Visual examination of the process was conducted on lexan, lucite and glass as work materials with the help of a movie camera. The mechanism o f material removal has been attributed to two modes: cutting wear mode that occurs at shal low angles of impact, and deformation wear mode which occurs at lower part of the cut at larger angles of impact. Nature of actual penetration o f abrasive water jet in solids has been reported by Hashish [June 1988].
Fig. 6.9 shows a relationship between penetration rate and depth of cut as a function of machining time estimated from the sequence of photographs. It is found that a number of steps are formed below the steady state interface at the top. The step formation takes place as the jet traverses over the workpiece. Size o f the step increases with the increase in the machined depth.
« 50
\
e
Ǥ 40
0) oc 30
oa
•*= 05 2 0c
0)a.
100
0.25
0.5
0.75
1.0
Tim e (s)
Fig. 8.9 Variation in depth of cut and penetration rate with machining time
PROCESS CAPABILITIES
AWJM process has been used to cut even thick materials (200 mm) with a narrow kerf. W idth o f the kerf depends upon the workpiece hardness. For hard materials, the kerf narrows down towards the bottom while the reverse is true in case of a soft material. Machined surfaces have not revealed [Benedict, 1987} the presence o f any thermal or mechanical damage to them. In case of machining, of glass, stray cutting may result in frosting.
APPLICATIONS
This process has been employed to cut a wide range o f materials including both metals (copper and its alloys, lead, tungsten carbide, aluminium, etc), and non-metals (graphite, silica, glass, acrylic, concrete, etc). The process has been applied to machine the sandwiched honeycomb structural materials currently used in the aerospace industries. Its advantage is the capability of omnidirectional cut ting having no burrs. The edges of structural aluminium plate have also been suc cessfully cut. This technique is getting acceptance as a standard tool for cutting materials in a number o f industries like aerospace, nuclear, oil, foundry, automotive, construction and glass. The specific advantages claimed by prom ot ers o f this technique are economic and environmental.
AWJ cutting has been employed for decommissioning nuclear facilities. The optimum performance has been reported with 0.38 mm jet at 200 M Pa with an abrasive flow rate of 0.54 kg/min. In some cases, this technique has proved to be economical as compared to conventional bulk material removal methods.
Slotting is one of the common applications of AWJC. Various steels (stainless steel, mild steel, special alloy steel, etc) have been cut into different shapes like plate, tube, corrugated structure, etc. K erf (slot) widths observed are in the range 0.75-2.25 mm. Roughness o f the cut surfaces varies with the machined depth and abrasive water jet cutting parameters. It is also reported [Hashish, Oct-Sept. 1984] that no embedded abrasives were seen while examining the machined surfaces under Scanning Electron M icroscope (SEM). Being a cold machining process, thermal stresses are not witnessed in the machined surfaces. Aluminium has also
BIBLIOGRAPHY
1. Benedict G.F. (1987), Nontraditional M anufacturing Processes, M arcel
Dekker Inc., New York.
2. Hashish M. (1982), The Application o f Abrasive Jets to Steel Cutting, Proc.
6th Int. Symp. Jet Cutting Technol., Guildford, England, pp. 447-464.
3. Hashish M. (1982), Steel Cutting with Abrasive W ater Jets, Proc. 6th Int.
Symp. Jet Cutting Technol., Guildford England, pp. 465-487.
4. Hashish M. (1983), Experimental Studies o f Cutting with Abrasive W ater
Jets, Second U.S. Water Jet Symposium, Rolla MO.
5. Hashish M. (1984), Cutting with Abrasive W ater Jets, Carbide Tool J., pp.
16-23.
6. Hashish M. (Sept., 1986), Aspects o f Abrasive W ater Jet Performance Opti
mization, Proc. 8lh Int. Symp. Jet Cutting Technology, Durham (U.K.), pp. 297-308.
7. Hashish M. (June, 1988), Visualization o f the Abrasive W ater Jet Cutting
Process, Exptl. Mech., pp. 159-170.
8. Hashish M. (April, 1989), A Model for Abrasive W ater Jet (AWJ) M achin
ing, Trans. ASME, J. Engg. Mater. Technol., Vol. 111, pp. 154-162.
9. Hashish M., Halter M. and M cDonald M. (May, 1985), Abrasive W ater Jet
Deep Kerfing in Concrete for Nuclear Facility Decommissioning, Proc. 3rd
U.S. Water Jet C on f, held at Univ. of Pittsburgh (Ed. Dr. N. Styler), pp.
123-143.
10. Malkin S. (1989), Grinding Technology. Theory and Applications o f
Machining With Abrasives, Ellis Horwood, U.K.
11. Momber A.W. and Kovacevic R. (1998), Principles of Abrasive W ater Jet Machining, Springer-Verlag London.
SELF-TEST QUESTIONS
Q l. W rite true (T) or false (F)
(i) AWJM can’t be applied to cut composite materials.
(ii) Physical and mechanical properties of workpiece material do not
affect performance of AWJM process.
(iii) MRR obtained during AJM is higher than AWJM.
(iv) Life of a nozzle made o f sapphire is as high as 400 hr.
(v) It is difficult to fabricate multiple jets central feed nozzle as compared to single je t side feed nozzle.
Q2. Choose correct answer from the given choices
(i) In AW JM, process intensifier is used to
(a) increase the pressure of water, (b) increase the flow rate of abrasives, (c) increase the velocity of water jet, (d) none o f these.
(ii) It is a finish machining process
(a) AWJM, (b) AJM, (c) USM, (d) AFM.
R E V IE W Q U E S T IO N S
Q3. W rite the names o f various elements of AWJC machine and explain them, in brief.
Q4. Why a need is being felt to develop a system which can directly use slurry instead o f mixing abrasives and water?
Q5. Answer the following questions, in brief.
(a) Some parts in an aerospace industry are made o f asbestos. Because of high level of asbestos in the air (harmful to the health of workers) at cut ting station, Osha shuts down Lockheed Georgia Com pany’s mechanical cutting operation section.
You as incharge of the section, are asked to suggest two o f the suitable advanced machining techniques so that the work can be resumed in this section.
(b) Composite plates are to be cut down into smaller pieces on large scale. Good finish is required. W hich process(es) will you recommend?
(c) 5 mm diameter hole is to be drilled in a concrete block o f a machine tool foundation. W hich advanced machining process(es) would you recom mend?
Q6. W rite the differences (in table form) between W JM, AJM, AFM, and AWJM processes (working principles, applications, limitations, and merits o f the