Abrasive Jet Machine

1. INTRODUCTION A set up for abrasive jet machining developed for the presents study. The proposed design of the set up minimizes commonly occurring deficiencies such as clogging of the nozzle, abrasive wastage, production of conical holes and low material removal rate. The effect of the process parameters has been experimentally analysed. In abrasive jet maching (AIM) material removal occurs on account of impact of high velocity stream of abrasive particles mixed with a iron brittle work piece made of material such as glass, ceramics, plastics, quartz, semi-conductors, super alloys, refractories, nylon, Teflon, resistors etc. Tiny brittle fractures occur and the carrier gas carries away the fractured fragments. AJM has been successfully used for cutting, drilling, etching. 2. DESIGN OF THE AJM SET UP Figure shows the schematic of the proposed experimental set up. A reciprocating air compressor is used to supply compressed air to a reservoir at a maximum pressor of 9 kg/cm2. On Opening the valve the air passes from the reservoir to the mixing chamber through a dehumidifier. High pressure air is also led to the top of an inclined hopper. The convergent divergent nozzle is attached at the inlet end of the mixing chamber. The nozzle diameters are selected such that sonic velocity exist at its throat. The exit nozzle is made of carbide and has a diameter of 1 mm. Finally ground powders of SiC and Al2O3 are used as abrasive powders. THE Abrasive hopper feeds controlled amount of abrasive into the mixing chamber through a controlled valve. All joints are made leak proof. In the present set up the work chamber covers not only the working area with work holding device and exit nozzle but also the abrasive hopper and the mixing chamber. The work piece is held in a work holding devie designed to move the work piece independently. During AJM the abrasive jet nozzle is kept stationary while the work piece is moved to maintain a constant stand off distance and required feed velocity. A vaccum dust collector is attached to the work chamber for carrying away the floating dust particles. 3. PARAMETRIC STUDY A systematic study through experiments has been carried out to investigate the effects of various process paramaeters on the material removal rate (MRR). The parameters considered were(i) air pressue, (ii) different abrasive grit sizes such as 90, 120 grits of SiC and 120 grits of Al2O3, (iii) stand off distance and (iv) nozzle sizes: 1mm and 2mm nozzle diameters. The following are the experimental observations: A. AIR PRESSURE : MRR increases with increase in air pressure as shown in figs. 2a to 2c. In the present setup the threshold pressure below which material removal ceases is about 3 kg/cm2. It is observed that MRR increases with increase in grit size and nozzle diameter. MRR is higher with SiC abrasive than with Al2O3 abrasive. B. GRIT SIZE : In figures 3a and 3b is given the effect of grit size on MRR. Final grit size produces higher MRR. This is attributed to the fact that final grit size has more number of particles per unit mass and this attend readily the velocity of air jet. C. EFFECTS OF SOD : Figure 4a and 4b show the effect of stand of distance (SOD)on MRR. According to these the MRR first increases with SOD and decreases beyond and optimum value. In this

present set up, the SOD is varied from 1mm to 10mm. It is observed that maximum MRR is obtained around 9 to 10 mm SOD. The air velocity is maximum at the nozzle exist, but abrasive particles being heavier tend to lag behind and attain maximum velocity some distance from nozzle exit. At this particular distance when the abrasive particles attain maximum velocity, MRR is also maximum. Above this SOD, the jet flares and particle velocity get decreases due to increasing in air resistance. Moreever due to flaring of jet, all the particles may not be impacting the work piece. The decrease in MRR is due to the above mentioned reasons. 4. ADVANTAGES : i) Ability to cut intricate holes shapes in materials of any hardness and brittleness ii) Ability to cut fragile and heat sensitive material without damage. iii) Low capital cost and power consumption. 5. DISADVANTAGES : i) Material removal is slow and hence its application is limited. ii) Process is not suitable for ductile material. 6. RESULT : It is also observed that optimum abrasive flow rate is about 25 gm/min. for SiC and 33 gm/min. for Al2O3. Maximum MRR is about 59 mm3/min. at pressure of 7 kgf/cm2 with SOD 9mm using 1.5mm diameter nozzle and 120 grit size SiC abrasive. 7. CONCLUSION : Experiment on present set up indicate that material removal rate (MRR) increases with air pressures, grain size and nozzle diameter. SiC was found to perform better than Al2O3. The present study shows that AJM is viable process for drilling of glass sheets and cutting counters. The geometric accuracy and surface finish is quite good. However optimization of process parameters in relation to material remove rate requires further investigation. 8. REFERENCE : i) Neema, M.L. and Pandey, P.C., “Erosion of glass when acted upon by an abrasive jet”. Proceeding of International Conference of Wear of materials, St.Louris, pp 387 – 392 (1977). ii) Purohit, B. K. and Deshingker, A.V. “Design and development of an abrasive jet machine unit”, Proc. IX ISME Conf. on Mech. Engg. Roorkee, pp 747 – 75 (1994).