Blasted divots....

[davek0974]

I've been messing around this morning punching 20mm holes in 4mm plate trying to get rid of those pesky divots.

So far I've tried about 20 different combinations of lead-in, over-cut and under-cut and so far all the results look the same

I can confirm that the issue is definitely caused by the end of the cut as I made a cut and killed the torch 3/4 of the way round, looking into the kerf you can clearly see that the walls are nice and clean.

The better tests seem to be radial lead-in with the torch turned off 1.5mm before the end and arc lead-in with the torch turned off 3mm before the end but neither of these are really good.

I have a Hypertherm 30XP and I'm running the finecut consumables at 30A

Any ideas?

[davek0974]

Anyone?

[CNCCAJUN]

I'm a newbie . . . but I am curious why an arc leadin with an arc leadout back into the hole won't work?

Set arc angle leading in with what is working & leadout with an arc that will run "almost" parallel to the circle . . . almost an intersecting inside offset.

Is it possible that when you kill the torch that the air flow is blowing molten metal?

Again, newbie here . .
Steve

[davek0974]

I haven't tried an arc lead out yet, will give it a go.

I've tried turning the torch off before the end of the cut but it seems to make little difference.

There is a half second pause after the cut ends before the torch moves to the next cut.

It just seems that whatever I try makes little difference.

[CNCCAJUN]

Make it as close to matching the perimeter without causing a long "divot".

Steve

[davek0974]

Here's a couple of pictures of the divots i see, this is on a 50mm hole in 4mm plate, no lead out.

[davek0974]

Just tried a couple more, 20mm hole, 4mm plate, 30A

This one is probably the best yet - 6mm perpendicular lead-in, 1.5mm arc lead-out...

This one is not too bad - 6mm perpendicular lead-in, 1.5mm arc lead out, torch off 1mm before lead-out starts.

I've tried arc-in and arc-out style leads but where the two overlap seems to cause a bit of burn on the rear face, probably arc-stretch due to the slug falling out as soon as the two arcs meet?

Now, the main issue for me is - what is good enough? Never having run a cnc plasma before i have no idea of what a good divot is, is there always a divot?

Anyone care to post some pics of what they deem acceptable?

[jimcolt]

If the divot is caused at the end of the cut...program an overburn that stays on the hole radius roughly .150" past the lead-in kerf. This is the best way to minimize the end of cut divot. On high end industrial cnc plasma's and overburn is used and the plasma off signal is co-ordinated (depending on thickness and power level) so that the arc is fully extinguished before the end of the overburn is reached. Called overburn with advanced off.

Keep in mind that an air or oxygen based plasma does not instantly extinguish. When the "off" command is sent to the plasma it takes a while for the exothermic reaction with the steel to stop. If the motion stops and the torch is sitting in one place while it is extinguishing....this will be the reason for the end of cut divot.

Using a lead-out will improve the divot....but will have a dramatic effect on consumable life. Plasma torch consumables rely on fully extinguishing while metal is underneath to improve consumable life. If you lead out into the center slug of a hole....the slug usually drops and the arc is forced off.....this is called a "Ramp Down Error" by Hypertherm engineers.

Jim Colt Hypertherm

[davek0974]

Thanks Jim,

That explains the improvement i see with just a small (1.5mm) arc lead-out. I will try an overburn with torch-off before the end and see what happens.

[davek0974]

Some pics, this is still a 20mm hole in 4mm steel @30A, finecut consumables.

This the same hole from two angles, very hard to photograph without macro gear.

I used a 4mm overcut and killed the torch 2mm before the end by using the "Before End" cut rule in sheetcam.

Still looks rough to me, in fact i think it's worse than the previous one made with just a 1.5mm arc lead-out into air.

Not sure why I can't seem to shift this little annoyance.

[SeanP]

I'll try the same on mine tomorrow Dave to give you something to compare to.

[davek0974]

Thanks, that would be very interesting.

