1/2" and 3/4" parts won't drop out

[Jhart]

I recently upgraded to a PM85 and am working (still) on the best parameters for cutting thicker metal. This particular problem involves the parts not falling free, due to the plasma not cutting completely through the final 1/8" or so of the cut. This is not a problem on 1/4" or 3/8", only 1/2" and up. I have watched the flame at the end of the cut and can see it jump past this final short distance, to the kerf created by the lead-in, then it will continue cutting through the plate for the remainder of the lead-out. I have tried every conceivable combination of lead-in, lead-out, but the results are the same regardless. Everything else is fine. Getting great edge angularity (3 degrees max). Not a lot of fun getting the parts removed when dealing with the weight of this plate. Any ideas would be welcomed.

Below is a photo of the back side of a couple cuts in 3/4", and a couple of photos of the final part after I beat it out of the plate and removed the dross.

PM85
85 amps with shielded consumables
Book specs for pierce height, delay, cut speed.
CANDCNC Electronics
Sheetcam
Mach 3

[dhelfter]

Does the arc extinguish before end of cut? Try and increase you speed. Also do an extreme lead out, like 1" just to see if this still happens. Make sure work clamp has a good connection to clean metal.

[plain ol Bill]

I'm sure Jim Colt will jump in here with suggestions - he can help you a LOT. Ugly looking dross on the bottom that should not be there. Are you using Hypertherm book parameters for the thickness material you are cutting?
Are you using Sheetcam? The Sheetcam TNG development version has an overburn setting that should help a lot with your cut not quite getting finished.

[TAP]

Not sure if this will help but...

With DEdge I would change the Gap At End Of Loop to a negative number to make sure parts would fall out or (to a positive number to keep the part from falling out).

[Jhart]

Dan,

The arc does not extinguish until the lead-out is complete. It just seems to have something to do with crossing the kerf. I've cut probably 25 of those squares and never had one that cut through at the end.

Plain ol Bill,

I know the dross is bad, but I am cutting at Hypertherm book specs and have verified the torch height by stopping mid cut and measuring the height with a feeler gauge. It is dead on, and I think the lack of edge taper is a good indication of that as well. I have tried every cut speed from 22-29 ipm, with no change in the dross. I have dropped my water level to the bottom of the table, sprayed the back side with Pam and still the nasty dross remains.

No dross problem on 1/2" or less, but as stated, the 1/2" won't cut all the way through at the end either.

Here is a better photo, with a 1" lead-out.

[jimcolt]

Get rid of the lead out and put a slow down at the end of the cut, maybe the last 1/4" or so. With some software you can stop right at the kerf crossing point (stop the motion) and add in a plasma off delay time. Either will solve your issue. Plasma has a natural lag angle in the arc, the bottom trails 10 to 15 degrees behind the top. Slowing allows the bottom to catch up, so does keeping the ar on momentarily after the motion stops. Software on industrial machines automatically handles this in many cases. Looks like you have some drive tuning issues as well from your picture!

Jim Colt

[dhelfter]

Jim,
Trying to learn here. I understand what you are saying about no lead out, and stopping motion before torch, but why does the arc "jump" across the metal at that point? With the huge lead out the arc lag I would think would be irrelevant?
Thanks
Dan

[Jhart]

Jim,

I have the same question as Dan. Hopefully you can provide some insight.

As far as the mechanicals are concerned, there really isn't any problem. That last photo I put up is of the bottom of the plate and was (obviously) cut without looping the corners. I think the flame was just dancing around as it came around the corners. I have attached a photo of what the top of the actual part looked like. The corners are perfectly square and the edges are straight. If I loop the corners, the bottom looks just like the top.

I took your suggestion and slowed the feedrate using an action point in Sheetcam just prior to the end of the cut. This did the trick and allowed the part to fall free. I'm still not sure why I have to do this. I would love to hear from others if they have to do the same thing, or if their parts fall free with just a normal lead out.

I still don't know what to do about the dross. As I mentioned, I have verified my cut height is exactly .06 and I cut every speed from 22-29 ipm in 1 ipm increments with no change in the amount of dross.

Plain ol Bill. I haven't had the chance to try the overburn/overcut feature to see if it works. I still need to download the newest development version and work with it. I will report back when I do.

Thanks for everyone's replies on this issue.

[jimcolt]

The Plasma is an electrical process. The arc is negative and the plate is the positive side of the circuit. When the top of the arc crosses the (already cut) lead in kerf there now is a break or a gap between the top arc attachment point and the bottom (remember that the botton of the arc is trailing 10 to 15 degrees). The gap requires more voltage (more voltage = longer plasma arc) to maintain arc attachment (our engineers call these cathode feet attachment), and the laws of physics for electricity rule that an electric arc with take the path of least resistance....so the arc jumps the gap and misses the cut on the bottom.

Slowing down: This allows the arc to "catch up" by reducing the lag angle. The bad things about slowing down? More heat input, more dross, wider kerf width.

Stopping motion at the end of the cut, or just very slightly past the end of the cut (called overburn) and keeping the plasma arc on after this motion stops: This allows the bottom of the arc to maintain its attachement, and catch up, finishing the cut on the bottom.

More power will drop cut thick materials easier. If you are having issues with an 85 amp plasma drop cutting 3/4"....and prefer not to use the above techniques...then upgrade to a 105 amp or 125 amp plasma......which will drop cut thicker material with their more powerful, higher voltage capable arcs.

If you ask me for a recommendation for cutting production (2 or 3 shifts a day) on 3/4" to 1" steel....I will recommend at least a 200 amp plasma, better yet a 260 amp plasma system. The air plasma systems that are far less expensive simply do not have the power to do what thier industrial cousins can do.

Jim Colt

[jimcolt]

Jhart,

I realize the picture is the bottom of the cut. I am seeing some weird shapes at the corners....indicating to me that either de-acceleration/ acceleration is shaky, or that there is some mechanical backlash or loading causing some motion roughness. If you say it is not an issue, then that is fine with me! I am just going with my 36 years of looking at plasma cut quality and making adjustments to improve it....

The arc does not just dance around, rather it reacts to the motion. If speed is changing (often with your hand on the torch you feel "roughness", especially when entering and exiting corners), then the arc will reach out and melt metal a bit off path.....this shows more often on the bottom of the cut than the top (energy density is highest just as the arc leaves the nozzle). Consequently if the motion is very fluid and stable during de-acceleration (into the corner) and acceleration (out of the corner) you will see almost the same shape on the bottom of the cut as the top, looping corners in not necessary in most cases. A small amount (a few thousandths) backlash can do it, drive tuning that is too "hot" can do it as well.

Jim Colt Hypertherm



"As far as the mechanicals are concerned, there really isn't any problem. That last photo I put up is of the bottom of the plate and was (obviously) cut without looping the corners. I think the flame was just dancing around as it came around the corners. I have attached a photo of what the top of the actual part looked like. The corners are perfectly square and the edges are straight. If I loop the corners, the bottom looks just like the top."

[Jhart]

Jim,

I obviously don't know a fraction of what you do about this process and didn't mean to come off as if I did. That's what I hate about emails and such, too often what I write is not exactly what I am trying to convey. It just seemed to me that with the flame lagging so far behind it would make sense that it would do some strange things on the bottom of the cut when it went to make the 90 degree turn. I will look into the problem further. Thank you for your responses.

[beefy]

Jhart,

I've had that exact same problem quite a few times. I've got the PM1250 retrofitted with the Duramax machine torch. I've also had a tab left on 90 degree corners which is strange because it's not jumping a gap and taking the path of least resistance. It effectively has to re-pierce around the corner.

It's always when using the 80 amp nozzles (the 1250 only goes to 80 amps) on thicker steel, 16mm was the last I remember.

Since I last had this happen, Sheetcam cutting rules has come along. I'll be playing with those when I do thick steel again to introduce short delays at the problem areas.

Keith

[Oldsarge]

I had a little problem with 1/2" doing the same thing on my 1250 with Duramax torch.... I increased the air pressure by about 5 psi and it cuts like a dream now... my 1/2" parts fall out like a puzzle piece. I love the overburn in the development version of Sheetcam, the holes I cut are much cleaner now!

Oldsarge

[beefy]

Thanks Oldsarge,

I thought I had my air pressure up high enough but I'll put it higher next time to see if it works.

Keith.

[tnbndr]

Just thinking out loud here. But has anyone tried the lead in in the middle of one side, then the finish of the cut won't have to cross a kerf it will flow right into one and with a long lead out should complete the cut. I don't have a table but will someday.

[_Ogre]

i notice on 10 ga mild steel that if i set the lead in/out at a corner that some pieces won't drop out
i move the starts out of the corner to a side and everything cuts like glass and drops out.
i'm new to this plasma table stuff, so take everything i say with a grain of salt

[Stout]

Referencing the two posts above, one showing the bottom with obvious flame path issues, and the other showing the top, with the Brown and Sharpe 90' angle tool:

I have seen this type of flame movement on magnetized plate. The top is the perfect looking cut and there significant change the actual flame path as it passes closer and further from the central magnetic field. The flame path also changes the closer you get to the end of the cut where the piece is free of the plate. Maybe something to investigate.