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# Cutting and Shield Gases Pressure Effects on Plasma Cutting Quality

Sébastien Chabert, Jean-Jacques Gonzalez, Pierre Freton

► To cite this version:

Sébastien Chabert, Jean-Jacques Gonzalez, Pierre Freton. Cutting and Shield Gases Pressure Effects on Plasma Cutting Quality. Welding Journal -New York-, 2024, 103 (03), pp.75-84. 10.29391/2024.103.007. hal-04468853

HAL Id: hal-04468853

https://hal.science/hal-04468853v1

Submitted on 20 Feb 2024

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Cutting and shield gases pressure effects on plasma cutting quality

Experimental observation of kerfs obtained with varying current and shield and
cutting pressures

S Chabert¹,², J J Gonzalez¹, P Freton¹

¹LAPLACE, Université de Toulouse, UMR 5213 CNRS, INPT, UPS, Toulouse, France

² SAS GYS, 1 rue de la Croix des Landes 53941 Saint-Berthevin, France

[chabert@laplace.univ-tlse.fr](mailto:chabert@laplace.univ-tlse.fr)

**I Abstract**

In this paper, the effects of the shield gas on cut quality in plasma arc cutting were quantified experimentally. Measurements were performed on plasma cutting kerfs (PACs) cut through a 4 mm (1/8in) S355 steel plates with a *Gys Neocut105* cutter equipped with a *Toparc AT-125* torch. This system uses compressed air as both cutting and shield gas. Separate circuits for shield air and cutting air were used. This way, the influences of the shield air and the cutting air could be studied independently. A full 3-factor, 3-level Taguchi design was followed. The studied factors are the cutting air pressure, the shield air pressure and the arc current. The measured responses are the removed steel surface and the right and left bevel angles. As expected, the current proved to have the greatest influence on kerf surface. The cutting air pressure showed significant influence on the kerfs' shapes while shield air flow rate proved less important but yet sensitive. Some negative bevel angles at high plasma, high cutting and high shield air pressures have also been observed.
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# II Introduction

## II.1 Background

Plasma arc cutting, used to cut metals, offers an interesting compromise between speed, accuracy and cost for intermediate thicknesses as compared to laser and oxycutting [1-2]. It uses a transferred arc and the plate to be cut is used as the anode. The process uses a cutting gas and a shield gas (both compressed air in our study). A hot plasma melts the metal and blows it away, thus creating a kerf. Kerfs display various qualities.

## II.2 Literature review

There are numerical studies about the kerf quality problem. For example, Hendricks [3] has numerically found that increasing current or reducing torch speed increases plate maximum temperature, kerf size and HAZ (Heat Affected Zone, surrounding a kerf and in which the heat modifies the metal crystal structure). Also Nemchinsky and Severance [4] [5] propose a numerical approach to determine top and bottom kerf widths and bevel angles depending on torch speed: increasing speed leads to narrower top kerf width and greater bevel angles, which means even narrower bottom kerf widths. The interaction between factors, i.e. mass flow and current is complex and leads to non-monotonic results with regards to kerf width [6].

Several authors [7-10] have also been interested in the experimental characterization of cutting quality. Mostly, the MRR (Material Removal Rate, the volume of metal removed during the cut process during a given time), bevel angles, rugosity, HAZ are studied through arc current, torch speed and torch height variations. High currents lead to high MRR and HAZ. Concerning the other parameters, the results vary according to the operating parameters and can adopt non-monotonic behaviours.

Illi et al. [11] and Nemchinsky et al. [12] have also shown that torch speed, torch height, gas pressure and nature were highly impacting factors over cut quality. Notably, dross formation is strongly depending on torch speed. There is a window of speeds allowing no-dross cuts. Low speed dross occurs when the cut speed subseeds that window and may be removed easily, whereas high
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speed dross is hard to break. In this study, the samples displayed some low speed dross but they
were cleaned effortlessly. Dross is not considered in this paper.

Shield gas is used in most modern plasma cutting systems to prevent molten metal projections from damaging the torch and helps cooling the nozzle down. It can also form a protective atmosphere in multi-gas cutting systems (which is not the case in this paper). However, its effects on the kerf geometry are not well documented.

II.3 Purpose

Even if several papers [7-11] deal with operating parameters impact on cut quality, this work is original in that it considers the plasma air pressure along with the shield air pressure and shows their influences on kerf shape.

