This study develops software that provides optimum lathe cutting conditions for specific materials and parameters. This is accomplished by developing a model, based on empirical and analytical relationships, which estimates the optimum cutting conditions, (i.e. spindle speed, feed rate and depth of cut) for a single pass, external turning operation. These parameters are optimized to yield the minimum production cost, while satisfying constraints imposed by workpiece specifications and equipment limitations. The equipment limitations considered include the available machine power, maximum workholding force of the lathe, range of spindle speeds, feed rates for the lathe and tool, and the range of depth of cut of the tool. The workpiece constraints are the surface finish specification for the part and the maximum allowable deflection of the part at each cross section. The effects of each of these limitations are discussed. The developed analytical model introduces manufacturing economics, along with the above constraints, into a decision making process which heretofore relied primarily on the lathe operator's experience and standard handbooks. Typically, the determination of the metal cutting conditions is based on the machinist's experience. This method of specifying cutting conditions tends to emphasize the requirements of the workpiece specifications - i.e. surface finish and dimensional tolerances - excluding economic considerations. On the other hand, a pure rate of production analysis would maximize the ratio of actual cutting time to total machining time without considering workpiece specifications or the implications of operating the equipment at the maximum production rate. While minimizing the production time generally reduces production costs, there is a trade off to be considered. A higher production rate requires an increase in spindle speed or tool feedrate resulting in a decreased tool life. This reduction in tool life adds the associated costs of additional tools and tool changing times to the production cost. Hence, it is necessary to find the operating conditions which will minimize cost while considering all aspects of the manufacturing process. With the increasing usage of CNC lathes which involve large capital expenditures, the use of an economic analysis combined with technical considerations becomes imperative for minimizing the overall production cost. Further, an effective optimization procedure allows low volume runs on many different part numbers with the first part being both cost effective and fit for function. The methodology used to develop the model was based on published literature, experimentation, and several well known and widely accepted equations defining tool-life and tool-workpiece relationships. Through the use of a statistically designed experiment, data was obtained and a set of equations was determined to estimate the cutting forces generated in the turning operation. The data compared favorably with the published equations for calculating cutting forces which were used in this model. The parameters for this experiment, which was conducted on an instrumented lathe at Renesselaer Polytechnic Institute, were feed rate and depth of cut. An additional experiment was conducted to determine the tool life corresponding to the maximum allowable tool flank wear for several feed rates. These values are unique for a tool-workpiece material combination. For the purpose of applying the model, the experimentation was restricted to the use of a carbide coated tool insert and a free machining stainless steel, AISI 416. The work was based on the actual needs and production tooling of a major company. The determination of the empirical constants for other tool-workpiece material combinations would extend the model's application. The optimization procedure is incorporated into a computer program to calculate the economical machining parameters in a finishing operation.
Library of Congress Subject Headings
Turning (Lathe work)--Mathematical models; Turning (Lathe work)--Simulation methods; Lathes; Machining--Simulation methods
Department, Program, or Center
Mechanical Engineering (KGCOE)
Bothner, Rose, "An Optimization model for selecting the economical cutting parameters in an external forward turning operation" (1990). Thesis. Rochester Institute of Technology. Accessed from
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