Turbomachinery must use superalloy microcomponents because of high stress and temperature. A superalloy possesses superior mechanical and metallurgical properties, but poor machining properties. Although various nontraditional techniques have been used successfully for superalloy microcomponents, current design of turbomachinery still requires some microcomponents to be manufactured by conventional machining techniques. Since frequent breakage of microtools during machining of superalloys is a concern, a novel technique to reduce machining stress and eliminate tool breakage must be developed. This study characterizes a micromist system and optimizes the different input variables for minimum tool wear using statistical approach.

IntroductionIntroduction

ResultsResults

S. Adera*, S. Chittipolu, S. Kajaria, W. Hung, and W. ZhanTexas A&M University

*Georgia Institute of Technology

Experiment

SummarySummary

Effect of cutting speed and pump frequency on surface finish is non-linear (Fig. 3). The best surface finish (Figs. 3, 4) could be obtained at high frequency (10 stroke/min) and low cutting speed (4000 rpm). For effective lubrication, the atomized lubricating particles must reach the rotating cutting tool surface and adhere to it, i.e., the adhesive force of the particles must be greater than the centrifugal force (Fadhesive > Fcentrifugal). The rotation speed must satisfy:

AcknowledgementAcknowledgementThis material is based upon work supported by the National Science Foundation under Grant No. 0552885. Support from Haas Automation and UNIST is greatly appreciated.

Cutting speed is the most influencing factor in this study. Micromist machining is most effective at low cutting speed

(4000 rpm) and high stroke frequency (10 stroke/min). Particle size, lubricant type, tool size, and cutting speed

determine the effectiveness of mist machining. Adhesive force must be greater than centrifugal force for

proper lubrication. Theoretical investigation should be done to determine how

the micromist particle travels and adheres to a rotating tool.

Four input variables from Haas OM2 system and UNIST micromist system were used: A (cutting speed), B (mist pressure), C (pump stroke length), and D (pump stroke frequency). Aluminum 6061-T6 was machined with carbide end mills (Ø1/8 inch, 2 flutes) and chip load 0.0127 mm/tooth under atomized mist from coolube 2210EP cutting fluid (Fig. 1a). Surface finish (Ra, Rz, and Rmax), measured with the Mitutoyo Surftest211 profilometer, was used to evaluate the effectiveness of machining. The first experiment was designed to identify the most influencing variables (Fig. 1b), and the second experiment optimized those influencing variables.

Fig. 2: Effect of cutting speed A, mist pressure B, pump stroke length C, and pump stroke frequency D on surface finish Ra.

Fig 3. Effect of cutting speed and stroke frequency on surface finish Ra.

Fig 4. Contour map of surface finish Ra measured in µm as a function of cutting speed and pump stroke frequency.

(a) (b)Fig. 1: (a) Experimental set up. Note the micromist needle at 45° to the cutting tool, and (b) Pareto ranking of influencing variables.

Similar trends were found for Ra, Rz, and Rmax. Only Ra is presented here. Figures 1b and 2 show machine speed A affects surface finish the most follow by the stroke frequency D while the stroke length having no significant effect. Only cutting speed and stroke frequency were used in subsequent tests.

40000 rpm

Ra [µ

m]

4000 rpm 3.1 bar 3.8 bar

.9 mm 4 stroke/min4.5 mm 10 stroke/min

8.5

40000

5.5 7 104

Freq [stroke/min]

31000

400004000 22000 4 10

Ra [µ

m]

Cutti

ng S

peed

[rpm

]

2242 mRNr

where r = radius of the micromist σ = surface tension of the mist m = mass of the micromist particle R = the radius of the cutting tool N = cutting speed

7

Freq [stroke/min]

22000

13000

4000

Cutting Speed [rpm]