As shown in Figure 3, the two dial gauges are fixed on the machine tool holder, so that the dial gauge probe touches the cylindrical surface of the cutter bar. Rotate the hob and check for a and b, which allows the amount of runout to be determined by the machine's machining accuracy. For example, a hobbing machine with a machining accuracy of 6 is allowed to have a maximum runout amount a of 0.005 mm and a b of 0.002 mm.
Hob installation and adjustment
The hob is installed according to the accuracy of the gear, and the hob with a relatively high precision is selected. Generally, the gear of the 6th grade or higher must use the AA precision hob. Use the adjusting washer to adjust the position of the hob relative to the center of the machine and clamp it. The clamping of the hob should be achieved by the static friction between the hob, the bushing and the end face of the adjusting shim. Even for hobs with axial keyways, in general, the keying only serves as a backup drive.
Hob adjustment For the machining of high-precision gears, the hob must be carefully aligned on the hob. As shown in Figure 4, the two dial gauges are fixed on the machine tool so that their contacts touch the straight faces on both ends of the hob. Rotate the hob to observe the beating of the dial gauge and eliminate the radial runout of the hob by adjusting the orientation of the hob, adjusting washer or bushing. The general requirement is that the maximum runout amount a is 0.006 mm and the b is 0.003 mm.
4 Effect of temperature on machining accuracy
The temperatures discussed here are machine temperature and ambient temperature. For large gears with long machining cycles, temperature changes have a serious impact on gear machining accuracy. During processing, changes in temperature cause a slight change in the position of each of the spurs relative to the workpiece. Practice has proved that the temperature has a great influence on the accuracy of the gear, especially the accuracy of the tooth direction, which will reduce the amount of contact spots of the gear meshing.
Generally, the CNC gear hobbing machine has the function of automatically adjusting the internal temperature of the machine and has a numerical display. Therefore, we must pay attention to the observation during processing, and focus on controlling the indoor temperature, it is best to keep the indoor constant temperature.
The hobbing process described in this paper is for high-precision gears. In practical applications, corresponding technical measures should be taken according to the specific conditions and different requirements of the workpiece, so as to avoid unnecessary costs.
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One of the most fascinating materials of our time is polycrystalline cubic boron nitride PCBN - the second hardest material after diamond. At CeramTec, PCBN is used as a cutting material from which indexable inserts are manufactured for machining.
PCBN is produced synthetically and is a "composite material" made of cubic boron nitride and a mostly ceramic binder phase. The ratio of cBN to binder content and the cBN grain size determine the application.
PCBN cutting materials are used in the machining of hard and/or highly abrasive workpiece materials. The high hardness and the extremely high hot hardness (up to temperatures above 1,000°C), the edge stability and inertness towards ferrous materials as well as the basically good chemical resistance give CBN cutting materials a high potential for use in machining. On the cutting material side, a distinction is made between high- and low-cBN cutting material grades.
PCBN (Polycrystalline Cubic Boron Nitride) blanks for cutting tools are composite ultrahard materials sintered by polycrystalline CBN and carbide alloy under ultra-high pressure and high temperature conditions. They combine the hardness, wear resistance, and thermal stability of CBN with the impact resistance and toughness of carbide alloy. PCBN blanks are highly suitable for cutting ferrous and non-ferrous metals with high hardness.
PCBN Composites and Inserts
Hans Super Abrasive Diamond Tool Co.,Ltd , https://www.hansuperabrasive.com