Guarantee the basic conditions of automobile fixture

by:Top Talent     2020-07-19

The design of automotive fixtures is a highly empirical and comprehensive technology. When designing, we must first determine the production program, be familiar with the product structure, understand the deformation characteristics, grasp the parts and assembly accuracy, and be familiar with the process requirements. Only by doing this can we design the welding fixture in all directions.
  
1. Production Program
  
The production program determines the automation level of the welding fixture and the configuration of the welding station, which is reflected through the production cycle. The production cycle is composed of fixture action time, assembly time, welding time, handling time, etc. The action time of the fixture mainly depends on the degree of automation of the fixture; the assembly time mainly depends on the accuracy of the stamping parts, the accuracy of the process parts, and the skilled procedures of the operator; the welding time mainly depends on the level of the welding process, the degree of automation of the welding equipment, and the selection of the welding tongs. The degree of rationalization, etc.; the handling time mainly depends on the degree of automation of handling, the degree of rationalization of logistics, etc. As long as we grasp the above points, we can reasonably solve the contradiction between the automation level and manufacturing cost of welding fixtures.
  
Second, the structural characteristics of the car body
  
The car body generally consists of an outer cover, an inner cover and a frame. The thickness of the steel plate of the cover is generally 0.8-1.2mm, and the thickness of the steel plate of the frame is mostly 1.2-2.5mm, which means that most of them are thin plate parts. For welding fixture design, it has the following characteristics:
  
1. The shape of the structure is complex and the composition is difficult
  
Automobile bodies are all space shells made of thin plate stamping parts. In order to have a beautiful appearance and a certain rigidity, the parts that compose itself are usually space curved bodies that are drawn and formed, and the structure is more complicated.
  
2. Poor rigidity and easy deformation
  
Formed sheet metal stamping parts have a certain rigidity, but compared with machined parts, the rigidity is much worse, and a single large stamping part is easy to deform, only after welding into a body shell, it has strong rigidity.
  
3. Dimensioning with spatial three-dimensional coordinates
  
The car body product diagram is marked with three-dimensional spatial coordinates. In order to indicate the position of the cover on the car and to facilitate dimensioning, the car body generally draws a coordinate network line every 200 mm or 400 mm. The bases of the three coordinates are: the front-rear direction (Y direction)-the center of the front wheel of the car is 0, the negative value is forward, and the positive value is backward; the vertical direction (Z direction)--the plane above the longitudinal beam It is 0, positive value is upward, and negative value is downward; left-right direction (X direction)-the car symmetry center is 0, left and right are positive and negative.
  
3. Assembly accuracy
  
The assembly accuracy includes two aspects: the appearance accuracy and the skeleton accuracy. The appearance accuracy refers to the gap surface difference after the door is assembled, and the skeleton accuracy refers to the three-dimensional coordinate value. The assembly accuracy of the truck direction is generally controlled within 2mm, and the car is controlled within 1mm. The design of the welding fixture must not only ensure the welding requirements between the process parts, but also ensure the overall welding accuracy, and meet the overall assembly requirements by adjusting the matching state between the process parts.
  
