Research Papers

Evaluation of Cutting Forces During Single Pass Microtrenching of Poly (Methyl Methacrylate)

[+] Author and Article Information
Thomas P. James

Department of Mechanical Engineering,
Tufts University,
Anderson Hall 025,
200 College Avenue,
Medford, MA 02155
e-mail: thomas.james@tufts.edu

Nathaniel B. Eckman, Amrit Sagar, Anil Saigal

Department of Mechanical Engineering,
Tufts University,
Anderson Hall 025,
200 College Avenue,
Medford, MA 02155

Unlike lathe turning, where feed velocity is normally constant, the feed velocity in microtrenching is interrupted, meaning it is set at the beginning of each trenching stroke and held constant throughout the stroke. For conciseness, the term feed is preferred here to undeformed chip thickness.

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received September 16, 2013; final manuscript received August 12, 2014; published online September 1, 2014. Editor: Jian Cao.

J. Micro Nano-Manuf 2(4), 041003 (Sep 01, 2014) (8 pages) Paper No: JMNM-13-1070; doi: 10.1115/1.4028319 History: Received September 16, 2013; Revised August 12, 2014

Research was conducted to evaluate a microtrenching process to create microchannels on the surface of poly (methyl methacrylate) (PMMA) for applications in tissue engineering. Experiments with a trenching tool included an exaggerated cutting edge radius (48 μm) to study the impact of a highly negative effective rake angle on forces during single pass microtrenching at subradius cutting conditions. During microtrenching, forces were measured by dynamometer and compared to a finite element (FE) model using an elastic-perfectly plastic material model for an undeformed chip thickness from 9 to 64 μm. During experiments, cutting was first observed when the ratio of undeformed chip thickness to cutting edge radius was 0.33. Measured and predicted values of thrust force exceeded cutting force up to an undeformed chip thickness equivalent to the cutting edge radius. The FE model predicted a linear trend in cutting force with feed (r = 0.99) and was substantiated by linear regression of experimental data (r = 0.99). However, at lower values of feed the model overestimated force, with a maximum difference of 42% at a feed of 22 μm. Thrust force was also predicted to be linear (r = 0.99), but at greater feed the experiments indicated a nonlinear decline in thrust force, resulting in a maximum difference of 27% at 64 μm. Finally, an analysis of nodal velocity plots from the FE model revealed a material stagnation zone developed along the cutting edge, rising from the workpiece surface in proportion to feed and then remaining fixed at 63 deg (stagnation angle) for all feeds greater than 35 μm. While the application of an elastic-perfectly plastic material model for PMMA was sufficient to predict microtrenching forces by the FE method, differences between predicted and measured thrust forces at greater undeformed chip thickness implies a more complex rheological model may add value.

Copyright © 2014 by ASME
Topics: Cutting , Thrust
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Fig. 1

(a) Schematic of custom microtrenching apparatus with key features labeled and (b) Tool carriage assembly and related components: (A) position encoder strip, (B) linear guide rail, (C) dynamometer with PMMA sample mounted, and (D) tool carriage

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Fig. 2

Typical PMMA sample showing two mounting holes and a partial trench cut in the surface of the workpiece

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Fig. 3

Measurements of cutting and thrust force during microtrenching PMMA at feeds from 9 μm to 83 μm for a constant cutting edge radius of 48 μm (0.88 mm tool width)

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Fig. 4

Comparison of measured and predicted cutting force in terms of feed during microtrenching PMMA

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Fig. 5

Comparison of measured and predicted thrust force in terms of feed during microtrenching PMMA

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Fig. 6

Representative contour map showing plastic strain for a 54 μm feed

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Fig. 7

Results from FE simulation showing the average depth of effective plastic strain (≥4) as a function of feed for the region directly below the cutting edge and extending to beneath the cut surface (see inserted graphic)

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Fig. 8

Vector plot of nodal velocities during FE simulation of microtrenching PMMA at an undeformed chip thickness of 30 μm. A separation zone is apparent at a height of approximately 18 μm above the tool tip. Below this depth, material is forced beneath the cutting edge rather than extruded upward into an eventual chip.

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Fig. 9

Analysis of nodal velocity plots to estimate the height of a separation zone above the tool tip as a function of fee




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