Discussion on the quality of the machining center of high speed drilling and tapping machining center with graphite electrode thread processing machine tool

The production process of common power graphite electrode high speed drilling and tapping machining center, long cycle, high energy consumption, many uncertain factors; because of external processing quality fluctuation of graphite electrode and joints caused by quality accidents shall be machined so It is often seen., strengthen the control of graphite products, focus on processing precision, and electrode joint with the precision and the reliability of the connection.

The electrode surface contact reaching 100%, the gap tends to 0; screw side all fit (contact), the other side and tip there is a reasonable gap. This connection can ensure that all thread surface uniform stress, connection strength greatly increased; in the process of using, broken and loose a few rate reduce electricity; the decrease of resistivity, thermal expansion of the gap decreases, but does not produce hot break, electrode consumption decreased and difficult to burn.

In the actual machining process, the machining precision, the connection end clearance, thread contact is not good, increase the resistance power, easy to produce local arcing and burning even burned; many teeth connected into little tooth contact, and point contact, the force strength decreased, the vibration power of the electrode is easy to be loosened and broken; connecting radial tooth surface and the gap when energized, the thermal expansion will make the gap is too small part of the local hot collapse, also greatly affects the connection strength, aggravated damage loss, loose and broken.

Therefore, in order to realize the ideal connection, the processing quality of the electrode and the joint must be improved simultaneously. Through the implementation of the equipment transformation, the existing problems are found and the process control is strengthened, and the actual state can be completely closed to the ideal connection.

1. The quality problem of graphite electrode processing

The poor processing quality of graphite electrode is mainly shown in the following three aspects: 1) the machining precision of the tapered hole thread of the electrode does not conform to the standard. It is not possible to ensure that any electrode is reliably connected to any one standard joint, that is, keeping interchangeability. 2) the outer axis of the electrode body is not concentric with the axis of the joint hole. The connecting axis appears "drift", the coaxality is not guaranteed, the edge can not reach contact. 3) the two ends of the electrode are not parallel or the trunk line of the electrode body is not perpendicular to one end face. When the two electrodes are connected, the axis appears "twisted", the perpendicularity is not guaranteed, the end face is tightly pressed, and the other side produces the gap.

It is particularly necessary to point out that because of the difference of working principles, the "American type" lathe can lead to 2 phenomena, 3) two phenomena occur, while the "Japanese style" lathe problem is mainly focused on the processing of conical thread holes 131. The common quality problems in the processing of the taper hole thread of the "Japanese" lathe electrode are as follows:

1) the plane degree "ten" in the direction of the so-called "good one, three points bad" and "three points good, a little bad";

2) the threaded hole ellipsoid (that is, the usual "flat");

3) the conical hole has a small segment of threaded tooth defect on the whole circle of the conical hole.

4) the threaded teeth are thin and can lead to "flower teeth" when they are serious.

5) the conical hole and the bottom thread of the hole are normal, and the other thread is incomplete.

6) abnormal tooth type. The periodic bright spot (commonly known as "watermelon skin") appears on the tooth surface of the thread after the joint is connected with the electrode hole.

7) "convex screw hole gas

2. Analysis of the causes

The quality of graphite electrode depends largely on the performance of machine tools. The various problems of machine tools may lead to the unqualified quality of processed products. The following are some common problems in the thread machining machine itself.

2.1 saddle bracket

1) improper adjustment, the support wheel is high or low. The premise of processing uniform size circle on the electrode, if the support roller is high, cause the electrode processing end rise, equivalent to increase the depth of thread processing, electrode thread hole enlargement processing out; if the support roller is low, cause the electrode processing end sink, electrode thread hole processing out of the Yeda. At the same time, the plane degree control of the end face will be affected because the axis of the electrode is not consistent with the actual rotary center line.

2) the rotation of the wheel is not flexible. The bearing is damaged or the wheel is rubbed with one side of the bracket.

3) worm and worm wear or damage, after bearing double side (or single side) sinking. During the rotation of the electrode, the threaded hole formed by the thread hole copying and the shape of the outer surface of the saddle bracket part (especially the one side sink) is ellipse. If the joint is swirled, the tooth surface even appears the "parabolic" mark.

2.2 ball screw pair

1) the clearance of 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 between mechanical transmission elements and the gap between guide pairs will increase. The compensation value of ball screws will inevitably change. Once the wear or maintenance is not timely compensated, the original position accuracy is not in the state when the factory is just out of the factory, making the machining precision discounted. During the processing, the clearance of the two axis screw rod exceeds the internal control standard, which can cause the screw hole (that is, the degree of bonding) to be small and small.

The clearance of the 2 axis screw rod exceeds the internal control standard, which can lead to abnormal results of the abnormal type of threaded teeth.

