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CNC Plasma Cutting - Part Warpage

Jwest7788

Joshua West
Administrator
Was working with @Janger over the weekend on the CNC Plasma table, cutting his stock (3/8" I believe it was) into strips.

What we noticed was that, even though we were using CNC to cut straight lines, as the cut proceeded the metal was warping due to the heat to the point that the final cut pieces really were not that straight.

@CalgaryPT , @kylemp what do you two reccomend for combatting part warping, presumably from the heat buildup? (Is this a relatively common issue with thicker stock? I haven't come across the issue with thin stuff.)

Possibly related, when we went to move the parts, some of them had bonded to the supports, like the slag had pinned the parts to the supports. Could this be related?

@Janger , could you snag some pics or measurements of those flat bars to help convey the issue?

JW
 

Bofobo

M,Mizera(BOFOBO)
Perhaps some directed compressed air for cooling attached to the cutting head or otherwise?
Just thinking about how the slag would take the path of least resistance, presumably in a conical pattern but I have only ever used a handheld one time for a rough cut.
What is the machine rated to cut? Is 3/8" above or below that?
 

Jwest7788

Joshua West
Administrator
"Low dross cuts on steel up to 5/8" with up to 3/4" maximum capacity (under ideal conditions)"
 

Jwest7788

Joshua West
Administrator
Nope, correct pressure. (Near the top end of the acceptable range)

We even tested different feed rates to find the one that looked the best on a test piece, but it promptly warped when actually cutting.
 

Jwest7788

Joshua West
Administrator
We were making several long cuts without breaks, maybe it needs cooling breaks next time?
--> Or potentially making the furthest left cut, then the furthest right cut, second left, second right, etc.?
 

Bofobo

M,Mizera(BOFOBO)
Like welding sheet metal, great idea! Or like the lines on a road and final pass is is a solid one...
 

CalgaryPT

Ultra Member
Vendor
Premium Member
I suspect there are posts on this in the old Pirate 4x4 forum from Jim Colt. He has all the answers.

Than being said I don't recall the issue with thick plate or anything 16 gauge or heavier Josh. Thinner stuff I programed pauses into the g code. I suspect Jim would tell us to check speed and amp settings. Technically the water doesn't cool, it is just there for dust collection; but I cut many sheets with water touching the bottom of the sheet and even with it on top. The steam tells me it is having some affect
 

CalgaryPT

Ultra Member
Vendor
Premium Member
Hi Josh. As usual I cannot sleep. So in addition to worrying about North Korean nukes, Trump, Hurricane Harvey/Irma, and NAFTA talks, I was thinking about your problem. I looked at four 2x4 foot 3/8" plate blanks I have in my shop where I cut out bumpers for a Jeep years ago. None of them are warped or distorted at all--and there was a lot of cutting on the blanks.

Can you post pics of your issue? Are you cutting A36 steel? I've cut hundreds of feet of A36 with plasma w/o warping so am baffled. Also, what are your settings for the plasma, feed rate, AVHC, etc?

And what is Kim Jong Un's end game???? Ooooppps...sorry. I got side-tracked.
 

Jwest7788

Joshua West
Administrator
Hi Josh. As usual I cannot sleep. So in addition to worrying about North Korean nukes, Trump, Hurricane Harvey/Irma, and NAFTA talks, I was thinking about your problem. I looked at four 2x4 foot 3/8" plate blanks I have in my shop where I cut out bumpers for a Jeep years ago. None of them are warped or distorted at all--and there was a lot of cutting on the blanks.

Can you post pics of your issue? Are you cutting A36 steel? I've cut hundreds of feet of A36 with plasma w/o warping so am baffled. Also, what are your settings for the plasma, feed rate, AVHC, etc?

And what is Kim Jong Un's end game???? Ooooppps...sorry. I got side-tracked.

We'll need to depend on @Janger for the photos and possible measurements, as he took all the stock and cuts with him when he left.
Seems we settles on a feed of 15 (The unit escapes me, but F15 in the gcode)

35 amps, around 85PSI

Was standard hot rolled steel, I cannot confirm the grade though.

I feel like it's from the metal bonding via slag to the slats underneath, but hopefully John gets some pics for us to confirm.


JW
 

CalgaryPT

Ultra Member
Vendor
Premium Member
15 ipm sounds way too slow for 3/8. Just checked my Hypertherm manual and at 45 Amps you should be at 32 ipm. Machines will vary, but most are pretty close unless you use fine cut consumables, which you shouldn't be on 3/8 plate. If you machine only does 35 Amps so you are going slower this could be your problem.


