Analysis of machine tool random error

The random error of machine tool is caused by external conditions and is greatly affected by external conditions.

It can be divided into: positioning accuracy error, geometric accuracy error, thermal deformation error and so on. Let’s take a brief look at these three errors.

1. Error caused by load deformation of the process system: when turning, the workpiece is often affected by cutting force, clamping force, inertia force, gravity, etc., resulting in corresponding deformation, which eventually destroys the correct relative position between the tool and the workpiece and reduces the machining accuracy of the workpiece. For example, when the stiffness of the workpiece is much less than that of the tool and fixture, under the action of cutting force, the workpiece will deform due to insufficient stiffness, thus reducing the machining accuracy. Conversely, when the stiffness of the workpiece is much greater than that of the tool and fixture, the tool and fixture will deform in the process of machining the workpiece, which will also reduce the accuracy of the workpiece.

Therefore, it is necessary to reasonably select the tool material, increase the rake angle and main deflection angle of the tool, and reasonably heat treat the workpiece material to improve its machining performance. At the same time, it is necessary to improve the stiffness of the process system, reduce the cutting force and compress their variation amplitude.

2. error caused by redistribution of internal stress: the so-called internal stress refers to the stress existing in the part without the action of external force. Once the internal stress is generated on the workpiece, it will make the workpiece in an unstable state of high energy level, so as to instinctively transform to the stable state of low energy level, accompanied by workpiece deformation, and finally make the workpiece lose the original machining accuracy. For example, after heat treatment, the workpiece produces internal stress due to uneven wall thickness and uneven cooling, which leads to deformation and finally reduces the machining accuracy.

Therefore, when designing parts, try to achieve uniform wall thickness and symmetrical structure, so as to reduce the generation of internal stress.

3. Error caused by thermal deformation: in precision machining and large workpiece machining, the thermal deformation of the process system has a great impact on the machining accuracy of the workpiece. Sometimes, the machining error caused by thermal deformation can account for 40% ~ 70% of the total error of the workpiece. Machine tools, cutting tools and workpieces are affected by various heat sources, and the temperature will gradually rise. At the same time, they transfer heat to the surrounding materials and space. Thus, the workpiece and the whole process system are deformed. When the heat input per unit time is equal to the heat dissipated, the process system reaches the state of heat balance. For example, in the process of turning large parts, the temperature of spindle bearing and z-axis lead screw increases due to excessive cutting force, resulting in large deformation, which affects the machining accuracy of the workpiece.

There are many kinds of machine tool errors. Today, with the continuous development of technology, we believe that the accuracy of our machine tools will be higher and higher, and more good products can be produced in a worry-saving and labor-saving production place.