Jennifer Doyle, Aaron Ramirez, Adrienne Watral Micro Milling Jennifer Doyle, Aaron Ramirez, Adrienne Watral

Outline What is micro milling Milling fundamentals Macro-scale physics Micro capabilities CAD/CAM Micromilled parts Tools Benefits Limitations Quality, cost, rate, accuracy

Summary Micromilling best process for prototyping Variety of material possibilities 3D machining in one step good for microfluidic applications Risks: burrs, material grain size issues, tool wear/breakage

What is Micro Milling?   Used to create 3D features in the range of a few microns to a few hundred microns  Fields currently used in include: optics, electronics, medical devices, telecommunications micro-holes for fiber optics, nozzles for high-temperature jets, molds and x-ray lithography masks

Fundamentals of Milling Process I Conventional Material Removal Chip formation Roughing and finishing Micromilling No roughing, finishing passes

Fundamentals of Milling Process II Conventional Material Removal Important parameters: Feed Speed Chip load/tooth Cooling Cutter/workpiece material Cutter geometry electron.mit.edu Additional Micro-milling considerations Minimum Chip Thickness effect Grain size effect Dynamic response Burr formation

Fundamentals of Milling Process III Conventional Milling Climb Milling electron.mit.edu electron.mit.edu Rubbing Work hardening Poor finish, in general Better finish Increased cutting forces 'Special Case' materials

Fundamentals of Milling Process IV Conventional micro-milling Climb micro-milling electron.mit.edu electron.mit.edu What about at the micro-scale? Rubbing Work hardening Poor finish, in general Digs into softer materials Damages cutters Better finish Increased cutting forces 'Special Case' materials  Vibrations Damages delicate features Source: D. Korn

Fundamentals of Milling Process V Thermal considerations Macro-scale Heat treatment Thermal stresses Fire  Micro-scale Thermal expansion Thermal conductivities Draper guy contradiction? Source: X. Liu, R.E. DeVor, S.G. Kapoor.

Dominant Physics at Micro Scale I Dynamic response Instability regions Dynamic Model  Optimal feedrates  Axial depth of cut Source: X. Liu et. al.

Dominant Physics at Micro Scale II Minimum Chip Thickness (MCT)  effect Poor finish Increased forces MCT material dependent MCT causing increased roughness, cutting forces. X. Liu, et. al.

Dominant Physics at Micro Scale III Grain size to chip thickness effect Erratic cutting forces + high frequency excitation Instability and tool breakage! Mitigate with smaller grains and isotropy Fig. 1: Grains in brass. M. Takacs, et al. Fig. 2: Comparison of grain sizes in macro- and micro- milling G. Bissacco, et. al

Micro Milling Capabilities

CAD/CAM Considerations Tool motion calculations Rounded toolpaths Rounding becomes useless below a certain value Low spindle speeds  limits maximum attainable feedrate  

Macro vs. Micro scale summary Well established Greater variety of tools Micro Minimum Chip thickness Dynamic considerations Surface effects Grain effects Elastic-plastic machining Significant burrs  CAM not as developed

Microlution 363-S 3 Axis CNC Micro Milling Machine http://www.microlution-inc.com/

What Can We Make? http://www.microlution-inc.com/

What Can We Make? http://www.microlution-inc.com/

Micromilled Features Source: www.nist.gov

Micromilled Features Source: www.nist.gov

Micro Mill Endmill Tooling Examples

Draper Lab: Drill bit: .006"

Draper Lab: Endmill: .001" 2 flute carbide

Slots Micromilled in Steel

Rectangular Features Micromilled in Steel

Tools: Focused Ion Beam Machining

Alternative Micro Manufacturing Technologies   Lithographic Methods MEMs-Based Methods Micro EDM (Electrical Discharge Machining) Laser Machining

Benefits of Micro Milling vs Other Technologies Relatively rapid manufacture of prototype devices Broad range of materials can be used Steel, brass, aluminum, ceramics, plastics, other polymers Ability to use solvent and heat resistant materials (i.e. stainless steel, tool steel)  True 3D geometry creation Other techniques such as lithography, only work in one plane - constructing a 3rd dimension requires multi-layering  Milling offers manufacturing of 3D structures in just one machining operation  

Benefits of Micro Milling vs Other Technologies Potential Microfluidics Applications:   Micromilling can be used to fabricate stamping dies or directly fabricate micro channels  Geometries useful for microfluidic devices Shaped walls In-channel features for fluid mixing Transitions between channel elevations

