A Model for Multi-Stage Machining Economics Including Cutting Fluid Related Costs

Lucas P. Keranen | MS | 2002

Abstract:

Cutting fluids are widely used in machining operations to remove heat, lubricate, removed chips from the cutting zone, and add temporary corrosion protection. The use of cutting fluids in machining operations has led to concerns regarding worker health, the environment, and machining economics. Worker contact with cutting fluids may produce dermatitis, and mist created during a machining operation may lead to illnesses ranging from respiratory inflammation to asthma or even cancer. Disposal of large volumes of cutting fluids poses a danger of contamination of groundwater, lakes, and streams. Historical data suggests that the costs of cutting fluid systems in high volume machining are one of the main sources of expenditures related to machining.

In spite of the concerns associated with cutting fluid usage, the overwhelming majority of manufacturers still use copious amounts of fluid, most often employing a flood application of the fluid in high production operations. In some situations, dry machining (no use of cutting fluid) has proven to be effective, but this technology has met with resistance from some decision makers because of poor process performance (e.g., low tool life, inadequate lubrication, and thermal distortions) or uncertainty in what might be the real impact of eliminating the fluid. As an intermediate alternative to dry machining and fluid flood application, the technology of minimum quantity lubrication or MQL has recently received attention. MQL may provide process benefits not available with dry machining, and address the health, environment, and economic concerns associated with a cutting fluid flood application.

One issue examined by this thesis is the difference in process performance between different fluid application strategies. Cylinder boring experiments performed on cast iron cylinder liners were conducted at various rates of fluid application. The surface error, machining forces, and temperature were measured. No difference in surface error was noticed when applying fluid at varying fluid flow rates. In addition, there was no fluid effect on the machining forces.

Traditionally, to reduce the cost of high volume machining, dedicated transfer lines have been used. High volume production of machined parts requires low cycle times and robust equipment for which the transfer line is well suited. However, the large initial investment required, limited production line flexibility, and dependence on flood cutting fluid application is motivating interest in more reconfigurable production equipment such as machining centers. Machining centers provide greater flexibility, lower initial investment, and can easily accommodate MQL Technology.

Some manufacturers are currently considering the use of MQL technology on transfer lines and machining centers as an alternative to flood cutting fluid application. One of the principal concerns with such a conversion is the cost of MQL technology relative to traditional fluid application procedures. Also, the differences in economics between transfer line and machining center production equipment has received little attention in the technical literature. With these thoughts in mind, this thesis examines the economics associated with multi-stage operations such as those found in high production industries. A cost model is established to describe this situation, and it should be noted that the model includes cutting fluid related costs.

A new machining economics model is proposed to compute the costs associated with multi-stage machining systems. The use of the multi-stage machining economics model for a typical production situation reveals that a 9% reduction in cost is achieved by employing an MQL fluid application instead of a fluid flood application. A cost comparison of a transfer line to a set of machining centers was also performed using the multi-stage machining economics model. For this analysis the transfer line shoed a 5% advantage in cost over the machining centers. Though, the cost benefit offered by the transfer line must ultimately be weighed against the inherent flexibility of machining centers.

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