EXPERIMENTAL INVESTIGATION INTO THE EFFECTS OF VARIABLE DEPTH AIR PADS ON AIR SPINDLE VIBRATIONS IN NANOMACHIN

The increasing demand for precision manufacturing of components for computers, electronics, nuclear energy and defence applications has caused the appearance of ultra precision machining, UPM, processes, as high speed rotation with small heat generation is possible for air spindles, due to the low viscosity of the air lubricant. It also gives rise to noise-free and smooth running, and does not add to the sound and vibration levels of the machine like high-speed ball bearings. UPMs are made in order to create very fine and accurate products. The main features of an UPM can be classified as: A machine tool structure with high loop stiffness, high thermal and mechanical stability, low vibrations, and high precision axis of motion. Air spindles and drive systems are important parts of ultra precision machines, because spindle motion error has a significant impact on the surface quality and accuracy of machined components. Spindles in ultra precision machines have high motion accuracy and rotational speed, and its vibrations directly affect the quality of the work surface. In order to achieve nanometer accuracies, the low vibration of air spindles is vital. Pressurized air is injected into the gap of the spindle in order to make it operative. Injected air may create a lack of stiffness for several reasons. Some of the parameters affecting air spindle vibrations are: Rotational speed, input hole diameter, parameters of air pads, air gap pressure, and etc. In this study, the air pad depth and rotational speed are experimentally investigated. The air pad depth is considered variable. 6 levels are selected for the air pads bottom mode: flat, conical, pyramid, spherical with 2 various radii, and constant depth. Also, for rotational speed, 3 levels are selected. Totally, 18 experiments have been undertaken. For accomplishing these experiments, air spindles are made using various production processes. The VibroTest 60 is used for studying air spindle vibrations. Then, experimental results are analyzed using the DOE method. The results show that the case of air spindles with air pads in a pyramid bottom, at low speed, has minimum vibrations.