Plasma Cutting of Structured Sheet Metals in Comparison with Laser Beam Cutting

Journal of Materials Science and Engineering B 5 (3-4) (2015) 135-144 doi: 10.17265/2161-6221/2015.3-4.004 D DAVID PUBLISHING Plasma Cutting of St...
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Journal of Materials Science and Engineering B 5 (3-4) (2015) 135-144 doi: 10.17265/2161-6221/2015.3-4.004

D

DAVID

PUBLISHING

Plasma Cutting of Structured Sheet Metals in Comparison with Laser Beam Cutting Leander Schleuss*, Thomas Richter, Ralf Ossenbrink and Vesselin Michailov Department of Joining and Welding, Brandenburg University of Technology Cottbus-Senftenberg, Cottbus 03046, Germany Abstract: One main problem of thin-walled, large-area sheet components for lightweight construction solutions is the inadequate stiffness. One approach for a solution is provided by structured sheets which have an increased flexural stiffness due to three-dimensionally incorporated structures up to 3 times compared with flat sheets. Since few systematic investigations have been conducted on structured sheets in the field of characterization, forming, welding and joining technologies and corrosion until now, there is a lack of knowledge in the field of cutting technologies. The aims of the presented work is to qualify the plasma cutting process for thin structured sheets as an alternative to laser cutting. The qualification of the process was carried out with roughness and straightness measurements and by assessing the absence of burrs. The results obtained show the good quality of plasma cutting on structured sheets and were compared with the results of qualified laser beam cuts. Key words: Structured sheet metals, lightweight, plasma cutting, distance control, cut quality, design, automation.

Nomenclature KW: ln: lr: l t: neg.SP: pos.SP: Ra: Rz5: s: SP: smooth. const.: vs: vcorr:

Key width Evaluated measuring length Single measuring length Total measuring length Negative structure location Positive structure location Arithmetic average roughness Averaged roughness of five measurements Distance between sheet and torch Structure position Smoothing constant Cutting velocity Correction velocity/correction rate

1. Introduction One main problem of thin-walled, large-area sheet components for lightweight construction is the inadequate stiffness. One approach for a solution is provided by structured sheet metals which have an increased flexural stiffness due to their three-dimensional structures. The stiffness increase of

*

Corresponding author: Leander Schleuss, Dipl.-Ing, research fields: cutting, joining and welding. E-mail: [email protected].

the considered structured sheet metals is up to three times compared with flat sheets. The application of the structured sheet metals in automotive and railway industry, civil engineering and vessel construction needs the investigation of the processing technologies. Since few systematic investigations have been conducted on structured sheets in the field of characterization, forming, welding and joining technologies and corrosion behaviour until now [1-8], there is a lack of knowledge about the further processing of these sheets by means of cutting technology. There is only one investigation in the field of laser beam cutting [9]. Plasma cutting is an important cutting process in industrial application. The advantages are low equipment costs, less maintenance requirements, easy automation and the possibility to cut non-conductive materials. The safety precautions are lower in comparison to laser beam cutting. Some process variants were developed [10-13]. Mostly conventional plasma cutting and new process variants or the handling of equipment (e.g. torch changing) are automized [14-17]. Some investigations on

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Plasma Cutting of Structured Sheet Metals in Comparison with Laser Beam Cutting

conventional and new process variants are done, e.g. about the emission rate on hot-wire-plasma-cutting [18] or the influence of cutting gas on quality of cutting processes [19, 20]. Plasma cutting is also used for finishing edges on components in shipbuilding, for marking [14, 16, 21]. Therefore, this process has widespread applications in many industrial sectors [22], e.g. in shipbuilding [22], in vessel construction [14] or in automotive manufacture [12]. The achievable cut quality is comparable to laser beam cutting [12].

2. Investigation Subject and Objective The investigation subject are structured sheet metals (material: DC04, sheet thickness 0.7 mm) whose regularly arranged honeycomb structure is produced by means of hydroforming. The honeycombs have a KW (Key width) of 33 mm and an average structural height of 3.0 mm. The bridges between the honeycombs exhibit a width of 2 mm and the sheet metals have two structure directions (Fig. 1). When the bumps are orientated to the plasma torch, the structure location is indicated as “positive”, otherwise “negative”.