The application of the four-point and six-point rule in body welding fixture
  
The 6-point rule refers to restricting the freedom of movement in several directions. When designing body welding fixtures, there are often two misunderstandings. One is that the 6-point positioning is not suitable for thin plate welding fixtures; the second is to see thin plate welding fixtures. There is a phenomenon of over-positioning on the fixture. The reason for this misunderstanding is that the freedom to restrict movement in 6 directions is understood to limit the freedom in 6 directions. The purpose of the welding fixture design is to restrict the freedom to move in 6 directions. The limitation not only depends on the positioning and clamping device of the fixture, but also on the mutual restriction between the parts. This principle can only be applied correctly if we correctly understand the characteristics of sheet metal stamping parts welding production and at the same time correctly understand the 6-point rule.
From the positioning principle, the support is indispensable for the thin plate, which can eliminate the deformation caused by the clamping force of the workpiece. Over-positioning makes the contact point unstable and causes interference in the assembly position, but as long as the support surface is carefully polished when adjusting the fixture, the adverse consequences caused by its over-positioning can be controlled within the allowable range.
In the 1980s, a large number of fixtures used for body welding were cut from the positioning surface of the stamping die, that is, positioning on the cavity of the stamping parts of the body. It was called 'positioning block', which was characterized by a large positioning area. According to statistics, the projected area is above 50×100. The positioning block is a machined part, and the rest of the supporting part is cast iron. The positioning block is used as a positioning pin after assembly and adjustment. In appearance, it has two styles: large-area positioning block, small-area pneumatic or manual indenter; large-area positioning block, large-area pneumatic or manual indenter. The former causes complicated processing of the positioning block, resulting in insufficient pressing force of the body parts, and the latter interferes with the spot welding operation of the welding tongs and the difficulty of assembly. The precision of the jigs used in production must ensure the requirements of the product assembly; the number of positioning surfaces is also conservative, rather than less. In addition, the assembly of each positioning block is all fixed on the welding support bottom plate with four screws in the counterbore. Due to the filling of the welding slag, it is difficult to repair and replace. Therefore, this kind of 'positioning block' type fixture that is a whole casting is a material that consumes energy, and its design, manufacturing cycle and cost are relatively high.
With the improvement of tooling manufacturing level and inspection methods, the positioning of body welding fixtures has been transformed into positioning plate positioning, and the thickness of the plate is selected in several levels of 16, 19, and 25. The entire fixture body is changed to welded joints, which shortens the cycle in manufacturing and assembly, and relatively reduces costs.
Each positioning pin hole of the positioning plate and the corner support, the corner support and the bottom plate is bored, and the deviation of the hole spacing is L±0.02. This method is adopted in processing. If the size of the body product changes slightly during commissioning (such as the dimensional error often found in stamping parts), the dimensional accuracy guaranteed in the machining is wasted. If the geometric accuracy of the car body is successfully installed on the fixture at one time, the stamping parts must not have large dimensional deviations, and the number of positioning points is relatively large. The adjustable positioning method with the right-angle block is the same as the above, except that the positioning plate and the indenter are transitioned by the right-angle block and the gasket. Its advantage is that the positioning plate and the indenter are easy to repair and adjust after being damaged; it is also relatively easy to form a standardized design and manufacturing (except the positioning block and the indenter on the indenter, other parts can be made into standard parts). However, the designer must solve the relationship between the boring pin hole and the gasket, and can not allow the positioning plate to be processed without tolerance. It also prolonged the assembly cycle, increased the size chain from the positioning profile to the detection pin hole, and the cumulative error increased. For example, there are about 900 right-angle blocks for the left/right side fixtures of the flat head body. It can be imagined that the workload of assembly adjustment is large, it is also exchanged at the cost of improving the processing accuracy. From the overall shape of the fixture, there are rules and regulations, but they are not beautiful.
The three cylindrical pins are used to position the parts and components. It reduces the machining accuracy of the above two positioning methods from L±0.02 to L±0.05, so that the jig is processed and assembled in production and the body welding accuracy is obtained. Guarantee. However, in the design and assembly, the use of positioning components should be considered, otherwise the accuracy will be lost with bumps and other undesirable factors.
  
5. Selection of body segmentation and positioning datum
  
The body welding assembly is generally composed of the bottom plate, the front wall, the rear wall, the side wall and the top cover. Different car models have different block division methods. When selecting the positioning reference, generally:
1. Ensure the assembly size of the door opening
  
When the assembly is welded without side wall sub-blocks, the door opening must be used as the main positioning reference. In the sub-assembly fixture, all sub-assembly welding related to the front and rear columns must be directly positioned with the front and rear columns, and from the sub-assembly to the assembly The positioning reference should be unified; when the assembly is welded with side wall sub-blocks, the door hole should be formed on the side wall welding fixture, and the door hole and the process hole should be used for positioning during assembly welding, and the positioning reference from the subassembly to the final assembly should also be unified.
  
