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Chip breaking and burr control of Sandvik Coromant auto transmission parts processing
Chip control is a matter of life and death in every factory. The biggest concern of the cutting tool industry is how to consistently maintain the chip breaking capacity of ductile steels (such as SAE 1018, 1020 and 8620) and control the burrs very well. Improper chip control can cause downtime, parts scrapping or rework, and even damage caused by tool breakage and factory downtime. These are the bottom lines that the factory cannot tolerate.
In general, there are chip control issues in semi-finishing, finishing, and variable depth of cut applications, and most turning operations in the automotive transmission industry require chip control . Starting Henry Ford (Henry Ford) and William · C · Durant (William C. Durant) era, this problem has plagued all of the car manufacturers. Despite the highly advanced technology in the automotive and cutting tool industries, the processing of these ductile materials continues to be a headache for operators, engineers and production managers. From the existing experience of the automotive industry and the expected peak production level, the factory needs to solve this problem anyway in order to comply with the trend of the times, meet production needs and maintain profitability.
Physical properties and economic costs are the first considerations when solving chip control problems. The methods we have been using for efficient machining, including the use of tools with positive rake angles and the use of the largest tip radius, are not ideal for practical applications. Because although these solutions can make the chips thinner, thus extending tool life and increasing production efficiency, it also greatly increases the difficulty of chip breaking. In addition, many parts have surface roughness (Ra) requirements, and only a large tip radius can be used to achieve acceptable surface quality.
The slender swarf produced by turning ductile steel can seriously damage normal processing and production. If the chip is not broken well, the chips will wrap around the tool and cause downtime. If the machine is always down, and the manual removal of the chips is required, the operator will be very annoyed. This is not only a waste of time, but also the chip temperature is very high, the edge is extremely sharp, it is easy to hurt people. In addition, the chips can move along the workpiece, scratching the workpiece, and eventually the parts are scrapped or reworked. If the chip is too long to wrap between the cutting edge and the part, more problems will occur, because if the tool "recuts" the chip, it will cause a knife break.
In automated operation, chip control can also cause problems for material handling robots or online measurements because the chips can interfere with the performance of the robot and give incorrect meter readings.
Master operating parameters
In mass production processing environments, part processing includes castings and forgings. Material cutting is usually not a problem. To improve the chip control of these machining operations, four factors need to be considered, namely the tool nose radius, the cutting depth, the feed rate and the top groove shape of the blade.
Our goal is to maximize the chip area while maintaining part accuracy and maintaining production cycle times. In order to increase the chip area, the depth of cut should not be less than the tip radius. When the one-sided depth of cut is 0.010 inches and the blade tip radius is 0.031 inches, the chips become thinner by about 60%, so chip breaking becomes more difficult.
In the past, the one-sided depth of cut must never be less than 66% of the tool nose radius, because at this position all the chips are no longer thinned (the amount of thinning is a function of the tool nose radius and depth of cut). In order to increase the feed rate and increase the chip area, it is necessary to adjust the spindle speed. Spindle speed is the cutting factor that has the greatest impact on tool life. Although depth of cut and feed rate are also important, the impact is much smaller.
Of course, based on other experience, increasing the feed rate will reduce the surface quality. In this case, it is best to use a wiper. When the wiper blades were first introduced, they were only used to improve the surface quality. However, we have now realized that by increasing the feed rate of these wiper blades, both surface quality and productivity can be improved.
Using blade technology
Most of today's turning inserts have a groove shape that is pressed into the rake face for easy chip control. There are now thousands of trough types available, and the factory can choose the best trough depending on the application and part material. When processing ductile steels, only the special requirements of these materials for the inserts are met to form the appropriate chips.
In most cases, a narrower blade with a margin line or a major cutting edge should be used. In the case of a one-time pass with a variable depth of cut, a groove shape with a major cutting edge of 0.004" to 0.010" is usually used. The key at this point is to ensure that the feed rate exceeds the width of the main cutting edge. When the feed rate exceeds the width of the main cutting edge, the rake angle of the insert groove and the chip forming ability can be fully utilized. The rake angle is to cut off the material, rather than squeezing the material through the edge of the blade. When the feed rate of the machining is smaller than the width of the main cutting edge, heat is generated, which is detrimental to the tool life. Optimized feed rates and edge widths make full use of the chip forming capability of the groove. The different angles of the groove and the "bulge" are designed to discharge chips into the chip breaking structure for automatic chip breaking. In the case of low feed applications, these chip flutes or projections need to be close to the cutting edge line, especially at the blade tip radius.
The hardest chip breaking is to deal with the shortest chips, the shortest chips just appearing above the cutting edge radius. Therefore, when optimizing the cutting, the depth of cut and the radius of the tip must be adjusted. It is more practical to use a smaller tool nose radius than conventional and with a wiper.
In some cases (such as high-strength materials), the deviation from the main cutting edge may be more conducive to chip breaking. The negative cutting edge of the main cutting edge creates a "back wall" outside the leading edge of the cutting edge. In this case, this is the only way to control the chip.
Tips for success - correct programming
Whether it is chip breaking or tool "deburring", programming correctly will result in very different results. To give a simple example, the off-surface machining of external turning is the tool path we usually use. When we used the CNMG blade for surface machining (ie, using the shoulder surface away from the centerline of the workpiece), the lead angle was 85 degrees. This angle is larger than the lead angle of a common high feed milling cutter, which is typically 75 to 80 degrees. The precondition for high feed milling cutters is to thin the chips as much as possible to increase production efficiency at a feed rate of 0.040 to 0.060 inches per tooth. Thus, when the blade is processed away from the centerline, long strips of thin chips are formed. Instead, a better method is to work inward or from the outer surface toward the centerline of the workpiece. Because of this we can use the lead angle of -5 degrees to increase the chip thickness.
Another challenge in the processing of ductile materials is profiling and forming a radius of the arc. In most cases, we can set the machine parameters correctly and select the correct turning geometry to find the best chip breaking position for the outer and end faces. However, in profiling, these same parameters and the same geometry create uncontrolled chips. In such cases, it is sometimes necessary to first optimize a portion of the machining feature. For example, in the radius of the arc of the car hole, first inserting some points at the radius will facilitate chip breaking and obtain a small arc radius.
Keep cool
The appearance of high pressure cooling has a great influence on the chip control of ductile materials. A 1000 psi coolant can break the titanium alloy chips into a coarse powder. This technology has been tested on typical automotive steels but has not been successful because the pressure needs to be above 2000 psi for effective chip control. Typically, high pressure pumps provide pumps with a water-based coolant pressure of 1000 psi and a pure oil pressure of 2000 psi. However, in the near future, these companies will produce higher pressure pumps to solve these problems.
Controlling chips and avoiding burrs is not a new problem when processing ductile steel. The new challenge is whether cutting tool companies and factories can work together to solve these problems. Sometimes, when a new product goes public, it promises to solve all the problems, but in the end it is found that it is limited to certain applications or only some of the problems.
No factory wants to have unnecessary downtime, or wasted time and money due to scrap or rework, and does not want to throw away broken tools or endanger employee safety. Knowing how to efficiently process these materials with the right cutting tool parameters, using the latest insert geometry and high pressure cooling technology to increase productivity and profitability will help meet the ever-changing needs of today's and future automotive parts processing. .