Limitations of Micro Milling Size/accuracy constraints imposed by limits of tool geometry Issues with surface quality and finish  Tooling wear and breakage Issues involving material grains Still less knowledge of appropriate machining techniques  and values (spindle speed, feed rate, etc) for different tasks Burrs

Quality There are quality concerns, particularly with regards to surface quality/roughness   If roughing and finishing are combined as advised, optimal quality may not be achieved However, vibration or fracture that can arise from multiple passes also damaging to surface quality For multiphase materials, significant variations in machining process arise when moving between grains  Affects cutting forces and causes dynamic excitation/vibrations of tool-workpiece system Can lead to uneven surface generation

Quality Burr Formation Disproportionate burr formation, on scale of a few microns in size up through 50 microns Fig. 1: Burrs in stainless Rounded cutting edges of tools - in the curved region, workpiece is compressed and forms burrs Reduced when using sharp diamond tools and increased when using worn out tools With multiphase materials, burr formation occurs between grains as chip formation gets interrupted Source: K. Lee Fig. 2: Correlation between feed per tooth and burr height

Quality Burr Formation Possible to reduce burrs, both during or after cutting During cutting: Protect surface with polymer coating such as cyanacrylate Works as shield to prevent burrs from growing, can be removed with acetone after cutting After cutting: Can also use polymer coating to embed burrs For steel, polymer not stiff enough to prevent burring Clean off burrs with electrochemical polishing  But need to make sure not to damage or reduce desired structures Source: Th. Schaller et. all

Accuracy Accuracy in principle in the 1E-3 to 1E-5 range (tolerance to feature size), compared to 10E-1 to 10E-2 range for most MEMs-based methods  Accurate features have been made to the following limits: Minimum 50 micron wide channels Minimum 8 micron wide walls Draper says their machine's tolerances are very accurate, though did not specify numbers           However, potential for tool variance even among tools of the "same" size On the order of +/- 5 microns Draper noted this variance even from tools in the same package They inspects all their tools and pick the "best" ones for the real jobs  

Accuracy Tooling wear impacts quality and accuracy Lack of tool sharpness promotes burr formation Increases cutting forces which lead to tool breakage Desired profiles of grooves and walls quickly become more rounded     Limited stiffness of mill tools can cause dynamic vibration - leads to problems with both surface quality and tool wear Source: Schaller et al

Cost Compared to other micro-manufacturing methods, micromilling is considered more cost-feasible for small series runs and prototyping Lithographic techniques are based on more costly and time consuming multi-layering methods Higher manufacturing costs associated with the masks required for exposure for lithographic methods Thus, less suitable for small batches or single-part production     More automation/computer control means less labor involvement  Draper says process is almost completely automated Tool breakage risks run higher with micro milling than with conventional milling Greater tool replacement costs

Speed Similarities between High Speed Machining and micromilling -- thus might assume fast speeds Small tool edge radii mean spindle speed is often too slow to produce high cutting feed Limits the maximum attainable feedrate For example, a feedrate of 100m/min with a 10mm cutter should require a spindle speed of approx 3200rpm (N = V/pi*D) Scaling down to 100 micron cutter would theoretically require a spindle speed of 320000rpm, which is currently unattainable  Combined roughing and finishing may reduce time because of fewer passes Machining time is on magnitude of hours to days

Works Cited X. Liu, R. E. DeVor, and S. G. Kapoor. "An Analytical Model for the Prediction of Minimum Chip Thickness      in Micromachining." J. Manuf. Sci. Eng. 128, 474 (2006) X. Liu, R.E. DeVor, S.G. Kapoor. "The Mechanics of Machining at the Microscale: Assessment of the       Current State of the Science." ASME 2004   X. Liu et. al. "Cutting Mechanisms and Their Influence On Dynamic Forces, Vibrations, and Stability in      Micro Endmilling." Proceedings of ASME International Mechanical Engineering Congress and       Exposition. November 13-20, 2004, Anaheim, California USA M. Takacs, B. Vero, I. Meszaros. "Micromilling of metallic materials." Journal of Materials Processing      Technology, Volume 138, Issues 1-3, IMCC2000, 20 July 2003 G. Bissacco, H.N. Hansen, L. De Chiffre. "Micromilling of hardened tool steel for mould making      applications." Journal of Materials Processing Technology, Volume 167, Issues 2-3 K. Lee, David A. Dornfeld. "Micro-burr formation and minimization through process control." Precision       Engineering, Volume 29, Issue 2, April 2005, Pages 246-252