Machining method of NC turning stainless steel thread
The influence on NC machining
The difficulty of NC cutting of different stainless steel is different due to the different mechanical properties and chemical composition. Some stainless steel in the cutting process, it is difficult to achieve a satisfactory surface roughness; Some stainless steel, although easy to meet the required surface roughness, but in the process of cutting tools are particularly easy to wear. The main reasons for the difficulty of cutting stainless steel are as follows: 1
The high thermal strength and toughness are not suitable for NC high speed cutting. The hardness and tensile strength of austenitic stainless steel and martensitic stainless steel are not high, which is only equivalent to No. 40 steel, but the elongation, section shrinkage and impact value are relatively high. For example, 1Cr18Ni9Ti elongation is 210% of 40 steel, which makes it difficult to cut off in the process of NC High-speed Cutting, and the work consumed during cutting deformation is quite large. In contrast, stainless steel has less strength reduction at high temperature, such as steel 45 at 500 ° The results show that the endurance strength is 7kg/m2, while 1Cr18Ni9Ti is 550 ° The results showed that the lasting strength of the composite was still 19-24kg / m2. It is proved that under the same cutting temperature, stainless steel cutting is more difficult to process than ordinary carbon steel, and its high thermal strength is an extremely important factor.
The machining hardening trend is bad for NC turning. In the process of NC high speed turning, the metal in the cutting area is deformed due to the extrusion of the workpiece material by the cutting tip, the sliding in the crystal, lattice distortion, dense structure, and mechanical properties change with the change of mechanical properties. Generally, the cutting hardness can also increase by 2-3 times. The depth of hardened layer can vary from tens to hundreds of microns after NC cutting. Therefore, the hardening phenomenon of the previous cutting hinders the cutting of the next cutting, and the high hardness of the machining hardening layer causes the tool to wear easily.
The high adhesion and thermal conductivity of chip have an impact on NC cutting. In the process of NC cutting, cutting debris is easy to adhere or melt firmly on the cutting tip and blade, forming chip lumps, which causes the surface roughness of workpiece surface to deteriorate, and increase vibration during cutting process and accelerate tool wear. Moreover, a large number of cutting heat can not be transmitted in time, even the heat generated by cutting can not be transmitted to the chip as a whole, which results in the total heat of the incoming cutting tool 3-5 times more than that of the ordinary carbon steel, which makes the cutting edge lose cutting performance at high temperature. In the process of NC cutting, a large amount of heat energy produced can not be discharged rapidly, which will be transmitted to the tool, which will increase the temperature of cutting parts. At the same time, because of the difficulty of chip removal, especially continuous chip, the chip cut off will be jammed, especially for machining the inner hole, the chip jam is more serious. In addition, the cutting tool is easy to vibrate due to the limitation of thread section shape and its own poor strength. The tip of the tool is easy to burn out due to the high local temperature or crack due to excessive vibration.
Problems and solutions of NC cutting stainless steel tools
The surface roughness of thread after NC cutting is too poor, such as fish scale and pitting, are the most common phenomena in stainless steel thread turning. The reasons for these phenomena are: (1) the back angle of both sides of thread turning tool is too small, the friction between the two sides and the back thread surface makes the machining surface worse. The effect of thread rotation angle on the actual rear angle of both sides of the blade must be considered（ 2) The front corner of the thread turning tool is too small, the edge is not sharp enough, and the chip can not be cut smoothly, but it is partially squeezed or torn down, which will cause the surface of the thread to be very rough. When the current angle is too large, the strength of the blade is weakened and easy to wear, crack and tie the knife, which is more likely to cause vibration and cause the ripple on the thread surface. Therefore, the proper front angle should be selected according to the different materials of stainless steel. When turning stainless steel thread for sulfuric acid resistance, the front angle should be smaller than that of 2Cr13 stainless steel thread. The cutting edge on both sides of the lathe tool should be equipped with narrow blade belt to avoid the sharp wear of the edge. When cutting 2Cr13, 1Cr17 and 4Cr13, the cutting edge should be as sharp as possible, otherwise it will not be easy to achieve the required good surface roughness（ 3) The cutting blade of thread cutter is dull, the front and rear angles in fact change from big to small, and the chip is squeezed seriously in the formation process, which makes the increase of cutting force and vibration and the surface deterioration serious. Therefore, when turning stainless steel thread, the blade must be kept sharp at any time and the tool head should be replaced in time（ 4) The vibration will be caused by the factors such as the fixed screw tool, too long extension of the cutter head, the rigidity of the tool rod, or the poor precision of the machine tool, the loose spindle and the loose part of the tool frame, etc., which will cause the ripple on the thread surface. Therefore, it is necessary to pay attention to the operation of machine tools, tools and workpieces in order to make the system rigid enough. When