proceeds smoothly, and the processing quality and efficiency of the workpiece are greatly improved.
In milling processing, the three support points define a plane, and the fourth support point cannot be absolutely coplanar with ABC. The four-point fixed surface is over-positioned. However, in actual production, multiple surfaces with better mutual position accuracy are often used as positioning benchmarks at the same time, forming an over-positioning method. This over-positioning method not only enhances the clamping reliability and system rigidity, but also improves the stress deformation of thin-walled workpieces, thereby better ensuring product processing quality. Removing the fourth point of support and eliminating over-positioning methods has the opposite effect.
In other words, some positioning methods are over-positioned from a formal point of view, but there is no substantial mutual interference or conflict between the positioning fulcrums with repeatedly restricted degrees of freedom, or although there is interference, it does not exceed the allowable limit of the workpiece. requirements, this kind of over-positioning is allowed. In other words, using a precision datum with high machining accuracy as the positioning datum, the error of the positioning datum is small, and the workpiece position can still float within a small range. This kind of over-positioning is only formal over-positioning and is allowed to occur [4].
When using positioning, you must pay attention to the following three points.
1) The error of the positioning reference determines the degree of undesirability of the over-positioning interference result. The greater the error of the positioning datum, the more serious the interference deformation and the greater the adverse consequences. Therefore, higher requirements must be put forward for the size and geometric accuracy of the positioning datum hole used as the workpiece to reduce the error of the positioning datum itself.
2) The force used for loading and unloading the workpiece must be appropriate, and its local deformation and contact stress must be controlled within the range allowed by the technical requirements.
3) In an over-positioning fixture system, the number of positioning parts affects the comprehensive deviation of the entire fixture system.
4 Application cases of three-pin over-positioning on one side
The stainless steel rotary plate mentioned earlier has a total height of 210mm and an I-shaped cross-section. There are N (odd number) coaxial and evenly distributed small holes of φD1 on both end surfaces, and a large through hole of φD2 in the center. This workpiece is a welded structural part, and there are high requirements between the upper and lower axes of the small holes, between the uniform circular axis and the axis of the large holes, and the position of the small holes relative to the large holes. When processing on a vertical machining center, the difficulty lies in the high coaxiality requirements for the small holes between the upper and lower layers. Using extended tool processing and boring from one end can ensure the technical requirements, but the lengthened boring tool requires many specifications, the tool cost is high, and vibration is prone to occur during processing, and the efficiency is not high. Therefore, a more feasible processing solution is to use Special fixture, U-turn processing, so only a small number of short knives are needed. The key to the success of the U-turn processing plan is that the clamping and positioning accuracy during turning processing must meet the technical requirements.
As mentioned before, when the fine datum is used as the positioning datum, over-positioning is allowed to improve the positioning accuracy. When using a vertical machining center to process the holes on the second surface of the rotary table, a three-pin positioning structure on one side can be used for clamping. The bottom surface of the tooling and the three cylindrical pin axes on it are used as the positioning datum, and the workpiece is based on the hole-pin clearance. Installed on the tooling base plate in a matching manner. The XY displacement of the workpiece and the rotation around the Z axis are simultaneously restricted by three pairs of pin hole positioning pairs. According to the above three conditions of use of over-positioning, a high-precision vertical machining center should be used to make the tooling base plate and process the small holes on the first surface of the rotary table to reduce the difference in pin spacing and hole spacing. The machining center has high positioning accuracy (positioning error ≤0.01mm). Therefore, the size difference between pin spacing and hole spacing, and the shape error can be ignored. The only factor that affects positioning accuracy is the matching clearance between pins and holes [5].
Continue to use UG NX software to simulate the process of positioning and clamping three pins on one side, and add contact constraints for the third pair of pin holes. As can be seen from the assembly navigator in Figure 4, the position status of the porous workpiece 2 is a "half black and half white" small circle, indicating that the workpiece 2 is in a partially constrained state. Click the constraint button on the assembly toolbar, move the cursor to the workpiece, press and hold, and rotate the mouse. The three small holes on the workpiece will each rotate around the contacting cylindrical pin at the same time. The workpiece is indeed in a non-fully constrained state. Obviously, with the help of UG NX software, it can be seen intuitively that when the workpiece in the three-pin structure floats, the diameter of the ring formed by the center of the small hole will not exceed the fitting gap, and the combined effect of the three restrictions makes the center of the workpiece larger. The hole can only float within a small range. So, what is the positioning error of the large hole in the center of the workpiece?
