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System parameter changes or changes, mechanical failure, machine tool electrical parameters optimization of the motor running, an abnormal machine tool position loop or improper control logic, is the production of CNC machine tool processing precision is a common cause of abnormal trouble, find out the related point of failure and processing, machine tool can be back to normal.
Production often encounter nc machine tools machining precision abnormal failure.Such strong concealment, fault diagnosis is difficult.The cause of such failure mainly include five aspects: (1) machine tool feed unit is changed or changes.(2) of each axis machine zero offset (NULLOFFSET) anomalies.(3) axial BACKLASH (BACKLASH) anomalies.(4) the abnormal motor running state, i.e., electrical and control part of failure.(5) mechanical failure, such as screw, bearing, coupling device shaft parts.In addition, the process of preparation, tool selection and artificial factors, also may lead to abnormal processing precision.
1. The system parameter changes or modifications
System parameters including machine tool feed unit, zero offset, reverse clearance, and so on.Such as SIEMENS, FANUC nc system, the feed unit has two kinds of metric and imperial.Some processing machine tool repair process, often affect the change of the zero bias and gap, fault handling should make timely completion of adjustment and modification;Due to mechanical wear, on the other hand, severe or loose connections may also result in changes of the parameters measured values, require corresponding changes to the parameters can meet the requirements of the machining accuracy of machine tool.
2. The machining accuracy of anomaly caused by mechanical failure
A THM6350 horizontal machining center, the FANUC0i - MA CNC system.In the process of milling turbine blade at a time, suddenly found the Z axis feed exception, at least 1 mm cutting error amount (Z to cut).Investigation to understand: fault happened suddenly.Machine tools in point, MDI operation mode under the shaft running normally, and back to the reference point is normal;No alarm prompt, electrical control part of the hard fault rule out the possibility.Analysis, mainly deal with the following aspects one by one.
(1) check the precision machine tool is running the processing procedures section, especially the tool length compensation, the processing coordinate system (G54 ~ G59) proofreading and calculation.
(2) under the point move way, repeated the Z axis motion, after see, touch, listen to the motion state diagnosis, found that Z motion abnormal voice, especially quick point, noise is more obvious.This judgment, mechanical aspects there may be a hidden danger.
(3) check the Z axis of the lathe precision.Pulse generator by hand movement Z axis, (will hand the pulse rate as the 1 x 100 place, every step that changes, motor feed 0.1 mm), cooperate with the dial indicator watching the movement of the Z axis.After the unidirectional movement accuracy to maintain normal positive movement as a starting point, hand every step pulse change, Z axis machine tool movement of the actual distance d = = d1 d2 = d3...= 0.1 mm, the motor running is good, good localization accuracy.And return machine, on the change of actual displacements can be divided into four stages: (1) machine tool movement distance d1 > d = 0.1 mm (slope is greater than 1);(2) show for d = 0.1 mm > > d2 d3 (slope is less than 1);(3) the actual machine tool is not move, show the most standard reverse clearance;(4) machine tool movement distance and hand vein (slope equal to 1) the given values are equal, return to the normal movement of machine tool.

No matter how to compensation of backlash parameters (1851), the show is characterized by: except the first stage (3) can compensate other paragraphs changes still exist, especially in the first stage (1) seriously affect the machining accuracy of machine tool.Found in compensation, the bigger the gap compensation, the first (1) section of the moving distance.
Analysis of the above checks, CNC technicians trained think there are several possible reasons: one is the motor of abnormal;2 it is mechanical failure;3 there is a certain gap.In order to further diagnosis fault, off the motor and screw completely, inspect the electrical and mechanical parts, respectively.Motor running normally;In mechanical parts found in diagnosis, plate moving screw by hand, the initial return movement has very obvious gap.But under normal circumstances, should be able to feel the bearing an orderly, smooth movement.After overhaul found that its bearing is damaged, and have a ball fall off.After the replacement machine back to normal.
3. Machine tool electric motor running abnormal parameters optimization
An nc vertical milling machine, configuration FANUC0 - MJ CNC system.In the process of machining, found that the precision of the X axis.If there is a certain gap, inspection found that the X axis and exist when the motor first starts, unstable phenomenon.Touch the X-axis motor feel serious jitter, and stopping time is not obvious, worldwide mode is more obvious.
Analysis, there are two reasons for failure, a mechanical backlash is larger;The second is the X axis motor abnormalities.Using FANUC system parameter function, debugging on the motor.First of all the existing gap compensation;Servo gain adjustment parameters and N pulse suppression function, a shaking of the X axis motor, machine tools machining precision returned to normal.
4. Machine tool position loop exception or control logic
A TH61140 boring and milling machine processing center, numerical control system for FANUC18i, full closed loop control mode.Machining process, found that the Y axis machine tool precision, precision of minimum error at around 0.006 mm, maximum error can be up to 1.400 mm.Machine tools have been set up in accordance with the requirements of G54 workpiece coordinate system.In the MDI mode to G54 coordinate system running a program that is "G90G54Y80F100; (;", wait for after the machine running shown on the display of the mechanical coordinates for the "1046.605", record the value.Then in manual mode, the machine Y points move to any other location, again in the MDI mode to perform the above statements, after being machine stop, found the machine mechanical digital display coordinate value for the "1046.992", compared with the number displayed value after the first execution by 0.387 mm.According to the same method, the Y axis points to move to a different location, repeatedly execute the statement, digital display of uncertain value.With a dial indicator to test the Y axis, found that the mechanical position error with digital display shows the actual error, so as to think that the cause of the problem repeat positioning error is too large for the Y axis.The reverse clearance and positioning precision of the Y axis carefully check, back to compensate, had no effect.So doubt grating ruler and system parameters, etc. There is a problem, but why have such a big error, but does not appear the corresponding alarm message?Further examination found that the shaft for vertical shaft, when the Y axis to loosen, spindle box fell down, caused the error.
PLC logic control program for machine tools made changes, namely in the Y to loosen, the Y axis can load, first to loosen the Y axis;When the clamping, the first axis clamping, remove the Y axis enabled again.After adjusting the machine fault is resolved.