3/8 Stainless not finishing a cut

[Capstone]

I have a job to cut 20 custom exhaust flanges... I have a Hypertherm PM45 with clean good air and just changed consumables.

I was able to get the first ten cut, albeit with a slight problem.

The cuts aren't finishing..

That's one issue, but now I can't even get the table to cut the piece any more...

2015-02-12 14.56.27.jpg (60.77 KiB) Viewed 2075 times

2015-02-12 14.56.27.jpg (60.77 KiB) Viewed 2075 times

I'm still struggling to figure out why all of a sudden the machine stops cutting all the way through after the initial pierce. You can see the half starts.

Any ideas where things are going wrong on both issues?

Thanks

[I Lean]

The first problem might be helped if you put in a little overburn on the inside circle. That's a problem I've had before--it's not usually a deal-killer, you can just break out the piece and use a die grinder to clean up the inside.

The problem of not even piercing, can't really help you on that.

[Shane Warnick]

Just a few things to check. I am sure you have already checked them, just might run through the list again.

1. Ground / work clamp? On the stainless, good connection, or laying off on the floor? Needs to be clamped directly to the stainless for best results. (We have ALL found it on the floor. At least once )

2. You stated you just changed consumables. Did you put the correct nozzle back in? Or did you put in the wrong size for the amps you are running? (Yup, been there done that as well.) If you put one in for say 45 amps and cut at 65, it will open it right up. If however you throw and 85 in and try to cut at 45, arc density really isn't so great.

3. Oring in the torch? Good, or no good? Also, swirl ring. It's not in upside down is it? Shield. Are all the holes open, it's clean etc?

4. Amps? You have the machine set on the correct amps? I can tell you (unfortunately from experience, and it looked a lot like your cuts) that a 65amp nozzle cuts great with 65 amps. With 45 amps, you really can't slow it down enough to make it not look like crap.

5. Speed. Didn't inadvertently bump the speed up by accident did you?

6. Height? Same story as 5. Pierce and cut good?

7. Material. I have been given material that was a little thicker than what I ordered by accident more than once, and tried to cut it and well, it doesn't work so well. Asked for 16g, got home with 14g. Asked for 3/16", got home with 1/4". Do you have a set of calipers or maybe a sheet gauge you could throw on there, just to check?

8. If all else fails, have you rebooted everything? If the table was fine, and was cutting fine, and now it's suddenly not, reboot, reload your settings and see what happens. Stranger things have happened. I would say if the torch, consumables, work clamp etc all check out, then your settings have been changed or corrupted somehow.

I agree with Ilean on the overburn, it will make the holes round (or at least minimize the little tit left where the lead in comes in).

Let us know if you get it sorted out.

Shane

[SeanP]

I have that little missed cut on the backside with thicker stuff sometimes, I find adding a action point in that area with reduced feedrate of say 60% works well, tried overburn but that didn't seem to help with mine.

[Capstone]

Thanks everyone for the replies. I'm going beck to check these things now. I think likely culprits are the grounding possibly, since I use a mag-clamp and the first cuts had the clamp snug against the edge, but really it was holding on to a slat.

I figured out the over-cut feature in sheet-cam on my own last night, so I'll add that to the g-code on the lead-out along with slowing it down some at the end as well.

Using a PM45 machine torch is definitely pushing the limits of the machine, but I'm using the HT settings so hopefully taking these additional steps and resetting the table will do the trick

[jimcolt]

Stainless cutting with an air plasma is a whole different process. When cutting steel, the air, which consists of roughly 20% oxygen, 80% nitrogen, and the oxygen content provides an exothermic boost to the high temperature cutting arc....this allows the arc to burn the metal with the arc cutting straight down through the plate.....you can see this in the lag lines of a properly cut steel part.

Stainless, since it does not easily oxidize like steel is strictly a melting (thermal, not exothermic) process. The fiction of the molten stainless causes the plasma arc to trail back as much as about 15 to 20 degrees. So....if you are cutting stainless above about 1/4", the bottom of the arc is behind the top of the arc.....and if your part program stops the cut right exactly at the end of the shape of the part....there will be a tab of stainless on the bottom that is uncut.

Most software has a delay function that you can set for stainless that keeps the arc on for a period of time after the machine motion stops, allowing the bottom of the arc to "catch up" to the top.

So, here are the normal ways to complete cuts on stainless:

- Draw you part with an overburn, go past the lead in by about .150" to .200". This usually works on stainless from about 3/16" through about 3/8" at amperages from 45 to 85 pretty well. On thicker stainless and at lower power....often the bottom of the arc will still skip the end of the cut.

-Some software allows you to do a slowdown at the end of the cut....generally it senses the end of the cut then decelerates, again allowing the bottom of the arc to catch the top.

- You can also cut the whole part at a slower speed, keeping the cutting arc more vertical, this will result in dross and a wider kerf, but should drop the part.

-End of cut arc off delay as mentioned above.

Jim Colt Hypertherm

[_Ogre]

try slowing down quite a bit from the book settings, i cut 3/8 at 65 amps and almost half the book speed
we cut a lot of ss flanges, it's not pretty compared to steel, slowing down will also help your bolt holes a lot
most of the time with mild steel i don't attach the ground clamp, with stainless i always ground it dirrectly

i had the same problem initially with ss, notice the uncut and holes incomplete cut



slowing way down everything now drops out and reduces the angularity of the plasma stream



one problem we have with ss is the slag will ball up in front of the torch and push the material around
or half way knock the torch off the mag lock base
i clamp the material securely and hold my hand on the mag lock base when i see a ball

at the price off stainless these days you could reduce your material cost by nesting your parts better
on our flanges i typically cut into the last cerf
like jim colt mentions your melting stainless vs the exothermic oxidization of mild steel

[Capstone]

Thanks again! So before I had a chance to see Jim's reply, I added the "over cut", slowed down the entire cut from 30ipm (HT Book) to 26ipm, and also added a 60% slow down snippet at the end of the cut. The other issue I found was that I had to re-level the material for each cut using shims because after each piece was finished, the remaining material might get out of level. This did the trick!

After all of these adjustments, I was able to cut 10 more very good pieces, with only one bad one that wasn't usable.

I'm really grateful for all of the support and advice.

Phil

[Dynatorch78]

Same thing is happening to me when I cut 4130 Chromoly. I will try the over burn.