See attached.
 

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Jwest7788

Joshua West
Administrator
Appreciate that. My machine can do up to 50, we just settled on 35 and F15. Will try 45 next time. Maybe that's the issue!

Thank you!

JW
 

Janger

(John)
Administrator
Vendor
It warped about 0.125 over two feet.
 

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Jwest7788

Joshua West
Administrator
John, how did they measure in the narrow dimension (width) from one end to the other?
--> In the picture you provided, the face looking at the camera, measured vertically from the camera's point of view.

It seemed to me that was the direction with the most severe warping, wasn't it?
 
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Janger

(John)
Administrator
Vendor
Turns out It warped worst across the plane of the stock. End to end width is out .07" on one piece and 0.04" on another over 2'. So turns out not much at all.

I could watch the metal moving apart at the kerf line on the table as the cutting was underway. But as the measurements show it did not make much difference. It was a relatively small 2 foot by 6" piece. Does something like that need to be held down with weights when cutting? Is the part of why it warped? Or is it just the cutting speed?
 

CalgaryPT

Ultra Member
Vendor
Premium Member
Hi guys. Hope everyone is well. A couple of observations:

1) As annoying as it is, the warping you have experienced isn't really that significant WRT its length. You could experience similar or worse results due to pressure deflection from transport. And as this is hot rolled, you can correct manually pretty easily.

2) There is no correlation of Dross to Warping. Dross is largely a dwelling issue, but ancillary impacts include speed, amperage--and occasionally, arc angle. Production plasma facilities (e.g., Torchmate/Lincoln Electric) actually include dross removal in their labour costs. In fact, a leading indicator of cut quality is the characteristic (and SO satisfying) "SNAP" you hear from hitting an old fashioned tombstone chip hammer (or twisted wire cone brush on an angle grinder) to the dross of cutouts you just produced. Proper dross is normal. It's tough to describe in writing...but once you hear--or feel it pop off a cutout--you think... "Oh...Snap....I dialed that in PERFECTLY!"

3) The purpose of a water table isn't to cool. That's a common misconception . Sure, it seems logical if you watch videos with all the dramatic steam etc. But water tables don't prevent warping because they do not cool. Their purpose is to catch dust. Period. Downdraft tables illustrate the issue: they capture the dust and suck it out. That's all.

Look/revisit Jim Colt's wealth of information on these topics. He explains the logic of it based on 30 years of experience with Hypertherm. If I look at posts from years ago, I too had similar problems. Jim maintains that plasma is a repeatable process. Once you dial it in, you can replicate the same results over and over again (hence the ability to program the settings into the AVHC and why this controller was designed for repeatable processes). Once set, the only variation (I) found is the surface quality of the plate--which is dependent on the manufacturer (rust, thickness variation, dirt, oxidation coatings, etc.) and is easily resolved with coarse steel wool, or a wire brush to ensure sufficient electrical contact for both the ground and the AVHC. Put another way--if the process required too much tweaking for every cut, industry wouldn't use it. Thus, you know there are standards, and these should be a good starting point.

Josh, somewhere in the docs I provided originally you should find Jim's recommendations. He says, if this happens, do this...etc. If you can't find his posts, check Pirate 4x4 or let me know. I think it is archived.
 
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Jwest7788

Joshua West
Administrator
Josh, somewhere in the docs I provided originally you should find Jim's recommendations. He says, if this happens, do this...etc. If you can't find his posts, check Pirate 4x4 or let me know. I think it is archived.
All great advice, Thank you!

I'll look for that documentation next time I am in the shop!

John, was the plate warped at all before we cut it? I never thought to check.
 

kylemp

Well-Known Member
I'd agree with @CalgaryPT, that is a small deviation for the distance. I would be interested if the material had that bend, since 3/8 isn't very likely to warm unless you're putting a lot of heat to it for a long time.

I can say I've cut 3/8 and 1/2 without any warping but I may be running higher current which keeps it moving faster. I'd recommend the highest current you can run to make that cut.

On another point, I don't know about the water table comment... I've used my machine with fully submerged cutting to prevent warping and it did do that job. It was -FULLY- submerged... which may change things to some degree, but no matter what it cuts the crap in the air out of the equation, I wouldn't run one without water personally after using it.
 

Janger

(John)
Administrator
Vendor
I don't think the plate was warped as some of the other pieces we cut were not warped as much.
 
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