Materials and Processes in Manufacturing

E. Paul Degarmo, J T. Black, Ronald A. Kohser

Chapter 21 Fundamentals of Machining

Machining Process Parameters

An understanding of what the process parameters are is necessary in order to design and operate machining processes, to specify machine tools and tooling and to use machining process models. Process models can be used to predict the effects of process parameter changes on process performance. So they are useful for process design and process improvement. An important aspect of using process models is to understand the relationship between the usually simplified model and its parameters and the real physical process being modeled. An example of using a realistic, but simplified, model is the use of the orthogonal machining model to describe the turning process.

Lathe turning has a simpler configuration than many other machining operations, yet includes all the important characteristics of most machining processes. To clarify the definitions of the process parameters a schematic of the turning process with the capability of changing cutting conditions is presented.

Problems: Turning process parameters are further emphasized in the following problems having to do with

- spindle speed and cutting speed,

- feed rate and machining time.

Material Removal Rate

The material removal rate, MRR, can be defined as the volume of material removed divided by the machining time. Another way to define MRR is to imagine an "instantaneous" material removal rate as the the rate at which the cross-section area of material being removed moves through the workpiece. The usefulness of this view can be seen in answering the following question. What is the material removal rate when turning a tapered shaft using a constant cutting speed?

Since the depth of cut is changing the material removal rate changes continuously during the process. In some cases this may be important. For example, if cutting forces and the resulting workpiece and tool deflections are of interest. The changing amount of material being removed along the tapered shaft means the cutting force and so the deflections will change during the process.

The work-tool contact area or cross-section area of work material being removed is shown in the presentation

Problems: Material removal rate and its effect on cutting force and power are the topics of the following problems

Chip Formation Model

At the most basic level, cutting forces depend on the properties of the work material and on how the work material is deformed in the chip formation process. The workpiece deformation is not directly controllable. However, work material deformation can be influenced by process parameters. Some of these process parameters are directly controllable while others are not. For example, cutting tool shape can be changed within reasonable limits so that the tool maintains sufficient strength. A less controllable process characteristic that influences deformation in chip formation, and also temperature, is the tool-chip coefficient of friction. While cutting fluids can be applied to change the coefficient of friction, a specified friction coefficient cannot be exactly and reliably specified and produced.

Process models of varying complexity have been developed to relate the dependent process variables or process outcomes such as shear zone characteristics, to the controllable or independent process variables.

A simple, but useful, model that captures some of the important characteristics of machining process is introduced

© 2001 by Barney E. Klamecki. All rights reserved.