Hard Turning Is Not Difficult
The main advantages of hard turning is no need for grinding, but it has some factories continue to believe that turning over 45HRC hardness level of parts and precision grinding process to achieve feasible. ?
Hard turning goal is to chip away with at least 80% of calories to maintain parts of the thermal stability. Reasonable system of hard turning and grinding can reduce or even eliminate the tools associated with high costs and processing time is too long. Hard turning process with a reasonable surface finish available 0.0028mm, 0.0002mm roundness and diameter tolerance ± 0.005mm, so the accuracy of pre-hardened workpiece in the “soft turning” on the same machine can also be achieved, thus maximizing equipment utilization. However, some factories due to an error in selecting the blade (or rather less expensive blades), or do not know whether the machines used in rigid enough to withstand twice the pressure in the normal turning, making the hard turning process not fully play its high impact. Hardinge Inc. (Hardinge) application engineers TomSheehy under hard turning in the company’s specific application, that the hard turning to be focused on the following eight areas.
1. Workpiece Although 45HRC hardness is the starting point for hard turning, hard turning, but more often 60HRC hardness of the workpiece on. Hard turning material usually consists of tool steel, bearing steel, case-hardened steel and chrome-nickel alloy, corrosion resistant nickel-based alloys, tungsten cobalt chromium alloy and other special materials. According to metallurgy, hardness in the depth of cut within the deviation is small (less than 2 HRC) of the material may show that the best course of predictability. Most suitable for hard turning of parts with a smaller aspect ratio (L / D), in general, no support for the workpiece L / D ratio less than 4:1, there is support for the workpiece L / D ratio does not greater than 8:1. Despite the slender part with tailstock support, but due to excessive cutting pressure is still possible to cause vibration knife.
Order to maximize the system of rigid hard turning, should try to reduce the overhang. Extended length of the arbor tool no larger than 1.5 times the height.
2. Machine tool Machine stiffness determines the precision hard turning. Nearly 15 to 20 years almost every machine made a very good rigidity, to withstand the hard turning. In many cases, the overall situation of the machine age is more important than the use of large, well-maintained lathe can also be used for hard turning. ?
Hard turning machine in order to increase the stiffness and damping characteristics, Hardinge Machine Tools will be many advanced features for a turning center, including the base polymer composites increase with the spring chuck (so close to the workpiece spindle support) and directly with the spindle and the static pressure guide.
System to maximize mean to minimize the suspension of rigid, tool and parts extend out, and removed the film and the gap adjustment ring, the aim is to keep all components as close as possible turret. ?
3. Blade Although cubic boron nitride (CBN) inserts are expensive, but CBN blades best suited for hard turning. CBN blade cutting process can be interrupted to maintain the same orientation, in a continuous process to provide a safe cutting tool wear rate. When the hard turning process with a reasonable time, CBN inserts in addition to smaller than in the control area diameter tolerance grinding, other properties are second to none.
CBN ceramics as wear-resistant, it is generally not used for the processing tolerance less than ± 0.025mm. Ceramic is not suitable for intermittent cutting, and can not add coolant, because thermal shock can cause the blade fracture. The blunt edge of blade geometry is the inherent characteristics of ceramic materials, this characteristic makes the cutting force increases surface roughness decreased. In addition, the ceramic blade edge fracture could be catastrophic, it could lead to all the cutting edge can not be used. ?
Metal-ceramic (cubic titanium carbide) cutting the continuous carburization hardening material is very effective, although it did not have the wear resistance of CBN, but the blade in most cases will be in proportion to the wear and tear without breaking. ?
Blade rake angle and cutting force due to its small, often used in machine tool rigidity is not high on hard turning. The most reasonable application on the blade, it is recommended close cooperation with tool vendors, particularly in the early stage, to quickly achieve the best cutting speed. ?
4. Coolant The biggest problem is with or without coolant. For the intermittent gear like cutting parts, the best “dry turning”, or feeding and retracting the thermal shock when the blade is likely to lead to rupture. For continuous cutting, head turning in the dry heat generated in the process sufficient ductility (softening) pre-cutting area, thereby reducing the hardness to make it easy to cut. This phenomenon illustrates the increased speed dry cutting is beneficial. At the same time, no way of cooling fluid has obvious cost cutting advantages. ?
In continuous cutting, the cooling fluid may help to prolong tool life and improve surface finish. The key is to make the cooling fluid to reach the head, high pressure coolant is the best way to solve this problem, because it does not easily evaporate at high temperatures. In addition, accumulation of high pressure can reduce the chip, thereby reducing the chip block on the impact of the cooling flow to the head. Another option is to release into the coolant while the top and bottom of the blade to ensure continuous cooling fluid to reach segment. ?
If you use the coolant must be water-based. Completely match the hard turning process of the formation of the chip can take away 80% to 90% of calories (cutting the highest temperature region 1700 ° F). So hot when exposed to low light chip cooling oil, the whole process will likely have been completely destroyed. If the open-type machine on hard turning, appropriate personal protective equipment must be increased to prevent the operator being burned chip. ?
5. Process Because most of the heat generated by hard turning away from the chip, the chip before and after processing check whether the coordination of the entire process can be found. Continuous cutting, the chip should be was blazing orange, and wildly flowing out like a ribbon. If the chip cool hand of a basic compression fracture, that chip away the heat is normal. ?
6. Whitening layer
“Whitening layer (heat affected zone)” may be annoying to appear in the hard turning and grinding operations, which formed in the surface invisible layer of very thin (typically 1 m) of the crust. In the hard turning process of the formation of bleaching layer, generally because of blunt blade to cause excessive heat transfer components within. Whitening layer often formed in the bearing steel and bearing ring for the class to sustain the high contact pressure of the parts is very harmful, over time, bleaching layer may peel and cause bearing
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