Use of Waterjet in Manufacturing Test Bars of High-Strength Steels.
Jenicek, Stepan ; Vratislav, Kotesovec ; Kalina, Tomas 等
Use of Waterjet in Manufacturing Test Bars of High-Strength Steels.
1. Introduction
In order to produce high-quality test specimens from high-strength
metal sheets, a new procedure that could deliver the desired quality and
effectiveness had to be developed and put in place.
Thermomechanically-processed materials of this kind are characterized by
high hardness, strength, and toughness [10],[11]. For this reason,
making specimens of these materials by classical chip cutting is
difficult and costly. Cutting plate is a difficult and time consuming
task, especially when the job calls for a tight tolerance or an unusual
shape. In those cases, can choose abrasive waterjet cutting [9],[4].
Abrasive waterjet cutting is an attractive process with many advantages.
The process is suitable for cutting steel of small thickness where it
provides a smooth and very precise cut. In terms of precision, waterjet
cutting can even outperform laser cutting, as the resulting edges are
smoother and free from melting, burning and thermal distortion [1].
Waterjet is now the most cost effective method to cut steels [9].
Therefore, an alternative machining process of abrasive waterjet cutting
was employed.
2. Available methods
Where materials are to be parted or cut, a decision must be made as
to the process to be used. Basically, one can choose from four primary
cutting processes: oxy-fuel cutting, plasma-arc cutting, laser cutting,
waterjet technologies or perhaps wire electrical discharge machining
(WEDM). Tensile test specimens must not develop any heat-affected zone
during manufacture because in this case the properties imparted by
thermomechanical treatment are to be measured. This is the reason why
all high-temperature cutting processes [2],[3] have been ruled out, i.e.
those which have severe heat effects on the cut area. Hence, the only
processes that are still available are waterjet cutting and wire
electrical discharge machining.
2.1 Abrasive waterjet cutting
Abrasive waterjet is an appropriate choice for cutting any dense
materials, ranging from very thin foils to parts of 300 mm or even
greater thickness. Cuts of the highest precision can be made with the
aid of draft angle compensation in materials of up to 50-100 mm
thickness, depending on the machine design. Rough cuts can be completed
in materials much thicker than 100 mm. [4] The process is suitable for
cutting steel of small thickness where it provides a smooth and very
precise cut. In terms of precision, waterjet cutting can even outperform
laser cutting, as the resulting edges are smoother and free from
melting, burning and thermal distortion. Waterjet is not constrained by
thickness, unlike laser and plasma technologies. Higher costs, on the
other hand, are a disadvantage. The investment cost of the equipment
exceeds that of plasma cutting equipment while being lower than in laser
equipment. By contrast, the operating costs of waterjet cutting are much
higher than laser cutting, which is mainly due to the price of the
abrasive. [1]
2.2 Wire electrical discharge machining (WEDM).
Wire electrical discharge machining can only be used for
electrically conductive materials. Sculptured products with a thickness
up to 500 mm can be machined [6]. The greatest advantage of the process
is its ability to machine electrically conductive materials regardless
of their hardness. The process does not generate any conventional
cutting forces. During cutting, only electric erosion forces occur. The
cut must be made through thickness of the material. The method is
well-suited for applications that required shape and dimensional
accuracy. With the introduction of NC systems, this technology became
one of the leading machining processes in terms of precision. The
contour accuracy in state-of-the-art machines is several micrometres and
the quality of the cut surface matches the parameters of ground
surfaces. The most common diameter of brass wire used for WEDM is 0.25
mm. Today's top-quality machines can use wires with diameters down
to 0.02 mm. Electrical discharge machining does not introduce adverse
stresses into the material. It can also produce parts with very thin
walls [5]. Nevertheless, the operating costs of WEDM are much higher
than those of waterjet, mainly because of its slow machining speed. The
investment costs of the equipment are approximately the same as those of
waterjet cutting equipment, provided that the cutting space in the
machine is considerably smaller.
2.3 Process selection
Optimized test specimens (Fig. 1) are thermomechanically treated in
a thermomechanical simulator [7]. They must be converted to
standard-shape specimens (Fig. 1) for tensile testing. For reasons
related to economy and speed of specimen preparation, abrasive waterjet
cutting was chosen. For this process, a workholding fixture for accurate
and firm clamping of the specimens with respect to the tool had to be
designed.
2.4 Process selection
The fixture is a very important item, as it has a major impact on
the test bar quality. The main reason for using a fixture is the need
for quality and dimensional, shape and geometric accuracy.
The requirements for the workholding fixture for waterjet cutting
were as follows:
* secure positioning
* firm clamping of the part
* clamping the part in a defined position with respect to the tool
The design (3 and 4) was developed using the SolidEdge ST8
software.
The design of the fixture allows up to 8 specimens to be cut out in
a single series. This solution is motivated by the economy of and time
savings in production. The test specimen is centred in the fixture with
V-grooves. Mating cylindrical surfaces are guided accurately in the
fixture. At the top, they are provided with threads for holding the
specimens in place. The transverse plane of the fixture is provided with
marks in the form of channels. These are used for centring the specimens
before tightening.
To verify the design and function of the fixture (Fig. 5), it was
manufactured of S235 steel. This material is not the optimal choice for
this fixture because it lacks corrosion resistance but it is fully
sufficient for verifying its function. Semi-finished products were also
made by waterjet cutting. The fixture was designed to be producible from
sheet metal.
