Strong concealment of the causes of the abnormal processing precision fault, diagnosis difficulty is bigger, summarizes the five main reasons: machine tool feed unit is changed or changes;Each axis machine tool of zero offset exception;Axial backlash exception;Abnormal motor running state, i.e., electrical and control part exception;Mechanical failure, such as screw, bearing, coupling device shaft parts.Another procedure formulation, processing cutting tool selection and artificial factors, also may lead to abnormal processing precision. 


First, the cause of machining precision abnormal failure
 
Strong concealment of the causes of the abnormal processing precision fault, diagnosis difficulty is bigger, summarizes the five main reasons: machine tool feed unit is changed or changes;Each axis machine tool of zero offset exception;Axial backlash exception;Abnormal motor running state, i.e., electrical and control part exception;Mechanical failure, such as screw, bearing, coupling device shaft parts.Another procedure formulation, processing cutting tool selection and artificial factors, also may lead to abnormal processing precision. 
 
Second, the principle of nc machine tool fault diagnosis
 
1. After the first external internal nc machine tools is the collection of mechanical, hydraulic, electrical, as one of the machine tool, so the failure will be reflected by the three integrated.Maintenance personnel should be outside-in on a screen, and try to avoid random unsealed, disassembly, otherwise it will enlarge the fault, make the loss of machine tool accuracy, reduce performance. 
 
2. After the first mechanical electrical in general, mechanical failure find more easily, while the numerical control system fault diagnosis is difficult.Before troubleshooting, first of all, pay attention to rule out a mechanical fault, often can achieve twice the result with half the effort. 
 
3. The first static after the move, first under the machine the stationary state of power, by understanding, observation, testing, analysis, identified as non destructive failure after, can give a machine electricity;Under the operation condition for dynamic observation, inspection and testing, to find fault.For destructive failure, must first, after eliminating the risk before electricity. 
 
4. After the first simple when there is a variety of complex fault interweave each other, at that time do not know how to start, should first solve the problem of easy, after solve difficult problems.Often after solve simple problem, difficult problem may also be easier. 
 
Three, numerical control machine tool fault diagnosis methods
 
1. The intuitive method: listening, asking and q - machine failure phenomenon, processing conditions, etc.;Look at - CRT alarm information, alarm indicator light, capacitor element, such as deformation smoky burning, protectors tripping, etc.;Listen - abnormal sound;Smell - electrical components and other anxious burnt flavour peculiar smell;Touch - fever, vibration, poor contact, etc. 
 
2. Method: parameters is usually stored in RAM, sometimes the battery voltage is insufficient, system without power for a long time can make parameter is missing or confusion or external interference, should according to the characteristics of the fault inspection and calibration of relevant parameters. 
 
3. The isolation method: some of the fault, is difficult to distinguish between CNC parts, or servo system and mechanical parts, isolation method is often adopted. 
 
4. Switch the similar method with the function of the standby board replace the suspected fault templates, or exchange will feature the same template or unit. 
 
5. Function test method for G, M, S, T, the function of all instructions write a small program, in the diagnosis of fault running these programs, the lack of judgment function.

Fourth, the machining accuracy abnormal fault diagnosis and process instance
 
1. Mechanical failure resulting in abnormal processing precision
 
Fault phenomenon: a SV - 1000 vertical machining center, using Frank system.In the process of connecting rod processing mold, suddenly discovered that the Z axis feed exception, at least 1 mm cutting error amount (Z direction). 
 
Fault diagnosis: investigation understands, fault happened suddenly.Machine in point, in the way of manual input data each axis running under normal operation, and back to the reference point is normal, no any alarm prompt, electrical control part hard fault rule out the possibility.Should be the main inspection on the following aspects one by one. 
 
Check the accuracy of machine tool when abnormal running process and especially the tool length compensation, processing coordinate system (G54 - G59) proofreading and calculation. 
 
In inching mode, repetitive motion Z axis, after see, touch, listen, the motion state diagnosis, found that Z motion abnormal noise, especially the quick point, noise is more obvious.This judgment, mechanical aspects there may be a hidden danger. 
 
Check the Z axis of the lathe precision.Mobile Z axis with hand pulse generator, (the ratio was 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.In one-way movement keep positive movement as a starting point, after every step pulse change, Z axis machine tool movement of the actual distance d = = d1 d2 = d3 =...= 0.1 mm, the motor is running well, positioning precision is also good.And return machine, on the change of actual displacements can be divided into four stages: (1) the machine movement distance d1 > d = 0.1 mm (slope is greater than 1);(2) show for d1 = 0.1 mm > > d2 d3 (slope is less than 1);(3) the machine body actually didn't move, show the most standard reverse clearance;(4) machine tool movement distance and pulse by the fixed value equal to (slope equal to 1), back to the normal movement of the machine tool.No matter what the reverse clearance compensation, its performance is characterized by: in addition to (3) phase compensation, changes in other paragraphs still exist, especially (phase 1) seriously affect the machining accuracy of machine tool.Compensation, compensation gap, the greater the (phase 1) moving distance. 
 
Analysis of the above check think there are several possible reasons: one is the motor have abnormalities, 2 it is mechanical has fault, three is screw clearance.In order to further diagnosis fault, off the motor and screw completely, inspect the electrical and mechanical parts, respectively.Check result is motor running normally;On mechanical parts found in diagnosis, plate moving screw by hand, the initial return movement has a great sense of vacancies.But under normal circumstances, should be able to feel the bearing an orderly, smooth movement. 
 
Troubleshooting: after remove the inspection found that the bearing is damaged, and a ball fall off.After the replacement machine back to normal. 
 
2. Control logic wrong lead to abnormal processing precision
 
Fault phenomenon: a machining center of Shanghai machine tool manufacturers, system is Frank. The machining process, found that the precision of the X axis machine tool is unusual, minimum error precision is 0.008 mm, 1.2 mm. The maximum fault diagnosis: inspection, machine tool has been set up in accordance with the requirements G54 workpiece coordinate system.Under the operate manual input data, with G54 coordinate system running a program that is "GOOG90G54X60. OY70. OF150; (;", wait for after the machine running shown on the display of the mechanical coordinates (X) "1025.243", to record the value.Then in manual mode, the machine tool point move to any other location, again under the way of manual input data operation run just program section, after being machine stop, found that the machine tool coordinate numerical display as "1024.891", compared to last time after the execution of the numerical difference between the 0.352 mm. In the same way, the X axis point move to move to a different position, to perform the procedures section, and shown on the display value is different (unstable).Check with a dial indicator on the X axis, found the actual mechanical position error with digital display, to think that the cause of the problem repeat positioning error is too large for the X axis.To check the backlash and positioning precision of the X axis, to compensate for the error value, the results have 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 X axis to loosen spindle box fell down, caused the error. 
 
Fault handling: PLC logic control program for machine tools made changes, namely in X axis to loosen, X can make loading first, then loosen the X axis;In X axis clamping, the X axis clamping, first remove the enabled again.After adjusting the machine fault is resolved. 
 
3. Lead to abnormal processing precision machine tool location problem
 
Fault phenomenon: a vertical CNC milling machine made in hangzhou, with Beijing KND - 10 m system.In the process of point or processing, detect the Z axis.