Sunday, September 14, 2008

High speed machining concepts for different materials

The first question is what does one understand by high speed machining. In innumerable articles reference is made to historical theories regarding temperature reduction with high speed cutting as described by Carl Salomon, who patented the process in the 1920s ( German patent #523594).

It is also common to find ballistic references consisting of firing projectiles against materials to simulate material deformation at high cutting speeds like those of Kronenberg in the 1960s.

There are also complex discussions related to chipping under different thermophysical conditions depending on the cutting speed magnitude. Each definition has its own nuances and motivation, valid within a certain application range. In almost all high speed reports, the same generalities are repeated (copying or ‘inspiration’ among authors is evident), many of which use the Solomon curves. You have to remember Solomon put forward his idea almost 15 years prior to modern cutting models, thus it was a highly intuitive invention. He performed milling experiments at speeds over 15,000 m/min. To sum up this patent put forward: “tool temperature and wear increase with cutting speed until reaching a maximum value called critical speed, from they decrease with speed. Critical speed depends on material, as shown in Fig. Thus results will be optimum when machining above this value”.

Today we know temperature and wear always increase, although they tend to stabilise when cutting speed is high enough. The McGee (1979) curve is considered more appropriate for aluminium, although it is just one more obtained by different researchers.

Solomon Curves.

Regarding other materials, it is difficult to verify whether machining steels withstands speeds of 1700 m/min well, because prior to reaching these values, the tools break, As we see in the figure, the HSS rapid steel only withstands up to 650ºC, and the hard metal tool approximately 850ºC. Forgetting the generalities which are always repeated and whose repetition sometimes only manages to this kind of ‘high speed’ term magic halo, cutting values understood today as high speed are reflected in Fig. 4, with orientative cutting speed in each case. To reach these speeds machines capable of doing so are required, since they may be limited by their construction and above all by main motion (spindle). As can be seen there is an area (marked in red) where the machine to be used is high speed, coinciding with today’s industrial offer of this type of machine. Another area (in blue) requires a very high speed machine which although extant today is still in the prototype phase.

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