High Speed Machining Of Hardened Steel With Micro Endmills

In this thesis, hardened steel cutting using coated carbide end mill of small diameter ( 2mm) has been investigated. Milling experiments were carried out respectively on mid-speed machining center (maximum rotating speed up to 8000rpm) and high-speed machining center (maximum rotating speed up to 20000rpm). Studies were made into the cutting mechanism, chip appearance , cutting forces of milling process, influence of cutting parameters on the milling process, chatter and noise during milling process, small diameter end mill tool wear and breakage pattern.In hardened steel milling using small diameter end mill, the chip appearance shows that there are apparent cut slippage and rather high cutting temperature, but none remarkable cut unstabilization was observed. Cutting temperature rises with rotating speed increase, but cutting forces keep constant without decrease and increase. "Critical cutting speed" wasn't found in high-speed milling of S136 hardened steel. Main influence factors on cutting forces involve feed rate, cutting depth, cutting tool (ball nose end mill and flat end mill) and patterns (slotting, side milling, cornering and etc.). Cutting speed within the used range in the experiment (15-110 m/min) has little influence on cutting speed, influence of feed rate on cutting force in side milling is greater than that in slotting.In the milling process of high speed milling of hardened steel, chatter plays an vital role. The chatter signals of the milling process show the multi-mode property of the cutting system. The frequency of strong chatter isn't unique, and several strong chatter critical zone should be avoided in the cutting process. In order to reduce the system chatter, in the cutting process, some specific cutting speed should be avoided to adopt for different machines. Cutting noise only relates with spindle rotating speed, and noise becomes strong abruptly under specific rotating speed.In high speed milling of hardened steel using small diameter end mill, the main failure patterns include wear, tool tip breakage, coating burnt and drop-off, flank micro-chipping, and breakage. The tool tips are the weakest parts of the end mill. So improvements of the obdurability of the tool tips and cutting edges are very important to better the cutting property of the end mill. Breakage often occurs during cutting into and out of the workpiece and cornering. Tool rigidity must be strengthened and surfacecoating quality should be improved. The cutting temperature rise with the increase of the cutting speed, so high-speed cutting conditions should be made sure not to bum the cutting tool. Chatter is in relation with the fixed system frequency, tool wear degree and various machining parameters. Chatter signals move toward low-frequency with the increase of the tool wear.The recommended technical parameters obtained for high-speed milling using flat end mill of diameter 2mm are as following: for slotting, the spindle rotating speed lies between 8000pm and 15000rpm, feed rate per tooth between 0.005 and 0.02mm/rev, axial cutting depth between 0.05 and 0.2 mm; for side milling, the rotating speed is near 6369rpm or greater than 12000rpm, fz is advisable between 0.005 and 0.02 mm/rew, Rd is smaller than 0.4mm.The post-process program in this thesis can control the acceleration process of linear and circular interpolation, by means of optimizing the NC codes by the parameters such as maximum feed rate and acceleration. So it is important to improve the stability in order to avoid gouging.