Machinery's Handbook, 31st Edition
MACHINE TOOL REQUIREMENTS 1157 pointed at a rotating precision plug gage. The spindle displacement is then recorded on a computer for further analysis and display in either frequency or time domain. Commercial laser systems can provide displacement readings to ± 0.1 m m resolution. An example of spindle runout is shown in Fig. 1b; the spindle runout of a Haas OM2 machine was measured with a Keyence laser system to be ± 1.25 m m. Care must be prac ticed to isolate vibration of the spindle or it would affect the sensor reading, and avoid direct eye contact with the reflected laser from the shiny plug gage. Machine tool spindle Laser on 3mm plug gage rotating at 10,000 rpm 2.0
1.0
Precision plug gage Laser displacement sensor Data acquisition system Fig. 1a. Setup for Spindle Runout Measurement.
0.0
–1.0
–2.0
0 2 4 6 8 10 Spindle Rotating Time (s)
Fig. 1b. Spindle Runout of Haas OM2 CNC Micromilling Machine.
Example 1, Spindle Speed for Macro versus Micro Machining: The speed and feed table on page 1118 recommends a milling speed of 585 ft/min (178 m/min) and feed of 0.004 in/tooth (0.1 mm/tooth) for end milling 316L stainless steel using an uncoated carbide tool. Macromachining: To have said surface speed for an Ø1⁄2 inch (Ø12.5 mm) end mill, the required spindle speed is . , N D V 05 585 12 4 469 rad/rev in ft/min in/ft rpm # # π π = = = ^ ^ ^ ^ h h h h Micromachining: To obtain the same surface speed for an Ø0.004 inch (Ø0.1 mm) micromill, the new spindle speed is . , N D V 0004 585 12 558 633 rad/rev in ft/min in/ft rpm # # π π = = = ^ ^ ^ ^ h h h h To turn, face, or bore a stainless steel microshaft of Ø0.004 inch (Ø0.1 mm) at this cutting speed, a lathe spindle would need to rotate at 558,633 rpm too. A machine tool with spindle speed exceeding 500,000 rpm is rare or simply not commercially available at this time. Applying the recommended macro feed of 0.004 in/tooth (0.1 mm/tooth) for an 0.004 inch (0.1 mm) diameter micromill would break the tool because the feed/tooth is as large as the microtool diameter. Microcutting Tools Tool Stiffness.— It is relatively easy to have a rigid turning or facing microtool, but it requires careful planning to maintain rigidity of a micromill or a microdrill. Geometries of macroscale and microscale drilling/milling tools are the same: tool diameter, number of cutting flutes, point included angle for microdrill, helix angle, web thickness, clearance angle, flute length, shank diameter, and overall length. A careful selection of microtools must consider the intended machined features and highest possible tool stiffness. The two most important geometries that affect the microtool stiffness are the tool diameter and flute length assuming the number of flutes has been chosen. It can be shown that the torsional stiffness of a mill/drill is proportional to (tool diameter) 4 and (flute length) − 2 . For a specific mill/drill tool dimension, the milling/drilling strategy must be adjusted accord - ingly to avoid tool breakage.
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