What are the causes of roundness errors on machining centers?

In order to avoid such problems, we analyze the causes of roundness error and how to suppress such problems in the actual machining process to ensure the machining accuracy of parts.

Roundness error refers to the variation of the actual circle on the ideal circle of the measured object in the same cross-section of the rotating body.

CNC machine tools in the long-term use of the process, ball screws, guides and bearings there is a certain degree of wear and tear, at the same time, due to the maintenance is not in place, lack of lubrication and large eater and other factors will intensify the wear and tear of mechanical components, resulting in accelerated deterioration of the geometric accuracy of CNC equipment, these are the factors that cause roundness error. CNC system dynamic response instability, servo optimization is not perfect, will also affect the roundness of the CNC machine tool error.

The roundness test function of Siemens 840Dsl CNC system can measure the actual and theoretical roundness error of the machine tool by simulating the machining through the CNC program without the test cut of the part, with the powerful calculation ability and position feedback function of the CNC system. Through the analysis of different graphics derived from roundness test, the reasons for different errors and the adjustment method of such errors can be obtained. Generally the main causes of roundness error are interpolation axis backlash, reverse oscillation, servo mismatch, two interpolation axes are not perpendicular and machine vibration, etc. The following is an example of Siemens 840Dsl CNC system. The following is an example of Siemens 840Dsl CNC system.

1.Reverse clearance error

The backlash error of the machine tool refers to a certain axis of the machine tool in the process of movement, from forward motion to reverse motion, in a certain angle, although the drive motor drive screw has been reversed, but the table has to wait for the screw and nut gap to be eliminated before the reverse motion, this gap is the backlash gap of the machine tool axis, generally reflected in the screw angle of the screw, in the semi-closed-loop CNC system, the axis in the reverse motion, the difference between the instruction value and the actual movement is the backlash error value of the axis.

(1) Influence of backlash on roundness error The existence of backlash will affect the positioning accuracy and repetitive positioning accuracy of the machine tool, reduce the machining accuracy of CNC machine tools, and cause roundness error in the process of milling roundness in machining centers. When the Y-axis is in reverse motion, the existence of reverse clearance will cause the Y-axis to reverse lag behind the X-axis for interpolation motion, and the roundness error will occur during the milling process as shown in Figure 1.

Fig. 1 Roundness error caused by backlash.

(2) Backlash measurement and adjustment The factors affecting the backlash error of machine tool axes are various, from the drive motor to the moving parts, there will be a gap in all the mechanical joints, whether the coupling of motor to screw is loose, the manufacturing error of ball screw, whether the preload of screw is too tight or too loose, whether the coupling of screw mother and moving parts is tight or not, all these are the causes of the backlash of machine tool axes. For the mechanical part of the gap can not be eliminated, the need to compensate for the backlash in the CNC system.

As shown in Fig. 2, measure the backlash, use a magnetic table base to fix a position, press the dial gauge on the table to be measured, zero the dial gauge scale, move the feed axis in the same direction for a certain distance, reverse the axis to the initial position, read the difference value A from the dial gauge, the average value obtained after 7 times of measurement is the backlash error of the axis, i.e. A = (A1 + A2). ＋(+ A3 + A4 + A5 + A6 + A7)/7. The backlash of this shaft can be eliminated by writing the calculated value A in the parameter MD32450 of the corresponding shaft. By compensating the backlash of the shaft, the accuracy of the feed shaft can be improved to ensure the accuracy of the interpolation motion and improve the roundness error effectively.