The frames of machine tools are made of cast iron or steel. Most of the machine tools for cutting metal are made of cast iron. However, this is not the best material for this purpose in relation to the stiffness obtained from 1 kg of material.
It is granite that has proved to be the best material so far and the best machine frames are just made of it. It is very stiff and has insignificant thermal expansion. It is not "flowing" with long-term load, but it is difficult to process and therefore few machines are built on its basis.
The only advantage of cast iron is the ease of its formation. It has excellent properties of casting and it is good for machining. Unfortunately, cast iron is much weaker and much less rigid than steel. To achieve a similar stiffness of frames made of cast iron and steel and having similar sizes and structures, the mass of the cast iron frame must be over 2 times greater.
This happens because as far as the compressibility of steel and cast iron is similar, the tensility of grey cast iron, which is usually used for casting machines, is almost three times greater than of steel. This means that applying the same tensile force to cast iron will cause elongation three times greater than to steel. What is more, the aging process of casting stress, which occurs in cast iron in natural conditions, takes several years, and in order to shorten it, extra heat treatments are necessary. In addition, cast iron is prone to "flowing", i.e. under the long term load it can be deformed more than a steel construction in the analogous situation.
Producing steel construction lasts much longer than the treatment of cast iron, which is supplied from the foundry in the annealed condition and we need only to machine contact surfaces. This treatment lasts less than 8 hours. In contrast, the manufacturing time and processing of steel construction comes to a week. Moreover, steel bodies, due to high-quality steel that we use, are always more expensive than analogous ones made of cast iron. However, they have one indisputable advantage - the size and dimensions of steel details can vary depending on your needs, while the performance of casting models for a single detail has no economic justification.
The manufacturing time of the steel machine body is many times longer (which is more expensive) than in the case of cast iron. And this is the main reason why machines are equipped with cast iron bodies more often than with steel ones.
Is it better to suppress vibrations or not to have them? We also manufacture machines of cast iron and in spite of more than twice the weight they are not significantly stiffer than steel machines. An example is a perfect machine made by the German company Chiron, which is made of steel. The machine weighs about 10 tons and its stiffness is much greater than other machining centres made of cast iron.
Producing steel construction lasts much longer than the treatment of cast iron, which is supplied from the foundry in the annealed condition and we need only to machine contact surfaces. This treatment lasts less than 8 hours. In contrast, the manufacturing time and processing of steel construction comes to a week. Moreover, steel bodies, due to high-quality steel that we use, are always more expensive than analogous ones made of cast iron. However, they have one indisputable advantage - the size and dimensions of steel details can vary depending on your needs, while the performance of casting models for a single detail has no economic justification.
"Welded steel bodies of machines are more rigid than cast iron bodies" - Podstawy konstrukcji maszyn (Fundamentals of machine design) Volume 2, Collective Work, Wydawnictwa Naukowo-Techniczne (Scientific and Technical Publishing), 1999 "Welded bodies ... allow for increasing static stiffness while using less material ... Improving the vibration damping capacity of welded bodies can be achieved by treatments such as ribbing walls ..." - Obrabiarki skrawajace do metali (Machine tools for metal cutting), Lucjan T. Wrotny, Wydawnictwa Naukowo-Techniczne (Scientific and Technical Publishing), 1979.
