Quality Waterjet Newsletter 05/22/2007

Deep Hole Drilling with AWJs


Drilling a 300-mm-deep, 19-mm-diameter hole in tungsten with EDM takes over 40 hours. But how would you do it otherwise? Hashish* proposed doing it with abrasive waterjets.

Three methods of deep hole drilling were illustrated in the diagram. A series of drilling tests were done with an oscillating jet and a rotating workpiece. A 30.5-mm-deep hole was drilled on a tungsten rod at the rate of 160 mm/hr, with a 0.330-mm-diameter jet at 345 MPa of pressure and 3.75 g/s of mesh 100 garnet. By manipulating the oscillation frequency and the rotational speed, the bottom face of the hole can take the shape between high-in-the-center to high-at-the-rim. A computer simulation study of the jet path coverage showed that, to achieve an even coverage of the entire drilling face, the ratio of the oscillation frequency and the rotational speed should not be an integer. An alternation to the first method is to rotate a radially offset jet instead of oscillation. It was found that the implementation was easier. Similar to the oscillation method, the ratio of the two rotational speeds should not be an integer. The geometry at the bottom of the hole can be manipulated to some degree by changing the offset value. Ideally the diameter of the drill should be roughly equal to the required radius of the hole. The jet had a 3° angle to the rotational axis in this study. Experiments with a non-rotary rectangular jet and a rotating workpiece were also done. Multiple parallel waterjets shot through a rectangular mixing tube. Different sizes of jets can be used to achieve a desired power gradient for geometrical purpose - a flat bottom is possible. A drilling rate of 115 mm/hr was achieved on tungsten with two 0.254-mm-diameter jets (equivalent to one 0.330-mm-diameter jet in power consumption).


Higher drilling rates can be achieved by increasing pressure and abrasive flow rate. Test results showed that drilling rate can reach 457 mm/hr for steels and 305 mm/hr for tungsten. With these methods, drilling holes with thin walls is possible. However to minimize the hole diameter variation, an adaptive control system is needed to match the advance speed to the material removal rate. A trial-and-error approach can also be used. The AWJ-drilled holes have a high quality and are relatively smooth with a typical Ra roughness of 2.2 micron.


* Hashish, M. (1996) Deep hole drilling in metals using abrasive waterjets, in Colin Gee (ed.), Proceedings of the 13th International Conference on Jetting Technology, Sardinia, Italy, October 29-31, pp 691-707.



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