Low Speed Orthogonal Hot Machining With a Virtual Quick Stop Device

Abstract

Cocquilhat first documented (1851) how heat in a tool shortens the life of the tool. Research since his time has generally concluded that the tool gives up a relatively higher percentage of its “cold hardness and toughness” as compared to the work stock. This interpretation suggests that if the workpiece itself were pre-heated it might alternatively lose a large percentage of its “cold hardness”. This thesis looks at the advantages and disadvantages which may be gained by pre-heating the work stock thereby preferentially shifting the ratio/percentages back toward the tool. A body centered cubic (BCC) steel and face centered cubic (FCC) aluminum were chosen to test common commercially machined crystalline structures. The materials were heat treated and/or aged to provide various levels of hardness within the crystalline structures. Both alloys at two levels of hardness/temper were machined with two different depths of cut and three tool rake angles using the Videographic Quick Stop Device (VQSD) originally developed at Auburn University. Steel parts were machined at three different temperatures. Aluminum samples were cut at four different temperatures. Parts were preheated immediately prior to the machining operations. Tensile specimens of all the samples were undertaken to establish the cold working flow stress values of the materials tested. Machining was conducted in a specially modified Cincinnati Horizontal Milling machine using an improved Videographic Quick Stop Device (VQSD) to capture the geometry of the cutting formation simultaneously with the forces in the X, Y and Z-axes using a standard Kistler force plate dynamometer. Utilizing the VQSD greatly increases the number of replicates available for statistical analysis by the metal cutting researcher. This allows for comprehensive multivariate analysis of the data with high confidence (> 95 %) in the meaning of the results obtained. Forces were collected and analyzed using low speed orthogonal machining equations. Wear was measured on the face of each tool using white light microscopy. The results of the data collection and statistical analysis are then used to populate the various historical models predicting the flow stress in metal cutting. The results indicate that one model is superior utilizing the criteria established by Kobayashi-Thomsen. The Normal force data indicated there is a definite advantage to be had in pre-heating the workpiece. This, combined with improved tool cooling methods, may result in all tools living a longer time while undergoing less chatter and/or deflection.