Machinery's Handbook, 31st Edition
MICROFABRICATION PROCESSES AND PARAMETERS 1193
f 2 R ---- 2
f 2 R --
2
f
(20)
≈
2
– –
0.125
4 – – --
R R 1
=
=
R t
R R
f 2 R -- 0.031553 ≈
f 2 R -- for
f R -- 0.1 <
(21)
k f R ( , )
=
R a
where R a = average surface finish (m, inch)
R t = peak-to-valley surface finish (m, inch) f = tool feed for each revolution (m, inch) R = tool nose radius (m, inch) Example 16, Turning Parameter Selection: Select tool and parameters to achieve R a = 1 m m when turning 6061-T6 aluminum. Solution: Use a single crystalline diamond tool with tool nose radius R = 1 mm (0.040 inch) and cutting edge radius of 80 nm (3 m in). Depth of cut: Depth must be between the nose radius and edge radius, so choose 0.5 mm. Cutting speed: Speed does not affect surface finish, so choose 303 m/min (1000 ft/min). Feed: Referring to Fig. 37b, from point A ( R = 1 mm), draw a vertical line to intersect with R a = 1 m m curve at B. Draw a horizontal line at B and intersect vertical axis at C, and read the feed = 0.18 mm/rev (0.007 in/rev). Alternatively, Equation (21) can be rearranged and used to compute the feed distance for each revolution as follows:
1
1 m mm
1000 1
mm
. R R 0031553 a
# µ
. 0178
mm
f
. 0031553 = Effect of Tool Nose Radius and Tool Feed on Surface Finish m µ = =
Tool nose radius (inch)
Tool nose radius (inch) 0.02 0.04 0.06 0.08 0.10
0.02 0.04 0.06 0.08
0.10
0.3
0.3
0.2
0.2
C
B
0.006 0.004 0.002
0.006 0.004 0.002
0.1
0.1
A
0.0
0.0
0.0
0.0
0.5 1.0
1.5 2.0 2.5 3.0
0.5 1.0 1.5 2.0 2.5 3.0 Tool nose radius (mm)
Tool nose radius (mm)
Fig. 37a. Peak-to-Valley Surface Finish R t . Fig. 37b. Average Surface Finish R a . Microdrilling/Milling Parameters: The combination of spindle runout and radial cutting force can deflect and break a tool. Finite element analysis (FEA) of a micromill shows that the critical area is the junction between the cylindrical flute and solid conical shank. Using 4.7 GPa and 93 GPa as the average values of measured flexural strength and elastic modulus of carbide, the analysis indicates that a Ø1 mm carbide micromill will break if deflected more than 34 percent of its diameter. Fig. 38 shows crash-test and calculated data for catastrophic tool failure when micro milling 316L stainless steel. The radial depth of cut and chip load are normalized to the tool diameter for ease of comparison. The data points form a line that divides the plane into two regions: tool failure and tool safe areas. A tool will break if milling parameters are chosen to be above this threshold line. Therefore, all milling parameters should be conservatively selected below the threshold for a production run.
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