Technology suitable for both serial and small quantity production
Perhaps the most versatile direct machining process in conventional manufacturing is milling. This would also seem logical at the microscale. Peripheral end-milling and slot milling present the most severe machining environment of any of the micromechanical processes.
The geometry of a diamond cutting tool, even when its width is small, is a relatively strong structure. At the extremity of the cutting edge, there is primarily compressive stress. The loading condition away from the cutting edge becomes bending but the section modulus becomes large quickly as the depth of the cutting edge region becomes larger. Micromilling however, requires the tool to perform as a cantilever beam which is the weakest structure of the three types of cutting tools for micro chip-making processes.
Development of the micromilling process began by mimicking a conventional four-fluted peripheral end milling tool. Milling tools are made using the focused ion beam process. The focused ion beam process (FIB) is widely used in the semiconductor industry for tasks such as mask repair and junction and metallization layer sectioning. Both tasks make use of the fact that the ion beam process can remove material at the atomic scale.
The micromechanical milling tools begin as a cylindrical piece of hard material such as tool steel or tungsten carbide. The tool blank can be nearly any diameter; however for the 22-micrometer tools described in this paper, the blank is a broken microdrill with a nominal cutting tip diameter of 25 micrometers. The blank is part of a center less ground and lapped mandrel. The blank-mandrel combination operates in a vee-block (see section on microdrilling) with four convex diamond pads. So long as the tool was originally ground in, and is used in vee-blocks calibrated to each other, the blank turns with no measurable eccentricity. The vee-block arrangement is commonly used for high precision rotation applications.
Micromilling is characterized by mechanical interaction of a sharp tool with the workpiece material, causing breakage inside the material along defined paths, eventually leading to a removal of the useless part of the workpiece in the form of chips. The tool edge radius must be in the order of the dimension of the cut thickness or smaller. Monocrystalline diamond is the most suitable tool material but it implies a limitation with regards to the workpiece materials because of its high chemical affinity with steel.
Tool fabrication is another important issue for the application of microcutting technology. For industrial applications, micro powder (0.3 µm particle sizes) tungsten carbide two flutes end mills up to 100 µm diameter are commercially available with an edge radius in the order of 1-2 µm, while smaller sizes are still at the research phase.
The most attractive advantage of micromilling is the possibility to machine 3D micro structures characterized by high aspect ratios and high geometric complexity. However, an important issue in microcutting is burrs removal. Since the dimensions of the machined parts make handling after machining difficult, conventional methods for burr removal are impossible to apply. Therefore special techniques for burr removal as well as burr-free machining strategies have to be developed.
| Micromilling Data Sheet | |
|---|---|
| Mould Materials | Tool steel up to HRC 62, Al7075, Cu |
| Cutting tool | Tungsten carbide end mill up to Ø0.1 mm |
| Machining of channels & ribs | |
| Machine | Ultra-precision milling machine 3-5 axes |
| Removal rate | 1-3 mm3/h |
| Minimum width | 100-110 µm |
| Aspect ratio | 10-15 |
| Accuracy | 5 µm |
| Roughness | 0.3 µm Ra |
| Machining of holes & pins | |
| Minimum diameter | 110-150 µm |
| Aspect ratio | 5 |
| Accuracy | 5 µm |
| Is a 3D freeform surface possible? | Yes |