Integrated robotic plasma cutting requires more than just a torch attached to the end of the robotic arm.Knowledge of the plasma cutting process is key.treasure
Metal fabricators across the industry – in workshops, heavy machinery, shipbuilding and structural steel – strive to meet demanding delivery expectations while exceeding quality requirements.They are constantly seeking to reduce costs while dealing with the ever-present problem of retaining skilled labor.Business is not easy.
Many of these problems can be traced back to manual processes that are still prevalent in the industry, especially when manufacturing complex shaped products such as industrial container lids, curved structural steel components, and pipes and tubing.Many manufacturers devote 25 to 50 percent of their machining time to manual marking, quality control, and conversion, when the actual cutting time (usually with a hand-held oxyfuel or plasma cutter) is only 10 to 20 percent.
In addition to the time consumed by such manual processes, many of these cuts are made around wrong feature locations, dimensions or tolerances, requiring extensive secondary operations such as grinding and rework, or worse, Materials that need to be scrapped.Many stores dedicate as much as 40% of their total processing time to this low-value work and waste.
All of this has led to an industry push towards automation.A shop that automates manual torch cutting operations for complex multi-axis parts implemented a robotic plasma cutting cell and, unsurprisingly, saw huge gains.This operation eliminates manual layout, and a job that would take 5 people 6 hours can now be done in just 18 minutes using a robot.
While the benefits are obvious, implementing robotic plasma cutting requires more than just purchasing a robot and a plasma torch.If you’re considering robotic plasma cutting, be sure to take a holistic approach and look at the entire value stream.Additionally, work with a manufacturer-trained system integrator who understands and understands plasma technology and the system components and processes required to ensure all requirements are integrated into the battery design.
Also consider the software, which is arguably one of the most important components of any robotic plasma cutting system.If you’ve invested in a system and the software is either difficult to use, requires a lot of expertise to run, or you find it takes a lot of time to adapt the robot to plasma cutting and teach the cutting path, you’re just wasting a lot of money.
While robotic simulation software is common, effective robotic plasma cutting cells utilize offline robotic programming software that will automatically perform robot path programming, identify and compensate for collisions, and integrate plasma cutting process knowledge.Incorporating deep plasma process knowledge is key.With software like this, automating even the most complex robotic plasma cutting applications becomes much easier.
Plasma cutting complex multi-axis shapes requires unique torch geometry.Apply the torch geometry used in a typical XY application (see Figure 1) to a complex shape, such as a curved pressure vessel head, and you’ll increase the likelihood of collisions.For this reason, sharp-angled torches (with a “pointed” design) are better suited for robotic shape cutting.
All types of collisions cannot be avoided with a sharp-angled flashlight alone.The part program must also contain changes to the cut height (i.e. the torch tip must have clearance to the workpiece) to avoid collisions (see Figure 2).
During the cutting process, the plasma gas flows down the torch body in a vortex direction to the torch tip.This rotational action allows centrifugal force to pull heavy particles out of the gas column to the periphery of the nozzle hole and protects the torch assembly from the flow of hot electrons.The temperature of the plasma is close to 20,000 degrees Celsius, while the copper parts of the torch melt at 1,100 degrees Celsius.Consumables need protection, and an insulating layer of heavy particles provides protection.
Figure 1. Standard torch bodies are designed for sheet metal cutting.Using the same torch in a multi-axis application increases the chance of collisions with the workpiece.
The swirl makes one side of the cut hotter than the other.Torches with clockwise rotating gas typically place the hot side of the cut on the right side of the arc (when viewed from above in the direction of the cut).This means that the process engineer works hard to optimize the good side of the cut and assumes that the bad side (left) will be scrap (see Figure 3).
Internal features need to be cut in a counterclockwise direction, with the hot side of the plasma making a clean cut on the right side (part edge side).Instead, the perimeter of the part needs to be cut in a clockwise direction.If the torch cuts in the wrong direction, it can create a large taper in the cut profile and increase dross on the edge of the part.Essentially, you’re putting “good cuts” on scrap.
Note that most plasma panel cutting tables have process intelligence built into the controller regarding the direction of the arc cut.But in the field of robotics, these details are not necessarily known or understood, and they are not yet embedded in a typical robot controller – so it is important to have offline robot programming software with knowledge of the embedded plasma process.
Torch motion used to pierce metal has a direct effect on plasma cutting consumables.If the plasma torch pierces the sheet at cutting height (too close to the workpiece), the recoil of the molten metal can quickly damage the shield and nozzle.This results in poor cut quality and reduced consumable life.
Again, this rarely happens in sheet metal cutting applications with a gantry, as the high degree of torch expertise is already built into the controller.The operator presses a button to initiate the pierce sequence, which initiates a series of events to ensure proper pierce height.
