Plasma cut versus drilled hole 2

[BridgeFixer]

I have a fabricator who is preparing some 850mm x 25mm x 650mm steel plates with a variety of holes/slots. I specified that the 34mm dia. bolt holes within the plate be drilled. The fabricator is wondering if it would be acceptable to plasma cut the holes, as he will already have the steel plate on the plasma table for some slots and large diameter holes. The fabricator has indicated that he has a hi-def plasma cutting table (not sure if that makes a difference). I am looking for opinions on whether high definition plasma cutting is an acceptable alternative to a drilled bolt hole. Are there concerns with the heat affected zone near a plasma cut bolt hole? CSA S16-09 (yeah, I am based in Canada) Clause 28.4.3 seems to indicate that machine guided thermally cut holes are acceptable for statically loaded structures. I assume that since this plate will be used in a bridge repair (part of the connection between the pile cap and steel pipe piles, so it's an important component), I am dealing with a cyclically loaded structure and Clause 28.4.3 does not apply. I reviewed CSA S6-06 (Canadian Bridge Code) and could not find anything about plasma cut holes. I don't think the fabricator would have any issues drilling the bolt holes as specified, but it would make his life a little easier to plasma cut the holes. Any feedback is appreciated.

 

[BAretired]

I don't know much about plasma cutting, so I looked at the video referenced below. The plasma cut holes in the video do not look very round and I would be concerned about stress risers unless they were ground smooth, but if that is required, it would seem cheaper to just use a drill.

BA

 

[BridgeFixer]

Those are some pretty gnarly freestyle plasma cut holes. The fabricator will be using something like machine featured in the link below. I don't know exactly what type of equipment he has, but it will be somewhat sophisticated.

 

[BAretired]

That looks a lot more sophisticated than the video I found. I can see no reason why that would not be acceptable.

BA

 

[IFRs]

Plasma cut holes do not have smooth edges. If this presents stress risers for you, use the plasma to cut the holes slightly undersized and then ream them out with a drill or reamer.

 

[berkshire]

If you have a heat affected zone that you are worried about, then as IFRs says cut undersized and drill or ream.
Be aware, that the local hardening on the edges of the hole will beat the heck out of the drill or reamer. so have several on hand.
B.E.

 

[IFRs]

Water jet instead, to avoid heat issues.

 

[SteelPE]

It's pretty typical in the steel fabrication business to use plates cut on CNC machines similar to the one shown in the post by BridgeFixer. In fact, they have plasma machines that cut holes, notch beams and prep edges all in one shot (Peddinghaus Ring of Fire). I wouldn't think twice about it for one of my buildings. I will say that I have never designed a bridge though.

 

[connectegr]

RCSC Specification 3.3, allows for thermally cut holes in statically loaded structures. Limitations on surface roughness are provided in the commentary. However, RCSC notes that cyclically loaded structures require approval of the EOR. I have seen restrictions on thermally cut holes and even punched holes in bridge structures. Thermally cut holes can result in residual stresses and cracks.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[BridgeFixer]

If the fabricator was to plasma cut undersized holes and burn up bits by drilling/reaming them to size (due to heat hardening), would he not be better off to just drill the holes to begin with?

connectegr, that RCSC spec is quite similar to our Canadian spec. I would be the EOR and I am not yet comfortable with the plasma cut holes. These will not be slip-critical connections, so I have concerns with slip and crack initiation due to the micro-roughness of the plasma cut holes. If the fabricator had a way to compare the roughness profile of a drilled hole with a hi-def plasma hole, I would be very interested in the comparison.

 

[BridgeFixer]

IFRs, I would be very interested in water jetting the holes, but I think there is a significant cost increase associated with water jetting.

 

[MintJulep]

Past discussion on the same topic.

thread404-210234

 

[BridgeFixer]

Thanks MintJulep. I did review that post and found a lot of similarities and some important differences between the two situations. I think rejecting the use of a manually flame cut hole (as opposed to machine controlled plasma cut hole) is a more cut-and-dried decision, in my mind.

 

[IFRs]

The heavy truck manufacturer Mack Trucks only supports drilling holes in their frame rails. They are high strength steel. Cyclic loading requires special attention. How close to the yield strength is the plate stressed to?

 

[BridgeFixer]

I dug out my design notes and it appears that the plate will be loaded to about 60% of it's yield strength, at the design loading.

I went to the fab shop for a demonstration of the plasma cut bolt holes in a 25mm thick steel plate. The holes are 34mm (to accommodate a 1.25" diameter fastener). The fabricator and I both agreed that we will stick with the specification to drill the holes. His equipment has "true hole" technology, which is supposed to create very nice holes, but it is only rated for plate up to 3/4 in. I have attached some photos of the sample piece (taken with my phone - so quality is lacking).

1. top of sample - holes look pretty good.
2. bottom of sample - holes do not look so good.
3. close up of not so good hole
4. "Overburn" on the inside of the hole.
5. "Overburn" visible on all four holes.

 

[jimcolt]

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