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Jim Colt, Hypertherm
"This new process addresses distortion in the holes that result from edge inclusions (dings and divots that make the top and bottom of the holes out of round)."
The new True Hole technology for mild steel produces much better holes than what was previously possible with plasma. The above cut samples show ½ in thick mild steel cut with (left) and without the new technology. The pin fits perfectly in the True Hole sample because the hole is almost perfectly shaped. The hole cut without True Hole is wider at the top than at the bottom, causing the pin to fit poorly.
The rapid growth in plasma cutting during the 1960s-70s was based on its ability to contour cut steel plate with the highest speed in the industry. The best way to improve productivity utilizing similar floor space (as compared to mechanized oxy-fuel cutting operations) was to use the plasma process for cutting plate in the ¼ inch to 1 inch thickness range, with the added benefit of the ability to cut non-ferrous materials such as stainless and aluminum.
Early plasma processes provided high speed but less than perfect cut quality in terms of dross formation (resolidified metal) on the bottom of the cut, as well as cut edge angularity that was acceptable for many, but not all, metal fabrication applications.
Cutting holes for bolts with plasma was a less than acceptable use of this process due to the distortion in the holes in the form of edge inclusions (dings and divots that made the top and bottom of holes out of round). These inclusions could cause stress fractures in high strength applications. The natural taper in the holes (smaller bottom dimension than the top) inherently produced during the process by the lag angle of the plasma jet also created tolerance issues that forced fabricators to perform secondary operations such as reaming or drilling to improve the overall hole quality.
Manufacturers of plasma cutting systems relentlessly pushed the laws of physics to extract the best performance. They reduced cut costs by increasing the life of the torch consumable parts and improving the cut edge angularity and minimizing dross formation. This created more applications for this high speed metal cutting process. Over time, other major improvements in plasma cutting followed:
• Oxygen plasma cutting allowed faster speeds at lower power levels and improved edge squareness, dross formation, and improved cut edge metallurgy (better weldability, less edge hardening).
• Long Life consumable technology dramatically improved the average “cost per foot” of the plasma cutting process, which was already better than other processes, by allowing the torch consumable parts to last 3 to 6 times longer.
• High Definition plasma cutting technology, basically a higher energy density, narrower arc with higher velocity, dramatically improved cut quality through better edge angularity throughout the thickness range of plasma.
The platform that the plasma torch depends on – the CNC cutting machine which controls torch motion – also benefitted from many technological innovations:
• PC-based CNC controls increased their processing speeds and gained more comprehensive control over the required functionality of all of the tools on the machine.
• User friendly controls made it easier to train cutting system operators and easier to maintain consistency in the cutting operation.
• Improved drives, motors, gearboxes, linear ways all contributed to the smoother motion, more accurate tracking, and high acceleration rates that improved the plasma cutting process.
• Faster, more accurate torch height control systems optimized plasma torch performance with the proper pierce height and accurate real time cutting height.
Yet even with all of these plasma cutting process advancements, the same issues still remained with hole quality, even though taper, dings and divots were all less pronounced. In fact, recent surveys of long time plasma cutting equipment users showed that their number one process issue was hole quality.
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