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The Machining Accuracy

- May 09, 2018 -

In the past 10 years, the machining accuracy of general-grade numerically-controlled machine tools has been increased from 10 μm to 5 μm, precision machining centers have increased from 3 to 5 μm to 1 to 1.5 μm, and ultra-precision machining precision has begun to enter nano-scale (0.01 μm).

In terms of reliability, the MTBF value of the domestic numerical control device has reached more than 6,000 hours, and the MTBF value of the servo system has reached more than 30,000 hours, showing very high reliability. In order to achieve high-speed, high-precision machining, the supporting functional components such as electric spindles and linear motors have been rapidly developed and the application fields have been further expanded.
Of course, there is a certain error in the actual processing, and the CNC machining error △ is added by the programming error △, the machine tool error △ machine, the positioning error △ set, the tooling error △ knife and other errors formed.

That is: △ number plus = f (△ △ + + machine + △ set + △ knife)
among them:

1, programming error △ compiled by the approximation error δ, rounding error. The approximation error δ is generated during the process of approximating a non-circular curve with a straight segment or an arc segment, as shown in Figure 1.43. The rounding error is an error generated by rounding off the coordinate values to integer pulse equivalent values during data processing. Pulse equivalent refers to the displacement of the corresponding axis for each unit pulse. Ordinary precision CNC machine tools, the general pulse equivalent value is 0.01mm; more accurate CNC machine tool pulse equivalent value is 0.005mm or 0.001mm.
2, machine tool error △ machine by the CNC system error, feed system error and other reasons.
3. The positioning error △ is determined when the workpiece is positioned on the fixture and the fixture is positioned on the machine.

4, knife error △ knife is determined when the relative position of the tool and the workpiece.
Impact editing

Open-loop control: Control without a position feedback device. The machining accuracy is generally around 0.02-0.05mm accuracy.

Semi-closed-loop control: means that an angular displacement detection device is installed on the open-loop control servo motor shaft. The displacement of the moving part is indirectly detected by the detection of the rotation angle of the servo motor to the comparator of the numerical control device, and compared with the input command, and the difference is used. Values control moving parts. Processing accuracy is generally around 0.01-0.02mm accuracy.
Closed-loop control: direct linear or rotary type detection device at the corresponding position of the final moving part of the machine tool, and feedback the directly measured displacement or angular displacement value to the comparator of the numerical control device to compare with the input command shift amount, and use the difference. The value controls the moving part so that the moving part moves strictly according to the actual required displacement. Processing accuracy is generally around 0.002-0.01mm accuracy.

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