Different types of stainless steel have different CNC cutting difficulties due to different mechanical properties and chemical compositions. Some stainless steels are difficult to achieve satisfactory surface roughness during cutting; while some stainless steels are easy to achieve the required surface roughness, but the tools are particularly easy to wear during the cutting process.
After summarizing, the main reasons why all kinds of stainless steel are difficult to cut are as follows:
High thermal strength and toughness are not suitable for CNC high-speed cutting. Austenitic stainless steel and martensitic stainless steel have low hardness and tensile strength, only equivalent to No. 40 steel, but elongation, section shrinkage and impact value are not high. relatively high.
For example, the elongation of 1Cr18Ni9Ti is 210 (percentage) of No. 40 steel, so it is not easy to be cut off in the process of CNC high-speed cutting, and the work consumed during cutting deformation is quite large.
Relatively speaking, the strength of stainless steel decreases less at high temperature. For example, the lasting strength of No. 45 steel is 7kg/mM2 at 500°, while the lasting strength of 1Cr18Ni9Ti remains at 19-24kg/mM2 at 550°.
Practice has proved that under the action of the same cutting temperature, stainless steel machining is more difficult to process than ordinary carbon steel, and its high thermal strength is an extremely important factor.
The strong tendency of work hardening is unfavorable for CNC turning. In the process of CNC high-speed turning, due to the extrusion of the tool tip to the workpiece material, the metal in the cutting area is deformed, the intragranular slip occurs, the lattice is distorted, the structure is dense, and the mechanical properties are also improved. With the change, the general cutting hardness can also increase by 2 to 3 times.
The depth of the work-hardened layer after CNC cutting can vary from tens of microns to hundreds of microns, so the work-hardening phenomenon produced by the previous pass hinders the cutting of the next pass, and the high hardness of the work-hardened layer causes the tool Especially easy to wear.
The strong adhesion of chips and poor thermal conductivity have an impact on CNC cutting. During the CNC cutting process, the cutting chips are easily adhered or fused to the tool tip and the blade edge, forming a built-up edge, causing the surface of the workpiece to be machined. Roughness deteriorates, while increasing vibration during cutting and accelerating tool wear.
Moreover, a large amount of cutting heat cannot be conducted in time, and even the heat generated by cutting cannot be conducted to the whole chip, resulting in the total heat input to the tool 3 to 5 times more than that of ordinary carbon steel, causing the cutting edge to lose cutting performance at high temperatures. . In the process of CNC cutting, a large amount of heat energy generated cannot be quickly discharged, and will inevitably be transmitted to the tool, which will increase the temperature of the cutting part.
At the same time, due to the difficulty of chip removal, especially the continuous chipping, the chipped chips are crowded, especially the inner hole is processed, and the chip jamming is more serious. In addition, due to the limitation of the shape of the thread section and the poor strength of the tool itself, it is easy to generate vibration during processing, and the tool tip is easily burned out due to excessive local temperature or cracked due to excessive vibration during the cutting process.
Problems and countermeasures of CNC cutting stainless steel tools
The surface roughness of the thread after CNC cutting is too poor, the phenomenon of fish scale mottled corrugation and knife gnawing are the most common phenomena encountered in cnc machining stainless steel. The reasons for these phenomena are:
- The clearance angle of the two sides of the thread turning tool is too small, and the friction between the two sides of the blade and the rear thread surface deteriorates the machined surface. The influence of the thread rotation angle on the actual clearance angle of the two sides must be considered during processing.
- The rake angle of the thread turning tool is too small, the cutting edge is not sharp enough, and the chips cannot be cut off smoothly, but are partially squeezed or torn off, which must cause the thread surface to be very rough. When the rake angle is too large, the strength of the blade is weakened, and it is easy to wear, crack, and tie the knife, which is more likely to cause vibration and cause ripples on the thread surface. Therefore, the appropriate rake angle should be selected according to the different materials of stainless steel. When turning stainless steel threads resistant to concentrated sulfuric acid, a smaller rake angle should be used than 2Cr13 cnc turning stainless steel threads, and the cutting edges on both sides of the turning tool should have narrow margins to avoid rapid wear of the cutting edges. When cutting 2Cr13, 1Cr17, 4Cr13, the cutting edge should be as sharp as possible, otherwise it is not easy to achieve the required good surface roughness.
- The blade of the thread cutter is blunt, and the actual rake angle and relief angle change from large to small, and the chips are severely squeezed during the formation process, which increases the cutting force during the cutting process and intensifies the vibration, and the machined surface deteriorates seriously. Therefore, when turning stainless steel threads, the cutting edge must be kept sharp at any time, and the cutting head must be replaced in time.
