The Machinability Index shown in most waterjet feed rate calculators defines the relative cutting rates of different materials. Materials with higher numbers cut proportionately faster than lower numbers. Mild steel has a baseline value of one. Stainless steel at 0.9 would indicate that it cuts about 10% slower than mild steel to achieve similar edge quality and tolerance results.

If you know the machinability indexes of two materials, you can estimate fairly easily a good cutting speed of one material from the other. We know that the Machinability Index of stainless steel is 0.9 and below we see that Aluminum’s Machinability Index is 2.9. You know that ½” stainless cuts at 5.5 ipm for the edge quality you want. You want to know the speed for ½” Aluminum. Divide the machinability of Aluminum by the machinability of stainless. 2.9 ÷ 0.9 = 3.2. Multiply the cutting speed for Stainless by 3.2 for the cutting speed in the same thickness of Aluminum. 5.5 ipm x 3.2 = 17.6 ipm. Therefore, 17.6 ipm would be a good place to start for ½” Aluminum.