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
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.
Machine Construction
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.
Spindle
The ideal spindle for micro-milling is a closed-loop or vector-controlled spindle, which supports a wide range of speeds and offers full torque at low speeds, rigid tapping capabilities and consistent spindle orientation. A well-designed vector-controlled spindle on a micro-milling machine will offer great flexibility along with the ability to cut even the most difficult material. A 50K-rpm spindle would be quite adequate for most micro-milling applications, which use smaller size tools.
Toolholder and Spindle Interface
HSK toolholders offer a number of advantages for high rpm spindles and thus are the preferred choice for micro-milling machines. HSK toolholders are retained in the spindle by a set of internal grippers located inside the spindle. As rpms increase, metal-to-metal contact between the toolholder and the spindle is maintained because centrifugal forces cause the internal grippers to expand within the toolholder, pressing it firmly against the inside of the spindle shaft.
HSK tooling is also a dual contact interface. It locates on both a shallow taper and a flange creating a precision fit. This precision fit allows the interface to have superior run-out conditions compared to steep tapered tooling. When working with very small cutters, run-out inaccuracies can cause premature cutter failure. Excessive run-out can also reduce the life expectancy of the spindle. Therefore, run-out inaccuracies for micro-milling machines should be kept at 1 micron or less.
CNC Technology
While adequately covering the topic of CNC technology is beyond the scope of this article, two important aspects need to be pointed out:
1. Control interface: the interface should be logically laid out and simple to use, yet flexible enough to handle even the most complex toolpath output from any CAD/CAM system.
2. Processing speed: motion control and feedback are crucial for precision micro-milling applications. The control must be able to quickly process high-density complex data and command the motion to the axis in a precise manner.
Auxiliary Components
Tools that can hardly be seen by the human eye are nearly impossible to measure mechanically. Using a laser measuring system, both the length and diameter of tools as small as 40 microns can be reliably and automatically measured.
Setting up a micro-milling machine can be made easier with the use of a touch probe. Automatic centering, part zeroing and part alignment can be used to quickly establish part orientation. Additionally, part measuring can be accomplished using many of the touch probe routines commonly found on controls that feature probing.
Machine Environment
The machine environment must have a controlled temperature and free from vibrations. If the machine is not properly isolated, even a heavy truck that passes outside the facility could generate enough vibrations to leave its marks on the surface.
Micro-Milling CAD/CAM Requirements
In order to guarantee the level of accuracy, smoothness and continuity that is necessary in the micro-milling environment, the CAD/CAM system must be carefully tuned and optimized to support the following requirements.
Data Translation
A translation error resulting in a 0.005mm gap between two surfaces might not be problematic in a large part, because the part can be polished. However, polishing is typically not an option in miniature molds or micro components, so a gap of the same size in a micro-milled part would be clearly visible. Therefore, the CAD/CAM software should be capable of reading native format files from other CAD systems to keep the integrity of the geometry. Data translation issues between separate CAD and CAM packages can also adversely affect machining accuracy. An integrated CAD/CAM package minimizes the risk of introducing such inaccuracies.
Tight Tolerances
Working with tight geometric tolerances when generating parting surfaces and creating geometry for slides, lifters and ejectors is essential in order to prevent gaps between surfaces, and keep C1 and C2 continuity. The software should support toolpath calculation with tolerances down to 0.01 micron while considering the constraints of the machine used. Such tight tolerances require special toolpath algorithms with greater accuracy and better point distribution in order to achieve a polish-free surface.
Machining Strategies
The software should support machining strategies optimized for micro-milling, such as maintaining a constant chip load for the rough and re-rough procedures. The ability of the software to support efficient roughing strategies with constant chip load and varying feedrates is critical for micro-milling. To generate a smooth toolpath, the software should support the use of multiple cutters with different spindle speeds, feedrates and cutting parameters in a single finishing operation.
Tool Motion
Rounding can become a challenge in micro-milling, which commonly features very small stepovers. Rounding smaller than the stepover will likely create a sharp motion, while round corners larger than the stepover could create ridges and gaps between sequential passes and generate excessive scallops.
To prevent such gaps and ridges and ensure high surface quality, support for tool motion techniques such as CBP (Clean Between Passes), CBL (Clean Between Layers) and Ridge Machining is a key requirement for micro-milling success.
Multi Axis
A growing number of micro-milling applications require multi-axis machining, such as the use of 5-axis machining for miniature impellers. Five-axis machines enable the utilization of small tools with tilt options that result in better surface quality while eliminating the need for multiple tools. Compared to conventional 5-axis applications, micro-milling applications require greater flexibility in controlling the tool orientation, as well as the ability to track the stock model across multiple dimensions.
