CNC SKILLS: 9 machining tolerences. Do you know?
Machining tolerence refers to the degree of deviation between actual geometric parameters (geometric dimension, geometric shape and mutual position) and ideal geometric parameters after machining. The machining accuracy is the degree of agreement between the actual and ideal geometric parameters after machining. The smaller the machining error is, the higher the coincidence degree is, the higher the machining accuracy is. Machining accuracy and machining error are two kinds of formulation of the same problem. Therefore, the size of the processing error reflects the level of the processing accuracy.
The main causes of machining tolerences
1.Manufacturing tolerence of machine equipment
The manufacturing tolerences of machine equipment mainly include spindle rotation tolerence, guide tolerence and transmission chain tolerence.
Spindle rotation tolerence is the actual rotation axis of the spindle moment relative to its average rotation axis of change, it will directly affect the accuracy of the workpiece. The main causes of spindle rotation tolerence are the coaxiality tolerence of spindle, the tolerence of bearing itself, the coaxiality tolerence between bearings, the spindle rotation tolerence, etc. . The guide rail is the basis of determining the relative position relationship of each machine tool component on the machine tool, and is also the basis of the machine tool movement. The manufacture tolerence of the guide rail itself, the uneven wear of the guide rail and the installation quality are the important factors that cause the guide rail error. Transmission chain tolerence refers to the relative motion error between the two ends of the transmission chain. It is caused by the manufacturing and assembling tolerences of each link in the transmission chain, and the wear in the use process.
2. geometric tolerence of cutting tool
Any tool in the cutting process is inevitable to produce wear, and thus cause the size and shape of the workpiece changes. The Influence of tool geometric tolerence on machining tolerence varies with the type of tool: When a fixed size tool is used for machining, the manufacturing tolerence of the tool will directly affect the machining accuracy of the workpiece; The manufacturing tolerence has no direct effect on the machining tolerence.
3.Geometric tolerence of fixture
The function of the fixture is to make the workpiece have the correct position, so the geometric tolerence of the fixture has a great influence on the machining tolerence, especially the position tolerence.
The positioning tolerence mainly includes the base non-coincidence error and the positioning pair manufacturing inaccurate tolerence. When machining a workpiece on a machine tool, a number of geometric elements on the workpiece must be selected as the processing positioning benchmark,if the selected positioning datum does not coincide with the datum used to determine the size and position of a certain surface on the part drawing, the tolerence of the datum will occur.
The workpiece locating surface and the fixture locating element form the locating pair, and the maximum position variation caused by the inaccurate manufacturing of the locating pair and the fit clearance between the locating pair is called the locating pair manufacturing inaccurate tolerence. The manufacturing tolerence of positioning pair will be produced only when the adjusting method is adopted, but not when the trial cutting method is used.
5.The tolerence of the process system caused by the force and deformation
Workpiece stiffness: In the process system, if the workpiece stiffness relative to the machine tool, tool, fixture, relatively low, in the role of cutting force, the workpiece due to the lack of stiffness deformation on the machining error is relatively large.
Tool stiffness: The tool stiffness in the normal y direction of the machined surface is very large, its deformation can be ignored. If the bore diameter is small, the rigidity of the tool bar is very poor, and the force and deformation of the tool bar have a great influence on the machining accuracy of the bore.
Machine tool component stiffness: machine tool component is composed of many parts, so far there is no suitable simple calculation method for machine tool component stiffness, at present, the main method is to measure machine tool component stiffness by experiment. The factors that affect the rigidity of machine tool parts are the influence of contact deformation of joint surface, friction force, low rigidity parts and clearance.
6.tolerences caused by thermal deformation of process system
The thermal deformation of the process system has a great influence on the machining tolerence, especially in precision machining and large piece machining, the machining error caused by thermal deformation sometimes accounts for 50% of the total tolerence of the workpiece.
In each process of machining, the process system must be adjusted in one way or another. Because the adjustment can not be absolutely accurate, the adjustment tolerence is produced. In the process system, the mutual position accuracy of workpiece and cutter on the machine tool is ensured by adjusting the machine tool, cutter, fixture or workpiece, etc. When the original precision of machine tool, fixture and workpiece is up to the technical requirement and the dynamic factor is not considered, the adjustment tolerence plays a decisive role in machining tolerence.
When the part is measured during or after processing, the measuring accuracy is directly affected by the measuring method, measuring tool precision, workpiece and subjective and objective factors.
The stress that exists inside a part without the action of an external force is called internal stress. Once the internal stress is generated on the workpiece, the metal of the workpiece will be in a state of high energy potential instability, and it will instinctively transfer to the low energy potential of the stable state, and accompanied by deformation, thus, the workpiece loses its original machining accuracy.