2. Ensure the position accuracy of the front and rear suspension holes
  
Suspension holes on the underbody of the vehicle body are generally stamped on the floor reinforcement beams, and the relative position of the suspension holes must be ensured during assembly and welding so that the vehicle body can smoothly fall onto the frame.
  
3. Ensure the assembly size of the front and rear wind windows
  
The front and rear wind windows are generally composed of an outer cover and an inner cover. Some are formed on the front and rear wall assemblies. Attention should be paid to the positioning of the sub-assembly fixtures. Some are formed on the assembly fixtures. Generally, they are formed on special window positioning devices. The windows are precisely positioned to ensure the assembly of the windshield glass.

6. Structure and positioning clamping characteristics of body welding fixture
  
1. Structural characteristics of body welding fixture
  
The body welding fixture is huge in size and complex in structure. In order to facilitate manufacturing, assembly, inspection and maintenance, the fixture structure must be decomposed, otherwise, the measurement cannot be performed. The body assembly has three assembly datums: the bottom plate, the left side wall and the right side wall, the reference grooves and coordinate lines are processed on their planes, and the positioning and clamping combination unit is assembled and tested according to their respective reference grooves. 3 Most of them are combined to become a complete set of fixtures.
  
2. Positioning characteristics of body welding fixture
  
Body welding fixtures are mostly based on the curved appearance of stampings, reshaped platforms on the curved surface, the steps drawn and bent, the trimmed windows and outer edges, assembly holes and process holes are located. To a large extent, it determines the shape of its positioning components is relatively special, and rarely use standard components. Welding fixtures should position each workpiece to be welded separately so that they do not interfere with each other. When designing positioning elements, we must make full use of the interdependence of workpiece assembly as a natural positioning support. Some workpieces are welded into a closed body, and positioning support cannot be set. Pre-punching platforms and flanging can be required as positioning control points during product design. In short, welding fixtures are not used for assembly with less strict requirements. On the body welding fixture, there are many plate-shaped positioning. The positioning plate is generally made of A3 and A5 steel plates with a thickness of 12-20mm. The distance between the positioning blocks must ensure the positioning accuracy and the convenience of the welding tongs. The positioning parts are dimensioned according to coordinates without tolerances.
  
3. Clamping characteristics of body welding fixture
  
After the body stamping parts are assembled, resistance welding is mostly used, and the workpiece is not subject to torsional torque. When the gravity of the workpiece is in the same direction as the pressure during spot welding, the welding pressure is sufficient to overcome the elastic deformation of the workpiece and still maintain the accurate assembly position and positioning reference. Fitting, the clamping mechanism can be omitted at this time.
Welding is usually carried out between two workpieces, and there are usually many clamping points. Resistance welding is an efficient welding process. In order to reduce the auxiliary time of loading and unloading workers, clamping should use efficient and fast devices and multi-point linkage mechanisms.
For sheet metal stampings, the clamping force action point should act on the support point. Only work with good rigidity is allowed to act in the plane composed of several support points, so as to avoid the clamping force from bending the workpiece or deviating from the positioning reference. The clamping force is mainly used to maintain the relative position of the workpiece assembly, overcome the elastic deformation of the workpiece, and make it fit with the positioning support or the conductive electrode. For steel plates under 1.2mm thick, the fit gap is not more than 0.8mm, each clamping The clamping force of the point is generally in the range of 300-750N; for stamping parts between 1.5-2.5mm, the fitting gap is not more than 1.5mm. The clamping force of each clamping point is in the range of 500-3000N.
Clamps have flat, vertical, and 45-degree clamps according to the clamping direction; bolt clamping, quick clamping, and handle screw clamping according to the operating mode; and manual, pneumatic, or hydraulic pressure. The screw clamp with compensation is the most commonly used. This kind of clamp increases elastic expansion and contraction in the cantilever, counteracts the lateral component force during clamping, to compensate for the deformation of the clamp itself and the gap during the insertion process, and to ensure that the clamping force is perpendicular to the force receiving surface. The clamping head is generally made of carbon steel, stainless steel, nylon material to suit different workpiece requirements. If equipped with two-point and three-point clamping bridges, workpieces at two positions with different heights can be clamped simultaneously. In addition, special clamping heads can also be processed according to the clamping profile.
  
