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 (15), U is the cutting speed (separation speed if Q=1), h material
thickness, MD mixing tube diameter, P_{w} pressure, OD orifice
diameter, M_{a} abrasive flow rate, and AM abrasive mesh number. The
constant C = 4.272x10^{4} for English unit system (U: inch/min, P_{w}:
kpsi, M_{a}: lb/min, OD, MD, h: inch). F, F_{n},
F_{a} 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, F_{n}, and F_{a}_{ }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 1921, 2007, Houston, Texas, Paper 3B. 

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