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Reduce "Blow Holes" at beginning and end of cut

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jimcolt
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Reduce "Blow Holes" at beginning and end of cut

Postby jimcolt » Tue Sep 16, 2014 9:29 am

The large holes are obviously caused by the plasma sitting still......which causes a larger burn hole at the beginning of the cut, and the motion and the plasma shut off are not well co-ordinated at the end of the cut. On closed paths this is generally not critical as you can start and stop (lead-in, lead-out) in scrap areas, however when you have open paths with the start and stop on the finished part...it is critical.

Beginning or pierce point:
-First, ensure that you are using the lowest amperage set of consumables that will cut the material you are cutting. With the Hypertherm Powermax65 that will be the Finecut consumables. Following the book specs will get you a nice cut, however, experimenting with faster speeds and lower amperage (run them down to 20 amps if you want) will get a narrower cut kerf.

-Ensure that pierce height and cut height are correct. Too high and the kerf gets wider. On materials thinner than 10 gauge it is ok to pierce and cut at the recommended cut height of .060", and may help with the pierce blowhole. Piercing closer on thin materials may have a slight effect on consumable life, but may produce better quality cuts...it may be a worthwhile tradeoff.

- Reducing the pierce delay time will reduce the blowhole size. If zero doesn't work.....I suspect there is some sort of delay created by the software/electronics on your machine that does not start the motion and the plasma simultaneously.....actually, the plasma start takes about 80 miliseconds or so....so a machine that will move instantly will take off before the pierce is all the way through.

- With some software you may be able to start the x-y motion before the plasma start command is issued. With some software....you can actually enter a negative number for pierce delay, which will start the motion before the plasma.


End of cut blowhole:

- The characteristics of an air plasma arc are that it electrically shuts down when the stop signal is issued, however, the exothermic reaction between the steel and the oxygen (20% of air) continues for a few miliseconds. If the motion stops at exactly the same time as the plasma off signal is issued, then expect an end of cut blowhole......more so if you are cutting at higher power levels. Maybe lower powered consumables running at lower amperage will solve the end of cut issue.

-Some softwares have an advanced off (plasma) at end of cut that can be set.

Keep in mind that when you change parameters for open path cutting.....you may be changing the parameters for closed paths as well, and some of them may be adversely affected.

Jim Colt Hypertherm

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Re: Reduce "Blow Holes" at beginning and end of cut

Postby TWIZTED181 » Sat Sep 05, 2015 1:54 am

Thanks for the info Jim....

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Re: Reduce "Blow Holes" at beginning and end of cut

Postby abmetal » Sat Sep 05, 2015 9:27 am

Jim,
We've always (8+ years) had the blow hole problem and have never been able to get it to the size I'd be satisfied with. We spent probably 30 minutes on the phone with George at Plasmacam just yesterday going through all the settings trying to find the "sweet spot", but without finding a resolution. George finally suggested I go to the Pcam owners group for a possible answer. I can get the actual pierce time down to a shorter time than what the chart lists, but then it starts cutting (x-y travel) before the torch can get down to the proper cut height. We've never had a problem at the end of the cut, just when it pierces.
What prompted the call to Pcam was when I replaced the consumables and made a test cut. EVERY TIME I install a new finecut nozzle, the first 3/4" doesn't cut through cleanly. I've seen a post on Pspider from somebody experiencing this same problem, but I couldn't find it. The pic shows some of the cuts made at various setting. These particular cuts were made on 14g P&O at different speeds, amperage, cut heights, etc. We've tried about everything we and George could think of. Usually, after a half dozen cuts or so with the new nozzle, it'll start cutting ok, but yesterday was a different story. Even after an hour or so of cutting, it'd occasionally "stumble" and fail to cut through for an 1/8"-1/4".

We're running a Samson 5x10 with a HP Powermax 65 with genuine HP consumables purchased from the local Airgas store. We're open to any suggestions.

Thanks.
Allen
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Re: Reduce "Blow Holes" at beginning and end of cut

Postby larrycameron44 » Fri Sep 25, 2015 3:43 am

Hi. Yes I have had the same problem now for the last year. Genuine hypertherm consumables are all I ever use.

The only way to combat the problem after a consumable change was to reduce speed to 86-88% of my normal cut speed for 45 pierces and increase amperage 4 amps. Then after approx 45 pierces and cutting I can reduce the amperage and increase the speed again until the next consumable change.

Larry
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Re: Reduce "Blow Holes" at beginning and end of cut

Postby beefy » Fri Sep 25, 2015 5:50 am

This is an interesting one.

Allen,
you said specifically when you change a NOZZLE. I'm assuming that means you are still using the already used electrode. If so that eliminates the electrode as the issue. Could you confirm it happens when ONLY the nozzle is changed.

Larry,
you said a consumable change (nozzle & electrode are both consumables). Are you in the same boat as Allen and it also happens when just the nozzle is changed.

Keith.
8 x 5 water table
Powermax 1250 with Duramax torch
Candcnc MP3000 / Mach3
Sheetcam
Geomagic Design 3D solid modelling
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Re: Reduce "Blow Holes" at beginning and end of cut

Postby abmetal » Fri Sep 25, 2015 8:45 am

I typically change both the electrode AND nozzle but I have occasionally changed just the nozzle, but it makes no difference. I've even switched out a new nozzle with another one from a different pack thinking it could be a bad nozzle but that also doesn't make any difference. It's a real head scratcher.

Allen

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Re: Reduce "Blow Holes" at beginning and end of cut

Postby beefy » Fri Sep 25, 2015 5:03 pm

So that seems to confirm it's the nozzle and not to do with the electrode.

Hey Jim, you reading this ??

Looks like a good one for the R & D department. I'm having a wild guess that maybe the nozzle orifice gets "smoothed" out after a few cuts. Maybe R & D could chop a new nozzle and a used one in half (at the point it came good), and look at the nozzle surface under a microscope.

I wonder also if the flowrate changes for a given pressure, i.e. when new could the nozzle orifice have some microscopic "roughness" which affects airflow, then IF the bore smooths out after a few cuts, does this allow a better/smoother more dense air flow ??

Like I say though, just wild guesses.

Keith.
8 x 5 water table
Powermax 1250 with Duramax torch
Candcnc MP3000 / Mach3
Sheetcam
Geomagic Design 3D solid modelling
Coreldraw X5

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Re: Reduce "Blow Holes" at beginning and end of cut

Postby larrycameron44 » Mon Sep 28, 2015 7:35 pm

Hi, yes both when the electrode and nozzle are changed. I always change both at the same time..
Larry Cameron
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