Citation: Kim, J.-U.; Kim, R.-W.; Kim,
S.-R.; Kim, H.-H.; Lee, K.-C. Nozzle
Condition Monitoring System Using
Root Mean Square of Acoustic
Emissions during Abrasive Waterjet
Machining. J. Manuf. Mater. Process.
2022, 6, 31. https://doi.org/10.3390/
jmmp6020031
Academic Editors: Arkadiusz Gola,
Izabela Nielsen and Patrik Grznár
Received: 8 February 2022
Accepted: 1 March 2022
Published: 7 March 2022
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Manufacturing and
Materials Processing
Journal of
Article
Nozzle Condition Monitoring System Using Root Mean Square
of Acoustic Emissions during Abrasive Waterjet Machining
Jeong-Uk Kim
1
, Roh-Won Kim
2
, Sung-Ryul Kim
2,
*, Hyun-Hee Kim
1
and Kyung-Chang Lee
1,
*
1
Department of Control and Instrumentation Engineering, Pukyong National University,
Busan 48513, Korea; kcup2001@naver.com (J.-U.K.); hhkim@pknu.ac.kr (H.-H.K.)
2
Precision Mechanical Process and Control R&D Group, Korea Institute of Industrial Technology,
Busan 46938, Korea; nasca01@kitech.re.kr
* Correspondence: sungrkim@kitech.re.kr (S.-R.K.); gclee@pknu.ac.kr (K.-C.L.)
Abstract:
Machining of difficult-to-cut materials such as titanium alloys, stainless steel, Inconel,
ceramic, glass, and carbon fiber-reinforced plastics used in the aerospace, automobile, and medical
industries is being actively researched. One non-traditional machining method involves the use
of an abrasive waterjet, in which ultra-high-pressure water and abrasive particles are mixed and
then ejected through a nozzle, and the thin jet stream cuts materials. The nozzle greatly affects the
machining quality, as does the cutting tool of general machining, so it is very important to monitor the
nozzle condition. If the nozzle is damaged or worn, or if the bore size increases or the bore becomes
clogged with abrasive, the material may not be cut, or the surface quality of the cut may deteriorate.
Here, we develop a nozzle monitoring system employing an acoustic emission sensor that detects the
nozzle condition in real time.
Keywords: abrasive waterjet; nozzle wear; acoustic emission sensor; monitoring system
1. Introduction
Waterjet machining is a cutting method in which a high-pressure water/abrasive
mixture is delivered to the material surface via a nozzle. Pure waterjets are used to cut soft
materials such as foods, medicines, fabrics, or wood. Such waterjets cannot cut metals or
hard composites, so an abrasive waterjet (AWJ) is required [
1
–
18
]. Water at 300–600 MPa
and abrasive particles (#50–200 mesh) are mixed, ejected at high speed via a nozzle (bore size
0.24–0.40 mm), and the material is cut by the jet stream. AWJ machining affords excellent
surface quality and precision when fabricating complex shapes. Such machining can be
integrated with industrial robotics and has benefited from advances in ultra-high-pressure
pumps and nozzles. AWJs are increasingly being used to handle difficult-to-cut materials.
AWJ machining does not require the (expensive) tools of conventional machines [
19
–
21
].
AWJ machining quality (surface roughness and cut depth, width, and shape) is affected
by various factors [
22
]. The nozzle is very important: wear, damage, and bore expansion
may cause abrasive clogging that reduces the quality of the cut surface. Nozzle condition
must be monitored in real time and damaged nozzles must be replaced in a timely manner.
Currently, nozzles are replaced entirely after 50–100 h of operation, based on the subjective
judgment of operators [22].
In general, the lifetime of an AWJ nozzle is affected by processing parameters, nozzle
properties, and any misalignment between the orifice and nozzle in the mixing head.
Nozzle wear (which is directly related to nozzle lifetime) affects the waterjet pressure,
orifice diameter, shapes and sizes of the abrasive particles, the abrasive feed rate, the bore
diameter, and the angle/depth of the inlet cone [
22
]. Impacting abrasive particles carried
by the waterjet create a wavy erosion zone, and the erosion propagates to the exit of the
nozzle bore. The high-pressure water passes through the orifice of the mixing head to form
a high-speed flow, and the abrasive particles are sucked into the chamber by the Venturi
J. Manuf. Mater. Process. 2022, 6, 31. https://doi.org/10.3390/jmmp6020031 https://www.mdpi.com/journal/jmmp
