What kind of casting does a machine tool need?

21-04-2020

Machine tool is not only the main product in equipment manufacturing industry, but also widely used machining equipment in various industries. With the overall development of automobile, aerospace, military and manufacturing industries, the requirements for machine tool are raised higher and higher. Nowadays, CNC machine of high rigidity and high precision has been the leading product in machine tool industry. While the R&D capability of CNC machine is improving, the technology gap between domestic or overseas competitors is gradually narrowed. All in all, what machine tool requires focuses on high rigidity and high precision. Great casting is the foundation of CNC machine to meet standard of high rigidity and high precision, no matter how advanced the manufacturing technology is. Thus the quality of casting highly affects CNC machine’s functions, such as the quality, service life and the preservation of precision, also it is the main factor of determining the quality of machine tool.

01 Why does machine tool need to release internal stress?

What is internal stress?

It is the internal forces that interact on each other between the parts inside the object when the deformation of the object occurs due to external factors, such as force, humidity, temperature field change etc, to resist the effects caused by external factors and try to make the object recover from deformation.

Mechanical internal stress is the internal forces that machine tool needs to release. It is generated from machine tool body and other components while thermoforming machining. The accuracy of precision machining is required to be micrometer even nanometer, thus deformation and deviation caused by internal stress while casting is not acceptable. That’s why the internal stress needs to release. The higher standard of precision, the more essential to release internal stress, in order to guarantee the precision and stability of machine tool.

There are 3 ways to release internal stress?

① Natural aging: Through sun shines, winds, rains for months even years, the casting which is put in the open air will go under extremely slow deformation to release internal stress. It is a common traditional method of casting process with sound effect but long period.

② Manual aging: To speed up the aging process, the casting will be put into tunnel kiln, heated up to 600℃ and keep it for a while for stress relieving. It is a common treatment for small casting with good internal stress relieving but high energy consumption.

③ Vibration aging: For small casting, it can be put into cylinder to knock against each other to release internal stress; For big casting, vibration exciter can be used to resonate for internal stress relieving. The methods is with short aging period, wide application and fast development.

The manufacture process of casting is a complex combination of integrated process, including many production flow and procedure, from the preparation of metal and non-metal material to alloy smelting, modeling, core making, combined casting, cleaning, thermal treatment for casting defect elimination, to get the qualified casting. The main process of casting manufacture is as following.

1. The preparation of moulding sand and core sand: The preparation process of moulding sand directly affects the quality of mould sand. The preparation of moulding sand is generally including preparation&inspection of raw material and preparation&quality control of moulding sand.

2 .Modeling: It is the method and process of using moulding sand, pattern and other technique equipemnts to manufacture sand mould. Methods of modeling vary and depend on the shape, size, technical requirements of casting etc.

3 .Smelting: It is the process and operation of meeting the specified requirements of metal temperature&composition by heating metal to turn it from solid to liquid and applying metallurgical reaction to eliminate the impure substance in metal liquid. There are many types of equipments for smelting metal. Cupola is generally chose for making machine tool casting.

4 . Sand mould drying: For bigger type or high-quality oriented castings, it generally applies dry casting, like large cast iron platform, machine body casting, machine work platform, machine tool casting etc.

5 .Casting: It is the operation of pouring melted metal from ladle to mould.

6 .Cleaning: It is the process of removing the sand adhesion, moulding sand and unnecessary metal from the casting.

7 .Thermal treatment for the casting: It is the process of heating the casting to a certain range of temperature, preserving the heating for a while and cooling it to proper temperature in specified speed, in order to get expected structure and function. The purpose is to eliminate casting stress. For example, cast iron platform, machine tool casting, machine work platform, all have to be machining via heat treatment.

CNC machining centers

03 The structural characteristic and performance requirement of machine tool equipment

The performance requirements of casting mainly fall on the following aspects:

(1) High compresive strength and tensile strength

(2) Excellent accuracy stability

(3) High modulus of elasticity(MOE)

(4) Excellent abrasion resistance

(5) Good shock-absorbing characteristic

(6) Excellent cutting performance

(7) Excellent casting performance

(8) Relatively high dimension accuracy, relatively low surface roughness

CNC lathes

So to manufacture machine tool casting, control has to be made from chemical composition, mechanics performance, metallographic structure, dimension accuracy, aging treatment etc. Only through such all-round control can we make good quality casting which is the basis of machine tool with high rigidity and high precision.

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