First, the torch performs a height-sensing procedure, usually using an ohmic signal to detect the workpiece surface.After positioning the plate, the torch is retracted from the plate to the transfer height, which is the optimal distance for the plasma arc to transfer to the workpiece.Once the plasma arc is transferred, it can heat up completely.At this point the torch moves to the pierce height, which is a safer distance from the workpiece and farther from the blowback of the molten material.The torch maintains this distance until the plasma arc completely penetrates the plate.After the pierce delay is complete, the torch moves down toward the metal plate and begins the cutting motion (see Figure 4).
Again, all this intelligence is usually built into the plasma controller used for sheet cutting, not the robot controller.Robotic cutting also has another layer of complexity.Piercing at the wrong height is bad enough, but when cutting multi-axis shapes, the torch may not be in the best direction for the workpiece and material thickness.If the torch is not perpendicular to the metal surface it pierces, it will end up cutting a thicker cross-section than necessary, wasting consumable life.Additionally, piercing a contoured workpiece in the wrong direction can place the torch assembly too close to the workpiece surface, exposing it to melt blowback and causing premature failure (see Figure 5).
Consider a robotic plasma cutting application that involves bending the head of a pressure vessel.Similar to sheet cutting, the robotic torch should be placed perpendicular to the material surface to ensure the thinnest possible cross-section for perforation.As the plasma torch approaches the workpiece, it uses height sensing until it finds the vessel surface, then retracts along the torch axis to transfer height.After the arc is transferred, the torch is retracted again along the torch axis to pierce height, safely away from blowback (see Figure 6).
Once the pierce delay expires, the torch is lowered to the cutting height.When processing contours, the torch is rotated to the desired cutting direction simultaneously or in steps.At this point, the cutting sequence begins.
Robots are called overdetermined systems.That said, it has multiple ways to get to the same point.This means that anyone teaching a robot to move, or anyone else, must have a certain level of expertise, whether in understanding robot motion or the machining requirements of plasma cutting.
Although teach pendants have evolved, some tasks are not inherently suitable for teach pendant programming—especially tasks involving a large number of mixed low-volume parts.Robots don’t produce when they are taught, and the teaching itself can take hours, or even days for complex parts.
Offline robot programming software designed with plasma cutting modules will embed this expertise (see Figure 7).This includes plasma gas cutting direction, initial height sensing, pierce sequencing, and cutting speed optimization for torch and plasma processes.
Figure 2. Sharp (“pointed”) torches are better suited for robotic plasma cutting.But even with these torch geometries, it is best to increase the cut height to minimize the chance of collisions.
The software provides the robotics expertise required to program overdetermined systems.It manages singularities, or situations where the robotic end-effector (in this case, the plasma torch) cannot reach the workpiece; joint limits; overtravel; wrist rollover; collision detection; external axes; and toolpath optimization.First, the programmer imports the CAD file of the finished part into offline robot programming software, then defines the edge to be cut, along with the pierce point and other parameters, taking into account collision and range constraints.
Some of the latest iterations of offline robotics software use so-called task-based offline programming.This method allows programmers to automatically generate cutting paths and select multiple profiles at once.The programmer might select an edge path selector that shows the cutting path and direction, and then choose to change the start and end points, as well as the direction and inclination of the plasma torch.Programming generally begins (independent of the brand of the robotic arm or plasma system) and proceeds to include a specific robot model.
The resulting simulation can take into account everything in the robotic cell, including elements such as safety barriers, fixtures, and plasma torches.It then accounts for any potential kinematic errors and collisions for the operator, who can then correct the problem.For example, a simulation might reveal a collision problem between two different cuts in the head of a pressure vessel.Each incision is at a different height along the contour of the head, so quick movement between incisions has to account for the necessary clearance—a small detail, resolved before the work reaches the floor, that helps eliminate headaches and waste.
Persistent labor shortages and growing customer demand have prompted more manufacturers to turn to robotic plasma cutting.Unfortunately, many people dive into the water just to discover more complications, especially when the people integrating automation lack knowledge of the plasma cutting process.This path will only lead to frustration.
Integrate plasma cutting knowledge from the start, and things change.With plasma process intelligence, the robot can rotate and move as needed to perform the most efficient piercing, extending the life of consumables.It cuts in the correct direction and maneuvers to avoid any workpiece collision.When following this path of automation, manufacturers reap rewards.
This article is based on “Advances in 3D Robotic Plasma Cutting” presented at the 2021 FABTECH conference.
FABRICATOR is North America’s leading metal forming and fabrication industry magazine.The magazine provides news, technical articles and case histories that enable manufacturers to do their jobs more efficiently.FABRICATOR has been serving the industry since 1970.
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Post time: May-25-2022
