Plasma cutting Why the combination of parts makes all the difference in plasma cutting

Author / Editor: Craig Walsh of Kerf Developments / Alexander Stark

UK — Plasma cutting is a flexible and cost-effective method of processing parts. The process can be used to cut aluminium, mild steel, stainless steel and wear-resistant materials from 1 to 60 mm. Whilst the plasma unit is central to the cutting process, it's the sum of the complementary parts that control, hold and move the plasma cutting torch that will ultimately define cut quality.

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Available in a range of sizes, the RUR is available with cutting widths from 1,500mm right the way through to 3,000 mm and to any customer specified length.
Available in a range of sizes, the RUR is available with cutting widths from 1,500mm right the way through to 3,000 mm and to any customer specified length.
(Source: Matthew James Wilkinson)

Early CNC profiling machines were fitted with single or multiple oxy-fuel cutting heads. These were built using large heavy castings and/or structural beams and driven around at relatively slow speeds using gearboxes with large motors — and such machine designs are still available today. Modern high-speed precision plasma cutting machines need to be able to accelerate, decelerate and change direction in a smooth controlled and vibration-free manner. This requires a more refined machine design that includes a collective of features.

The key components to form the “ultimate” plasma machine design would include:

  • A rigid bridge assembly that is strong but not excessively heavy, as the task of the bridge is to move the torch assembly around the cutting table by precisely following a predefined NC toolpath.
  • A reliable and predictable CNC control and motion system that can position the cutting torch exactly where it needs to be in a smooth and controlled manner. The controller must provide feedback to the user and communicate with the plasma unit to set power settings, speeds feeds and gas pressures where appropriate.
  • Precision electronic torch height control that can maintain the exact pierce and cut height for the process. This needs to be vibration-free, as any movement or vibration in the torch will transmit to the cut profile.
  • Safety breakaway device for the torch. This needs to be ‘rigid’ but also able to protect the torch over the life of the machine. These are typically either magnetic or maintenance-free pneumatic systems. Whichever style is selected, the torch needs to maintain rigidity throughout the cutting process.
  • Independent downdraft cutting tables that are not connected in any way to the motion system. By remaining stand-alone, the table ensures vibration is not introduced into the motion control system whilst cutting.
  • Advanced CAM software that can take base CAD geometry and apply appropriate technology to the part to ensure consistent high-quality cutting. This should include an intelligent selection of cutting speeds for holes, slots and other internal features together with intelligent strategies to pierce and lead in and lead out of the profile. Failure to do so will impact consumable life and cut quality.

As the saying goes ‘a chain is only as strong as its weakest link’ and that is very much the case with a plasma cutting machine installation. Now, all you need do is find a manufacturer to deliver a solution that incorporates all of the above.

Kerf Developments have developed a wide range of profile cutting machines that include solutions for oxy-fuel, plasma and waterjet cutting applications. The key to success has been the belief that equipment should be selected on merit. The company believes that all of its machines utilise best in class elements that offer excellent performance and reliability. The independent team can evaluate new products and establish if they are suitable, reliable and cost-effective for the industry. Kerf offers new products when its engineers are completely satisfied that they meet in-house standards. When applying these standards to the specification for the “ultimate” plasma machine, Kerf has highlighted the reasons for recommending a specific solution.

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The key components that form the ‘ultimate’ machine design would include …

A rigid bridge assembly that is strong but not overweight: The bridge on Kerf plasma machines is strong portal frames manufactured from steel that is fully welded, stress relieved and machined. The bridge provides an excellent solid base for mounting the drive systems. The design of the RUR and RUM machines are such that other aspects can be replaced or upgraded if required to support new technology as and when it is available - a good example of this being the development of the Ultra Sharp precision plasma cutting technology.

A reliable and predictable CNC control and motion system that can position the cutting torch exactly where it needs to be in a smooth and controlled manner. Due to their reliability, Kerf has supplied a range of Burny controller and drive systems. The modern systems provide reliable control and drive systems that feature ease of use and reliability. The spring-loaded Easy Glide drive system used in conjunction with the Burny system eliminates backlash and reduces power consumption throughout the motion control system.

Precision electronic torch height control that can maintain the exact pierce and cut height for the plasma process. The Kerf choice of electronic torch height control is the Inoav. The unit is stable and provides precision height control. The unit lowers the torch assembly down to the workpiece and sets the initial pierce height. Having pierced the material, the unit then maintains the correct cut height above the plate and is constantly being adjusted as the head moves over the plate.

Safety breakaway device for the torch. This needs to be “rigid” but also able to protect the torch over the life of the machine. Kerf offers both a magnetic or pneumatic torch protection system. Given the environment in which plasma machines operate, a maintenance-free sealed pneumatic type is advised. The unit is linked to the e-stop system and once activated stops the machine immediately should the torch come into contact with anything on the machine cutting bed.

Independent downdraft cutting tables that are not connected in any way to the motion system. The heavy-duty Kerf tables have been designed to efficiently collect dross in bins and at the same time extract the fumes generated by the cutting process. The bins are approximately 500 mm wide and controlled by pneumatic rams that open and shut baffle plates. By extracting fumes from the area immediately under the cutting torch and surrounding area, the tables are highly efficient.

Advanced CAM software that can take base CAD geometry and apply appropriate technology to the part to ensure consistent high-quality profiles are cut. Kerf works with leading independent global suppliers whose systems are not tied to any particular process or machine type. This allows customers the freedom to invest in new equipment at a later date without having to compromise on selecting a specific machine manufacturer or scrap their previous investment in CAD and CAM databases. The value of your production database should not be underestimated. It is worth significantly more than the initial cost of the software.

Kerf has worked with several independent CAD/CAM suppliers over the years. There are systems on the market that offer differing levels of user control and automation. For installations where customers already have a CAM installation, the company works with customers and their suppliers to create a machine efficient post-processor.

For new installations or where UltraSharp cut quality is required, the most popular system is the Lantek Expert system. In addition to industry-leading nesting algorithms, the software offers fully automatic nesting, remnant and material management, automatic tool path selection, intelligent lead-in and lead-out strategies and strong data import facilities.

Plasma units

Kerf has installed over five hundred machines with a range of different plasma units. Early high definition systems included systems from Hypertherm, Inner Logic and Kaliburn. More recently, the leading system for high-performance plasma cutting machines to Kerf Developments has been Lincoln Electric. These are our system of choice.

As the World’s largest supplier of welding and cutting systems, Lincoln Electric has delivered excellent products and support to Kerf and its customers. The team at Kerf does not doubt that the Lincoln Electric Spirit II systems fitted to its machines offer unrivalled price to performance ratios and have proven reliable operation.

A high-performance plasma cutting machine can reward users with returns of over a hundred pounds an hour if they can produce high-quality parts. To counter that, a machine that lays idle waiting for spare parts, an engineer to arrive or simply for a call back from the supplier to resolve an issue will be costing them a similar figure plus the additional costs that downstream disruption causes. Furthermore, the knock-on effect with customers and the supply chain can be even more costly to your business.

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