Models of machine tool efficiency and specific consumed energy
Introduction
Correct selection of cutting parameters in machining is very important in order to achieve the prescribed quality, economical and productive process. Cutting parameters are obtained by various methods based upon assurance of required accuracy of the workpiece, with a maximum production rate, minimum production cost or maximum profit rate [5], [6].
In context, one major problem concerns the link between specific consumed energy in machining and the parameters which could have an influence on it. These parameters deal with the power at main spindle of the machine tool, the machine tool efficiency and the amount of material removed.
There are lot of machine tools, powerful ones—having tens or hundreds kW power and very many of small or medium power, all of them working with little efficiency—even smaller than 0.2, with a large specific consumed energy and a very low productivity. The main reason for which one cannot establish the specific consumed energy is the lack of knowledge on the efficiency model.
If the model of machine tools’ efficiency is known, then the specific consumed energy could be determined for establishing cutting parameters and the consumed energy necessary for removing a certain quantity of chips. The available information on this problem is limited, the matter of machine tools’ efficiency is almost ignored by researchers.
It is necessary to carry out studies on machine tools efficiency, because of the great number of machining processes and of the increase of machine tools nominal power. A pioneer work in this field was done in 1981 [2], for face milling of the aluminium alloys on a vertical-milling machine. Later, researches regarding the machining of pieces made by different materials, on different machine tools were developed [5], [6].
Section snippets
Modelling of machine tool efficiency
Machine tool efficiency could be defined as the ratio between necessary cutting power, Pc, and consumed power, Pmc, absorbed from the power network by the electric motor [7], i.e.:where Pm1 is the power loss in machine tool and electric motor.
Cutting power could be written as a function of machining parameters, that is why machine tool efficiency can be considered as a function of these parameters.
Working parameters of machine tools could be: spindle speed, n, the torque at
Influence of cutting parameters on tangential cutting force and on machine tool efficiency
For a given D diameter of tool, the spindle speed, n, can be expressed as a function of cutting speed, v, and the cutting torque, Mt, as a function of tangential component, Ft, of cutting force, so that
The tangential cutting force, Ft, can be expressed as a function of milling parameters [1], [2]. At incomplete face milling (when the contact length of milling tool, B, is smaller then the tool diameter, D), the tangential cutting force, Ft, can be expressed aswhere v
Specific consumed energy
The efficiency of machine tool gives no information about how this power is used for cutting. For instance, the machine tool can be loaded at a maximum efficiency, without machining, functioning with rotation and torque at the main spindle the same as on machining operation, the torque being obtained by braking the main spindle.
In order to appreciate how the consumed energy absorbed from the power network is used for cutting, it is necessary to divide this power by the quantity of material
Effect of cutting parameters on specific consumed energy
Every component of relation (9) is a function of different cutting parameters, and can be expressed related to them. Relation (9) makes possible the study of influence of cutting parameters upon specific consumed energy for different cutting processes.
For incomplete non-symmetrical face milling, e.g., it can be obtained:where D is the diameter of the mill (mm), sz the feed per tooth (mm/tooth), t the depth of milling (mm), B the contact length of the milling tool (mm), z
Consumed energy
The consumed energy, Ec, can be determined by multiplying the specific consumed energy, Ecs, and the total volume of material removed, Y, i.e.:Under usual cutting conditions, with standard tools, for economical cutting parameters, mean specific consumed energy can be determined for cutting different materials, on different machine tools. Using those specific consumed energy, it can be determined the consumed energy for a certain quantity of material. That is of great importance for
Conclusions
For determining the electric energy consummation for machining, it has to be known the mathematical model of machine tool efficiency. It is possible statistic modelling of machine tool efficiency as a function of their working parameters, using experimental data and Response Surface Methodology.
The machine tool workload has a great influence upon the machine tool efficiency, which has its maximum value close to the level of the machine tool nominal power.
For the same type of machine tools, the
Acknowledgements
The authors are very grateful to Professor Michael Hillery, from University of Limerick, for his precious support.
References (7)
A study of high efficiency face milling tools
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