Elsevier

Wear

Volume 254, Issues 7–8, April 2003, Pages 763-773
Wear

A study on recycling of abrasives in abrasive water jet machining

https://doi.org/10.1016/S0043-1648(03)00256-4Get rights and content

Abstract

This paper reports the effect of recycling of local garnet abrasives (origin: southern India) while cutting aluminium using abrasive water jet machining. The influence of pressure, traverse rate, and abrasive flow rate on American Foundrymen’s Society grain fineness number, average particle size, depth of cut, top kerf width, bottom kerf width, kerf taper, and surface finish obtained using a specially formulated optimised abrasive test sample have been studied. The performance of the test sample has been compared with that of commercial grade abrasive of mesh size 80. Recycling studies, undertaken with used abrasives after screening out particles less than 90 μm size and also with all particles without screening are reported. It is found that the test sample performed better than mesh size 80 abrasives, in terms of achievable depth of cut and surface finish. Recycled abrasives reduces kerf taper, improving the parallelism of cut surface. These results indicate that the proper selection of abrasive particle size distribution is necessary for achieving improved results. The reusability percentage of test sample of the local abrasives that can be recycled is determined as 81%.

Introduction

Abrasive water jet machining (AWJM), an emerging technology is experiencing continuous growth. Its industrial usage depends on cost effectiveness. In general, the overall cost of AWJM systems remains quite high compared to traditional machining techniques, despite the thrust by the industry to reduce the equipment cost and increase the system reliability. System operating costs have been held steady for many years at a high level [1]. The largest component of operating cost is that of abrasive, constituting nearly 75% of the total operating expenses. When abrasive disposal is included, this percentage can be even higher [1], [2]. The cost of abrasives has restricted many opportunities and usage of this technology. This cost, however must be considered along with abrasive performance. Good abrasive performance is more important than the cost of abrasive, since any disadvantage in higher abrasive purchase cost can be outweighed by the higher cutting speed achieved with a better performing abrasive. Therefore cost of abrasive should be weighed against its performance and the most cost-effective abrasive should be selected [3]. The cutting efficiency is influenced by the particle size, particle size distribution, and shape of the abrasive particles.

Abrasive particles disintegrate during the acceleration and focusing processes and also after cutting. During the cutting process, the breakdown of abrasive particles occurs in two stages: (1) particle/particle, particle/water jet and particle/wall collisions in the mixing chamber/focusing tube assembly; (2) particle/particle and particle target collisions [4]. With proper cleaning and sorting, an important portion of sludge may be recycled and fed back to the cutting process. Only the remaining portion, the microchips of the workpiece material and the used abrasive material of finer size particles usually less than 90 μm are disposed [5]. Recycling of the abrasives makes the process more economical, effective and environmentally friendly. Realising the importance of recycling, fully automated systems for abrasive recycling have been recently introduced into the market.

Natural abrasives are often mined from riverbeds or sand deposits. Impurities are removed to improve the performance of cutting. The abrasives are subsequently sized. This multistep process uses metal screen sieves to remove very fine and oversized particles [3]. Among the abrasives, the industries frequently use garnet, as it demonstrated effectiveness of its hardness, sharp edges, flowability, availability, and reasonable cost [6]. Comparison of garnet, silica and steel grit indicated improved performance of garnet [6], [7]. However, different types of garnet, even when chemically and physically similar, perform quite differently [8].

Literature indicates that only limited attempts were made to study the influence of particle size distribution parameters in AWJM. For the first time Momber and Kovacevic [9] studied the influence of two different particle size distribution parameters; size modulus of 150–400 μm and a distribution modulus of 1–4, while machining of aluminium. The distribution parameters are derived from a Rosin–Rammler–Sperling grain size distribution. The influence of these parameters on the depth of cut is not significant in the selected parameter range, whereas surface finish in the smooth cutting zone is sensitive to changes in both of the particle size distribution parameters.

In our ongoing research, garnet abrasives obtained from southern India are being tested for utility. Preliminary research on AWJM with these abrasives [10], [11], [12] has concentrated on depth of cut, top and bottom kerf width, kerf taper, surface finish, and fragmentation of abrasive particles measured by American Foundrymen’s Society grain fineness number (AFS no.) as well as average particle sizes (a.p.s.) proposed by Guo et al. [2]. Since commercial grades of mesh sizes supplied by various vendors vary in their particle size distribution [4], [13], a specially formulated test sample with five equally distributed particle sizes rather than single or three equally distributed sizes is recommended for use based on optimisation studies [12].

The present work attempts to study the recycling capabilities and reusability of local abrasive particles with different particle size distribution. Specially formulated optimised abrasive test sample of five equally distributed particle sizes was compared with abrasive particles of commercial grade mesh size 80 having AFS no. and a.p.s., similar to test sample. Recycling studies are undertaken with used abrasives after screening out particles less than 90 μm and also with all particles without screening.

The target parameters considered are depth of cut (d), top kerf width (KWT), bottom kerf width (KWB), kerf taper (KT), surface finish (Ra) (Fig. 1, Fig. 2), and the fragmentation of abrasives during various stages of recycling studied using AFS no. and a.p.s. The AFS no. is defined as the sum of product (consists of weight of abrasive particles retained in each sieve in percentage multiplied by previous sieve mesh number) divided by the total percentage of abrasives retained in the set of sieves and the pan. The a.p.s. is calculated based on momentum method; defined as the sum of product (consists of particles retained on each sieve in percentage is multiplied with average mesh size of the sieve) divided by the total percentage of abrasives retained in the set of sieves and the pan.

Section snippets

Experimental set-up and procedure

An injection jet type abrasive water jet machine consisting of pressure intensifier, an abrasive machining head, an xy positioning system and a catcher tank has been used for experimentation. The equipment details are given in Table 1. In this type the jet is formed by accelerating abrasive particles through contact with a high velocity water jet. The water jet is formed in an orifice on top of the head, while the abrasives enter the head through a separate entry. The mixing of abrasives,

Experimental results and discussion

The following subsections detail the results of recycling studies with particles more than 90 μm size and also with all particles.

Reuse of abrasives

Tests have been carried out to estimate the amount of reuse of local abrasives of test sample and mesh size 80, when the limit for the reusable size is 90 μm. Fig. 14a shows the recycling capacity of test sample after every use. It indicates that 81% of abrasives can be reused after the first cut, 49% after the second cut, 26% after the third cut, and 15% after the fourth cut. This result indicates that test sample of local abrasives is found to be having superior recycling capacity than the 60%

Summary and conclusions

This paper reports the findings of research on recycling of garnet abrasives available in southern India. Tests conducted on aluminium using optimised abrasive test sample, indicate improved performance of the abrasives compared to commercial abrasive with mesh size 80. The results are summarised as follows:

  • Particle size distribution, if controlled will yield improved performance of fresh as well as recycled abrasives. The test sample showed advantages in cutting as well as recycling. These are

Acknowledgements

The authors express their sincere thanks to Science and Engineering Research Council, Department of Science and Technology, Government of India, for the financial support for the equipment. They also acknowledge the financial assistance for research interactions with German counterparts under the DST-DAAD project based personnel exchange program 1999.

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