Effect of fixture dynamics on the face milling process

Accurate prediction of dynamic machining forces is essential in order to estimate the product and process quality, tool life and stability of the machining process. To achieve such a goal, all the aspects affecting the machining process dynamics should be considered. An integrated dynamic model that takes into consideration the dynamic effects of different machining process elements on the chip load was developed. This model considers the effect of cutter geometry, cutter initial position errors, spindle tilt, workpiece geometry, machine tool dynamics, and workpiece/fixture system dynamics. The open literature has no information on the effect of fixture dynamics on the chip load and the machining process stability.

Proper modeling of the workpiece/fixture contact requires the modeling of the friction conditions because friction forces can be utilized to reduce the number of fixture components, thereby exposing more of the workpiece features to machining operations. Also, it provides a damping mechanism to dissipate input energy from machining forces out of the workpiece/fixture system. The literature lacks research on the tribological aspects of the workpiece/fixture contact. Since many factors influence the coefficient of friction, an experimental investigation was carried out to study the tribological aspects of the workpiece/fixture contact. Based on the findings of this study a velocity limited friction model (VLFM) was incorporated in the finite element analysis of the workpiece/fixture contact.

The model utilizes NURBS curves and surfaces for the geometric modeling of the tool cutting edges and the workpiece geometry. A grid based meshing scheme for the finite element analysis was developed based on the NURBS iso curves.

An in-house finite element code was developed for the analysis of the workpiece/fixture dynamics. Simulated cutting forces showed good agreement with the experimental validation. Two case studies are presented to demonstrate the practical application of the developed methodology. The integration of all of the above in one model provides an off-line tool to simulate and optimize the machining parameters and the fixture configuration. This cuts production time and cost.