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:

o optics, electronics, medical devices, telecommunicationso 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:o Feed Speedo Chip load/tootho Coolingo Cutter/workpiece materialo Cutter geometry electron.mit.edu

• Additional Micro-milling considerationso Minimum Chip Thickness effecto Grain size effecto Dynamic responseo Burr formation

Fundamentals of Milling Process IIIConventional Milling

electron.mit.edu electron.mit.edu

Climb Milling

• Rubbing• Work hardening• Poor finish, in general

• Better finish• Increased cutting forces• 'Special Case' materials

Fundamentals of Milling Process IVConventional micro-milling

electron.mit.edu electron.mit.edu

Climb micro-milling

• 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

What about at the micro-scale?

Source: D. Korn

Fundamentals of Milling Process VThermal 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.

Source: X. Liu et. al.

Dominant Physics at Micro Scale I• Dynamic response

Instability regions• Dynamic Model • Optimal feedrates • Axial depth of cut

Dominant Physics at Micro Scale IIMinimum 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 IIIGrain size to chip thickness effect

Fig. 2: Comparison of grain sizes in macro- and micro- millingG. Bissacco, et. al

• Erratic cutting forces + high frequency excitationo Instability and tool breakage!

• Mitigate with smaller grains and isotropy

Fig. 1: Grains in brass.M. Takacs, 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

Macro• 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

o Steel, brass, aluminum, ceramics, plastics, other polymers

o Ability to use solvent and heat resistant materials (i.e. stainless steel, tool steel)

•  True 3D geometry creationo Other techniques such as lithography, only work in one

plane - constructing a 3rd dimension requires multi-layering 

o Milling offers manufacturing of 3D structures in just one machining operation

 

Benefits of Micro Milling vs Other TechnologiesPotential Microfluidics Applications:•   Micromilling can be used to fabricate stamping dies or

directly fabricate micro channels • Geometries useful for microfluidic devices

o Shaped wallso In-channel features for fluid mixingo 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 o 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 o Affects cutting forces and causes dynamic

excitation/vibrations of tool-workpiece system Can lead to uneven surface generation

Quality

• Disproportionate burr formation, on scale of a few microns in size up through 50 microns

Burr Formation

o Rounded cutting edges of tools - in the curved region, workpiece is compressed and forms burrs

o Reduced when using sharp diamond tools and increased when using worn out tools

o With multiphase materials, burr formation occurs between grains as chip formation gets interrupted

Fig. 1: Burrs in stainless

Fig. 2: Correlation between feed per tooth and burr heightSource: K. Lee

Quality

• Possible to reduce burrs, both during or after cuttingo 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

o 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 o But need to make sure not to damage

or reduce desired structures

Burr Formation

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:o Minimum 50 micron wide channelso 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" sizeo On the order of +/- 5 microns o 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 accuracyo Lack of tool sharpness

promotes burr formationo Increases cutting forces which

lead to tool breakageo 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 o Lithographic techniques are based on more costly and time

consuming multi-layering methodso 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 o Draper says process is almost completely automated

• Tool breakage risks run higher with micro milling than with conventional millingo 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 o Limits the maximum attainable feedrate o For example, a feedrate of 100m/min with a 10mm cutter

should require a spindle speed of approx 3200rpm (N = V/pi*D)

o 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