Ph.D. Thesis Defense by Jose Francisco Hurtado
Tuesday, April 3, 2001

(Dr. Shreyes Melkote, advisor)

"A Methodology for the Design and Optimization of a Flexible Machining Fixture"


In order for a machining line to be truly flexible, it must be able to process small workpiece batches consisting of parts of varying shapes and size.  A flexible/conformable fixturing system removes the major bottleneck to implementation of fully agile and flexible machining lines.  Suitable metrics are required to quantify how well a flexible mechanical work-holding device (e.g., pin-array fixture) conforms to an arbitrary workpiece geometry.  Such metrics will enable comparison of different designs of fixtures/robotic grasps in terms of their ability to hold/handle a given part family. Furthermore, the metrics can be used in process planning to create new part families that can be assigned to a flexible work-holding device capable with a known maximum degree of conformability. In addition to conforming to a part family, an effective flexible work-holding device must ensure stable equilibrium during machining, assembly or manipulation, and also minimize part feature error due to elastic deformation of its structural components.

This thesis addresses the fundamental relationship between conformability and stability of parts of arbitrary shape held in a flexible mechanical fixture.  Specifically, it presents a model to predict the effect of fixture-workpiece conformability on static stability in flexible work-holding.  Metrics to quantify the conformability between an arbitrary workpiece and the conformable fixture/grasp are developed.  The model is experimentally verified using a pin-array type flexible fixture and is further used to analyze the effects of global and local conformability on fixture/grasp static stability for compliant parts.

The thesis also presents a model for part feature tolerance-based stiffness optimization of machining fixtures. The model enables determination of the optimal fixture stiffness required to keep the contribution of fixture elastic deformation to the total part feature tolerance less than a pre-determined value.  Experimental data taken from a pin-array type flexible-machining fixture is used to validate the model.  Finally, use of the conformability-stability model and the tolerance-based stiffness optimization model for optimal synthesis of pin-array type flexible machining fixtures is discussed.