I was forwarded this thread, and after reading it I thought I should ad a few notes and comments.
There are a lot of different plasma cutting processes and technologies available today. Many of the typical plasma processes that have been used for cutting steel over the past 5 to 10 years have been superceded with better technology with regards to cut edge metalurgy and cut part accuracy. An area that has recieved considerable work in the last few years is with regards to small plasma cut features such as "bolt ready" holes. New technology that involves the process gases, power level, motion control and lead-in and lead-out part geometry is in fact changing the way structural steel fabricators look at producing parts that need critically loaded slots and bolt holes.
Suppliers of cnc structural machines that produce flat plate and structural shapes are adopting the latest technology (as opposed to integrating secondary drilling operations) based on the processes accuracy, the integrity of the bolt holes from a stress and metalurical point of view, as well as from a time an cost saving point of view. Think about it....how would your drill a slotted hole, and how easy is it to drill multiple holes sizes on a single part. Plasma can do it quickly, with perfectly acceptable accuracy, and with excellent hole integrity.
-The latest technology from the major industrial plasma system supplier is called True Hole technology. It is the culmination of a 3 year internal proces engineering project to improve plasma cut bolt holes. In a nutshell it uses advanced CAM processing software to analyze part drawings, effectively identifying holes that fit within a certain area. The CAM software then uss its vast hole cutting data to automatically apply the correct shape lead-in, lead in speed, hole dimensional profile (including automatically changing kerf compensation when speeds and process gases are changed). The process uses Oxygen as the plasma gas and compressed air as the shield gas to pierce structural sections, then before tha cut profiile starts the shield gas is switched to pure oxygen. The oxygen shield increases the aggressiveness of the plasma cutting process and achieves virtually no taper in the hole....something no other plasma process can do. The holes end up have little or no taper, are round top and bottom, have virtually no lead-in and lead-out "ding or divot" that we are all familiar with with plasma cut holes. The bottom of the holes will appear slightly out of round, however the process is designed to bia any anomalies to the outside...so that bolts will drop through without secondary operations such as grinding or reaming.
-This True Hole technology requires proper cnc machine equipment. It must be used on machines with superior motion control, and with tightly integrated cnc, plasma torch height control and plasma systems that have the ability to cut with the Oxygen/oxygen process.
-Contrary to normal plasma hole expectations: The holes are typically as smooth or smoother as compared to most drilled holes. Metalurgical studies that test for stress fractures have shown favorable results with thes plasma cut holes vs drilled holes on structural plate.
-Also contrary to popular belief....cut edges that are cut using modern high definition class plasma systems (such as Hypertherm's HPRXD series) have very little edge hardening, and can be drilled and even tapped immediately after cutting. This relatively unaffected edge is the result of a small heat affected zone (plasma speed as well as the high density arc) and the lack of nitriding on the plasma cut edge.
I have been involved in the development of technology with plasma cutting systems for over 35 years with Hypertherm in Hanover NH. I'd be happy to answer any questions about the use of this new capability...or any plasma questions for that matter.
Best regards, Jim Colt