The objective of the investigations is to qualify the plasma cutting of the described structured sheet metals based on cut criterions: straightness, burr-freeness and roughness of cut face. In addition, recommendations for users should be given. The three-dimensional structure requires special approach to ensure a constant distance between plasma torch and the structured sheet metal. It was carried out by two approaches: a pre-programmed nonlinear cut path according to the structure topology and a self regulating distance control by use of the cutting voltage [11, 15]. The advantage of the first option is the nearly constant distance between torch and sheet metal which depends mainly on the accuracy of the programming and the handling system. The disadvantage is the needed accurate positioning of the work piece, the extensive programming process for different components resulting in less flexibility. The advantages of the second option are flexible cutting paths with reduced programming effort and higher valid tolerances for positioning of work pieces. Both options are compared according to their influence of the torch distance on the cutting quality. It is one of the most important influencing factors [11, 12, 15, 20, 21]. Finally, the comparison of the cutting quality with that of laser beam cutting of structured sheet metals allows the evaluation of plasma cutting relating to its applicability.

3. Experimental Setup The cutting experiments were carried out with a robot KUKA HA 30 with the control system KRC2. For the plasma cutting experiments, the equipment Kjellberg Hi Focus 160i with the plasma torch Percut 170 was used. A high speed camera with frame rate of 5,000 fps monitored the effect of the distance control on the

Fig. 1 Geometric dimensions and characteristics of structured sheet metals.

cutting process. A synchronized laser light with adapted frequency illuminated the cutting process. Fig. 2 shows the setup with the high speed camera.

Plasma Cutting of Structured Sheet Metals in Comparison with Laser Beam Cutting

The length of the cutting samples is 585 mm. The complete sheet metal is fixed with a clamping fixture to avoid the deflection during the cutting.

4. Operation and Distance Control

Parameterization

of

The plasma cutter integrates a cutting voltage gauge. A voltage divider converts the measured voltage to an analogue signal in a range from 0 to 10 volts. A measuring amplifier was used to monitor this analogue signal. The signal is transferred into the robot control and digital converted. Fig. 3 illustrates the control loop. The following equation corrects the distance between plasma torch and sheet metal (Eq. 1). ANIN ON $TECHIN[i] = FACTOR × CORRECTION + OFFSET (1) The parameter CORRECTION represents the input signal off the analogue clamp (cutting voltage). The

Fig. 2

Experimental setup for high speed imaging.

Fig. 3

Scheme to device communication and data flow.

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range of the signal is from 0 to 10 V. Due to an automatically executed normalization of the signal in the analogue clamp (range from 0 to 1), a factor with the value 10 is used to scale the values to the real measured values. The variable OFFSET allows the specification of an intended distance between plasma torch and sheet metal. After that the command, ANIN ON $TECHIN[i] assigns the calculated signal to the system variable $TECHIN[i]. In this way, the controller internal variable $TECHIN[i] is described cyclically with the values of the analogue clamp and transferred to the distance correction program. The distance control is working with a correction velocity depended on the controller internal variable $TECHIN[i]. For the distance control a function is required, which assigns the cutting voltage to this correction velocity. A linear function was used. The average cutting voltage (OFFSET neglected) is 5 V for which no correction of the distance is carried out. At lower voltages, the distance between torch and sheet metal must be shorter and at higher voltages than 5 Volts the torch must be positioned closer to the sheet. Fig. 4 shows the function. The X-axis represents the cutting voltage and the Y-axis the correction velocity in mm/s. The correction velocity is declared with the variable SCALE OUT. The calculated correction velocities from point to point are smoothed by the “smoothing constant”. The setting up of the distance control is carried out taking into account the cutting velocity.

Fig. 4

Characteristic of distance control.

