Gray iron castings adopt lost foam, resin sand casting process, equipped with large gas tempering kiln, artificial aging, quenching and tempering to release internal stress, etc., to improve the accuracy of castings after processing. In the finishing process of the bed, the machine tool will cause thermal deformation of the spindle due to long-term cutting, which will affect the final machining accuracy. For high-precision machine tools and large-scale machine tools, the machining error caused by thermal deformation accounts for about 40~70%. Because thermal deformation can only be improved but cannot be eliminated, when the machine tool is determined, it is generally only possible to control the cutting conditions of the machine tool. To achieve the purpose of stabilizing the temperature of the machine tool, so as to avoid the influence of temperature changes on the machining accuracy. The main measures to control temperature changes during the processing of the bed are as follows:

1) The machine tool must go through a period of dry running before finishing. After the temperature rise is stable, the entire machine tool system reaches thermal equilibrium, and the position of each part of the machine tool is relatively stable before finishing;

2) After semi-finishing, because the cutting volume of the machine tool is large, the temperature rise is large, so it can not be directly finished, and it should be finished after running for a period of time;

3) In the course of several passes of finishing machining, in order to ensure the same temperature rise, it is necessary to ensure that the cutting amount is basically the same.

(1) Higher compressive strength and tensile strength.

(2) Good accuracy stability.

(3) High elastic modulus.

(4) Good wear resistance.

(5) Better shock absorption.

(6) Good cutting performance.

(7) Good casting performance.

(8) Higher dimensional accuracy and lower surface roughness.

The acceptance criteria of large gray iron castings are to be accepted after pouring:

1. The chemical composition standard of castings;

2. The mechanical performance standards of castings, tensile resistance and hardness;

3. Metallographic standards of castings, carbide content, pearlite content, graphite length, etc.;

4. Size requirements of castings;

5. The performance requirements of castings, such as pressure, leakage, etc.;