The following section introduces the studied torch geometry and the way it works. Then the
prototype used in this study to control the plasma and the shield air pressures separately is
presented. Afterwards, the design of experiments is described. It defines the scope of the study with
regards to the factor's range. Then the equipment used during the experiment as well as the cutting
protocol are described. The measurements protocol is also detailed. Next, the calculations through
which measurements were processed is outlined.

Finally, the results are discussed, particularly the effects of shield air pressure on kerf's shape.

III Experimental procedure and setup

### III.1 Base torch

The plasma cutter used is a *Gys Neocut105* which can generate an arc up to 105 A. It was equipped with a *Toparc AT-125* torch (which is designed to operate up to 125 A). This system uses pressurized air as cutting and shield gases. Figure 1 shows the original torch head lower parts along the air effluxes circuits and their names (in bold and between brackets). The hose that feeds air and current to the torch and the torch head upper parts are not represented. A settable pressure regulator inside the generator body feeds the air to the torch. The swirl ring air (1) is the sum of the cutting air (1A) and rear cooling air (1B). It is granted a swirl component [13] and is injected all around the
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electrode by the swirl ring. It also separates the electrode from the nozzle when fed, allowing lift arc
ignition. The electrode is equipped with an erosion-resistant hafnium insert. The electrode feeds
current to the plasma. It gets constricted by the nozzle and blown on to the cut material. The rear
circuit (1B) helps cooling the electrode down. The retaining cap holds the components together and
channels the effluxes. They are ten shield injections holes inside the retaining cap that feed air to the
shield air circuit (2). This circuit is subdivided into two sections: the external shield air (2B) that
helps protecting the torch from molten metal projections, and the central shield air (2A) that greatly
helps cooling down the nozzle. It is the shield cap that channels the 2A and 2B effluxes along with
shielding the inner parts of the torch.

### III.2 Shield cap prototype

The aim here is to study the influence of the air pressures on the cut quality. Mostly, it is
about to determine whether the shield gas also has an influence or not. However, the original system
as seen in Figures 1 and 2 (a) is not designed to have the inputs to these circuits controlled
separately. This if how it was transformed to do so:
First, the ten 0.5 mm (1/50 in) diameter shield injection holes (Figure 1) were filled, thus making
the cutter pressure regulator only feed air to circuit (1). Second, a modified shield cap displayed on
Figure 2 (b,c) was used. It allowed to inject shield air from a separate source: an independent
pressure regulator connected to the pressurized air network.

The shield air is originally injected axially (along X as seen in Figure 1) through ten "shield
injection holes. In this paper's case, shield air is fed radially (Y) through two 4mm (1/8 in) Ø holes.
Thus, one concern about this prototype setup was that the air flow mechanics might have been
different from the original design. In order to lift this doubt, comparative kerfs measurements were
performed. Three kerfs were cut with the original setup and then, three other kerfs were cut with the
prototype setup. The pressure was set as explained in the following section to ensure flows rates
compliance in both cases. Then the measurements made on these kerfs were compared, leading to
similar results in both cases.

### III.3 Design of experiments
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The aim was to replicate normal operating conditions flow levels according to the specifications, i.e., when 5 bar (73 psi) is supplied by the cutter pressure regulator to the original torch. In both original and prototype configuration, let "P₁" the pressure set on the cutter. Only in the prototype setup, let "P₂" the pressure which is fed to the prototype shield cap lateral inlets. Q₁ is the mass flow rate in the swirl ring air circuit whereas Q₂ is that of the shield air circuit (Figure 1). Simultaneous pressure and flow rate measurements were performed. Then two flow rates values above and below these references were chosen to establish the studied pressure domain. This led to the settings presented in Table 1. The flow rate unit used in this paper is NL/min which is normal liter per minute, normal referring to the pressure (1.013 bar / 14.69 psi) and temperature (0°C / 32°F) of the gas. The flow rates were measured with a SMC PF2A751-04-67 sensor.

Most authors [7-11] describe current as a predominant parameter with regards to cut quality. Therefore, current was also taken as a varying factor in this study. Increasing the arc current decreases the cutting air flow rate at constant P₁ because plasma is viscous. As a consequence, the higher the current (and thus the plasma radius) the higher pressure losses it induces. In this paper, the pressures are set but not the air flow rates. The shield gas flow rate is roughly unaffected by the arc current (it only gains 2 NL/min when the arc is shut down from 105 A).

With P₁, P₂ and I, a full 3-factor 3-level plan (Table 2) was followed. It led to the 27 sets of parameters. This is the terminology used in this paper with regards to the experiments:

* A **set of parameters** is a combination of P₁, P₂ and I.

