Steel or cast iron?

The bodies of the cutting machine tools are made of cast iron or steel. Most metal cutting machines are made of cast iron. But not because it is the best material for this purpose in terms of stiffness obtained from, say, 1 kg of material.

Granite is the best material so far and it is used to make the bodies of the best machines. It is very stiff, has negligible thermal expansion, does not “flow” under long-term loading, but it is troublesome to process and therefore few machines are built on its basis.

The only advantage of cast iron is that it is easy to form. It has excellent casting properties and is easy to process. Unfortunately, cast iron is much weaker and much less stiff than steel. In order for bodies made of cast iron and steel of similar size to have similar stiffness, the weight of the cast iron body must be more than 2 times greater.

This is because, while the compressibility of steel and cast iron is similar, the extensibility of gray cast iron, which is most often used in cast machines, is almost 3 times greater than that of steel. This means that applying the same tensile force to cast iron results in an elongation 3 times greater than that of steel.

In addition, the aging process of casting stresses occurring in cast iron in natural conditions lasts up to several years and in order to shorten it, additional heat treatments are necessary. In addition, cast iron has a tendency to “flow”, ie under prolonged load, it can deform many times more than in a similar situation of a steel structure.

The time of making a machine body made of steel is many times longer than in the case of cast iron (i.e. more expensive) and this is the main reason why machine tools more often have cast iron bodies than steel ones.

Is it better to suppress vibrations or not to have them? We also manufacture machines from cast iron and despite their more than twice the weight, they are not much stiffer than steel ones. An example is the excellent machine tool of the German company Chiron, which is made of steel. The machine weighs approx. 10 tons and its stiffness is much greater than that of other cast iron machining centers.

The execution of a steel structure takes much longer than the processing of a cast iron casting, which is delivered from the foundry in an annealed condition, and it only remains to process the contact surfaces. This treatment takes less than 8 hours. However, the execution and processing time for the steel structure is up to a week. In addition, steel bodies, due to the quality steels we use, are more expensive than similar ones made of cast iron. However, they have the indisputable advantage that the sizes and dimensions of steel details may change depending on the needs, while the production of casting models for single details does not have an economic justification.

“Machine bodies welded from steel materials are characterized by greater stiffness than cast iron bodies” – Fundamentals of machine construction Volume 2, Collective Work, Wydawnictwa Naukowo-Techniczne 1999

“Welded bodies … allow to increase static stiffness with less material consumption … the ability to damp vibrations of welded bodies can be achieved by such procedures as wall ribbing … “- Metal cutting machines, Lucjan T. Wrotny, Scientific and Technical Publishing House 1 979.