The production process of ordinary power graphite electrode is many, the cycle is long, the energy consumption is high, and the uncertainty factors are many; the quality accidents caused by the fluctuation of the external processing quality of graphite electrode and joint are common. Therefore, the mechanical processing control of the graphite product should be strengthened, and the processing precision, matching precision and reliability of the connection between the electrode and the joint should be emphasized.

The contact between the electrode end face and the gap are close to 0; the thread is all connected (face contact), and there is a reasonable gap between the other side and the tooth top. Such connection can ensure that all thread surfaces are uniformly stressed and the connection strength is greatly increased; in the process of use, the probability of breaking and loosening is reduced; The resistivity decreases after power on, and the gap decreases with thermal expansion, but it will not cause thermal collapse, and the electrode consumption will decrease and it is difficult to burn off.

In the actual process, due to the poor machining accuracy, the gap between the end faces after connection is large, the contact of thread teeth is not good, the resistance increases after power on, which is prone to local arc striking, burning damage or even burning off; the multi tooth connection becomes less tooth contact, and it is point contact, and the stress intensity decreases. The electrode is easily loose and broken due to the vibration after the power on; When the clearance between radial and tooth surface is large and small, the thermal expansion after power on will cause local hot crushing of the parts with too small clearance, and also greatly affect the connection strength, which aggravates the damage of burning, loosening and breaking.

Therefore, to realize the ideal connection, the processing quality of electrode and joint must be improved at the same time. Through the implementation of equipment transformation, the existing problems can be found and the process control strengthened, which can make the actual state infinite close to the ideal connection.

1. Quality problems of graphite electrode processing

The poor quality of graphite electrode processing mainly includes the following three aspects: 1) the machining accuracy of the thread of the conical hole of the electrode does not meet the standard. It is not guaranteed that any electrode is reliably connected with any joint of the same specification (i.e. keep interchangeability); 2) the outer return axis of the electrode body is not concentric with the axis of the joint hole. After connection, the axis appears "drift", the coaxiality can not be guaranteed, and the edge cannot be contacted; 3) the two ends of the electrode are not parallel or the center line of the body body of the electrode is not perpendicular to one end face. After the connection of the two electrodes, the axis appears "twisted", and the verticality can not be guaranteed. The end face is in tight contact with pressure on one side and the other side has a gap.

It is particularly necessary to point out that, due to the difference of working principle, American lathe is easy to cause two kinds of phenomena, 2) and 3), while the "Japanese" lathe mainly focuses on the processing of conical screw holes 131. The following mainly analyzes the problems that are easy to appear in "Japanese" lathe. The common quality problems in the process of machining the taper hole thread of electrode in "Japanese" lathe are as follows:

1) In the direction of flatness "ten", the so-called "one point is good, three points are not good" and "three points are good, one is not good";

2) Ellipsoid of threaded hole (i.e. generally referred to as "flat");

3) There is a small segment of thread tooth defect in the whole circle of conical hole;

4) Thread tooth is thin, serious can lead to "flower tooth";

5) The conical orifice and the bottom thread of the hole are normal at one end, and the other end is incomplete;

6) Abnormal tooth shape. After the connection with the electrode hole, periodic bright spots (commonly known as watermelon skin) appear on the thread surface;

7) Threaded hole "middle boss

2. Analysis of the causes

The quality of graphite electrode processing depends on the machine tool's performance to a large extent. Various problems of the machine tool itself may lead to the unqualified quality of the processed products. The following are some common problems of thread machining machine tool itself.

2.1 saddle bracket

1) Improper adjustment, support roller is high or low. On the premise of the same size of the outer circle of the electrode, if the supporting roller is high, the electrode processing end will rise, which is equivalent to increasing the cutting depth of thread processing, and the processed electrode thread hole will be larger; otherwise, if the supporting roller is low, the electrode processing end will sink and the processed electrode thread hole will be large. At the same time, because the axis of electrode is not consistent with the actual rotation center line, it will also affect the flatness control of the end face.

2) The idler is not flexible. The bearing is damaged or the idler is rubbing against the bracket on one side.

3) Worm gear is worn or damaged, and it sinks on both sides (or unilateral) after bearing. During the process of electrode rotation, the machined screw holes are oval because of the profiling of screw holes and the shape of the outer surface of the electrode at saddle bracket (especially the unilateral sinking). If the tooth surface even appears "parabola" mark after the joint is screwed.

2.2 ball screw pair

1) The gap between the service feed system.

2) The change of screw pitch error. After long-term use, due to the objective existence of mechanical wear, the clearance of mechanical transmission components and guide rail pair will inevitably increase, and the compensation value of ball screw will inevitably change. Once the wear or repair is not compensated manually in time, the original position accuracy is no longer in the state of the factory, which makes the processing accuracy greatly reduced. In the process of machining, the clearance of the two-axis screw exceeds the internal control standard, which can cause the screw hole (i.e. the degree of joint) to be large or small;

The gap between 2-axis screw exceeds the internal control standard, which can lead to the abnormal thread tooth pattern and other serious consequences.

