Journal of Materials Processing Technology 189 (2007) 292–295

A study of taper angles and material removal rates of drilled holes in the abrasive water jet machining process Mahabalesh Palleda ∗ Mechanical Engineering Department, NH-4, Bypass, S.J.M. Institute of Technology, Chitradurga 577502, Karnataka, India Received 11 May 2006; received in revised form 20 December 2006; accepted 29 January 2007

Abstract The objective of this paper is to study the effect of using different chemicals on material removal rate, with varied stand off distances and chemical concentration in abrasive water jet machining. The use of such chemicals on the taperness of drilled holes is also studied. The study reveals the use of polymer can reduce the taper of the holes drilled. © 2007 Published by Elsevier B.V. Keywords: Abrasive water jet machining; Material removal rate; Stand off distance; Polymer; Acetone; Phosphoric acid

1. Introduction Greater advances have been noticed in the material, which are harder and difficult to machine. The traditional methods have become ineffective for machining of hard and brittle materials such that productivity cannot be achieved. Due to this some non-traditional machining processes came into existence and the latest addition to that series is the abrasive water jet machining (AWJM). Abrasive water jet machining makes use of the principles of both abrasive jet machining and water jet machining. In abrasive water jet machining, a small stream of finegrained abrasive particles is mixed in suitable proportion, which is focused on a work piece surface through a nozzle. Material removal occurs due to erosion caused by the impact of abrasive particles on the work surface. Non-contact of the tool with work piece, no heat affected zone, low machining forces on the work surface and ability to machine a wide range of materials has increased the use of abrasive water jet machining over other machining processes. Previous investigations [1–4] indicate that even though some efforts have been made to increase the material removal rate (MRR), the taperness of the drilled holes was not being reduced. Now an attempt has been made to increase MRR and to decrease the taperness by varying stand off distance ∗

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0924-0136/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2007.01.039

(S-O-D) with different chemical environments and chemical concentration. 2. Experimental set-up The experimental set-up of abrasive water jet machining consists of a main cylindrical tank with a semispherical bottom, a storage tank, an air compressor, abrasive water jet cutting head and work holding arrangements. The working tank has provision for slurry inlet and outlet and also for highpressure air inlet. The cutting head with a nozzle is mounted on a stationary work table. The S-O-D is adjusted by the vertical movement of the cutting head and is measured by the graduated scale fitted to the cutting head. The cutting head has a provision to accommodate nozzles of different orifice diameters (Fig. 1).

3. Working procedure The abrasive is mixed with water in a suitable proportion in the slurry tank and stirring should be done continuously till all the slurry from the slurry tank is passed to the working tank. After passing the slurry mixture into the working tank, compressed air is also being passed into the working tank. This compressed air pressurizes the slurry mixture and also avoids the settlement of abrasive particles in slurry. The pressurized slurry is let out to the cutting head through a pressure hose and then through the nozzle will make to impinge on the work surface. The impact of high-pressure abrasive particles on the work surface lead to erosion of the work material, such that material removal takes place. Abrasive water jet machining with pressurized slurry is essentially an erosion process, which involves two, distinct

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Fig. 1. Abrasive water jet machining set-up.

mechanisms depending upon weather the eroded material is brittle or ductile in nature. Ductile erosion is defined as a process in which the abrasive particles progressively cut the eroded materials, eventually causing volumetric material removal. Brittle erosion is described as a cracking process in which material is removed by the propagation and intersection of cracks ahead of and around the abrasive particles. 4. Results and discussions A series of experiments were conducted taking abrasive particle as Silicon carbide of 180 grit size which are of irregular spherical in shape, on a glass as a specimen, of thickness 4 mm. The working pressure used here is of 60–80 N/mm2 . The results are discussed to analyze the effect of S-O-D and chemical concentration on material removal and taperness. Analysis was made on:

It can be noticed from Fig. 2 that maximum material removal was observed at an S-O-D of 4.5 mm using plain water in the slurry, at 4 mm for acetone with slurry and at 3.5 mm for the slurry mixed with phosphoric acid. Comparison of material removal obtained with plain water slurry, acetone with slurry and slurry mixed with phosphoric acid shows higher material removal indicating more effectiveness. But the fourth curve which is due to the slurry added with polymer shows highest material removal than the other three slurries. The trend observed in the fourth curve (where polymer is used) indicate an increase in material removal with an increase of S-O-D. The reason for the increase of MRR with increase of SO-D may tend to increase in momentum of impacting abrasive particles on the work surface creating craters of more depth. This leads to more erosion of work material and higher material removal can be expected.

(1) S-O-D versus MRR, S-O-D versus taperness. (2) Chemical concentration versus MRR, chemical concentration versus taperness. 4.1. Effect of variation of S-O-D In the present work, experiments have been conducted under different chemical environments like acetone, phosphoric acid and polymer (polyacrylamide) in the ratio of 30% with 70% of water. By varying S-O-D, MRR is measured by noting the initial and final weight of a specimen keeping blasting time 2 min, then the following observations were made.

