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How Are Tolerances in Machining Created?

Parts in the processing process, due to various reasons, processing and manufacturing of parts size can not be exactly the same as the size of the parts drawing design, the degree of deviation between the actual geometric parameters and the ideal geometric parameters for the parts processing error.

Tolerance is the mechanical design and manufacturing, the actual parameters of the part value of the allowable amount of change. The larger the allowable range of variation, the greater the tolerance, and the greater the acceptable machining error, and vice versa.

Parts of different manufacturing processes, the reasons for deviation are different, to machining, for example, the main reasons for error are as follows:

01 The manufacturing accuracy of the machine tool

The manufacturing error of the machine tool mainly includes the spindle rotary error, guideway error and transmission chain error.

Spindle rotary error refers to the spindle moment of the actual axis of rotation relative to the average axis of rotation of the amount of change, which will directly affect the accuracy of the processed workpiece. The main reasons for the spindle rotary error are the spindle coaxial error, the bearing itself, the error between the bearing coaxial error, spindle winding and so on.

The guideway is the benchmark for determining the relative position of each machine component on the machine tool, and it is also the benchmark for the movement of the machine tool. The manufacturing error of the guideway itself, the uneven wear of the guideway and the installation quality are the important factors causing the error of the guideway.

Transmission chain error refers to the transmission chain at the beginning and end of the two ends of the transmission components of the relative movement of the error. It is caused by the manufacturing and assembly errors of the components of the transmission chain, as well as wear and tear in the process of use.


02 Geometric error of the tool

Tool wear is inevitable in the cutting process and therefore affects machining accuracy. Different types of tools have different effects on machining errors.

03 Geometric error of fixture

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

04 Positioning errors

Positioning error mainly includes datum non-coincidence error and inaccurate manufacturing error of positioning vice.

In the machine tool on the workpiece processing, the need to select the workpiece on a number of geometric elements as the processing of the positioning reference, if the selection of the positioning reference and the design of the reference (used to determine the size of a surface in the parts drawing, the position based on the basis of the reference) does not coincide with the benchmark does not coincide with the error will be generated.

Workpiece positioning surface and fixture positioning elements together constitute the positioning sub, due to inaccurate positioning sub manufacturing and positioning sub gap caused by the maximum position of the workpiece changes, known as positioning sub manufacturing inaccuracy error. Positioning vice manufacturing inaccuracy error is only produced when using the adjustment method of machining, in the test cutting method of machining will not be produced.


05 process system deformation error generated by force

Workpiece stiffness: process system if the workpiece stiffness relative to the machine tool, tool, fixture is relatively low, under the action of the cutting force, the workpiece due to the lack of stiffness and deformation caused by the machining error is relatively large.

Tool stiffness: external turning tool in the machining surface normal (y) direction of the stiffness is very large, its deformation is negligible. Boring a small diameter hole, the toolbar stiffness is very poor, the toolbar force deformation on the hole machining accuracy will have a great impact.

Machine tool component stiffness: machine tool components are composed of many parts, machine tool component stiffness so far there is no suitable simple calculation method, at present the main method to determine the stiffness of machine tool components with experimental methods.

Factors affecting the stiffness of machine tool components are the effect of contact deformation of the bonding surface, the effect of friction, the effect of low stiffness parts, the effect of clearance.

06 Error caused by thermal deformation of the process system

Process system thermal deformation on the processing error is relatively large, especially in precision machining and large parts processing, caused by thermal deformation of the processing error can sometimes account for 50% of the total error of the workpiece.

07 Adjustment error

In each process of machining, the process system should always be adjusted this way or that way. As the adjustment can not be absolutely accurate, thus generating adjustment error. In the process system, the workpiece, tool on the machine tool mutual positional accuracy, is through the adjustment of the machine tool, tool, fixture or workpiece to ensure.

When the original accuracy of the machine tool, tool, fixture and workpiece blank, etc. have reached the process requirements without considering the dynamic factors, the adjustment error plays a decisive role in machining error.

08 Measurement error

Parts in the machining or machining after the measurement, due to the measurement method, gauge accuracy and workpiece and subjective and objective factors have a direct impact on the measurement accuracy.

09 internal stress

The stress that exists inside the part without external force is called internal stress. Once the internal stress on the workpiece, it will make the workpiece metal in a high-energy level of instability, it instinctively to the low-energy level of the stable state of transformation, and accompanied by deformation occurs, so that the workpiece loss of the original processing accuracy.

In the field of machining, tolerance is an indispensable concept because the accuracy of a part depends on the complex interactions between the machine tool, the tool, the fixture, the workpiece and many other factors. Tolerance exists to tolerate the inevitable errors in machining, which can originate from a variety of sources, including the manufacturing accuracy of the machine tool, geometric errors of the tool, geometric errors of the fixture, positioning errors, deformation of the process system from force and heat, adjustment errors, measurement errors, and internal stresses, among other factors.

In this field of precision machining, Richconn is a precision manufacturing company that specializes in providing high quality CNC machining services. We pride ourselves on having state-of-the-art equipment and a team of specialized technicians to ensure that parts are machined to the smallest tolerances possible. Not only are we committed to meeting our customers' precision machining needs, but we also pay attention to detail and quality control to ensure the accuracy and performance of the final product. At Richconn, we believe that precision manufacturing is an art, and we will work tirelessly to continually improve our processes to meet our customers' needs for high precision parts.

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