Machine tools - Main organs

Selecting the machine tool components that are essential for correct machine operation requires a brief reminder of the basics of both mechanics and metal cutting.

The behavior of a machine tool during operation is the result of the interaction between the cutting process and a complex mechanical structure.

It is clear from the outset that the static characteristics of the cut, which receive most of the attention and are the only ones presented in traditional texts, will not be sufficient to explain the behavior of the machine in operation.

Similarly, the machine's static characteristics alone will clearly be insufficient to explain well-known phenomena such as chatter, or cutting instability, which can easily occur on all machines, and particularly those with continuous cutting, such as lathes or boring machines (or machining centers, when they carry out a boring operation).

To understand the behavior of a machine tool in operation, we need to establish a model. The same applies to metal cutting. A good match between the simulated behavior of these models and the actual behavior of working machines should then make it possible to clearly show the parameters and variables on which good machining depends.

This last sentence shows that the machine's suitability for its task can only be judged in terms of topology (the characteristics of surfaces and the relationships between them, independently of their dimensions) and the metrics of machined surfaces.

The machine will therefore have to meet a specification drawn up on this basis. While this practice is mandatory for special-purpose machines, which by definition have to machine parts of given shapes, dimensions and tolerances, it is completely unusual for general-purpose machines. As a result, the performance claims made by their manufacturers are generally meaningless, since in addition to the usual speeds, power, strokes, etc., they simply give a positioning accuracy without indicating the conditions under which this is achieved (cutting forces, speeds, feed rates, table load, temperature reached by machine components under different operating conditions, etc.).

To avoid the kind of criticism we've just made, we'll need to give a brief outline of the specifications for a universal machine, so that we can, at least qualitatively, deduce its main characteristics, which will then serve as the starting point for its design.

The article "Machine tools" is the subject of several articles:

• Presentation

• [B 7 121] Main bodies

• Examples of machines

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