3. Experimental
The process was tested in the WJ 2830-2Z-Cobra-PJ60 machine. The
experiment compared the alignment of cutouts from the original, and the
final shape. A total of 23 specimens were tested (Tab 1). The
measurement was carried out using the CMM Carl Zeiss Prismo 7 Navigator
3D measuring machine. A total of four parameters were measured (Fig. 5):
specimen width--dimension A, the distance between the right edge of the
cutout from the initial contour--dimension B, the distance between the
left edge of the cutout and the initial contour-- dimension C, and the
angle between the centreline of the initial specimen and the centreline
of the newly-cut specimen.
Test arrangement:
* PTV JETS 3.8/60-COMPACT high pressure pump (max. operating
pressure 415 MPa, max. flow 3.8 l/min)
* X-Y cutting table PTV WJ 2830-2Z (working space of 3000*2000 mm)
* Continuously variable cutting speed 0-20 m/min
Experimental conditions
* Operating pressure [MPa]: 400
* Abrasive type: GMA Australian garnet
* Abrasive particle size [MESH]: 200
* Mass flow of abrasive--[g/min]: 30
* Wateijet nozzle diameter [mm]: 0.10
* Abrasive nozzle diameter [mm]: 0.508
* Standoff distance [mm]: 4
Workpiece material
* Material--EN10131 DC01
* Thickness [mm] 1.5
As evidenced by statistical characteristics of the set of results,
the fixture meets the requirements for clamping precision (Tab. 1).
Since the cutting error is caused by random factors, the two sigma rule
can be applied. According to this rule, an interval of two standard
deviations around the mean value will contain 95.4% of the actual
measured values according to Gauss distribution. In this case, an
accuracy of 0.03 mm from the mean value is achieved with 95.4%
confidence. As a consequence of inaccurate setting of kerf width
compensation, the deviation of the mean width of the test specimen from
the required value was 0.15 mm. In a subsequent run, this deviation was
eliminated by precise measurement of the first piece and by resetting
the kerf width compensation function. The difference of 0.4 mm between
the B and C values was due to inaccurate movement to the zero reference
point. This error was also eliminated by an improved fixture positioning
procedure in the machine. In no case did the misalignment between the
initial and the final contour exceed the critical limit of 1[degrees].
4. Conclusion
As many roads lead to the final destination, each product can be
manufactured in various ways. When producing of the product, the goal
was to produce the product for cutting tensile tests specimens in such
way, which will full fillnot only all reguirements of drawing
documentation, but also will be produced in the cheapest way. Another
aim was to demonstrate practical experience of making the product with
the technology of waterjet cutting.
Waterjet cutting was used for making a fixture for preparing test
specimens for tensile testing. As the materials in question were AHSS
steels, the specimens were impossible to be prepared in required
quality, quantity and at the required cost by classical machining
routes. The fixture for making eight test specimens was tested. It was
found that all requirements for the test specimen quality have been met.
In the future, we will focus on optimizing cutting conditions and
their influence on surface integrity.
DOI: 10.2507/27th.daaam.proceedings.032
5. Acknowledgements
The present contribution has been prepared under the project LO1502
'Development of the Regional Technological Institute under the
auspices of the National Sustainability Programme I of the Ministry of
Education of the Czech Republic aimed to support research, experimental
development and innovation.
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This Publication has to be referred as: Jenicek, S[tepan];
Kotesovec, V[ratislav]; Kalina, T[omas] & Masek, B[ohuslav] (2016).
Use of Waterjet in Manufacturing Test Bars of High-Strength Steels,
Proceedings of the 27th DAAAM International Symposium, pp.0219-0224, B.
Katalinic (Ed.), Published by DAAAM International, ISBN
978-3-902734-08-2, ISSN 1726-9679, Vienna, Austria
Caption: Fig. 1.--Specimen shape
Caption: Fig. 2--Modified specimen shape
Caption: Fig. 3: Visualized fixture with eight test specimens
Caption: Fig. 4--Plan view from bottom
Caption: Fig. 5--Specimen dimensions of interest
Tab 1.--Results of measurement
Specimen Dimension Dimension Dimension Angle
number A [mm] B [mm] C [mm] [[degrees]]
1 3.181 10.335 10.697 0.77
2 3.174 10.385 10.696 3.27
3 3.182 10.336 10.678 0.23
4 3.177 10.351 10.674 1.10
5 3.186 10.269 10.788 0.45
6 3.163 10.325 10.721 7.22
7 3.178 10.322 10.767 3.19
8 3.168 10.335 10.732 5.38
9 3.199 10.360 10.740 4.33
10 3.142 10.412 10.640 0.52
11 3.202 10.252 10.840 0.03
12 3.180 10.364 10.690 1.42
13 3.168 10.228 10.856 4.97
14 3.182 10.370 10.745 2.05
15 3.186 10.318 10.787 0.55
16 3.190 10.312 10.734 4.43
17 3.196 10.303 10.763 2.35
18 3.175 10.412 10.664 0.45
19 3.207 10.286 10.865 3.27
20 3.193 10.407 10.683 4.85
21 3.189 10.255 10.773 1.97
22 3.209 10.376 10.677 3.88
24 3.186 10.354 10.734 7.83
Mean value 3.183 10.333 10.737 2.805
Standard
deviation 0.015 0.050 0.060 2.218
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