installing the tool, except to ensure that the installation is firm and free from any looseness, the tip of the lathe tool shall be slightly higher than the center of the workpiece 0.2-0.5mm, and it shall not be lower than the center to avoid the knife binding（ 5) The straight in method should be avoided when turning thread. Because the contact length of the chips on the left and right sides is long, it is easy to produce vibration, which makes the load on the blade tip increase, causes vibration and increases the resistance when removing the chip, and scratches the processing surface. Therefore, the best way to machining stainless steel thread is to turn the thread by cross. This method adopts alternate lateral feed, especially for cutting large pitch thread and viscous material, which is the most effective measure to solve the vibration problem. Because the cutting edge is used in the left and right cross, the wear is even and the service life of the tool can be prolonged（ 6) The matching degree of cutting amount directly affects the efficiency of machining in the process of NC cutting thread. Too small cutting quantity will cause the tool to wear more, too large will cause the tool to break, so the number of feed and feed per tool will have a decisive impact on the turning thread. In order to obtain the best tool life, the diameter of workpiece should not be greater than 0.14mm of thread diameter, and the feed rate of each tool less than 0.05mm should be avoided. The total cutting amount of machining shall be set at about 0.1mm. The first cutting depth shall be 150-200% of the radius of the tip (R), and the maximum cutting depth shall not exceed 0.5mm. For austenitic stainless steel, the feed rate of each tool less than 0.08mm shall be avoided. The ordinary blade with small tip radius shall be used for internal thread. The cutting times may increase correspondingly with the reduction of the cutting depth.
After NC cutting, the thread size is unstable, and the thread ring gauge is used to measure whether the "through end" of the external thread can not go in or the front and rear tightening is inconsistent and the "end stop" part passes.
The causes of these disadvantages are:
(1) The thread is not in the right shape. Even if the thread diameter has reached the specified size, the thread ring gauge and plug gauge may not be screwed.
(2) Thread reversed. When measuring with thread gauge, there is often a phenomenon of directionality restriction, that is, the screw is tight from one end, and the other end is loose, even the phenomenon that "through end" is not passed but "stop end" passes instead.
(3) Too small for the inner thread bottom diameter car or too large for the external thread bottom diameter, the thread gauge can not be screwed in. This is due to the blunt wear of the lathe tool and the extrusion phenomenon during the cutting process, which results in the burr extrusion of the outer diameter or inner diameter of the thread.
(4) When turning the inner thread with smaller diameter, the rigidity of the tool rod is poor due to the size limitation, so it is easy to generate "let the knife" in the turning process, and even the larger four parts size causes local over tolerance.
(5) When turning slender screw, due to the poor rigidity of the workpiece, deformation occurs in the turning process, resulting in the size error on the thread.
(6) When turning the inner and outer threads of thin-walled workpiece, the workpiece will deform locally due to the influence of the force and cutting temperature, and also will cause the local deviation of the thread. Therefore, to solve the problem of "thread gauge cannot go in", we must take corresponding measures for the above reasons, mainly from the correct installation and the proper installation of workpiece.
The subtle role of coolant in NC cutting thread
Reasonable use of lubricating fluid can improve cutting conditions to achieve the effect of twice the effort.
When NC cutting stainless steel thread, attention should be paid to:
Understand the special requirements for the coolant.
(1) Because of the toughness of stainless steel and the difficulty of separation in cutting, it is required that the coolant should have high cooling performance to take away a lot of heat.
(2) Because of its high viscosity and high fusibility, chip deposits are easily produced in the process of cutting thread, so the coolant should have high lubrication performance.
(3) It is required that the coolant has good permeability, and can penetrate into the micro gap line of metal area in cutting, so that the chip can be easily broken.
(4) It also has some washing function.
Take some suitable coolant
(1) Vulcanized oil has good cooling performance and lubrication function. According to the different preparation methods, it can be divided into two types: direct and indirect. The formula of direct fluidization oil is: mineral oil 98%, sulfur 2%. The formula of indirect vulcanization oil is: mineral oil 78% ~ 80%, black oil and vegetable oil 18% ~ 20%, sulfur 1.7%.
(2) F43 oil is suitable for the cooling lubricant for stainless steel cutting, and the best effect is for stainless steel thread. The formula is: 83.5% of high speed oil 5, 4% calcium petroleum sulfonate, 4% barium phosphate, 4% barium oxide soap, 4% zinc dithiophosphate, 0.5% disulfide.
(3) Vegetable oil such as soybean oil can obtain better surface roughness and prolong the service life of the tool when turning thread.
In conclusion, whether it is martensitic stainless steel, ferritic stainless steel or austenitic stainless steel, or even austenitic + ferritic stainless steel, corresponding processing methods should be formulated according to different metallographic structure to achieve ideal processing effect and create more economic and social benefits.
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