This part of the abrasive water jet machining is certainly the most crucial. Ordinarily the abrasive particles are fed into the side of the jet, sped up by the jet and delivered to the work piece. Conceptually the following system, Fig. 19, would work and similar designs, Figs. 20-22 have been studied extensively by Hashish [1].

Fig. 19 Abrasive Particle Water Jet Mizing Design

However there are several considerations. First the velocity profile across the jet is not uniform and hence the particles would tend to enter the side of the jet where the velocity is slowest. If the abrasive particles are not moving to begin with they will act as drag on the jet. As they enter into the jet they tend to increase the turbulence of the jet. Therefore much effort has gone into the Abrasive Particle Delivery System

Here are several other potential designs, two of which try to give the particles speed either due to gravitational force or air pressure, and the other being a design using a premix of the water and particles before jet formation.

Fig.20 Particle Water Premix [ 2 ]

Fig.21 Abrasive Particle Gravity Feed [ 3 ]

Fig.22 Forced Air Particle Feed [4]

For illustrative purposes Fig. will be used since it is a basic model that combines the jet former, the mixing chamber and the collimator (which is also known as the focus tube). The top section is the exiting straight portion of the jet former. Here the jet exits the nozzle and enters into the mixing chamber. From Bernoulli’s equation one can approximate the jet speed into the chamber as:

 ....(8)

where P is the pressure developed in the straight portion of the jet former and r _W is the density of the water. Here it is assumed that the pressure in the jet former is significantly greater than in the mixing chamber and the velocity of the exiting jet into the mixing chamber is much greater than the fluid velocity in the jet former. With these assumptions then the formula above is a good approximation to the jet speed as it enters the mixing chamber.

The abrasive particles are supplied from the right inlet tube and are pulled in by a pressure differential created by the moving fluid past the inlet port (similar to the lift developed on an aircraft wing as the air above the wing moves faster than the air under the wing bottom thus creating an upward pressure potential or lift.) From a simple conservation of momentum, the speed of the resulting abrasive particle loaded water jet can be estimated as

 ....(9)

where m_a and m_w are the mass flow rates of the abrasive particles and jet water respectively and V_a is the speed of the abrasive water jet as it enters the bottom portion of the mixing chamber. In applying the conservation of momentum it has been assumed that the impact of the jet stream water and the abrasive particles is a perfectly inelastic collision.

The purpose of the final reduced cross-sectioned nozzle is to collimate or focus the abrasive laden water jet before it exits just above the material to be machined. This is to insure that all the abrasive particle velocities are directed toward the work piece. Some energy however is lost because some of the abrasive particles do collide with the focusing tube wall. Hence there is some overall velocity reduction as the jet exits the focusing tube (from 3000ft/s to 1000ft/s).

Several problems occur in the abrasive water jet-mixing chamber. First upon impact of the abrasive particles by the high speed jet there is the potential that the particles will be fractured and hence reduced in size. Secondly the entrainment of the particles in the jet stream is by no means uniform. In fact most of the abrasive is in the peripheral section of the jet with very little in the center of the jet. The center of the jet is of course the area of highest speed, so the particles are in the slower boundary region of the jet. This entrainment of the particles within the jet periphery is not bad since as will be seen later on, the actual cutting by the jet results from the interaction of the jet surface with the material.

Upon exiting the collimator the abrasive water jet hits the work piece material, and it is the repeated action of the individual particle impacts that perform the machining operations. Because of the size of the abrasive particles the impact on the part of an individual particle is small but together the total number of particles erode the material.