Virtual fixtures and its benefits compared with physical fixtures

Abstract

Manufacturing companies in the automotive industry have difficulties predicting deviations in their products. It is required to measure the produced parts in order to gain knowledge of the geometry outcome and to control the manufacturing processes. The sheet metal parts are assembled into a car body. Deviations in the sheet metal parts can result in problems fulfilling the set functional and esthetic requirements of the car. This means that the geometry of the sheet metal parts is important. Measurement fixtures are used when controlling if the parts fulfill their geometry requirements. It takes time to construct, produce, and adjust the measurement fixtures before they can be used to measure sheet metal parts with accuracy. In addition, manufacturing the measurement fixtures also requires materials leading to an increased environmental impact. These fixtures are quite expensive and can only be used for a specific sheet metal part. When a new car project begins, a new set of measurement fixtures needs to be designed and produced. With car plants and suppliers spread in the world, transportation of parts and fixtures is not to be neglected, when studying the environmental impact. An alternative to using physical measurement fixtures has been developed. The new method is called Virtual Fixture (VF). Instead of using measuring fixtures, three supports are used to hold the part while scanning the whole shape. With scan data of the part, the spheres, and the floor (the gravity plane), the change of shape due to gravity can be estimated and compensated for. You are left with a part without external forces, just as if it were in space. The model without external forces can then be placed in a virtual fixture and the clamped shape be estimated. VF-models and scan data can be sent via the internet around the world. Furthermore, the three supports can be re-used for different parts and projects. Less need for physical measurement fixtures will decrease cost and the environmental impact. Thus, the study will consider if the measuring fixtures can be replaced with VF. Another question will also be how it will affect the economic, ecological, and social aspects when replacing the measuring fixture with the VF. The results show that when comparing the measuring fixtures with the VF, the deviation will be approximately ± 0.2mm. VF has better repeatability compared to measuring fixtures. By using the VF, the cost will decrease and, at the same time, have a much less environmental impact.