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The distance control was tested with parameters for flat sheets. The regulation of the distance was evaluated with high-speed recording. The distance between torch and sheets was nearly constant and hence the cutting quality was comparable with conventional cutting results by teaching start- and end point, following called “linear cut path”. Using the same settings for structured sheet metals, the distance control produces insufficient results due to slow response times. As a result, the distance controller has to be investigated. The study of the distance control was carried out by varying the key parameters − OFFSET, SCALE OUT and smoothing constant. First, the relationship between specified OFFSET and real torch distance was determined. For a given torch distance, the cutting voltage was measured. By rearranging the Eq. 1, the OFFSET can be calculated using the measured values. Due to the average cutting voltage of 5.0 V, the values are calculated according to Table 1. The second step was to determine the influence of the parameter SCALE OUT. The cutting parameter set from the previous experiments was used. The clamp voltage was documented by means of a measuring amplifier. For a correction velocity (scale out) of 50 mm/s, a periodic voltage curve occur. The frequency of the curve corresponds to the structure of the sheet metal. The distance between the torch and sheet metal is reflected by the amplitude of the graph. In comparison, the correction velocity of 50 mm/s, 100 mm/s and 120 mm/s showed different amplitudes. The smallest amplitude occurred for 50 mm/s. The amplitudes of 100 mm/s and 120 mm/s are almost the same (Fig. 5). High-speed recordings were conducted to evaluate the torch distances. It was found that for a SCALE OUT of 50 mm/s, almost no distance correction took place. The cutting path was in the middle of the structure height and the torch collides with the bumps (Fig. 6). The gradual increase of the correction velocity above 50 mm/s up to 200 mm/s improves the

cutting result and the collisions were reduced. The cutting speed was limited to 200 mm/s because at higher correction velocities collisions of the torch took place at the start of cutting. The high-speed imaging showed for a decreased cutting speed of 2.5 m/min a significantly improved distance control. Subsequently, the influence of structure position was studied. At constant cutting and control parameters, both structure positions were cut (Fig. 7). It turned out that both structure positions were suitable for cutting. Slightly better results were achieved in the negative structure position. Fig. 8 shows the distances at optimized parameters. However, the visual observed cutting quality is similiar in both cases. Furthermore, the influence of smoothing constant was determined. When the filter is less than the duration of an interpolation (clock pulse of robot control 0.012 s), the correction is unfiltered. To evaluate the effect of the smoothing constant, a sensitivity analysis was performed. Various smoothing Table 1 Measured voltage and calculated offset for various torch distances Torch distance 1.00 mm 1.50 mm 2.00 mm

Fig. 5

Measured voltage 3.70 V 3.90 V 4.00 V

Calculated offset 1.30 V 1.10 V 1.00 V

Clamp voltage at various Scale Out.

Fig. 6 Cutting path at Scale Out of 50 mm/s and cutting velocity of 4 m/min.

Plasma Cutting of Structured Sheet Metals in Comparison with Laser Beam Cutting

Fig. 7 Cutting path at Scale Out of 200 mm/s in structure location, (a) positive and (b) negative.

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Despite the parameter optimization of the distance control, a delay in the distance control remain. There are several reasons that explain the delay. The times for the transmission of the signal from the plasma system and forwarding inside the robot can be neglected. The A/D conversion time of the input terminal is 4 ms and is also negligible. Another reason is the interpolation. The main reason for this is the axis filter of the robot controller. Delays of more than 132 ms can occur - this is the equivalent of a cut length of about 9 mm for a cutting speed of 4 m/min. This delay is also seen in the high-speed imaging. The result of the delay is a slight contact of the torch at times.

5 Quality of Cut Face

Fig. 8 torch distances at a Scale Out of 200 mm/s in structure location negative and 2.5 m/min, (a) maximum distance about 3.5 mm and (b) distance about 1.5 mm.

Fig. 9

Clamp voltage for various smoothing constants.

constants were adjusted by a constant correction velocity. The graphs in Fig. 9 show that an increased smoothing constant decreases the cutting voltage and nearly no control is performed. The best results are obtained with increased Scale Out, low smoothing constant and low cutting speed. Thus, the following settings have been determined as the best values for the distance control: smoothing constant 0.012 s (unfiltered signals); cutting velocity 2.5 m/min; Scale Out 200 mm/s (maximum value).

The quality of the cut face must comply with the requirements of further processing. The further processes are forming and welding processes. Studies on the GMA welding show that there is a small gap bridging ability of about 0.1 mm [23]. Therefore, the absence of burrs, cutting straightness and roughness of the cut face will be considered in more detail as essential quality parameters for the quality of the cut face [11]. Based on these parameters, the different strategies for plasma cutting of the structured sheet metal were evaluated (Table 2). 5.1 Burrs and Solidified Droplets For flat sheets which were cut with linear cut path, it was found that with increasing torch distance burrs developed on the cut face. When exceeding a torch distance of 2 mm, significant burr formation occurred (Table 3). By using the distance control, the quality of cut face was improved. The cut face on all torch distances showed no burrs or solidified droplets. The distance control compensates distortions of the thin sheet metal during cutting. As a result, the distance control increases the quality of the cut face even for flat sheets (Table 4).

