Jig form selection, design, processing

by:Top Talent     2020-02-29

Keywords: fixture design, selection, engine The machining center is one of the most widely used machining equipment in the automotive engine industry. The following details the process of designing and manufacturing fixtures for a machining center, including fixture selection, fixture design, and fixture processing. 

 Choose the form of the fixture In order to select a suitable fixture, a comparative analysis of the advantages and disadvantages of known forms of fixtures is required first. The advantages and disadvantages of various types of fixtures are shown in the following table. According to the cycle and other conditions, the hydraulic automatic clamping fixture in the form of external oil circuit is finally selected. Design fixture details The general idea of the design is to facilitate the clamping and reduce the difficulty of operation. The position after clamping can meet the processing content of the product, minimize the clamping deformation, and facilitate the arrangement of the oil circuit. 

 The first fixture (milling the top surface) 

 Because the parts need to be lifted and installed on the fixture as a whole when clamping, in order to reduce labor intensity, the horizontal clamping is selected. Part mounting state when processing the top surface Place the product's most edge close to the edge of the workbench, so that the product can be processed effectively with a fixed-size milling cutter, and it is closest to the operator's clamping position. The second set of fixtures (machining the grooves and holes on the bottom surface) 

The positioning points of the parts are three rough datum datums on the right side of the side, and three rough datum datums on the bottom side. Attachment posture of the machined parts In order to facilitate the clamping, the product can be translated into the fixture to reduce the labor intensity of the operator when picking up the product. The top surface is selected as the positioning surface and the longitudinal clamping is selected (if the horizontal clamping is performed, the operator needs to raise the product too much above the cylinder and then drop Into the fixture support, increasing difficulty). Clip-specific design Before designing the clamp, we must first consider the machine tool used for part processing, so as to design the mounting position of the screw hole according to the table spacing of the machine tool. The specific size of the clamp must meet the workpiece clamping requirements, but it must not be too large to prevent interference when the worktable is not moved to the extreme position and the workpiece processing position is not in place because the clamp is in contact with the contour of the machine tool, which in turn causes parts. Cannot effectively install and process. At the same time, the specific outer edge of the clamp should be as close as possible to the machine door, so that the operator has the shortest distance when taking the workpiece and minimizes the force. Selection and design of oil cylinder According to a simple and reliable idea, the oil cylinder chooses a compact single-acting oil cylinder. The compression type is up and down compression (because there is no positioning pin to limit the position during the entire processing of the part, so the part is prevented from moving during the compression process, and a rotary compression structure cannot be selected). The type of the oil cylinder is selected according to the clamping force and the external dimensions of the oil cylinder. 

Cylinder Block Design Cylinder Block Design The external oil circuit needs to be designed with a cylinder base to simulate the direction of the oil circuit in the fixture daughter board to achieve hydraulic clamping. In order to make the external oil path of the fixture more regular and reasonable, the oil inlet and outlet ports on the cylinder seat should be chosen in the same direction as the design direction of the side oil cylinder. The design of the cylinder seat is shown in Figure 3. In addition, the design of the cylinder seat needs to consider the installation height after the cylinder is installed. The basic principle is that its height should be suitable for the installation height and processing stroke of the processed parts, and to prevent the difficulty of clamping due to too high, and the relative increase in the height of the positioning support device when the cylinder assembly is too high. Positioning scheme and device design Positioning accuracy is very important for dimensional accuracy, and positioning is also related to labor intensity and clamping efficiency. The first set of fixtures (machining the top surface) Since this set of jig only processes the top surface, it only needs to ensure the positioning accuracy in the height direction. 

Three rough top datums on the top surface are used as the height datum when positioning, and three auxiliary limiting devices (square boxes in Figure 4) are designed at the same time to facilitate part clamping and part limiting to prevent parts from being biased. Figure 4 Machining the top surface fixture The three auxiliary limiting devices are higher than the outer contour of the part, and are about 2mm from the edge of the part to play a limiting role. The flatness of the three height references is guaranteed to be within 0.02mm. 

Reference design of fixture height The second set of fixtures (machining the grooves and holes on the bottom surface) The second set of fixtures is designed according to the 'six-point positioning principle'. The bottom surface is the height reference, and the three blank references on both sides. In addition, in order to facilitate the clamping, a limit device is designed. To ensure the accuracy of lateral positioning, an auxiliary clamping device is designed for lateral clamping. Processing the bottom surface fixture Design of positioning device (support) The positioning device (support) structure is the same as the height reference structure of the first set of fixtures. The first set of fixtures (processing the top surface) has no special requirements. There are three supporting structures with a height difference of 0.02 mm. The second set of fixtures (processing the bottom surface) Slots and holes), in order to facilitate clamping, there are 9 support structures, 6 of which have the same support height for positioning, and the other 3 are slightly lower for the convenience of clamping (the second set of clamps uses workpiece translation and loading) Solution, if there are only 3 supports, because there are many holes in the cylinder head, it is easy to get stuck during translation). Platen design According to the principle of minimizing cost, simplifying structure and maximizing versatility. The left pressure plate is designed with an indenter mounting device for easy replacement and adjustment; the right pressure plate is designed as an arc head according to the structural constraints of the parts to reduce interference. According to the fixture scheme, the machine tool for processing the cylinder head is used to process the fixture directly during processing, and the installation and commissioning are directly performed after processing; the positioning pin holes are processed at one time during fixture processing to ensure the accuracy of the positioning of the positioning pin holes with each other; General high-speed steel taps, then use manual taps to tap thread holes of various sizes. Under such a scheme, the product quality meets the requirements, which indicates that the fixture structure and accuracy are reasonable.                                

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