- The thread turning tool is not fixed firmly, the tool head is extended too long, the rigidity of the tool holder is not enough, or the machine tool accuracy is poor, the spindle is loose, the tool holder is loose and other factors will cause vibration and cause ripples on the thread surface. Therefore, care must be taken to operate the machine tool, tool and workpiece during operation, so that the system has sufficient rigidity. When installing the turning tool, in addition to ensuring that the installation is firm and not loose, the tip of the turning tool should be slightly higher than the center of the workpiece by 0.2 to 0.5mm, and must not be lower than the center to avoid sticking the tool.
- The straight-forward method should be avoided when turning threads. Due to the long contact length of the chips on the left and right sides, it is easy to generate vibration, which increases the load on the tool tip, causes vibration and increases the resistance during chip removal, and scratches the machined surface. hurt. Therefore, for the processing of stainless steel threads, it is best to use the method of cross-cutting threads. This method adopts alternate side feed, especially for the cutting of large pitch threads and viscous materials, which is the most effective measure to solve the vibration problem. Since the cutting edges are used crosswise on the left and right, the wear is uniform and the service life of the tool can be extended.
- The matching degree of cutting amount in the process of CNC thread cutting directly affects the processing efficiency. If the cutting amount is too small, it will increase the wear of the tool, and if the cutting amount is too large, the tool will be broken, so the number of feeds and the amount of feed per cut will have a decisive influence on the turning thread.
For optimum tool life, the workpiece diameter should not be greater than 0.14mm larger than the major thread diameter, and feeds less than 0.05mm per tool should be avoided. The total cutting amount of processing should be set at about 0.1mm, the first depth of cut should be 150 ~ 200 (percent) of the nose radius (R), and the maximum should not exceed 0.5mm. For austenitic stainless steel, it should be avoided. If the feed per tool is less than 0.08mm, the ordinary edge inserts with small nose radius for internal threading may increase the number of cuts as the depth of the tool decreases.
After CNC cutting, the thread size is unstable. After thread processing, the “through end” of the external thread cannot be measured with a thread ring gauge, or there are phenomena such as inconsistent tightness before and after and the “stop end” part passing through. The reasons for these ills:
- The thread profile is incorrect. Even if the pitch diameter of the thread has reached the specified size, the thread ring gauge and plug gauge may still not be screwed.
- Thread undercut. When measuring with a thread gauge, there is often a phenomenon that is limited by directionality, that is, it is tightened from one end and loosened from the other end, and even the “through end” fails to pass but the “stop end” passes Phenomenon.
- If the bottom diameter of the internal thread is too small, or the bottom diameter of the external thread is too large, the thread gauge will not be screwed in. This is because the turning tool wears and becomes dull, and there is extrusion during the cutting process, which makes the outer thread of the thread. The result of extruding burrs on the diameter or inner diameter.
- When turning an internal thread with a small diameter, the rigidity of the turning tool holder is limited by the size, and it is easy to produce “bend the tool” during the turning process, and even the large size of the four parts causes local over-tolerance.
- When turning a slender screw, due to the poor rigidity of the workpiece, deformation occurs during the turning process, resulting in a dimensional error on the thread.
- When turning the internal and external threads of thin-walled workpieces, local deformation of the workpiece will occur due to the influence of force and cutting temperature, and local tolerances of the threads will also occur. Therefore, in order to solve the problem of “the thread gauge cannot enter”, corresponding measures must be taken for the above reasons, mainly from the aspects of correct installation and suitable workpiece clamping.
The Subtle Effect of Cooling Lubricant in NC Cutting Thread
Reasonable use of lubricating fluid can improve cutting conditions and achieve multiplier effect with half the effort. When CNC cutting stainless steel threads, pay attention to:
(1) To understand the special requirements of cooling lubricants
- a. Due to the high toughness of stainless steel and the inability to be separated during cutting, the coolant is required to have high cooling performance to take away a large amount of heat.
- b. Due to the high viscosity and high fusion, it is easy to generate built-up edge in the thread cutting process, so the coolant should have high lubricating performance.
- c. The coolant is required to have good permeability, which can penetrate into the fine crevice line of the metal area during cutting, so that the chips can be easily broken off.
- d, also have a certain washing function.
(2) Take several suitable coolants
- a. Vulcanized oil has good cooling performance and lubricating performance, and can be divided into direct and indirect according to different preparation methods. The formula of direct fluidized oil is: mineral oil 98 (percent), sulfur 2 (percent). The formula of indirect sulfurized oil is: mineral oil 78 (percent) to 80 (percent), black motor oil, vegetable oil 18 (percent) to 20 (percent), sulfur 1.7 (percent).
- b. No. F43 engine oil is suitable for cooling and lubricating fluid for stainless steel cutting, and has the best effect on stainless steel threads. Its formula is: No. 5 high-speed motor oil 83.5 (percent), calcium petroleum sulfonate 4 (percent), barium petroleum phosphate 4 (percent), oxidized petroleum barium soap 4 (percent), dioxane Zinc thiophosphate 4 (percent), disulfide 0.5 (percent).
- c. Vegetable oil such as soybean oil is beneficial to obtain better thread surface roughness and prolong the service life of the tool when turning threads.