7. Precision control of welding fixture
  
Welding fixture accuracy standards are formulated by the design unit, which specifies the shape and accuracy requirements of the base plate reference groove and sitting line; the size and shape tolerance requirements of the positioning pins and other positioning supports, the contracting unit shall detect, judge and carry out according to the requirements After adjustment, the positioning pin is fixed after passing. With the improvement of machine tool machining accuracy, in order to reduce positioning errors and improve machining accuracy, the manufacturing accuracy of fixtures is more demanding. The positioning hole pitch precision of high-precision fixture is up to 0.01mm/±5um, the verticality of the support surface of the fixture is up to 0.01mm/300mm, and the parallelism is up to 0.01mm/500mm. German demmeler (Dai Meile), the company's 4m long and 2m wide hole series welding fixture platform, the contour height error is ± 0.03mm; precision parallel pliers parallelism and perpendicularity within 5um; fixture repeated installation The positioning accuracy is up to ±5um; the repeated positioning accuracy of Swiss EROEA flexible fixture is up to 2-5um. The precision of machine tool fixtures has been improved to the micron level, and world-famous fixture manufacturing companies are precision machinery manufacturers. Admittedly, in order to adapt to the needs and economy of different industries, the clamp has different models and different grades of precision standards for selection.
  
8. Modules and combinations
  
The modularization of fixture components is the basis for the combination. The use of modularized serialized and standardized fixture components to quickly assemble various fixtures has become the basis of fixture technology development. Labor saving, time saving, material saving and energy saving are reflected in the innovation of various advanced fixture systems. Modular design lays the foundation for the computer-aided design and assembly of fixtures, using CAD; technology, can establish component library, typical fixture library, standard and user archives, optimize fixture design, and assemble fixtures for users' three-dimensional entities. Simulating the cutting process of the cutting tool can not only provide users with correct and reasonable fixture and component matching solutions, but also accumulate experience, understand market needs, and continuously improve and perfect the fixture system. The Combined Fixture Branch is working with Huazhong University of Science and Technology

The industry technology website provides a common platform for information exchange, fixture product consultation and development for the fixture industry, and strives to achieve the universalization of fixture design and services, remote informationization, and e-commerce management.
  
9. General, Economic
  
The versatility of the fixture directly affects its economy. A modular and modular fixture system is adopted, and the one-time investment is relatively large. Only the fixture system is reorganizable, reconfigurable and extensible. It has a wide range of applications, good versatility, high fixture utilization, and fast return on investment. In order to show good economy. Germany demmeler (Dai Mei Le), the company's hole series combination welding fixture, only using a variety of components with few specifications, that can be assembled into a variety of welding fixtures. The function of the components is strong, making the versatility of the fixture good, the components are few and precise, the supporting cost is low, and the economy is practical.
  
Experts suggest that the combined fixture industry should strengthen the collaboration of production, learning and research, accelerate the pace of transformation and upgrading of fixture technology with high and new technologies, create a fixture professional technology website, make full use of modern information and network technology, and innovate with the times. Develop fixture technology. Actively contacting foreign fixture manufacturers, seeking joint ventures and cooperation, and introducing technology is a more effective way to transform and develop my country's combined fixture industry.
  
10. Conclusion
  
The design of automotive fixtures is very closely related to the structure and accuracy of stamping parts and process parts, and is full of rich particularities. In designing, in addition to following the general rules, we must also deal with specific issues.
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