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Discussion on Processing Quality of Graphite Electrode Comb Machining Machine

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Media coverage
Release time:
2018/03/27 18:45
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Ordinary power graphite electrodes have many production processes, long cycle, high energy consumption, and many uncertain factors.
Ordinary power graphite electrodes have many production processes, long cycle, high energy consumption, and many uncertain factors. Quality accidents caused by fluctuations in the external processing quality of graphite electrodes and joints are also common. Therefore, the mechanical processing of graphitized products should be strengthened. Control, focusing on the machining accuracy, fit accuracy and reliability of the joints of the electrodes and joints.
 
The electrode end face contact is close to 100%, the gap is close to 0; the thread side is all fitted (surface contact), and the other side and the tooth tip have a reasonable gap. This connection ensures uniform stress on all thread surfaces, and the connection strength It greatly increases; during use, the probability of breaking and loosening is reduced; after electrification, the resistivity decreases, and thermal expansion reduces the gap, but does not cause hot crushing, electrode consumption decreases and it is difficult to blow.
 
In the actual processing process, due to poor processing precision, the gap between the end faces after connection is large, the contact of the thread teeth is not good, the resistance is increased after energization, and local arcing, burning or even burning is easy to occur; the multi-tooth connection becomes less tooth contact. , and it is a point contact, the strength of the force is reduced, and the vibration after the energization, the electrode is easily loosened and broken; the radial and the tooth surface gap are large and small, and the thermal expansion after the electric power also causes the gap to be too small. Local hot crushing also greatly affects the joint strength and aggravates the damage of burning, loosening and breaking.
 
Therefore, in order to achieve an ideal connection, it is necessary to simultaneously improve the processing quality of the electrodes and joints. By implementing equipment retrofits, finding problems and strengthening process control can completely bring the actual state infinitely close to the ideal connection.
 
1, graphite electrode processing quality problems
 
The poor quality of graphite electrode processing is mainly manifested in the following three aspects: 1) The precision of the taper hole thread processing of the electrode does not meet the standard. It is not guaranteed that any one electrode can be reliably connected with any one of the same size joints (ie, it is interchangeable); 2) The external axis of the electrode body is not concentric with the axis of the joint hole. After the connection, the axis appears “drift”, the coaxiality is not guaranteed, and the edge contact is not; 3) the upper and lower end faces of the electrode are not parallel or the center line of the trunk of the electrode body is not perpendicular to one end face. After the upper and lower electrodes are connected, the axis is "twisted", the verticality is not guaranteed, the end face is pressed tightly, and the other side has a gap.
 
In particular, due to the difference in working principle, "American" lathes are likely to cause 2), 3) two phenomena, and "Japanese" lathe problems are mainly concentrated on the processing of tapered threaded holes 131, the main Analyze the problems that are likely to occur with “Japanese” lathes. The common quality problems in the processing of "Japanese-style" lathe electrode taper hole thread are as follows:
 
1) The so-called "one point is good, three points are not good" and "three points are good, one is not good" in the "ten" direction of flatness;
2) Threaded hole ellipsoid (that is, commonly referred to as "flat");
3) There is a small thread-shaped defect on the entire circumference of the tapered hole;
4) The thread type is thin and can cause "flower teeth" when it is serious;
5) The tapered hole and the bottom thread of the hole are normal at one end, and the thread top of the other thread is incomplete;
6) Abnormal tooth shape. After the joint is screwed to the electrode hole, periodic bright spots appear on the thread surface (commonly known as "watermelon skin");
7) Threaded hole
 
    2, the cause analysis
 
    The processing quality of graphite electrodes depends to a large extent on the performance of the machine tool. Various problems of the machine tool itself may lead to unqualified quality of the processed products. The following are some of the common problems with threading machines.
 
    2.1 saddle bracket
 
1) Improper adjustment, support roller is high or low. Under the premise that the outer circumference of the electrode is the same, if the supporting roller is high, the electrode processing end is raised, which is equivalent to increasing the depth of the thread processing, and the threaded hole of the processed electrode becomes larger; The lower the electrode, the sinking of the electrode is processed, and the threaded hole of the processed electrode is also large. At the same time, since the axis of the electrode is inconsistent with the actual center line of rotation, the flatness control of the end face is also affected.
2) The rotation of the roller is not flexible. The bearing is damaged, or the roller and the bracket are rubbed on one side.
3) The worm gear is worn or damaged, and the double-sided (or single-sided) sinks after bearing. During the rotation of the electrode, the machined threaded hole is elliptical due to the shape of the threaded hole and the shape of the outer surface of the electrode of the saddle bracket (especially the one side sinking). If the joint is screwed, the tooth surface even has a "parabola" mark.
 
2.2 Ball Screw Pair
 
1) The gap between the servo feed system.
2) Change in screw pitch error. After long-term use, due to the objective existence of mechanical wear, the gap between the mechanical transmission components and the gap of the guide rail pair must be increased, and the compensation value of the ball screw inevitably changes. Once the wear or repair is not compensated in time, the original positional accuracy is not in the state of the factory, which makes the machining accuracy greatly reduced. During the machining process, the clearance of the two-axis screw exceeds the internal control standard, which may cause the threaded hole (ie, the degree of bonding) to become large and small;
The 2-axis screw clearance exceeds the internal control standard, which can lead to serious consequences such as abnormal thread shape.
 