2.3 machine guide

1) Normal wear. In the long-term use, because of the friction between the two contact surfaces, the friction pair surface has different degrees of wear, which affects the machining accuracy and production efficiency.

2) The contact surface is seriously ash accumulated. Lathe maintenance and lubrication are not in place.

2.4 chuck

1) The claw clamp is not tight. The three claws are not on a concentric circle, and the ash deposit of chuck is serious.

2) The air pressure is not enough. The compressed air has high moisture and low pressure. When processing thread, the claw does not clamp the electrode, which causes the electrode to move away from the original position or the electrode speed lags behind the tool walking phenomenon.

2.5 flying knife body and bearing seat

1) The spindle taper hole and the comb knife handle are not reliable. It is possible that the material and taper do not match and the handle is replaced improperly.

During the cutting process, the cone hole and the handle of the comb cutter will expand radially under the centrifugal force. The expansion increases with the increase of the rotation speed, and the contact and stress state are very complex between the mating surfaces. When the spindle taper hole and handle are of the same kind of material, the former expansion is always greater than the latter, which reduces the reliability of the connection; if the material properties of the spindle taper hole and the comb knife handle are not matched properly, the consequences will be more serious. The above changes are generally ignored at the common speed, but when the speed is high, the radial expansion caused by centrifugal force will reduce the contact stress between the connecting surfaces, which will lead to the change of the position of the comb tool handle relative to the spindle taper hole and make the tool bend under the action of radial cutting force, and the direct connection affects the machining accuracy and the surface roughness of the thread.

The radial clearance between the spindle taper hole and the comb handle increases with the increase of rotation speed in a square relation. On the whole contact cone, the change is uneven; moreover, the taper handle of the flying cutter body is long, so it is difficult to realize the full-length gap free coordination. The gap at the big end of the handle of the comb cutter is larger than that of other parts. The change of radial clearance is horn mouth shape. With the increase of the speed, the trend of the bell mouth is more obvious. The air between the two parts will not only cause the radial circle run out of the cutter, but also affect the dynamic balance of the tool system; The connection between the spindle and the handle of the comb cutter will be loose, which will cause the handle of the comb cutter to move towards the back of the spindle under the action of axial clamping force, and cause the axial positioning error of the handle. Therefore, generally only 70% contact with the front section is required, and the latter section will often have some air.

If the clearance between the spindle and the handle is offset by increasing the interference amount to ensure that the spindle taper hole and the comb knife handle are still in reliable contact at high speed, the excessive interference amount needs to pull the tool mechanism to produce a great pull force, which is very unfavorable for tool change, and also makes the end of the spindle expand, which has bad influence on the front bearing of the spindle.

2) The spindle bearing of the comb cutter is not adjusted properly. The main performance index of spindle components is rotation accuracy, rigidity and speed adaptability. It is particularly important to note that the precision of rotation depends on the manufacturing and assembly quality of the spindle and bearing. The clearance is too large, causing shaft to move; the clearance is too small, and the bearing is hot.

3) The plane knife is not fixed firmly.

2.6 cutting tools

1) At present, the cutting tools (comb cutters) of the taper hole thread of electrode mainly include high-speed steel, 45. steel inlaid cemented carbide or all cemented carbide. In contrast, the durability of the comb cutter made of high speed steel is poor.

2) Deformation after grinding: the grinding of the cutter with cemented carbide comb cutter is generally first ground with artificial diamond grinding wheel, and then the whole surface is ground with ordinary grinding wheel. Because the expansion rate of the carbide part of 45 # steel inlaid carbide comb cutter is lower than that of the tool holder, if the control of cutting depth, cutting speed and tool amount is improper during grinding, after 4 times and 5 grinding, the comb cutter will be deformed. The processed electrode thread hole is obviously damaged by plug gauge (Fig. 2); Theoretical taper actual taper day

3) The comber and the flying knife body do not match, and the dynamic balance is poor.

2.7 electrode body

During the process, the electrode is beating out badly. The main reason is that the graphitization degree is not enough; the weight is relatively light.

2.8 others

1) Insert (plug). Mainly wear or deformation, causing looseness or crawling.

2) Pressing plate (Crochet). After wear, the machining will produce jitter.

3. Solutions

1) The error compensation function of the NC system is used to compensate the pitch and pitch error of servo feed system in a limited range.

2) The machine tool shall be inspected regularly, and the guide rail and other friction pairs shall be scraped and repaired manually to keep the proper accuracy.

3) Check the running state and lubrication of the spindle bearing of the comb cutter frequently, adjust the bearing clearance, and select the grease reasonably according to the seasonal change.

4) Strictly control the process, and effectively implement the re inspection and confirmation system of the first electrode quality inspector, and increase the self inspection frequency of the operators.

5) The whole process management, periodic inspection and forced scrap of the comb knife for thread processing are carried out.

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