Nine mistakes for Aluminum alloy machining

Machining error refers to the degree of deviation between the actual geometric parameters (geometric size, geometric shape and mutual position) and the ideal geometric parameters after machining parts. The coincidence degree between the actual geometrical parameters and the ideal geometrical parameters is the machining accuracy. The smaller the machining error, the higher the degree of conformity, the higher the machining accuracy. Machining accuracy and machining error are two terms of a problem. Therefore, the size of the machining error reflects the level of machining accuracy.

The main causes of machining errors

(1) Manufacturing errors of machine tools

The manufacturing errors of machine tools mainly include spindle rotation error, guide rail error and transmission chain error.

Spindle obtation error refers to the actual rotation axis of the spindle at each moment relative to its average axis of rotation, it will directly affect the precision of the workpiece. The main reasons for spindle rotation error are spindle coaxiality error, bearing error, coaxiality error between bearings, spindle winding and so on. The guide rail is the benchmark to determine the relative position of each machine tool component on the machine tool as well as the benchmark of machine tool movement. The manufacturing error, uneven wear and installation quality of the guide rail are the important factors causing the guide error. Transmission chain error refers to the relative motion error between the beginning and end of the transmission chain. It is caused by the manufacturing and assembly errors of the components of the transmission chain and the wear in the process of use.

(2) the geometric error of the tool

Any tool in the cutting process is inevitable to produce wear, and the resulting size and shape of the workpiece change. The influence of tool geometry error on machining error is different with different tool types. When machining with fixed size tool, the tool manufacturing error will directly affect the machining accuracy of workpiece. But for the general tool (such as turning tool, etc.), its manufacturing error has no direct influence on machining error.

(3) geometric error of fixture

The role of fixture is to make the workpiece equivalent to the tool and machine tool has the correct position, so the geometric error of fixture has a great impact on the machining error (especially the position error).

(4) Positioning error

The positioning errors mainly include datum noncoincidence errors and positioning pair manufacturing errors. When the workpiece is processed on the machine tool, several geometric elements on the workpiece must be selected as the positioning datum during processing. If the positioning datum selected does not coincide with the design datum (the datum used to determine a surface size and position on the part drawing), the datum non-coincidence error will occur.

The workpiece positioning surface and fixture positioning elements together constitute the positioning pair. The maximum position variation of the workpiece caused by the manufacturing inaccuracy of the positioning pair and the matching gap between the positioning pair is called the manufacturing inaccuracy error of the positioning pair. The manufacturing inaccuracy of positioning pair can only be produced in the process of adjusting method, but not in the process of trial cutting method.

(5) Errors caused by stress and deformation of the process system

Workpiece stiffness: in the process system, if the workpiece stiffness is relatively low compared with machine tools, tools and fixtures, under the action of cutting force, the workpiece deformation caused by insufficient stiffness will have a greater impact on machining errors.

Tool rigidity: the rigidity of the outer turning tool on the normal (y) direction of the machining surface is very large, and its deformation can be ignored. Boring the inner hole with small diameter, the cutter bar stiffness is very poor, the deformation of the cutter bar has a great influence on the machining accuracy of the hole.

The machine tool component stiffness: the machine tool component is composed of many parts, the machine tool component stiffness has no suitable simple calculation method so far, at present mainly uses the experimental method to determine the machine tool component stiffness. The factors affecting the stiffness of machine tool components include contact deformation of joint surface, friction force, low stiffness parts and clearance.

(6) Errors caused by thermal deformation of the process system

The thermal deformation of the process system has a great influence on the machining errors, especially in precision machining and large machining, and the machining errors caused by thermal deformation sometimes account for 50% of the total errors of the workpiece.

(7) Adjust the error

In every process of machining, the process system must be adjusted in one way or another. Adjustment errors occur because adjustment cannot be absolutely accurate. In the process system, the mutual position accuracy of workpiece and tool on the machine tool is guaranteed by adjusting the machine tool, tool, fixture or workpiece. When the original precision of machine tools, cutters, fixtures and workpiece blanks all meet the technological requirements without considering dynamic factors, the adjustment error plays a decisive role in machining error.

(8) Measurement error

When measuring parts during or after machining, measurement accuracy is directly affected by measuring method, measuring tool accuracy, workpiece and subjective and objective factors.

(9) internal stress

The stress that exists in parts without external force is called internal stress. Once the internal stress on the workpiece, it will make the workpiece metal in an unstable state of high energy level, it instinctively to the stable state of low energy level transformation, and accompanied by deformation, so as to make the workpiece lose the original machining accuracy.