Micro-machining by mechanical machine tool, or micro mechanical machining, is an emerging fabrication technology that broadens applicable material ranges including metals and plastics. One of problems in this micro machining, however, is relatively large runout which is comparable to the diameter of tools. A series of experiments were conducted to identify Total Indicator Reading (TIR) and dynamic run-out of micro-machining on aluminum workpieces. Results presented in this paper indicate the ratio of run-out to toll diameter ranged between 2.2% and 10%. To compensate for the runout in the design stage of micro features, a Java-based Design for Manufacturing (DFM) tool was developed. This tool can assist designers to design micro channels accurately considering the effect of run-out. Furthermore, the feasibility of micro-injection molding was examined with micro-machined channels. Micro-molded parts were made from ABS plastic showing negligible shrinkage from the molding process.


Geometry of micro cutting tool
Total Indicator Reading (TIR) of three different tool lengths (Lo), 12, 20 and 28 mm, were measured with a dial gauge located near the end of cutting flutes. For this measurement, a spindle speed of 10 rpm was used without cutting the workpiece. Length of tool (L) was 38.1 mm, diameter of shank (df) was 3.175 mm, and Micro-Drop Coolant was used during machining.


Total Indicator Reading (TIR) as a function of tool length from the tool holder
Measured TIR as a function of tool length for the Mori Seiki machine. As described in the Experimental section, TIR is the measured run-out without cutting the workpiece material. For various tool diameters, run-out increased with longer tool length and became as large as 15 mm. Considering the length of micro end mills (shank + flute), 12 mm was practically the minimum tool length at which TIR had minimum values (6.5~9 mm). To be consistent, 12 mm tool length was selected as the standard tool length to minimize TIR for all the following experiments.


Total Indicator Reading (TIR) as a function of tool length from the tool holder
When channels were actually machined with the end mills, the width of machined channels was larger than the tool diameter by about 20mm. Tool diameter has been measured by the optical CMM. Tool diameter tolerance was 7mm. Figure 8 shows run-out values versus tool diameter using the Mori Seiki machine. According to the results, runout to tool diameter ratio was negligible (2.2%) for the large 762 mm tool, but the error was about 10% for the 127mm tool. The effect of runout on the accuracy of machined features would be even greater for smaller tools such as 50 mm. Using Air Turbine Tools spindle, the runout was slightly larger, but surface finish was better than that achieved with the Mori Seiki machine.


Run-out as a function of tool diameter