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

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

3. 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

4. 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

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

6. 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

7. 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

8. 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

9. 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.

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

11. 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.

12. 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

13. Micro Milling Capabilities

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

15. 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

16. Microlution 363-S 3 Axis CNC Micro Milling Machine

17. What Can We Make?

18. What Can We Make?

19. Micromilled Features
Source:

20. Micromilled Features
Source:

21. Micro Mill Endmill Tooling Examples

22. Draper Lab: Drill bit: .006"

23. Draper Lab: Endmill: .001" 2 flute carbide

24. Slots Micromilled in Steel

25. Rectangular Features Micromilled in Steel

26. Tools: Focused Ion Beam Machining

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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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 rpm, which is currently unattainable
 Combined roughing and finishing may reduce time because of fewer passes
Machining time is on magnitude of hours to days

38. 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