2.3 machine tool guide

1) normal wear. In the long-term use process, there are varying degrees of attrition due to the friction of the two contact surfaces, which affects the machining accuracy and production efficiency.

2) serious accumulation of ash in the contact surface. Lathe maintenance, lubrication is not in place.

2.4 chuck

1) the claw is not tight. The three claw is not in a concentric circle, and the chuck has a serious ash accumulation.

2) the pressure is not enough. The compressed air is water and the pressure is low. When the thread is processed, the claw does not clamp the electrode, causing the electrode to move out of the original position longitudinally in the course of rotation or the electrode speed is lagging behind the knife.

2.5 flying cutter body and bearing seat

1) spindle taper and comb knife handle unreliable connection. It is possible that the material, the taper mismatch and the replacement of the handle of the knife are unsuitable.

In the process of cutting, taper of spindle and comb knife handle will occur under the action of centrifugal force in the radial expansion, increase the size of expansion with the rotating speed increases, with the surface showing a stress and contact state is very complex. Are used in the taper hole of the main shaft and handle similar materials under the condition of expansion of the former is larger than the latter, which reduces the reliability of the connection; if the taper of spindle and comb knife handle material properties, properly, the consequences will be more serious.

The change in speed is generally ignored frequently, but at higher speed, the centrifugal force generated by the radial expansion will reduce the coupling between surface contact stress, leading to comb knife handle the relative change of spindle hole position and makes the tool bends in the radial cutting force, directly affect the machining accuracy and surface roughness of thread degree.

Improve the taper of spindle and cutter handle connected with the rotating speed of the radial clearance square relation between growth and the contact cone, this change is not uniform; and the knife body taper shank is longer, it is difficult to achieve full length with no gap. Comb knife handle the gap at the end than in other parts of the large gap, changes of the radial clearance is in horn shape, and with the increase of rotational speed, bellbottom trend more obvious, the clothes will not only cause the cutter radial runout, influence of dynamic balance tool system; and the connecting shaft and the winner of the comb knife handle loose, cause comb knife handle in an axial clamping force to the spindle end mobile, axial positioning error caused by the handle. Therefore, it is generally only required to cooperate with the front section of 70% or more, and the latter will often have a certain air - air.

If by increasing the amount of interference to the gap between the handle and the spindle rotates at a high speed offset, in order to ensure the taper of spindle and cutter handle connected at high speed is still a reliable contact, excessive interference to broach mechanism has great tension on the knife is very unfavorable, will make the spindle end of the expansion. Have a bad effect on the spindle front bearing.

2) the adjustment of the spindle bearing of the combing cutter is not reasonable. The main index of the performance of the spindle is the adaptability of the rotation accuracy, stiffness and speed. It is particularly important to point out that the rotation accuracy is determined by the spindle and the quality of the manufacturing and assembly of the bearings. The clearance is too large to cause the shaft to move; the clearance is too small and the bearing is hot.

3) flat knives are not fixed.

2.6 cutting tool

1) at present, the main material of the cutting tool (carding knife) with the cone hole thread of the electrode includes high speed steel, 45. steel inlaid hard alloy or all hard alloy. In comparison, high speed steel carding knives have poor durability.

2) deformation tool grinding: inlaid with hard alloy combing knife sharpening general use artificial diamond grinding hard alloy, and then the whole surface grinding ordinary wheel. Because of the hard alloy 45# steel inlaid with hard alloy cutter the expansion rate is lower than the rest, in the grinding process, if the cutting depth, cutting speed and feed control properly, after 4 to 5 times after grinding, often leads to comb electrode deformation processed with screw holes plug detection obviously feel "convex" (figure 2) tooth breakage; taper taper on the actual theory

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

2.7 electrode body

In the process of processing, the electrode is beating badly. The main reason is that the degree of graphitization is not enough; the weight is lighter.

2.8 other

1) insert (Sai Tie). It is mainly worn or deformed, resulting in loosening or creeping.

2) pressure plate (Gou Tie). After wear, the processing produces jitter.

3. Measures to solve

1) using the error compensation function of the numerical control system, the distance and pitch error of the servo feed system are compensated in a limited range.

2) regular inspection of machine tools, manual scraping and grinding, repair of guide rail and other friction pairs, so that the accuracy of the machine should be maintained.

3) often check the running state and lubrication of the spindle bearing of the cutter, adjust the bearing clearance, and choose the grease according to the seasonal change.

4) strict process control, quality inspection personnel to review the first electrode confirmation system, increase the frequency of self operating personnel.

5) the whole process management of the thread cutting tool, periodic inspection, and compulsory scrapping.