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Table 2

Plasma Cutting of Structured Sheet Metals in Comparison with Laser Beam Cutting

Cutting variants at flat and structured sheets

Flat sheets Linear cut path Distance control

Structured sheets Programmed robot path Distance control

Table 3 Programmed torch distance and resulting cut face quality as well as images of the cut face for flat sheet metals. Flat sheet with linear cut path Programmed torch distance Visual evaluation 1.00 mm Burr- and droplet-free 1.25 mm Burr- and droplet-free 1.50 mm Burr- and droplet-free 1.75 mm Scattered burrs, droplet-free Irregular cut face with burrs 2.00 mm and droplets

Table 5 Programmed torch distance and resulting cut face quality as well as images of the cut face for structured sheet metals. Structured sheet metals with programmed path Programmed torch Visual evaluation distance 1.00 mm Burr-free 1.25 mm Burr-free Predominant burr-free, scattered 1.50 mm droplets Predominant burrs, irregular cut face 1.75 mm in combs and webs Predominant burrs, irregular cut face 2.00 mm in combs and webs

Table 4 Parameters of the distance control and resulting cut face quality as well as images of the cut face for flat sheet metals. Flat sheet with distance control Offset/aspired torch distance 1.30 V / 1.00 mm 1.20 V / 1.25 mm 1.10 V / 1.50 mm 1.05 V / 1.75 mm 1.00 V / 2.00 mm

Visual evaluation Burr-free Burr-free Burr-free Scattered burrs Burr-free

Then the cut face on structured sheet metals by a programmed cut path is investigated. For a torch distance of 1.25 mm, the cut face is the same quality compared with flat sheet metals. With increasing torch distance, the quality of the cut face is impaired in the same way as for flat sheets. On bump and bridge, there were only small differences in the cut quality. The best results were obtained for torch distances of 1.00 up to 1.25 mm (Table 5).

Subsequently, the distance control has been studied for cutting structured sheet metals using the determined parameters (smoothing constant 0.012 s, cutting velocity 2.5 m/min, Scale out 200 mm/s) for the distance control with reduced cutting velocity of 2.5 m/min. As already mentioned, the distance control enables only partially a constant torch distance. Nevertheless, solidified droplets and burrs were not present on the cut face. In the positive structure position, the desired torch distance was achieved for the bumps, while the torch distance in the bridge was temporary 3 times larger. In the negative structure location, the deviation from the desired torch distance is not achieved at the bumps while it is in according at the bridges (Table 6). 5.2 Straightness The straightness of the cutting line is also important for the gapless positioning of the sheets. Accordingly,

Plasma Cutting of Structured Sheet Metals in Comparison with Laser Beam Cutting Table 6 Parameter of the distance control and resulting cut face quality as well as images of the cut face for structured sheet metals. structured sheet metals with distance control (vs=2.5 m/min, Scale Out 200 mm/s) Structure position Visual evaluation positive burr-free negative burr-free

the measurement of the straightness was carried out by means of a 3D-CNC-coordinate-measuring-machine Eclipse, Carl Zeiss AG. The measuring length was 600 mm. From obtained measurement, data was mathematically determined a straight line by linear regression with least squares method. In this way, the deviations of straightness (according to DIN EN ISO 12780-1) were obtainable. On flat sheet metals with the cutting variants “programmed cut path” and “distance control”, similar curves with deviations of up to 0.14 mm were obtained (Fig. 10 and 11). The measurement results of both cutting variants show no significant influence of the torch distance to the straightness of the cuts. It should be noted that the distance control for flat sheet metals keeps a significantly more constant torch distances. The structured sheets samples were cut with the same parameters. When cutting with a programmed cut path, the samples show larger deviations of 0.2 mm of straightness compared with flat cutting samples (Fig.12). This is due to tolerances in the positioning of the structured sheet metals relative to the plasma torch.