* A **run** consists in the cutting of a **kerf/sample** at a given set of parameters. There is a total of 27×3=81 runs.

* For each set of parameters, one **batch** of three runs were conducted, so there are 27 batches. The data used in the Taguchi's analysis are the averages over the runs within each batch.

The studied responses are described in section III.5.

### III.4 Cutting procedure
The plasma cuts were performed on a 4 mm (1/8 in) thick S355 steel plate. The plate was placed on an *AUTO.TEC* Uni-cut plasma cutting table which also allows the torch to be moved along three axes. The cuts are made on a single 750x500x4mm (2.5ft x 1.6ft x 1/8 in) S355 steel plate.

Besides the varying factors seen in Table 2, here are the constant cut parameters:

* Arc voltage=160 V (arc voltage is regulated through torch height control)

* Cutting speed=1000 mm/min (3.3ft/min).

The torch was equipped with fresh consumable parts at the start of the experiment and were used for every 81 runs. The kerfs are 60 mm (2.4 in) long. This is a sufficiently low workload for the consumable parts wear not to have any impact on the results. **Erreur ! Source du renvoi introuvable.** shows the kerfs layout. Each kerf is separated from the other kerfs in the same batch by a 6 mm gap and each batch is separated from the next in the same column by a 12 mm gap.

It should be underlined that, Salotinis and Vatousianos [9], whose cutting parameters were of the same order of magnitude, measured a HAZ of 0.5 mm (1/50 in). Therefore, the 6 mm (1/4 in) separation between each sample is sufficient to avoid cutting inside other kerf's HAZ. Thus, there was no delay between each cut and the plate was not allowed to cool down. The total time between each cut is due to the torch displacement and represents around 3s. After the plasma cuts, mechanical cuts were performed using a *Promac* SX-823DG metal band saw. These secondary cuttings were made perpendicular to the thermal kerfs and roughly at midlength to avoid performing measurements within the torch acceleration zones (10 mm (3/8in) from each end).

The aim of these secondary cutting operations was to give access to direct optic measurement by placing the microscope objective in the axis of the kerf. A final task to be performed before making measurements on the samples was to clean them: First, the cutting oil and dross had to be removed, before the kerfs got cleared of the small chips that can appear after a saw cut as seen in Figure 4. It should be noticed that, in Figure 4, the right and left sides are given according to the cutting direction, this is why they are flipped around with regards to the page layout.

The following section describes how information was gathered on the samples.

### III.5 Measurements procedure

After preparing the samples, measurements were done using a *Keyence* VHX-6000 microscope with a X30 zoom. The objective focused on the surface of the plate to obtain the most
accurate image. Three quantities were then measured using the built-in measurement tools of the microscope, as shown in Figure 5:

RSS is the removed steel surface (mm²) (1mm²=0.0016 in²)

RBA is the Right bevel angle (°) (right and left being defined with regards to cut direction). A null bevel angle means a vertical kerf.

LBA is the Left bevel angle (°)

These three measurements are used as responses in the Taguchi design of experiments. These measurements were made by placing the points manually on the Keyence tool. The way to process these measurements is the subject of the subsequent discussion.

# IV Results

To conduct this study, Taguchi's method [14] was used. This statistical method consists in studying one or several responses to runs conducted at varying factors. It can highlight the effect of each factor independently and also their synergistic effects (interaction). The studied factors are the cutting pressure P₁, the shield air pressure P₂ and the arc current I.

The studied responses are the removed steel surface RSS and the left and right bevel angles LBA and RBA. An overview of the overall results, encompassing all 81 runs, is presented in Table 3.

## IV.1 Calculation of the effects of each factor and their interactions

The effects of the factors and their interactions on a response can be calculated and plotted according to Equations 1 and 2 from Taguchi's method's. These equations are applied to each response independently. It is worth reminding that the overall averages are calculated over all 81 runs.

### Effect of factor A at level i (E<sub>Ai</sub>)

$E_{Ai}$ = Average of responses when A is at lvl i - Overall average

Equation 1
Effect of the interaction of factors A at level i with B at level j (Int$_{Aij}$)

Int$_{AiBj}$ = Average of responses when (A=i and B=j) - Overall average - E$_{Ai}$ - E$_{Bj}$ Equation 2

In that case there are 6 possible effects, 3 from each factors and 3 from their interactions (Table 4).

Let n ∈ N and 1≤n≤6.

Here, the magnitude of the effects was calculated using Equation 3.

