Distortion from plate internal stresses

[beefy]

A couple of weeks ago I way cutting some 200 x 200 (mm) square tube along its length. Got a big surprise when it started getting a slight banana shape. I could only conclude internal stresses pulling it into this shape. Lesson learnt there and sad to have wasted my big box section.

Now I've just read a thread here about 14 ga distorting even on a water table and someone posting it can be caused by the sheets internal stresses being released as it's cut. I'm talking sideways distortion, not the lifting up off the table type.

Has anyone ever experienced this, perhaps with lower quality steel sheets. The thought would never have crossed my mind before reading that thread.

[abmetal]

For me it's a common occurrence especially when I cut long lengths. I cut a lot of flanges from 3/16" hot roll and when I trim off the length of the plate it always "crawls" and after I've cut a lot of flanges from a 4x8 plate, it'll gradually raise and sometimes twist. I also cut a lot of 12ga. P&O and it will do the same, sometimes bad enough that I have to put weight on it to hold it in place. I've never cut cold roll on the plasma table so I can't say the same for it but I suspect it'd do the same.

Allen

[Marty]

A couple of things come to mind. First , is warping a new problem since you tried cutting tubing instead of sheet?

[beefy]

A couple of things come to mind. First , is warping a new problem since you tried cutting tubing instead of sheet?

Thanks for the replies lads.

Marty I'm a bit confused by the question, so I hope I'm giving the right answer here. I've been cutting 3mm (11 ga) on my table and haven't had any warping problems yet, but that could be because of the shapes I've cut.

The tube cutting was just an experience that taught me I can't simply make straight C-channel out of box section due to internal stresses. Then I read the thread about internal stresses causing a similar situation with plasma cut sheet, so I thought for future reference it would be a good idea to ask if the quality of sheet metal can cause a difference in the cut.

I've also read a thread here about steel being made in China, and one of the guys posting said it cuts like crap. I live in Australia so it wouldn't surprise me if I ended up with sheets that were made in China.

So no, I haven't had any problems but always good to get feedback from any others that may have.

[plain ol Bill]

When cutting tubing or pipe or long flat sheets it will help prevent warping if you leave a short section uncut in several parts of the cut. Come back and cut the short sections when the part is cooled off. Won't prevent all warping but will sure help.

[Marty]

Beefy

I see that you are talking about lateral warp (sideways) not so much about vertical warp........and your question about "internal stresses" in the makeup of the metal causing warpage....

I have only ever cut flat sheets so have only dealt with vertical warp........however I would add that

Once or twice I have suspected I had some sub grade metal sheet due to the way it cut from tine to time.....the metal was "grainy" / "porous" on the inside.......so I changed suppliers and no more quality issues..........but by far the problems I have had with warpage were due to cutting too fast for the gauge of material and comsumable I was using, or not having the water level high enough im my water table

I recently battled a problem with the torch not cutting all the way thru, which led to the need to lower my cut speed 20% below the manufacture's book value, which led to warping, which led me to finally notice my water level was way down. I now keep the water so that it touches the material. Joy is back in the shop again, getting clean proper cuts and quieter running, and no warping

So since you are having issue with warping, either the material is suspect as you may be thinking or, may need to speed up the feed rate, since the water will not help with cooling when you are cutting on top of a tube. Maybe just needs to cut faster? Maybe apply some water to the top of the tubing just ahead of the cutting ?Also another thought, perhaps by nature of cutting into a tube, there is mechanical energy release at the cut point that is causing the warp.....the metal may be OK quality but since it is a formed shape, it want to expand when there is a release.

I have no expierence cutting tubing with plasma, but I do have expirence welding tubing and angle iron, so I do like plain ol bill's suggestion of leaving uncut sections and then come back and cut them after cooling has occured. In the welding world, the same technique is used to minimize warping, IE tack or very short weld sections intermittantly along the length of the material, the fill in the un welded sections gradually

My take away on this is that the next time I need C Channel, I will not try to cut a tube! I will remember your expierence:) Is C Channel difficult to get locally?

Good Luck!

Marty

[beefy]

Thanks very much for that detailed reply Marti. That one's going into my cnc archives.

Interesting to hear poor quality steel can affect the cut, even if it doesn't cause any lateral warping. That could have someone scratching their head for a while wondering why they can't get the settings right.

I've got the water tray (in 2 peices) for my table but haven't got round to installing it yet. Have to do a few other modifications to the table first but ran out of money. I think for now I'll just make a mini tray to see how life is on the other side of the fence. So many have said how good the water table is for reducing distortion. I've especially noted what you've said about the water touching the underside of the plate and from what you are saying that also helps quieten the noise. That's of particular importance to me because I don't want to upset any neighbours, I'm even thinking about making some sort of shroud to help dampen the air jet noise.

C-channel is freely available but I'm cash strapped at present (terrible business year 2012). Had a couple of 3m lengths of 200mm square tube lying around so desperate men do desparate things. Next time I'll just stay desperate

Cheers,

Keith.