How to solve the problem of stainless steel thread turning?

2019/08/01 14:00
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Different types of stainless steels have different CNC cutting difficulties due to their different mechanical properties and chemical compositions. Some stainless steels are difficult to achieve satisfactory surface roughness during machining. Some stainless steels are easy to achieve the required surface roughness, but the cutters are particularly prone to wear during the cutting process.

Thread is one of the common geometric features in mechanical engineering and is widely used. There are many processing techniques for threads, such as rolling and twisting based on plastic deformation, turning, milling, tapping and threading, thread grinding, thread grinding, etc. based on cutting.

Different types of stainless steels have different CNC cutting difficulties due to their different mechanical properties and chemical compositions. Some stainless steels are difficult to achieve satisfactory surface roughness during machining. Some stainless steels are easy to achieve the required surface roughness, but the cutters are particularly prone to wear during the cutting process. It is concluded that the main reasons why various types of stainless steel are difficult to cut are as follows:

1 high heat strength and high toughness

Austenitic stainless steel and martensitic stainless steel have low hardness and tensile strength, which is only equivalent to No. 40 steel, but the elongation, section shrinkage and impact value are relatively high, so it is not in the CNC high-speed cutting process. It is easy to be cut, and the work consumed when cutting deformation is quite large. Relatively speaking, stainless steel has less strength reduction at high temperatures. For example, 45 steel has a permanent strength of 7 kg/mm ​​2 at 500°, while 1Cr18Ni9Ti maintains a permanent strength of 19 to 24 kg/mm ​​2 at 550°. Practice has 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.

2 Strong work hardening trend

In the process of CNC high-speed turning, the metal in the cutting zone is deformed due to the extrusion of the workpiece to the workpiece. The crystal is slipped, the crystal lattice is distorted, the structure is dense, and the mechanical properties change with 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 several hundred microns, so the work hardening caused by the previous pass prevents the cutting at the next pass, and the high hardness of the work hardened layer leads to the tool. Particularly easy to wear.

3 Chips have strong adhesion and poor thermal conductivity

In the CNC cutting process, the cutting debris is easily adhered or fused to the cutting edge and the blade edge to form a built-up edge, which causes the surface roughness of the workpiece surface to deteriorate, and at the same time increases the vibration during the cutting process and accelerates the tool. abrasion. Moreover, a large amount of cutting heat cannot be transmitted in time, and even the heat generated by the cutting cannot be transmitted to the whole of the chip, resulting in a total heat of the incoming tool being 3 to 5 times higher than that of ordinary carbon steel, so that the cutting edge loses cutting performance at high temperature. . In the CNC cutting process, a large amount of thermal energy generated cannot be quickly discharged, and is inevitably transmitted to the cutter to raise the temperature of the cutting portion. At the same time, due to the difficulty in chip removal, especially the continuous chipping, the chips that are cut are congested, especially the inner holes are processed, and the chip congestion is more serious. In addition, due to the limitation of the shape of the thread cross section, the tool itself is inferior in strength, and vibration is easily generated during processing. The tool tip is easily burned due to excessive local temperature during the cutting process or is cracked due to too much vibration.

4 Reasons for the difference in thread roughness and countermeasures

The surface roughness of the thread after CNC cutting is too poor. The scaly corrugation and trowel phenomenon are the most frequently encountered phenomena in stainless steel thread turning. The reasons for these phenomena are:

(1) The back angle of the blade on both sides of the thread turning tool is too small, and the friction between the two sides of the blade and the back thread surface deteriorates the machining surface. The influence of the thread rotation angle on the actual back angle of the two sides of the blade must be considered during machining.

The front corner of the thread turning tool is too small, the cutting edge is not sharp enough, the chip can not be cut smoothly, but is partially squeezed or torn, which must cause the thread surface to be very rough. When the current angle is too large, the blade strength is weakened and it is easy to wear, crack, and burr, which is more likely to cause vibration and cause ripple 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 for concentrated sulfuric acid, a smaller front angle should be used than for turning 2Cr13 stainless steel threads. The cutting edges on both sides of the turning tool should have a narrow cutting edge to avoid rapid wear of the cutting edge. When cutting 2Cr13, 1Cr17, 4Cr13, the cutting edge should be as sharp as possible, otherwise it is not easy to achieve the desired good surface roughness.

The thread cutter blade is blunt, and the actual front and rear corners are greatly reduced. The chips are severely squeezed during the forming process, which increases the cutting force and increases the vibration during the cutting process, and the machining surface deteriorates severely. Therefore, when turning stainless steel threads, you must keep the sharpness of the blade at all times and replace the cutter head in time.