Knowledge of Remaining Stock
Knowledge of actual remaining stock throughout the entire process allows the software to adjust the feedrate to actual tool load, in order to shorten machining time while protecting the delicate tools from breaking. As the workpiece shape dramatically changes during the roughing operation, the software simulates the remaining stock after each layer. This enables the tool to go into locations that were cleaned by previous layers, allowing shorter tools to cut into deep areas.
To reduce tool load during the finishing passes, the software should be able to address re-rough operations within the re-machining procedure, while using the remaining stock knowledge.
Geometry Mending
Almost every CAM programming job requires some geometry mending procedures. In many cases, only during the programming process does it become clear that a certain geometry modification is required. Surfaces must be extended to protect the areas that will be machined in another setup, and a draft angle should be applied. CAM software that includes built-in CAD capabilities and provides the assisting geometry with the appropriate accuracy and tangency enables this design-for-tooling mending to be performed by a toolmaker who understands the machining process, such as the NC programmer.
Micro-systems and micro-milling bring about new opportunities for moldmakers who are seeking to differentiate themselves, generate business in an emerging and lucrative market segment, and be better positioned against lower-wage competitors. New materials, new machining capabilities and innovative CAD/CAM software are available now to help moldmakers step up to the challenges and capitalize on the growing demand for micro-milling.
In order to achieve the quality and accuracy required in micro-milling while meeting economic and commercial constraints, the entire manufacturing chain must be optimized and synchronized. Vendors of CNC machines, cutting tools, holders, clamping tools, and quality control equipment are all required to provide the right solution at a competitive cost.
Following is a checklist of the major issues that should be addressed in a micro-milling environment:
1) Cutters, Holders & Spindles
Small-sized cutters are the enablers of micro-milling. Depending on job size, they can be as small as 0.1mm, and will probably get even smaller in the future. Availability and cost of cutters must be considered before committing to a micro-milling project.
While using small cutters, high speed spindles are critical. A spindle with 10,000 RPM and 0.1mm cutters means a cutting feed (Vc) of only 3.3 m/min — this is much too slow!
With RPM levels at 20,000-150,000, a fully balanced combination of spindle and shrink holders, with “zero” runout, is a must. Otherwise, surface quality will be compromised, and tool-life will be dramatically shortened.
2) Fixtures, Clamping Systems, and Manufacturing Processes
In most cases, the production of micro-milling parts should be done with a single setup. For example, combining EDM and milling would likely result in unacceptable mismatches and marks.
3) Machine & Shop Floor
It goes without saying that the machine must be consistently accurate and sensitive to the 4th decimal digits (mm controller).
Micro-milling can make good use of 5-axis milling. The ability to tilt a tool away from the material enables using shorter tools. However, since continuous 5-axis milling is currently less accurate than 3-axis milling, machine specifications and actual performance must be carefully validated when using continuous 5-axis for micro-milling.
Machine environment must have a controlled temperature (software compensation might not be enough) and free from vibrations. If the machine is not properly isolated, even a heavy truck that passes outside the facility might generate enough of a vibration to leave its marks on the surface.
4) Milling technology
Depending on part geometry, micro-milling might require special machining strategies that go beyond simply scaling everything down. In many cases, for example, conventional milling (and not climb milling) would be the preferred milling strategy.
Accuracy of micromilled molds play an important role in complex process chains enabling mass production of polymer micro components, such as lab-on-chips, fabricated by micro injection molding. Surface footprint of micromilling is defined as the technological signature left by machining process on the generated mold surface. This is sensitive to selected tools and machining parameters and, when not controlled properly, can badly affect mold topography and functionality (e.g. part demoldability). In case of complex mold geometry, the impact of micromilling footprint increases, in particular during the demolding phase due to the friction generated by the polymer shrinking around cores. This work studies these effects on molds characterized by sub-millimetric cylindrical cores. A physical and statistical modeling was developed to provide deep insights about the effects of milling strategies and cutting parameters on the generated footprint on the mold cores. These effects are investigated by machining cylindrical pins whose roughness and surface form errors, caused by static deflection of tool and parts, were controlled in the range of Sa = 150–400 μm and ΔRmax = 1–10 μm (profile radial deviation), respectively. Micro injection molding experiments proved that mold topography has a relevant effect on the ejection force. The demolding force generated by a specifically developed polystyrene micro part reached the highest value with the mold machined with the most unfavorable milling conditions. Proper controlling of machine parameters and conditions led to a reduction greater than 60% of the demolding force peak, confirming the feasibility of the conjunct approach to processes optimization. The results of this work move a step forward into the integrated optimization of micro manufacturing process chains.
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