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What is random error on machine tool

The random error of the 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 the three errors below.
1. Errors caused by the deformation of the process system under load: When the workpiece is turned, it is often affected by cutting force, clamping force, inertial force, gravity, etc., resulting in corresponding deformation, and ultimately destroying the correct alignment between the tool and the workpiece. The relative position reduces the machining accuracy of the workpiece. For example, when the rigidity of the workpiece is much smaller than the rigidity of the tool and fixture, the workpiece will deform due to insufficient rigidity under the action of cutting force, thereby reducing the machining accuracy. Conversely, when the rigidity of the workpiece is much greater than the rigidity of the tool and the fixture, the tool and the fixture will be deformed during the processing of the workpiece, which will also reduce the accuracy of the workpiece.
Therefore, it is necessary to select the tool material reasonably, increase the rake angle and the entering angle of the tool, and conduct a reasonable heat treatment on the workpiece material to improve its processing performance. At the same time, it is necessary to improve the rigidity of the process system, reduce the cutting force and compress their variation amplitude.
2. Errors caused by internal stress redistribution: The so-called internal stress is the stress that exists inside the part without external force. Once internal stress is generated on the workpiece, the workpiece will be in a high-energy unstable state, which will instinctively transform to a low-energy stable state, and along with the deformation of the workpiece, the workpiece will eventually lose its original machining accuracy. For example, after the heat treatment of the workpiece, due to uneven wall thickness and uneven cooling, internal stress will be generated, which will lead to deformation and ultimately reduce the processing accuracy.
Therefore, when designing parts, try to achieve uniform wall thickness and symmetrical structure to reduce internal stress.
3. Errors caused by thermal deformation: In precision machining and large parts processing, the thermal deformation of the process system has a greater impact on the machining accuracy of the workpiece, and the machining error caused by thermal deformation can sometimes account for 40% to 70% of the total error of the workpiece . Machine tools, tools, and workpieces are subjected to various heat sources, and the temperature will gradually rise, and at the same time they transfer heat to the surrounding material and space. As a result, the workpiece and the entire process system are deformed. When the heat input per unit time is equal to the heat dissipated, the process system reaches a thermal equilibrium state. For example, in the process of turning large parts, the temperature of the spindle bearing and the Z-axis screw increases due to the 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 our machine tools will have higher and higher precision, which can save worry and effort to produce more good products.

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