Active Tool Deflection Compensation using Magnetic Bearing Spindles during Precision Engineering Operations

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Abstract

Research Method

Results

Abstract

An open-loop technique has been developed here at the PEC to compensate for deflection of small milling tools (diameter < 1 mm). This open loop technique involves predicting the cutting and thrust forces, applying these forces to the tool, calculating the tool deflection, finding the magnitude of the error and creating a new tool path to eliminate this error. However, the tool and part position must be known exactly for the open-loop technique to be effective. Ideally, closed-loop feedback is desired and is currently being researched. The objective of this research is to develop a closed-loop control system to compensate for tool deflection. The prime mover for this active control scheme is a magnetically-suspended, 100,000 rpm spindle. Such a spindle is capable of creating the high surface speeds needed for miniature milling tools, can adjust the center of rotation to compensate for tool deflection, and has the high bandwidth necessary to provide active chatter compensation. The correction scheme has two modes: low-frequency path correction for tool deflection and high-frequency compensation for chatter.

Research Method

This project includes two main areas: one area involves the implementation of the closed-loop control feedback into the existing Nanoform 600 diamond turning machine and the second area involves the dynamics and control of the magnetic bearing spindle. The tool deflection will then become part of the control loop to re-position the DTM axes or the magnetic bearing spindle accordingly. The magnetic bearing spindle will be used both as a sensor and an actuator. The spindle has 6 eddy current sensors than have sub-micrometer resolution along with 2 axial and radial actuators. Based on the tool and bearing stiffness, tool deflection can be calculated based on the deflection of the spindle. This closed-loop machining will allow dies for optical and mechanical components to be fabricated at a faster rate while retaining the crucial part shape and features.

Results

Open-loop experiments were conducted and the error was reduced by an order of magnitude from 20-50 µm to 2-5 µm. However, these results can only be obtained by accurately knowing the part and tool position. Force feedback will be able to compensate for tool deflection on-the-fly independent of tool or part position.

The following faculty, students, and PEC affiliates are involved in this project: