Machinery's Handbook, 31st Edition
1168
Microcutting Tools
f d T V g b c
(4) Chipping is generally not acceptable since a chipped tool generates excessive burr and a very rough surface. By reducing depth of cut and feed, then chipping should be eliminated assuming micromachining with a quality tool and machine tool. When stable parameters are applied, then the only damage mechanism is thermal and tool life can be predicted with Equation (3). It has been shown that flank wear due to abrasion is directly proportional to the magni tude of acoustic signal or feeding force. An increase of 300 percent in micromilling feeding force from an initial value was established as a threshold for reaching the tool life. A reduction in feeding force, however, might indicate gradual failure of a microtool due to fatigue crack propagation. Indirect monitoring of tool wear by monitoring feeding force for both micromilling and microdrilling would be a preferred technique since this does not interfere with the machining process and reduce productivity. In the absence of a sensitive commercial system that can reliably and accurately monitor tool force and tool life in micromachining, direct tool wear monitoring should still be a popular practice. Traditional tests using the Taylor approach would machine at the same cutting speed until reaching the predetermined tool failure criteria. Such tests can be time consuming if a chosen speed is too low, and only applicable to turning since a constant cutting speed is required. In reality, a part must be machined with the same tool in different directions and speeds to obtain the final profile and surface finish. Several techniques were developed to accelerate the testing method since turning tests alone are tedious, expensive, and do not reflect actual part machining. The cumulative wear technique, assuming that the abrasion wear mechanism is the same at different cutting speeds, is more flexible and can reduce the testing time and cost. The proposed cumulative tool life testing technique: • Is flexible. If an initial speed is too slow, testing speed can be increased and the cumu lative time and tool wear recorded. • Is simple. Manual machines can be used instead of CNC machines. The same rpm on a manual lathe can be used for the turning test until tool failure. Times and cutting speeds for all passes are used to calculate the equivalent time and speed. • Is more cost-effective. Both turning and facing can be combined to completely con sume an expensive workpiece material. • Is order independent. The level of cutting speed is not important if providing the same tool wear mechanism. Experimental data for macromachining shows no difference of tool life if changing cutting speeds from low to high, or in reverse order. Consider a tool that machines at cutting speed V and stops after machining time Δ t before reaching its tool life T . The tool then cuts at different speeds and times until reaching the tool life criteria—for example, 50 m m flank wear on a carbide microendmill. The fraction of tool life when cutting at each speed and time is Δ t / T , and the total tool life fraction is (5) The theoretical value of the total tool life fraction Q should be one. Experimental values for Q were found to be in the range 1.2–1.5. When combined with Taylor Equation (3), then Equation (5) becomes (6) After machining with a tool at different times and speeds in different conditions (e.g., different tool coatings), it is necessary to compare the tool performance by calculating its i t V QC i i n 1 ⁄ k 1 = / ∆ = n 1 ⁄ C a ′ = = T t T t T t T t Q i = k k i i = / 1 k 1 1 2 2 f + + + = ∆ ∆ ∆ ∆
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