Micro-Milling Opportunities and Challenges
We are all familiar with the phrase “the world is getting smaller.” However, it is not just that the world is getting smaller; practically everything we use is getting smaller.
Not only are things getting smaller, they are packed with more components to provide added power and functionality. Micro-size components have a wide variety of applications in almost every industry, including aerospace, automotive, electronics, healthcare, information technology and telecommunication.
The increasing demand for micro-scale components and products presents moldmakers with new and diverse challenges—ranging from the use of new materials to special mold coatings, milling parts with 0.1mm diameter tools and achieving sub-micron-level accuracy.
At the same time, it is the inherent complexity of micro-components that brings about new opportunities for moldmakers. At a time when production of simple and medium complexity molds is shifting to countries with low labor cost, U.S. and European moldmakers can turn to more advanced technologies such as micro-molds and micro-milling to maintain their competitive edge.
An integrated approach to the design and machining of micro-milling components is key for moldmakers that are looking to capitalize on this growing opportunity.
Micro-Milling Machine Requirements
With multiple elements working in tandem, a machine is only as good as its weakest individual component. Less forgiving than traditional milling, micro-milling requires each machine component to be suitable for the unique requirements of the task.
Machine geometry determines the machine’s stiffness, accuracy, thermal stability, damping properties, throughput and ease-of-use. The most popular vertical machine geometry types are bridge and C-frame construction. With the spindle or Z-axis being the only moving axis, a C-frame construction offers the best stiffness qualities. Since stiffness directly affects accuracy, this design is highly suitable for micro-milling.
One of the challenges when milling delicate and accurate parts is minimizing vibrations. Machine tools with greater damping will absorb more of the vibrations induced by cutting. The most suitable machine frame material for micro-milling is polymer concrete, which provides up to 10 times higher absorption of vibrations than cast iron. Polymer concrete also provides superior dynamic and static rigidity and has substantially better thermal stability than cast iron, all crucial properties for small part accuracy.
Guide Way System
The machine tool way system includes the load bearing components that support and guide the movement of the spindle and table. The two most common guide way types are boxways (sometimes called hydrodynamic ways) and linear guides. The boxways used in a large percentage of machines today are problematic in applications that require frequent axes reversal and low friction motion for extreme accuracy. Linear guide ways that offer low static and dynamic friction are the better choice for a micro-milling machine.
Drive and Motion Technology
How small of a part you can successfully machine depends greatly on the drive and motion technologies built into the machine. A ball screw driven by servo motor is the axis drive mechanism used in most machine tools, and is also the most suitable for micro-milling machines. Most important, though, is how the drive and servo motor work together to provide precise and accurate motion in order to produce miniature size 3-D features.
To ensure the most precise axis position, micro-milling applications require glass scales to be placed close to the guide ways in order to provide additional feedback to the control. Micro-milling applications will most likely necessitate the use of 0.1 micron glass scales rather than the commonly used 0.5 micron version.