abstract
-
This thesis deals with an adaptive control constraint system for end milling with constant force. The control criterion is to hold the force acting on the cutter at a value safely below the force which would break the cutter. This criterion is applicable to finish milling of complex shapes, typically of die cavities. The controlled variable is the command feed rate.
Two other constraints were considered for the adaptive control system, namely: chatter and overload of the cutting edge.
The work presented in this thesis was based on a homemade Computer Numerical Control (CNC) and Adaptive Control (A/C) System consisting of an NC retrofitted No. 4 vertical milling machine and an HP 2100A minicomputer.
The operating characteristics of this system were examined both experimentally and by digital computer simulation. The analytical analysis of the A/C system concentrated on its behaviour as a servomechanism, i.e. mainly with respect to the speed of its response to step inputs and to its stability. In this analysis both the Laplace transform approach as well as a numerical simulation (State-Space approach) were used.
In order to improve the system response to collision or step changes in cutting loads caused by abrupt changes in work surface contact area, special action strategies are presented. These strategies are allowed to operate for a limited time during impact before the system switches back to a slower acting (lower gain) strategy. This leads to improved reaction time whilst maintaining system stability.
The A/C experiments were conducted using two different types of dynamometers for the on-line measurement of cutting force. Two different workpiece materials were used: Al. Alloy (95 BHN) and steel AISI 1020 (155 BHN). High Speed Steel end mills were used in all experiments.
An experimental investigation of the chatter constraint was also carried out as well as a review of the basic features of the phenomenon of cutting edge overload.
It is shown that the flexibility of the end milling cutters is beneficial in attenuating the overload in a sudden transient situation and it is also beneficial in attenuating chatter. These benefits are obtained for a certain range of diameters and lengths of cutters. Within this range constraints on feed rate for edge overload must be considered and outside of this range the chatter constraint has to be included.