In micro milling, the tool bit with diameter as small as 0.1mm is held in a high speed spindle rotating at 20,000 to 150,000 rpm, and used to mill steel, brass and aluminum with depth of cut at about 30 microns and feed rates of 120mm/m to 240mm/m to provide surface quality finishes as good as 0.2 microns.
While micro milling has been successfully applied in manufacturing bio-medical components, embossing dies and micro encoders, the breakage of the tool bit has been identified by many users as a teething problem.
Why does the tool bit break so easily in micro milling as compared to conventional milling?
There are 3 main reasons:
Firstly, when metal is removed by machining, there is a substantial increase in the specific energy required as the chip thickness decreases. This means that in the case of micro machining, as the chip gets thinner with smaller depths of cut, the micro tool bit will be subject to greater resistance when compared to conventional machining. It is as if the workpiece material becomes harder during micro machining. This resistance force is strong enough to exceed the bending strength limit of the tool bit even before the tool experiences any significant wear, and leads to the breakage of the tool bit. One way to prevent this is to make the chip thickness smaller than the edge radius of the tool bit.
Secondly, a sharp increase in cutting forces and stress from chip clogging during the micro milling process would cause the tool bit to break. In most micro milling operations using miniature micro tool bit with two cutting edges, each cutting edge removes the chips from the machining area only within half a rotation. However, if chip clogging occurs, the cutting forces and stresses will increase beyond the bending strength limit of the tool bit within a few tool rotations, and the tool bit will break. Some users prefer high speed steel tool bits as these are very much more flexible and tolerate clogging better than carbide tool bits.
Third, the tool bit tends to lose its cutting edge due to built-up edge and cannot machine efficiently. As the workpiece starts to push on the tip of the tool bit, the tool bit will deflect slightly. The increase in tool deflection and the stress generated by the milling with every rotation will eventually cause the breakage of the tool bit. This process is also called extensive stress-related breakage.
In view of the above phenomena occurring in micro milling, most micro milling machines are sold with sensors to measure the forces acting on the tool bit, and advanced CAM software to predict the chip load throughout the micro machining process. In this way, precision manufacturers seeking a niche in micro milling could try to keep their machines running smoothly with minimal machine downtime.
