Abrasive air jet micro-machining of highly curved surfaces

Ali Nouhi

doi:10.32920/ryerson.14649972.v1

Abstract

In the abrasive jet micro-machining (AJM) process, a jet of high-speed particles is directed through a micro-nozzle which is used to erode a wide variety of materials. The micro-machining of small curved devices made of brittle and ductile materials is required in optical and biomedical equipment. This dissertation aims at fabricating axial grooves and helical micro-channels in stationary and rotating curved targets, respectively, using AJM. In addition, a model is proposed to predict the shape of machined channel profiles in glass and PMMA rods. Since the air driven jet is divergent, the edges of the desired features are usually defined using a mask which is attached to the surface of the target material. This thesis presents an alternate technique using shadow masks that can be moved over the surface. It is demonstrated that this apparatus can be used to direct write features on the surface. This dissertation proposes a modification to the existing surface evolution models for predicting the channel profiles machined on highly curved and tilted surfaces. It is shown that considering the change in local nozzle standoff and the divergence angle of each particle trajectory in the jet plume results in more accurate predictions. Computational fluid dynamics (CFD) modeling showed that the jet footprint difference on the flat and curved surfaces was not due solely to the expected conical divergence in the jet, but also due to differences in the erosion caused by secondary impacts of rebounding particles. This observation has important implications for the surface profile modeling of curved surfaces. Finally, a model for the prediction of the volumetric removal of material during the machining of rotating and translating PMMA and glass rods is presented. In addition, a new experimental procedure is proposed for machining helical micro-channels in glass and PMMA rods using a cylindrical steel spring as a mask. This method of machining provides a convenient means of fabricating helical micro-channels with different aspect ratios and radii of curvature means of fabricating helical micro-channels with different aspect ratios and radii of curvature. Such helical micro-channels may have applications in inertial microfluidic devices where they can be used to aid liquid mixing and the separation of particles from a flow.

Highlights

4.2 Experiments4.2.1 Jet and footprint measurements4.2.2 computational fluid dynamics (CFD) modeling
A typical Abrasive jet micro-machining (AJM) setup consists of following: a Micro-Abrasive Blaster, operating at 200-500 kPa air pressure that passes through a 0.46-1.5 mm diameter nozzle; a dehumidifier, a refrigeration air dryer, and a desiccant filter on the inlet air that minimizes the moisture in the powder reservoir; a variable-speed rotary electric mixer to avoid powder bed compaction and particle agglomeration inside the reservoir (Fig. 1.1). 10-50 μm granular particles are used as the abrasive for machining
This paper introduces a new adjustable shadow mask system which is attached to a stationary nozzle, producing a narrow stream of abrasive particles to machine a target which is scanned in front of it, in order to produce the desired pattern

Summary

Introduction

4.2 Experiments4.2.1 Jet and footprint measurements4.2.2 CFD modeling. 5.2 Experiments5.2.1 AJM apparatus 5.2.2 AJM lathe5.2.3 Unmasked lathe machining ix5.2.4 Spring-masked helical channels on rods5.3.1 Prediction of surface evolution for masked helical channels5.3.2 Prediction of volume of removed material in unmasked AJM lathe operations 106. An inexpensive and convenient means of fabricating helical micro-channels on small sized highly curved glass and PMMA rods has potential application in microfluidic devices, e.g. flow mixing and particle separation from flow. Because the size of the jet footprint is typically on the order of several millimeters, a potential drawback of AJM is the need to apply erosion resistant masks in order to etch micro-scale features. Li et al [33] studied the radial-mode abrasive waterjet turning process on 59 mm diameter steel rods They concluded that the machined depth increased as the jet incidence approached the surface normal, and when a higher water pressure, a higher surface speed, or a lower feed rate were used. The surface evolution model to predict the cross-sectional profiles of channels machined in initially flat brittle materials using AJM, where only normal component of particle velocity is relevant, was presented in [9] as.

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