Effect of spindle speed control of air turbine spindle on milling for difficult-to-cut materials

In precision machining, machining efficiency is improved by high-feed, high-rotation milling using small-diameter ball end milling. Additionally, static air bearings having excellent properties including low friction, low vibration, high accuracy, and rapid response have been used widely to maintain cutting speed. However, insufficient investigations have been conducted of the stabilization of rotational speed during processing with an air turbine spindle used to drive the main shaft. We have been unable to conduct a systematic evaluation of the effects of changes of the rotational speed during processing on a machined surface with periodic cutting force. Therefore, this study controlled the rotational speed of the air turbine spindle using a high-precision, quick-response (HPQR) pneumatic pressure regulator to examine its effects on the material surface roughness. 1) Rotation-controlled milling was conducted with aluminum alloy and steel alloy, which has good machinability. Then its surface roughness was investigated. Aggravation of the surface roughness was prevented by maintaining the rotation speed: the cutting speed. 2) Rough milling for difficult-to-cut materials by the air turbine spindle without control was impossible. In the worst case, the rotation stopped because the spindle torque was insufficient. However, the rotation speed control of air turbine spindle resolved the torque shortage. An appropriate cutting speed was maintained throughout the milling process. These results demonstrated that the surface roughness was improved compared with non-controlled surface roughness.