2.3 machine guide
1) Normal wear and tear. In the long-term use process, due to the different degrees of friction between the two contact surfaces, the friction pair surface has different degrees of wear, which affects the processing precision and production efficiency.
2) The contact surface is heavily dusty. Lathe maintenance and lubrication are not in place.
 
2.4 chuck
 
1) The jaws are not tight. The three claws are not on a concentric circle, and the chuck is heavily dusted.
2) The air pressure is not enough. Compressed air has large moisture and low pressure. When the thread is machined, the claw does not clamp the electrode, causing the electrode to longitudinally disengage from the original station during the rotation process or the phenomenon that the electrode rotation speed relatively lags behind the knife.
 
2.5 flying knife body and bearing seat
 
1) The coupling of the spindle taper hole and the comb shank is not reliable. Material, taper mismatch, and improper tool change are possible.
During the cutting process, both the spindle taper hole and the comb shank will expand radially under the action of centrifugal force. The amount of expansion increases with the increase of the rotating speed, and the mating surface presents a complicated contact state and Stress state. In the case where the spindle taper hole and the shank are made of the same type of material, the former's expansion amount is always greater than the latter, thereby reducing the reliability of the joint; if the material properties of the spindle taper hole and the comb shank are not properly matched, the consequences will be more serious. The above changes are generally neglected at the usual rotational speeds, but at higher rotational speeds, the radial expansion caused by the centrifugal force reduces the contact stress between the coupling faces, resulting in a change in the position of the comb shank relative to the spindle taper hole and the tool in the radial direction. Bending under the action of cutting force directly affects the machining accuracy and the surface roughness of the thread.
The radial clearance of the coupling between the spindle cone and the comb shank increases in a square relationship with the increase of the rotational speed, and the variation is uneven over the entire contact cone; plus the flying knives have a longer taper It is difficult to achieve full length without gap fit. The gap at the big end of the comb shank is larger than that of other parts, and the change of the radial gap is bell-shaped, and with the increase of the rotational speed, the trend of the bell mouth is more obvious, and the air drying will not only cause the radial rounding of the tool, Affecting the dynamic balance of the tool system; and loosening the connection between the main shaft and the comb shank, causing the comb shank to move toward the rear end of the spindle under the axial clamping force, causing axial positioning error of the shank. Therefore, it is generally only required to match more than 70% of the front section, and the back section tends to be aired.
If the gap between the spindle and the shank during high-speed rotation is offset by increasing the interference, to ensure that the spindle taper and the comb shank are still in reliable contact at high speed, the excessive interference requires the broach mechanism. Producing a large pulling force is very unfavorable for the tool change, and also causes the end of the spindle to expand, which has an adverse effect on the front bearing of the spindle.
2) The adjustment of the comb spindle bearing is unreasonable. The performance of the spindle components is mainly the rotation accuracy, stiffness and speed adaptability. In particular, the accuracy of the rotation is determined by the quality of the spindle and the manufacturing and assembly of the bearing. The clearance is too large, causing the shaft to move; the clearance is too small and the bearing is hot.
3) The flat knife is not fixed firmly.
 
2.6 cutting tool
 
1) At present, the cutting tool (comb) of the electrode taper hole thread is mainly made of high speed steel, 45. steel inlaid hard alloy or full hard alloy. In comparison, high-speed steel combs are less durable.
2) Deformation after tool grinding: The sharpening of the inlaid carbide comb usually first grinds the hard alloy part with a synthetic diamond wheel, and then smooths the entire surface with a common grinding wheel. Since the expansion rate of the hard alloy part of the 45# steel inlaid carbide comb is lower than that of the tool holder, in the grinding process, if the depth of the knife, the cutting speed and the amount of the cutting are not properly controlled, after 4 After 5 times of grinding, the comb is often deformed, and the threaded hole of the processed electrode is detected by the plug gauge. It is obviously “convex” (Fig. 2). The actual taper is the theoretical taper.
3) The comb knife does not match the flying knife body, and the dynamic balance is poor.
 
    2.7 electrode body
 
During the processing, the electrode beats badly. Mainly because the degree of graphitization is not enough; the weight is lighter.
 
2.8 other
 
1) Striped strips (plug iron). Mainly worn or deformed, causing loose or creeping.
2) Platen (hook iron). After wear, the processing produces jitter.
 
3, the solution
 
1) Using the error compensation function of the numerical control system, the pitch and pitch error of the servo feed system are compensated to a certain extent within a limited range.
2) Regularly test the machine tool, manually scrape and repair the guide rails and other friction pairs to maintain the proper precision.
3) Always check the running state and lubrication of the combshaft spindle bearing, adjust the bearing clearance, and select the grease according to the seasonal changes.
4) Strict process control, earnestly do a re-examination confirmation system for the first electrode quality inspection personnel, and increase the self-test frequency of operators.
5) Carry out the whole process management, cycle inspection and forced scrapping of the comb for thread processing.
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