Machinery's Handbook, 31st Edition
1164
Microcutting Tools
Microtool
Laser displacement sensor
Plastic membrane with marked center A
h 5 µ // m
z
x
-A-
Workpiece
Fig. 8a. Microtool Offset and Microtool Height Detection Using Laser.
10 15
5 0
Paper & center drill Paper & 4-insert cutter Laser & membrane
–5
–10 –15
0
10
20
30
40
Trial Number
Fig. 8b. Superior Accuracy and Repeatability of Laser Offset and Height Detection Technique Compared to Paper Technique.
When a part is small or does not have a large surface for the fixture to rest on, then an indi rect technique to find vertical tool offset for a microdrill or micromill is recommended. The following example illustrates this. Example 7, Vertical Tool Offset Calculation: A vice or collet is used to clamp a micropart for drilling. The micropart protrudes upward a distance h 1 = 0.1000 inch. If the vice surface has been qualified as a reference, it can be used to find the vertical offset of a microdrill tip (Fig. 9). i) Measure the fixture height at the reference mark using the laser sensor, h 2 = 0.3500 inch. ii) Position the fixture on top of the vice. iii) Lower the microdrill onto the reference mark of the membrane. Stop when the membrane is slightly deflected which can be detected easily with the laser sensor. iv) Calculate the required drill vertical offset: Vertical offset = h 2 − h 1 = 0.3500 − 0.1000 = 0.2500 inch Tool Damage.— Tool damage can be categorized by the relative size of the damage, rang ing from submicron to hundreds of microns, as indicated in Table 2. The tool failure mech anisms include damages due to mechanical, thermal, and chemical effects, and adhesion. Examples of microtool damages are illustrated in Fig. 10a through Fig. 10d. Mechanical effect is the most common source of tool damage. Abrasive wear is caused by sliding of hard particles from workpiece or tool against the cutting tool surface. Attrition wear is larger than abrasion wear; it occurs when one or a few grains of the tool are weakened at their grain boundaries and are dislodged from the tool. Microchipping and chipping are larger chunks of tool being removed due to mechanical or thermal shocks upon loading and unloading. Machining at optimal parameters and with a rigid setup will reduce vibration, shock, and mechanical damage to a microtool.
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