In Chapter 1, “Cutting Characteristics,” under the section “Creation of the abrasive waterjet stream”, we discussed how the abrasive waterjet stream is created. The cross-section of the cutting head is shown again here.

<img class="size-medium wp-image-6973 aligncenter" src="http://www.wardjet.com/wp-content/uploads/2022/03/closeup-of-waterjet-cutting-head-300x225.jpg" alt="closeup-of-waterjet-cutting-head" width="450" />

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As abrasive is added to the waterjet stream, the abrasive particles are accelerated to near the speed of the waterjet, approximately 2200 miles per hour (almost three times the speed of sound). This speed imparts momentum to the abrasive particles so that they can erode the material. Adding more abrasive gives more energy to the process and erosion occurs faster. Eventually a saturation point occurs where adding more abrasive robs speed and power from the waterjet stream and cutting speeds will start to decrease. Each waterjet manufacturer goes through extensive testing with various orifice and nozzle combinations to find the optimum abrasive amount, balancing cost and cutting speed.

The following graph shows what happens to cutting speed for ½” stainless steel as abrasive is added. Speeds are shown for the two most common orifice/abrasive nozzle combinations, 0.010” orifice with a 0.030” abrasive nozzle (orange line) and 0.014” orifice with a 0.040” abrasive nozzle (blue line).

Starting with an abrasive flow of zero pounds per minute, there would be absolutely no penetration of the material except for maybe a very light etching of the top surface of the material. At this point cutting speed is zero. Cutting speed increases as more and more abrasive is added. For the larger nozzle combination, speed increases until around 1.5 pounds per minute. At this point, cutting speed starts to decrease as too much kinetic energy is removed from the waterjet stream by the abrasive. A similar thing happens with the smaller orifice/nozzle combination, but at slower speeds and lower abrasive amounts.

<img class="size-full wp-image-2961 aligncenter" src="http://www.wardjet.com/wp-content/uploads/2021/11/Waterjet-University_Nozzle-n-Orifice-Abrasive-Amount-and-Cutting-Speed-Chart.jpg" alt="Waterjet University_Nozzle n Orifice Abrasive Amount and Cutting Speed Chart" width="617" height="343" />

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The optimum cost point may be slightly below what looks to be the apex of the speed curve. The “Law of Diminishing Returns” becomes evident. As the maximum speed is approached, each additional unit of abrasive that is added results in an ever smaller increase in speed. In the case of the orange line, increasing the abrasive from 0.7 pounds per minute to 1.0 pounds per minute yields an increase in speed of only 0.2 inches per minute. This 3.6% increase in speed results in a 4.7% increase in cost per inch.