well as a groove angle relative to the top surface of the workpiece and angle of the torch. Figure 9 illustrates the concept of offset angle gouging as viewed from the top. In summary, a wider gouge can be accomplished by using the offset angle gouging technique. Optimum profile shapes can be gained by experimentation, the maximum allowable tangent surface being determined to suit the application. Additionally, the effect of the torchto workpiece distance (TWPD) can be cited as a key influencing factor on the gouge shape. In essence, the closer the torch consumables are to the workpiece, the deeper and narrower the gouge will be. This is because the energy density will be higher as it reaches the workpiece with a smaller TWPD. Conversely, the further the plasma travels without the aid of an impinging device (i.e., a nozzle), the more diffuse it will become. To reiterate, the relationship of arc constriction to distance traveled to the workpiece is the enabler of plasma gouging. This working distance is commonly referred to as arc stretch. Varying the arc stretch has a direct effect on the gouge profile shape and the ability to remove material. However, all plasma systems place a limit on arc stretch as a function of arc voltage. This is mainly attributable to the duty cycle associated with the power supply and its ability to manage heat and energy loads on the system components. Therefore, when the arc reaches its maximum, the power supply will automatically break the circuit and the arc will disconnect from the workpiece. At this maximum distance, the operator should be able to achieve a theoretical maximum width gouge at minimum depth for a given amperage and primary torch angle. Examples 1, 2, and 3 in Fig. 10 show the variations in profile shape that are typical when varying the TWPD. It should also be noted that similar effects can occur (though to a lesser extent) by varying system amperage and linear travel speed. Width X, depth Y, and groove angle , as well as the scaling factors, will vary significantly by make of system and its associated system parameters. Lastly, some discussion of the starting transition region of the gouge is required. This region can be referred to as the lead-in region. When gouging with plasma, the process requires some linear distance to reach its full depth. Since arc attachment occurs at the surface of the workpiece and is somewhat diffuse, a delay in complete penetration is to be expected. Variations in system make and process parameters will vary this effect. Furthermore, the angle tangent to the curve of the lead-in region is typically about one-third the primary torch angle, depending on process parameters. This effect is most evident in mechanized applications when a starting shim is used. Conclusion In today’s welding and fabrication markets, the operator has many choices when it comes to selecting a gouging solution. Established technologies such as carbon arc gouging and grinding are still today’s preferred solutions for many operators. However, user feedback suggests gouging with plasma offers a safe, efficient, and cost-effective way to make a gouge. Standard and best practices for plasma gouging are only now developing. To address this need, this article has presented the basic relationships between these key process parameters: • Amperage • Torch positioning • Torch motion. With a basic understanding of how these process parameters interact, a user now has the building blocks to design a plasma gouging process to address specific applications. 42 WELDING JOURNAL / DECEMBER 2016 WJ CLIFF DARROW (clifford.darrow@hypertherm. com) is engineering leader, Light Industrial Torches and Consumables, Hypertherm, Inc., Hanover, N.H. Fig. 9 — Offset angle method of gouging, top view (Z direction). Fig. 10 — Typical gouge profile shapes as a function of TWPD. The relationship of arc constriction to distance traveled to the workpiece is the enabler of plasma gouging
Welding Journal | December 2016
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