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In the cutting variant with distance control for both structure locations (positive and negative), the straightness was measured. The deviations of straightness were up to 0.35 mm and between the structure locations no significant differences were detected. After about 100 mm measuring length, the distance control stabilized for both structure locations. The deviations are then reduced to 0.25 mm. Furthermore, a periodic variation of the deviations was observed. The period length is 35 mm and corresponds to the length of the bump. This behaviour

Fig. 10 Straightness on flat sheet metals with linear cut path for various torch distances.

Fig. 11 Straightness on flat sheet metals with distance control at various aspired torch distances.

Fig. 12 Straightness on structured sheet metals with programmed cut path at various torch distances.

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occurred in positive and negative structure location. The reasons for the periodic straightness can be found in the structure caused by the different distances of torch due to the distance control (Fig. 13). Unequal distances between torch and sheet metal cause changes of the arc power and its distribution which influence the straightness [11]. 5.3 Roughness For the roughness measurements, the Hommel-Etamic meter T 8000 was used. At a distance of 170 mm from the cutting start as well as cutting end samples were taken with a length of 20 mm. According to DIN EN ISO 4288 [24, 25], a tracing length of 15 mm are provided as a function of the expected roughness of approximately 30 µm. For structured sheet metals no or only slightly straight measurement lines on the cutting edge are present. Therefore, only a tracing length of 4.8 mm could be realized (Fig. 14).

Fig. 15 Roughness Rz of flat sheets, (a) cut with linear cut path and (b) cut with distance control.

Fig. 13 Straightness on structured sheet metals with distance control in both structure locations.

Fig. 14

Sample extraction for roughness measurements.

Fig. 16 Roughness of structured sheet metals, (a) cut with programmed cut path and (b) cut with distance control.

Plasma Cutting of Structured Sheet Metals in Comparison with Laser Beam Cutting

On cut faces of flat sheets, the roughness values Rz were between 19 and 26 µm at torch distances of 1.00 to 1.75 mm (Fig. 15a). When using the distance the roughness was reduced to 8-21 µm (Fig. 15b). The slightly worse results at the cutting variant linear cut path can be attributed to the fact that the sheet metal of 0.7 mm is deflecting during cutting. The cuts on structured sheets with programmed cut path showed similar roughness on the cut faces as the flat sheets with distance control and the cuts with distance control showed better results negative as in positive structure location (Fig. 16).

6. Summary Plasma cutting of structured sheet metals requires high demands on the automation technology because distances between the sheet metal and the torch affects the cutting quality. To ensure an almost constant torch distance, self regulating controls could be applied using the cutting voltage as a control variable. For plasma cutting of structured sheet metals with a thickness of 0.7 mm additionally increased demands exist: a high cutting speed is necessary and the bumps produces significant differences in height of 3.0 mm at a periodic interval of 35 mm. Despite the optimization of the parameters of the distance control remained an delay in the distance control (because of the axis filter). According to Ref. [17], the control precision is significantly improved when using the KR C4 control of KUKA Robot via the RSI (Robot Sensor Interface). Also, the use of an additional external axis is conceivable. The robot controller receives the data for the cutting path and the external axis does a fine positioning of the torch distance. The control of the external axis can be directly derived by the cutting voltage. The independent simple logic of the external axis make the fine adjustment faster. The performed plasma cuts on flat and structured sheets were carried out with the linear or programmed cut path and distance control. Despite of the delay of

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the distance control the cutting faces show good quality. The absence of burrs was achieved for structured and flat sheet metals for torch distances of 1.00 to 1.75 mm. The deviations of straightness of flat sheet metals were from -0.1 to 0.1 mm and of structured sheet metals from -0.2 to 0.1 mm. For structured sheets slightly tighter tolerances -0.125 to 0.125 mm were obtained with laser cut samples [9]. The highest roughness of all samples was 25.65 µm. In this case, no correlation between the torch distance and roughness could be determined. Also no differences in the roughness of structured sheet metals according to the bump and bridge areas could identified. Compared to laser beam cutting a slightly higher roughness in the plasma cut samples was determined. The roughness values obtained can be ranked to class 2 according to DIN EN ISO 9013. The recommended torch distance should be 1.00 to 1.5 mm.

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