(Dr. Shreyes Melkote, advisor)
"Characterization and Modeling of a Magnetic Workholding Device"
Magnetic workholding devices (or magnetic chucks) are used to hold ferromagnetic parts in grinding/machining operations where workpiece accuracy is of critical importance. These devices offer several potential advantages over conventional mechanical workholding methods such as locator pins and strap clamps, and three- and four-jaw chucks. In particular, elastic distortion of the workpiece due to mechanical loads is virtually eliminated. They also offer a high degree of flexibility with regard to the variety of workpiece shapes they can hold with little or no modification. As a result, workpiece set-up and changeover times are considerably lower when using these devices.
The influence of workpiece surface roughness, texture and workpiece form errors on the normal and frictional holding forces generated by a magnetic chuck is not well understood. Accurate knowledge of these effects will allow selection of a magnetic chuck and/or the operating parameters necessary to produce sufficient workpiece holding forces to prevent it from slipping against the chuck surface during machining/grinding.
The objectives of this thesis are: i) to experimentally study and characterize the influence of workpiece surface attributes such as surface roughness, texture and form errors on the normal and tangential holding forces produced in a magnetic chuck, and ii) to develop a theoretical model to predict the holding force as a function of the significant workpiece surface attributes.
The results of this study show that surface flatness error has a significant effect on the normal and tangential holding forces for a radial pole electromagnetic chuck. In contrast, the effect of specimen surface roughness on the normal holding force was statistically insignificant for the range of roughness considered in this study. However, the effect of surface roughness on the chuck-workpiece static friction, and hence the tangential holding force, was found to be important. Ploughing of the soft chuck surface at high normal loads was observed to play an important role in determining the limiting tangential (frictional) force for workpiece slip. Results show that the Coulomb friction model may be used to predict the limiting tangential holding force provided the coefficient of static friction accounts for the effect of high normal force produced by the chuck's magnetic field. Both analytical and finite element models for the normal holding force were developed and compared with measured forces.