Quality Waterjet Newsletter 09/04/2007

Machinability Determination in AWJ Cutting


Machinability is an important parameter in an AWJ cutting process. Some limited information is available in the literature and from some AWJ machine manufacturers. If that is not enough, a test to determine machinability or estimation is often necessary. Zeng* presented a paper on the methodology of determining machinability on the 2007 WJTA conference.


A mathematical definition of machinability for AWJ cutting was given in Zeng’s paper. Several methods, that use this mathematical definition to calculate the material machinability,  were described. With a single cutting test, the following equation can be used to calculate the machinability N: 


where Q is the quality index (1-5), U is the cutting speed (separation speed if Q=1), h material thickness, MD mixing tube diameter,  Pw pressure, OD orifice diameter, Ma abrasive flow rate, and AM abrasive mesh number. The constant C = 4.272x10-4 for English unit system (U: inch/min, Pw: kpsi, Ma: lb/min, OD, MD, h: inch). F, Fn, Fa are respectively the Unknown Factor, Nozzle Index, and Abrasive Index (for lack of information, they can be set to 1).


To determine machinability more accurately, Zeng suggested that two tests be done with: one reference material with a known machinability and one subject material with an unknown machinability. The other process parameters are to be kept identical or as close as possible. Ideally the two tests should be done one after another so that the factors F, Fn, and Fa are also the same. As a result, the above equation turns into a relation between the ratio of machinability and the ratios of cutting speeds and some other parameters that are not identical.


Five different testing methods were described. The preferred method is to make multiple 2” long trial cuts on a flat part until the separation speed is found. After the first cut, the cutting speed is adjusted up or down to make the next cut. Testing is completed if two speeds are found, which are different by 5% or less, one separating the material and the other not. The separation criterion is that the sum of width of any remaining “bridges” at the bottom of cut is less than 1/16” --- “bridging free” criterion (see picture).


If testing is not possible, an educated guess can be made using this equation (where Rb is Rockwell B hardness):

It is also useful to know that the two versions of machinability values provided by OMAX Corporation and Flow International Corporation are different by a factor of 24 (i.e. OMAX’s # is about 24 x Flow’s #).


* Zeng, J. (2007) Determination of Machinability and Abrasive Cutting Properties in AWJ Cutting, in Mohamed Hashish (ed.), Proceedings of 2007 American WJTA Conference and Expo, August 19-21, 2007, Houston, Texas, Paper 3-B.



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