BIW WELDING FIXTURE_brief.docx

BIW WELDING FIXTURE UNIT BIW welding fixtures are used for holding the sheet metal or part assembly together in space while the components are sometimes aligned to a convenient the car-line plane of BIW for robotic or manual weld guns can have easy access to weld the components together. So, designing a fixture in this plane is often a complex process, considering the variations in coordinate systems and basic rules such as maintaining few elements in specific orientations. There is deformation of body assembly due to the welding process effect and resulting variation due to part dimensional variation, when you are designing a fixture you should eliminate them and another objective is achieving body quality within the required tolerances, so you can achieve this purpose by planning location layout. Location layout is the selection of the positions and numbers of locators and clamps for assembly of fixture components into a final configuration on a fixture base. Designing a welding fixture is based on the location layout. It involves numbers of fixture units and due to the complexity and shape of the component geometry, the choice of elements as well as configuration of each unit tends to change from case to case. Fixture unit to hold the body parts securely and composed of following item:  Locating pin  Locating surface  Clamping device  Base Locating pin Typically, the locating pins are used to lock a body part in a position sufficiently accurate to execute a welding process. The body part is constrained by a set of appropriately locating pin so total constraint by locating pin should be fully constrained at all times to prevent any movement. Usually two locating pins are enough to properly locate the body part on one plane. One of the pins is round, and another one is diamond shaped. This configuration will allow easier loading of body part. The end of each pin has a round or conical shape or is chamfered for the purpose of maintaining workability such as ease of loading and quality such as prevention of damage. The taper lead angle is large, 60 degrees or less, for small-size body parts and manual operation, but it is frequently between10 and 30 degrees for large-size body parts that are difficult to handle.

The difference between the locate hole in the panel or body part and the locating pin diameter should be 0.2 mm and a tolerance for locating pin should be 0,+0.05. The dimension from the panel to the locating pin leading end (straight area) should be 5 mm. Locating pins are available in numerous shapes and sizes in a design. There are two configurations for position of locating pin:  Keep two dowel pins for positioning as far apart from each other as possible  Set the dowel pins in symmetrical positions Locating surface One of the principal purposes of a locating surface is to locate and support the panel surfaces for performing a welding process. This is usually done with the datum surfaces that are reference for locating surface. The dimensions of datum surface are decided by selecting a necessary crosssection from the part drawing. The shape of locating surface is a form of cross-section of panel. Locating surface is configured to receive panel for setting in a correct position and locating a body part from any surface such as flat, curved, or have an irregular contour. In most applications, locating surface devices locate a panel by its external flat surfaces and flat area, such as: edges, flanges, steps, faces, shoulders, and slots. Clamping device In every welding operation for body assembly, clamping of panel is an essential requirement. A clamp can be defined as a device for providing an invariant location with respect to an external loading system. In other words a clamp is a force-actuating mechanism of a fixture unit. The forces exerted by the clamps hold a panel securely in the fixture against all other external forces. When a welding process is producing a load or moment on the body part, it is necessary that a sufficient clamping force must be exerted to withstand such actions. The generalized requirements of clamping device can be summarized as: To provide a suitable fixing for achieving the stability of location To produce sufficient frictional effects but without causing any undesirable effects to the body parts such as distortion or surface damage The clamping forces hold the body part in its located position and should not

cause any positional displacement or excessive distortion under the action of the clamping forces Clamping forces should be directed towards supporting and locating elements on thin sections of the panel Clamping force should be transmitted to the rigid sections of the body frame of the base unit There are two configurations of clamping device:  Manual clamp  Air clamp cylinder Base The base for BIW welding fixture should be attached with fixture unit such as locating pin, locating surface and clamping device to maintain the accuracy of body parts. The surface of base should be flat smooth and the base should be rigid enough for easy welding and machining. The framework of the base is usually made of shape steel. Load condition and configuration are not uniform, so it is impossible to set a base shape in general.