Machinery's Handbook, 31st Edition
1158
Microcutting Tools
Example 2, Stiffness of Microtools: If a drill diameter of 0.8 mm is selected instead of 1.0 mm, then the 20% reduction of diameter will result in a reduction in torsional stiffness E of: . . . % E D D D 10 08 10 59 1 4 2 4 1 4 4 4 4 ∆ = − = − =− ^ ^ h ^ h h Similarly, if a flute length of 1.2 mm is chosen instead of 1.0 mm, the 20% change in flute length will lead to a decrease in torsional stiffness E of: . . . % E L L L 10 12 10 30 1 2 2 2 1 2 2 2 2 ∆ = − = − = − − − − − − − ^ ^ h ^ h h Tool Sharpness.— The tool edge radius is critical in micromachining. Fig. 2a through Fig. 2d shows two scenarios for the same microcutting tools with edge radius r . The tool can be either a turning, facing, or boring microtool that linearly engages a workpiece material at a certain depth of cut. A similar tool can move in a circular path as a microdrill or micromill, and engage a workpiece at a certain chip load (feed per tooth). If the depth of cut (or chip load) is too shallow, the tool simply plows the material and pushes it away elastically. This elastic material layer just springs back after the tool passes by. If the depth of cut is substantial (recommended), then a chip is formed and a new machined surface is generated with negligible spring back. Chip load is commonly used interchangeably with feedrate for a cutting tool with multiple cutting edges (teeth) such as in milling or drilling. Chip load is defined as tool feed distance for each tooth and represents the chip size forming for each tooth. Chip load can also be interpreted as the radial depth of cut for each tooth in milling. The following equation converts chip load of a cutting edge to feedrate of a multiple-edge cutting tool: f c nN L = where f = feedrate of tool (mm/min, in/min) c L = chip load of a cutting edge (mm/tooth, in/tooth) n = number of cutting flutes or cutting edges (#teeth/rev) N = rotational speed (rpm) Example 3: A two-flute uncoated carbide end mill with diameter Ø1 mm (Ø0.040 in) is used for micromilling pure titanium. Table 13b suggests a chip load of 17 m m/tooth and cutting speed of 90 m/min. The rotational speed is computed as:
90
(
m/min
)
N D V
, 28 600
rpm
= =
=
rev rad
π
. ( ) m
0001
a
k
π
#
The feedrate for this operation is:
972 38 m mm in . .
tooth m µ
rev teeth
mi rev
µ
f c nN 17 = =
2
28,600
972,400
`
j
a
k
`
j n
=
min
min
min
#
#
L
Typical fine grain carbide tools are first sintered from submicron carbide particles in a cobalt matrix, and then ground and lapped to final geometry. Optimal edge radii of 1–4 m m (39–156 μ inch) are typically designed for sintered tools to balance edge sharpness and edge strength. Only single crystalline diamond tools can be ground and lapped to edge radii within the nanometer range. The threshold for minimum depth of cut has been investigated theoretically and verified experimentally by many researchers. It varies from 5 to 40 percent of the tool edge radius depending on the workpiece material and original rake angles. The threshold depth of cut or chip load, therefore, can be conservatively set to be 50 percent of the tool edge radius. When machining below this threshold, a microtool just rubs and plows the surface with negative effective rake angle and deforms it elastically during the first pass. This results in high cutting force, high specific energy, fast tool wear, rough surface finish, and significant burrs. In subsequent passes when the cumulative depth is greater than the critical depth of cut, then a tool can remove materials as chips and the cycle repeats.
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