Magnitude of effect E<sub>m</sub> (MagE<sub>m</sub>)

$$
MagE_m = \text{MAX}(E_m) - \text{MIN}(E_m) \qquad \text{Equation 3}
$$

Next, the contribution of each of the 6 effects (3 from the factors and 3 from their interactions) and
of the average on the responses was calculated using Equation 4.

Contribution of effect E<sub>m</sub> (C<sub>E<sub>m</sub></sub>)

$$
C_{E_m} = \frac{E_m}{\sum_{n=1}^{6} E_n + \text{Overall average}} \qquad \text{Equation 4}
$$

Finally, the contribution of the overall average was calculated in the same way using Equation 5.

Contribution of effect Overall average (C<sub>overall avg</sub>)

$$
C_{\text{Overall avg}} = \frac{\text{Overall average}}{\sum_{n=1}^{6} E_n + \text{Overall average}} \qquad \text{Equation 5}
$$

The contributions calculated through Equations 4 and 5 are plotted in Figure 6. This show how each factor (or factor's interaction) affects the response, with regards to its average. As a reminder, the factors in this study are P₁ (cutting air pressure), P₂ (shield air pressure), and I (arc current).

A response where the contribution of the factors and their interactions tends towards 0% (and the average towards 100%) would mean that these factors do not impact the responses. For example, it can be seen in Figure 6 that LBA is the most influenced response (by the studied factors and their interactions) and its average has the smallest contribution. Conversely, RBA is the least influenced response, while RSS is mostly affected by the arc current. Nonetheless, RBA shows a significant influence due to the sole factor P₂ (shield air pressure). This study focuses on the effects of shield air pressure, specifically on LBA. The sole factor P₂ has a weak effect, but its interactions with P1
and I are important and contribute up to 18% of the LBA response. This will be the subject of
deeper discussions but the two upcoming section briefly deals with RBA (and why not much
attention is given to it) and RSS.

IV.2 RBA, the less studied side

Plasma arc cutting kerfs always bring a more vertical side, depending on the cut direction. In this study, every kerf was cut along the same direction and, as a consequence, RBA is the bigger bevel angle. In an industrial context, this side of the kerf would have little interest and be discarded because most applications need the straightest possible side. As a result, the effects on RBA are not discussed much further in this paper.

IV.3 Effects on Removed Steel Surface

As other authors' claim [9-10], the current proved to be the most impactful parameter to RSS.
Figure 7 represents the effects of each factor on RSS calculated with Equation 1. This is how to read
Figures **Érreur ! Source du renvoi introuvable.**, and then 8, 10 and 11: The abscissa axis
corresponds to the factors level (see Table 2) whereas the ordinate axis represents the effects on the
response (RSS in this case). Here is how to read the current curve: at factor level 1, the effect is
around -3 mm² (-0.0047 in²). It means that runs performed at current level 1 (45A) show kerfs
smaller than the overall average by 3mm². On the other hand, any factors at level 2 involves almost
no effect in comparison with the overall experience because these effects are close to 0 mm².

It can be seen that increasing the current leads to significant increase of the kerf width. It is because
higher currents lead to wider and hotter arcs, melting more metal down. Increasing P₁ has the
opposite, but much less strong effect. The right amount of pressure (according to the arc current) is
required to remove the molten metal efficiently. In this case it could be reduced slightly if the aim
was to optimize the material removal rate. P₂'s influence on RSS is negligible (as also seen in
Figure 6).

IV.4 Effects on Left Bevel Angle

The bevel angle is a major quality criterion. It must be as close to 0 as possible. As mentioned in
*IV.2 RBA, the less studied side*, only the LBA is given importance in this paper.
With regards to the factors effects (Figure 8 calculated with Equation 1), increasing the cutting air pressure is the most significant way to reduce LBA. This may be because increasing the cutting air flow rate leads to faster ejection of the molten steel, thus reducing asymmetry due to molten steel thickness between the top and the bottom of the plate [4].

It is noteworthy to remember that most bevel angles are positive. The only negative bevel angles occur on LBA on batches 24, 26 and 27 (Figure 9). Except for these three batches, a negative effect on the bevel angle means a more vertical side. Batches 24, 26 and 27 factors level [$P_1, P_2$ I] are respectively [3 2 3] [3 3 2] and [3 3 3]. It appears that all three factors must be high in intensity to bestow enough LBA reduction to bring it negative. However, according to Figure 8 **Erreur ! Source du renvoi introuvable.**, an isolated high $P_2$ or I brings only a very small decrease in LBA (green and orange lines). This indicates that this is $P_2$ interactions with the other factors that allows a greater LBA reduction (as seen in Figure 10). This is the subject of the two upcoming sections.