The thread turning tool is not fixed firmly, the cutter head is extended too long, the rigidity of the cutter bar is not enough, or the machine tool has poor precision, the spindle is loose, and the tool holder is loose, which causes vibration and corrugation on the thread surface. Therefore, it is necessary to pay attention to the operation of machine tools, tools and workpieces during operation to make the system rigid enough. When installing the turning tool, in addition to ensuring that the installation is firm and there is no looseness, the cutting tool tip should be slightly higher than the center of the workpiece by 0.2 to 0.5 mm, and must not be lower than the center to avoid tying the knife.

Straight-cut method should be avoided when turning the thread. 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. Therefore, it is better to use a cross-turning thread for the machining of stainless steel threads. This method uses alternating lateral feed, especially for large pitch threads and viscous material cutting, which is the most effective measure to solve the vibration problem. Since the cutting edges are used for the left and right crosses, the wear is even and the tool life can be extended.

The matching degree of cutting amount in the process of CNC cutting thread directly affects the processing efficiency. Too small a cut will cause the tool to wear out. If it is too large, it will cause the tool to break. Therefore, the number of infeeds and the feed per tool will have a decisive influence on the turning thread. In order to obtain the optimum tool life, the workpiece diameter should not be larger than the thread diameter 0.14 mm, and the feed per knife should be avoided less than 0.05 mm. The total cutting amount of machining should be set at about 0.1mm. The first cutting depth should be 150~200 (percent) of the tool nose radius (R), and the maximum should not exceed 0.5mm. For austenitic stainless steel, avoid it. For a feed per knife of less than 0.08 mm, a normal blade with a small nose radius for internal threads may increase the number of cuts as the depth of the tool decreases.

5 Reasons and countermeasures for unstable thread size of CNC machining

After the thread is machined, the threaded ring gauge is used to measure the "open end" of the external thread or the inconsistency of the front and rear tension and the passage of the "stop end" portion. The reasons for these ills:

The thread shape is wrong. Even if the thread diameter has reached the specified size, the thread ring gauge and plug gauge may not be able to be screwed.

Threaded teeth. When measuring with a thread gauge, there is often a phenomenon that is limited by the directionality, that is, it is tightened from one end and tightened from the other end, and even the "pass end" is passed but the "stop end" is passed. phenomenon.

If the internal thread bottom car is too small, or the outer thread bottom diameter is too large, the thread gauge will not be screwed in. This is due to the blunt wear of the turning tool and the squeezing phenomenon during the cutting process, so that the outer diameter or inner diameter of the thread The result of squeezing out the burrs.

When the internal thread with a small diameter is turned, the rigidity of the turning tool holder is limited due to the size limitation, and the “knives” are easily generated during the turning process, so that the four sizes are large and the local tolerance is large.

When the slender screw is turned, the rigidity of the workpiece is poor, and deformation occurs during turning, causing dimensional errors on the thread.

When turning the inner and outer threads of a thin-walled workpiece, the workpiece is locally deformed due to the force and cutting temperature, and the local tolerance of the thread is also generated. Therefore, in order to solve the problem of "thread gauge can not enter", it is necessary to take corresponding measures for the above reasons, mainly from the aspects of correct installation and proper loading of the workpiece.

6 The subtle effect of cooling lubricant in CNC cutting threads

Proper use of lubricating fluids can improve cutting conditions with less effort. Pay attention to the CNC cutting of stainless steel threads: to understand the special requirements of cooling lubricant

Due to the high toughness of the stainless steel and the difficulty in separating the cutting, it is required that the cooling liquid has a high cooling performance to take away a large amount of heat.

Due to the high viscosity and high meltability, it is easy to produce built-up edge during the cutting thread, so the coolant should have high lubricating performance.

The coolant is required to have good permeability and can penetrate into the micro-gap line of the metal zone during cutting to make the chip easily break off.

Also have a certain washing function.

7 Recommend several suitable coolants

Sulfurized oil has good cooling performance and lubricating function, and can be divided into direct and indirect according to different preparation methods. The formula of direct fluidized oil is: mineral oil 98 (percent) and sulfur 2 (percent). The indirect vulcanized oil formula is: mineral oil 78 (percent) ~ 80 (percent), black motor oil, vegetable oil 18 (percent) ~ 20 (percent), sulfur 1.7 (percent).

F43 oil is suitable for cooling lubricant for stainless steel cutting, and it is the best for stainless steel thread. Its formula is: No. 5 high-speed motor oil 83.5 (percent), petroleum calcium sulfonate 4 (percent), petroleum yttrium phosphate 4 (percent), oxidized petroleum saponin 4 (percent), dioxane Zinc thiophosphate 4 (percent), 0.5 (percent) disulfide.

Vegetable oils such as soybean oil help to achieve better thread surface roughness and extend tool life when turning threads.

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