Fig. 2. Effect of variation of S-O-D on material removal.

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M. Palleda / Journal of Materials Processing Technology 189 (2007) 292–295

Fig. 3. Effect of variation of S-O-D on taperness. Fig. 4. Effect of variation of chemical concentration on material removal.

The increase in material removal obtained using the chemical environments like acetone and phosphoric acid indicate the effect of hydrogen (chemo-mechanical effect) [4] in enhancing of cracks formed due to the impact of abrasive particles on work material. The polymer in the slurry creates cohesiveness [5,6] in the jet, bonding the water molecules along with abrasive particles leading to narrowing down of the beam of the jet. This avoids spreading of the abrasive particles as they come out of tip of the nozzle and almost all the abrasive particles are expected to take part in impacting on the work surface leading to highest material removal, the hole diameter of 2.041 mm and the length of 4 mm is made. Referring to Fig. 3, it can be noticed that there is a considerable reduction in the taper of the drilled holes as the S-O-D is being increased. The reduction in taper is assumed to be due to more momentum gained by the abrasive particles with an increase in S-O-D and also more number of abrasive particles might take part in impacting on the work surface and eroding the work material. Fig. 3 indicates that the hole taper observed to be less in case of phosphoric acid combination with slurry than the plain water slurry and the slurry with acetone combination. But hole taper observed with the use of polymer seems to be almost nil. This drastic reduction in the hole taper is attributed to be due to bonding of water molecules along with abrasive particles together leading to a narrow beam of the jet making effective impact on the work surface. Under this situation, it is expected that almost all the abrasive particles available in the jet might take part in erosion of the material leading to greater reduction in taper. 4.2. Effect of variation of concentration of different chemicals The effect of variation of concentration in ratio of 8:1of different chemicals on material removal can be observed referring to Fig. 4 and 3 mm of S-O-D is maintained. From Fig. 4, it is evident that at lower concentration of chemicals used, material removal seems to be less. The material removal is observed to be increasing with increase of chemical concentration of acetone and phosphoric acid in the slurry up to a certain level and then receding. But in case of polymer

Fig. 5. Effect of variation of chemical concentration on taperness.

used with the slurry shows a continuous increase in material removal. The reasons for increase in material removal from lower concentration to higher concentration up to a certain limit in case of acetone as well as phosphoric acid combination with slurry might be due to increased chemo-mechanical effect. Well after the peak limit, even though concentration is being increased, there is a decrease in trend in material removal indicating saturation limit of the chemical concentration used with such proportions of abrasive slurry. In case of abrasive slurry mixed with polymer, the continuous increase in material removal being assumed to be due to increase in bonding effect of water molecules and abrasive particles available in the jet which makes effective impact on the work material automatically enhances the material removal. The effect of variation in chemical concentration on taperness of the holes drilled can be well observed from Fig. 5. In case of phosphoric acid combination, the taper of the hole is comparatively less than acetone combination. The taper in case of polymer combination is observed to be very less or it can be said as nil, due to effective impact of narrow beam of slurry jet on the work surface leading to higher material removal eliminating the taper. 5. Conclusions Based on the experiments conducted and observations made, following conclusions are drawn:

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(1) The material removal increases with the increase in S-O-D, up to certain limit and further increase in the S-O-D beyond the limit results in decrease of the material removal. (2) The material removal was found to be more in presence of chemically active liquids such as acetone and phosphoric acid rather than plain water in the slurry. (3) The material removal was identified to be the highest in the case of a slurry mixed with polymer (polyacrylamide) rather than other two chemical environments used in the experiments. (4) The slurry with a polymer combination shows a continuous increase in material removal with a variation in the chemical concentration. (5) The chemical concentration was observed to be having an influence over the taper of the holes produced. The hole

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taper in case of polymer combination showed almost nil taper. References [1] Y. Enomoto, Sliding fracture of soda-lime glass in liquid environment, J. Mater. Sci. 6 (1981) 3365–3370. [2] M. Hashish, A Model of Abrasive Water Jet Machining, J. Eng. Mater. Technol. Trans. ASME (1989) 154–162. [3] M. Hashish, Pressure effect in AWJ machining, J. Eng. Mater. Technol. 3 (1989) 221–228. [4] A.R.C. Westwood, Control and application of environment sensitive fracture processes, J. Mater. Sci. 9 (1974) 1871–1995. [5] W. Glenn Howells, Polymerblasting—A Chemist’s Point of View, Berkeley Chemicals Research, Inc., Berkeley, California 94709. [6] W. Glenn Howells, Polymerblasting with Super-Water from 1974 to 1989: A Review, Berkeley Chemicals Research, Inc., Berkeley, California 94709.