[SvrdRam]

I'm having the same issue, how can I turn the torch off prior to the end of cut on sheetcam?

[m79b01]

I'm having the same issue, how can I turn the torch off prior to the end of cut on sheetcam?

You can enter a cut rule. However I have not had very good luck with this. I think timing is an issue.

[SeanP]

Sorry Dave I missed getting back to you on this.

It does work, very well, I use it on thinnest material up to 6/8mm and it will near enough eliminate the burnt divot as the torch turns off, not the leadin/out divot that's a different story.

These are the settings I use, you can tweak the torch off distance or even the overcut to fine tune.
What it's doing is switching the torch off at the same time as the path starts it's lead out or as in my case .05mm before that point, leave 'include leadout' box unchecked.

I set leadout as usual as that keeps moving out of the way as the torch shuts off.

I'll try and find some photos.

[SeanP]

These are 4mm thick with a 20mm dia hole.

Topside:

Underside:

[pertneer]

Thanks for sharing this tip. I will give this a try on my next part being cut.

[davek0974]

Thanks for that,

I will try them at the next opportunity.

What current were you running for the tests??


Dave

[SeanP]

45a there Dave.

Thanks for that,
What current were you running for the tests??

Dave

[vmax549]

Dave I do not believe the fine cut consumables would be the best approach to cutting what you are cutting. your cuts look fairly rough. Have you tried a standard tip it may have a better power density and help you cut cleaner/smoother with the smaller power supply. I use the FC parts a great deal in CERTAIN areas of cutting steel AND alum but what you are cutting I would not be using the FCs.

I normally use a straight lead in long enough to to get a clean edge on the part and NO leadout.

Just a thought. (;-) TP

[davek0974]

Yes, this was some time ago now, I have since started using the std consumables on the 4mm stuff, I do use the finecuts on 3mm aluminium though as it gives a better detail.

I also made some notes a while ago and have indeed tried the torch off on lead out method but found it made little difference, also tried overburn as well.

In post one of this thread, I killed the torch manually mid-cut and could still see the start was ok, then I got busy elsewhere and not much more was done about it. Looking into the hole you could clearly see it, but I could not get a picture of it.

I tried arc lead-ins but they made the end suffer as the flame passed into the larger cavity made by the arc.

I have some time so I will need to some more tests, maybe it's just time to get a PM45 and not worry about the finecuts?

[Brand X]

I get a little divot at the start point/ end point, but use standard straight lead in/no lead out. Just 16 ga... Just using 20 amp drag tips on my 30 amp low-end of my A-120.. (I think the machine goes a bit lower then advertised) No nozzle wear to speak of.. What I do like is the 70 ipm speed settings on 16 ga. (Can be less on a circle) I just made a few cuts, but 30g kerf with a .1200 cut height. Probably would get a bet smaller cutting lower.Little less top dross in spots, but comes off with a fingernail.

[beefy]

Dave,

what is your controller software, are you using Mach / Candcnc.

[davek0974]

Hi

I'm using Mach3 and the system is CandCNC MP3000-DTHC, the new model as I only fitted about six months back, it's the serial not Ethernet version, seems to control z height very nicely most of the time, it suffers a little with the finecuts sometimes as the cut height is so low (0.5mm) there is so little headroom and a bit of dross under the tip will cause a dive and cut-out. Not so bad on the standard consumables though.

The plasma is the 30XP and the machine is home-brew and very small (625x625mm cut area) using steppers and belts.

[beefy]

That's what I've got but an older version.

The reason I asked is that I believe in my version you get a pause in motion (it's a Mach thing) when you turn THC on or off, or run a macro while the axis are moving. As far as I'm aware the M05 used to turn off the torch is a macro and so may cause this momentary pause in motion, thus causing a divot. Carefully check your torch movement remains smooth when the torch is turned off (during motion). With my system, when I switch off THC during motion, the pause is quite obvious.

I'm pretty sure Tom got rid of this issue with future upgrades, so if your system is only 6 month old it should be fine.

Keith.