I've been screwing around with building an abrasive waterjet for cutting sheetmetal. The concept is fairly simply, accelerate abrasive particles (garnet or silica sand) to a high velocity using water and slam them into the material you want to cut. A useful site for understanding waterjets is the Waterjet Web Reference

So I did some calculations and built a cutting head for use with my 1400 psi pressure washer and 40lbs. sand blaster...alas it was not enough water pressure to accelerate the particles adequately.

Here is a picture of the head I designed and built:


Here is a picture of the finished unit: