Cut off question

[tinman13]

I have been having problems with parts not cutting off completely. It looks like the plasma flame jumps over a small tab of steel at the bottom of the cut as it gets to the lead out, leaving it tabbed to the plate.
I am using a radius lead in on these parts but it seems to do it with a straight lead in as well.
Has anyone else experienced this and if so what have you found to fix it?

This is with a Powermax 85.
These parts were cut at 85 amps with specs. per the book. It seems to do the same at lower amps as well. 10 or 14 gauge mtl. doesn't seem to be a problem, 3/8 occasionally will do it.

Thanks, Steve

[jimcolt]

Air Plasma, when cutting at the speed that produces minimal dross...has about a 5 to 15 degree lag angle in the arc.....meaning the bottom of the arc trails behind the top. On most cnc machines (I don't know what you are using) there is a function that holds the plasma arc on after the machine motion stops at the end of the cut....it is a timed function that should be set differently for different materials and thicknesses. On some machines it is called "plasma off delay".

Another way to solve this "drop cut" issue is to program a lead out at the end of the cut....so that the part program crosses the starting curve and drives past the end of the cut about .150" or so. Another way to minimize the issue is to simply slow the process down a bit...however this often produces a bit more dross (called low speed dross.)

jim Colt Hypertherm

[tinman13]

Thanks Jim, I have CandC controls using Sheetcam for programming.
I have tried lead out and no lead out with little success, it still jumps to the lead in cut area leaving a small tab.
These parts were cut with a radius lead in of .500 and a straight lead out of .100 witch seems to leave the least amount of tab.
Even with no lead out and a pause at end of cut of 1 sec. it will leave the tab.

It seems like a slowdown at the last .100 or so would fix it but I don't know if that's possible...

As you can tell from the edge quality my machine isn't as rigid as it should be..... (it's a cantilever machine)... Would this have an effect or am I grasping at straws?

Thanks, Steve

[jimcolt]

The machine rigidity will affect cut quality going in and out of corners and on smaller holes....but probably won't affect the drop cut.

Question...are you using the speeds (and consumables) from the mechanized cutting charts in your manual? There are recommendd and maximum speed listed...if you are running at speeds above the recommended speed...there is far more lag angle in your arc.....or if you are using the shield for hand cutting and the speeds for hand cutting...again, the speeds are too fast and will cause excess arc lag.

Even if you are using a hand torch...use the consumables for the mechanized torch as well as the cut charts for the mechanized torch. Using the hand shield will make your torch to work standoff too high...that shield is for drag cutting.

Jim

[tinman13]

I am using a machine torch with the correct consumables listed in the manual.
I have not had good luck with the maximum speed settings, but the recommended settings seem to be spot on!

I was hoping someone else has had this happen and there was an "easy" answer! I'll do some more experimenting and see if I can find a solution!

Thanks for your response.
Steve

[Torchhead]

Originally Posted by tinman13 I am using a machine torch with the correct consumables listed in the manual. I have not had good luck with the maximum speed settings, but the recommended settings seem to be spot on! I was hoping someone else has had this happen and there was an "easy" answer! I'll do some more experimenting and see if I can find a solution! Thanks for your response. Steve

Do you have any delay in SheetCAM (end of cut)? It keeps the torch from picking up instantly at the end of a cut. It may take more on thicker material.

Are you running with the Tip Saver (anti-dive) on of off? What are the settings in the Cut Profile for the tip saver percentage?

The natural tendency for a torch under THC control is to dive into the cut as it starts to get starved for metal. The Tip Saver will stop that issue.

It is possible to predict the end of a cut and possibly program in a feedrate slowdown BUT I think that will just be masking a problem that would best be solved buy finding the cause.

TOM caudle
www.CandCNC.com

[tinman13]

Yes I do have a delay in Sheetcam.
I don't have tip saver or antidive. (MP 1000C)
This happens as the cut passes over the original cut and into the lead out area.

[WSS]

Hey Steve, have you added anything magnetic to your table? Is your ground a mag type (as opposed to a spring clamp type)?

A magnet will pull on the arc (sometimes really strong) and give me that same effect when I use a magnetic ground and get the torch within two inches of it.

Just a thought.

Tommy

[tinman13]

No, nothing like that..... It looks like the arc just jumps over the last bit on the bottom leaving an uncut tab.

[Torchhead]

Check to make sure your torch is perpendicular in both planes to the work. Make sure your air pressure is correct...you may need to crank it up some on the thicker stuff. Make sure you have the workclamp on the material so you have maximum current flow at the cut. Normally the flame does not jump to an open space it does just the opposite...it jumps to metal where it can get current flow. Electricity takes the path of least resistance and that is to a spot that has lower resistance. Air has high resistance. If the torch becomes starved for metal (resistance starts to climb) it will look for an easier path. Normally that would be off to the sides (kerf gets wider) It should not "jump" unless the piece you are cutting is losing conduction (contact) with the workclamp. That would typically not happen as long as it is still attached. Just for grins get a set of jumper cables and clip one end to the workclamp and as the torch approaches the end of the cut touch the other end of the cable to the workpiece that is falling out and see if it's a loss of resistance problem. Normally the table grid would act as the connection but rusty metal does not conduct well and big blobs of slag do not either. The jumper cable trick will at least prove or disprove the theory you are losing current flow in the cut piece.

Since the problem is on the bottom of the cut and the torch and flame is reacting to the top of the cut the top and bottom needs to be pretty close to each other. The current flow is from the tip to the closest conductive material. Depending on the direction of the cut the bottom should actually be closer to the end than the top because of flare. The flame does "bend". The flame swirls (rotates) and the leading edge cuts different than the trailing edge. You can see that by reversing the cut direction and note the bottom flair is different.

Tom Caudle
www.CandCNC.com

[tinman13]

Part tab update.......
In reading through the forums on CNC Zone I stumbled across a thread by stout about angle
kerf cuts and realized I have been having kerf problems for a while also. So on
a whim I changed the swirl ring and WOW....
I think I just fixed it!

I can't tell if it is the air holes in the swirl ring a little plugged or it
looks more like the fiber material has worn from the electrode traveling up and
down allowing the electrode to move from side to side during the cut, resulting
in a jagged looking kerf. At any rate I ran an order of 65 parts with NO tabs
and better edge quality!

As stout said.... I haven't changed it..... It doesn't wear out! Now I know better!

Thanks for the reply's..... That's what make these forums work!

Steve