### IV.4.1 $P_1-P_2$ Interaction on left bevel angle

The Taguchi's method allows not only to determine the effects of isolated factors, but also that of their interactions. The $P_1-P_2$ interaction effects on LBA with **Erreur ! Source du renvoi introuvable.** which results were calculated with Equation 2 are now detailed.

As expected from the discussion in the preceding section, there is a $P_1.P_2$ interaction that strengthen the LBA reduction of high $P_2$ (Figure 8) when $P_1$ is also high: at high $P_1$, increasing the shield air pressure $P_2$ leads to an even lower LBA, i.e. to a more vertical side. This is shown by the green line on Figure 10. It is interesting to note that this tendency is opposite (but weaker) at lower $P_2$ pressures. A possible explanation would be that, at lower $P_1$, the central shield air (Figure 1) which is fed not axially hinders the axial flow of the cutting air. At higher $P_2$, the cutting air might have enough momentum not to be disrupted by the shield air and to pump axially into the kerf, thus having it contributing more efficiently to molten metal removal.

As a consequence, shield pressure should be increased along with cutting pressure to attain better verticality. However, at intermediate cutting pressure, lowering the shield pressure might be considered because it does not hinder verticality. This could be relevant if a strong protection effect is not mandatory (apart from the piercing phase) and that the shield gas has to be spared (in case of a multi-gas system using an expensive shield gas).
IV.4.2 I-P₁ Interaction on LBA

Figure 11 shows that, just as with the P₁P₂ interaction, having high plasma pressure (P₁) allows the current's increment to reduce the left bevel angle, while the opposite effect is seen for lower P₁ values. It means that current should be increased along with P₁ to attain better verticality (this is not commonly mentioned in the cutters or torches' user's guides). This is another manifestation of the importance of Current-Pressure balance in plasma arc cutting as discussed in IV.2 *Effects on Removed Steel Surface*.

Figure 12 shows an example of the shield air pressure P₂ effect at low current and medium plasma air pressure P₁. The difference between these three samples is the shield air pressure. The red and (resp.) green lines help visualizing LBA and (resp.) RBA. The fact they are not parallel shows the influence of P₂ over these angles. In this case, having P₂ go from 0 to 0.28 bar decreases LBA and increases RBA. The difference between 0.28 and 0.97 bar is less sensitive.

V Conclusion

In this paper, effects of both the cutting and shield air pressures were studied independently. As current is a major criterion when it comes to kerf shaping, it was also taken into account.

A 3-level 3-factor full plan spanning over current, cutting air and shield air pressures was followed.

A prototype had to be made to allow setting the cutting and shielding pressures separately.

However, it was verified that this prototype led to results similar to those obtained with the base torch (with standard current and air flow rates).

Taguchi method was used to interpret the results. Kerfs were cut through a steel plate with various sets of parameters and then their dimensions were measured with a microscope. Results were averaged to smoothen scattering. This study concerns the removed steel surface, the right bevel angle and mostly the left bevel angle.

Naturally, the removed steel surface is largely increasing with higher current values. The air pressures proved less important to this response.

Regarding the bevel angle, a major quality criterion, each of the studied parameters affects it, either directly or through an interaction with another parameter. To attain the straightest left sides (the retained sides, right sides being discarded), a combination of high levels of all three parameters is
required. The synergistic effects are interesting because the P₁-P₂ (resp. P₁-current) interactions effect tendencies get inverted between level 1 and 2 of P₂ (resp. P₁) and level 3.

As a result, to attain better verticality, shield and cutting pressures shall be increased together. This is usually done in existing cutting systems and this study confirms that it is beneficial. In case of low cutting pressures, if the protection from the shield gas does not need to be high and that this gas has to be spared (if the results are extrapolated to a multi-gas cutter), the shield gas pressure can be reduced even further without affecting verticality.

Also, increasing the cutting pressure along with the current may also help getting straighter kerfs.

The effect of the shield air pressure P₂ is undeniable because it is required to attain the few negative left bevel angles observed in this study. Indeed, it requires all three factors (including shield air pressure) at high levels to subtract enough bevel angle value so it turns negative. However, strong P₂ fed at low P₁ lead to skewer kerfs. To conclude, the shield air pressure, apart from its importance in shielding the torch, has also effects on kerf verticality. In existing cutters where it is set along with the cutting pressure, good results can be expected. Finer settings could involve the shield air pressure when possible.

## Acknowledgments

Funding, experimental setups and technical support provided by SAS Gys.
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