problem with peircing steel (oxy-acet)

[marmon]

not really a mechanical eng problem, but i figured someone here might be familiar with burning tables. Anyways the table i program is capable up to 12" plate, but we rarely run anything over 2-3". The machine is wonderful it can cut any shape to perfection, but it seems to be limited to peircing, Whenever a production run is done, the tips get dirty after just a few peirces, taking time and adding frustration. Also with heavy plate, say 2" its almost impossible to peirce without messing up the tip. (the molten steel has nowhere to go but up, into the tip). I know theres shops that can easily peirce 3-4 inch plate all day long but how? I am stumped on how to do it, ive tried progressing forward slowly as the peirce begins but no luck. Pre-drilling is also not an option as it add tremendous amounts of labour, especially when runs of 200+ parts is common. Is anyone familiar with this, or has anyone seen it done?

 

[MikeHalloran]

I'm >>>conjecturing<<< that moving the torch rapidly in a small circle might help... or make a bigger mess.





Mike Halloran
Pembroke Pines, FL, USA

 

[marmon]

i see what your saying, but the servo motors are somewhat clumsy, making it difficult. the table has one servo for the x-axis and one for the y. I have tried "soft-start" valve for the cut oxygen but they are hard to tune and did little for the blow bacik. i was thinking of adding an air hose that blew the slag away as the torch peirced, but looking at other companies tables i see nothing like this attached. i have yet to approach another company and just ask, but i feel its kind of a trade secret or something along those lines.

 

[MikeHalloran]

I used to work for a place that has a table rigged for both plasma and oxy cutting, but the old timer who really knew how to use it is long gone, and I don't think they have the manuals for it anymore. Sorry.





Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Among the flackery on this page, you may find a pearl:

http://www.torchmate.com/qa.htm

Another page I found suggested cutting squares or polygons, fast, instead of circles, but suggested that it might be hard on your gantry.



Mike Halloran
Pembroke Pines, FL, USA

 

[IFRs]

From Mike's URL:

A bit more on the subject of plasma vs. oxy-fuel: When you mount a plasma torch in a machine, the piercing capacity is reduced by about 50%, because you can't tilt the torch head. You just have to blast your way straight through. If the plasma cutter doesn't have the amperage to do this, molten metal splashes up and fouls your electrode and nozzle. Starting at a drilled hole or an edge allows you to pierce at full capacity.

 

[bingopin]

Check the tip size and tip to plate distance. As important, make sure there is sufficient preheat. I would not be hesitant to ask what the other shops you mentioned are doing.

 

[metengr]

Also with heavy plate, say 2" its almost impossible to peirce without messing up the tip. (the molten steel has nowhere to go but up, into the tip). I know theres shops that can easily peirce 3-4 inch plate all day long but how?

You need to select the proper torch tip size for cutting. The cutting tip provides for preheat and a jet stream for pure oxygen to cut. Preheat the spot, angle the torch, add oxygen to blow the metal out of the hole.

 

[marmon]

ok, but in an automated process, there is no control on the angle of the torch tip. To manually do it would also be impossible as you would never get the torch square to the plate again. I have tried the reccomended tip along with smaller and bigger sizes. I also should mention these are triple seat tips. meaning theres a seperate solenoid and line for cut oxygen, and preheat oxygen.

IFR, you are right, we also have a plasma head mounted, and when you mess those up, the arc ruins the consumables and in the garbage they go. But if you cut only what your amperage can handle, they work great and can peirce all day long, and they never get dirty from general use. MAybe its the 85 psi of air running through it keeping all that popping scale away?


Also as you mention edge starting eliminates a lot of problems, but again in multiple runs of the same part its not always possible. With larger parts, leaving an open edge also creates a stress releif point were the part is no longer contained and can move a large amount during the cut.

Mike, thanks for the link,and your time. I'll check it out.

 

[berkshire]

marmon (Mechanical)
I do not know if this is a practical solution for your problem.
Years ago I worked for a company that used a three head oxy fuel profile cutter (magnetic follower type), when they needed to do piercing, a hand torch was used to blow a 1" dia hole through at three points on the plate. The cuts were then edge started from there. The operator hand guided the cutters back to the profile then let the machine run.
The operator also had a handful of metal strips the thickness of the cut, which he inserted into the kerf as the cut progressed, this reduced the distortion of the part.
B.E.

 

[Darken99]

Marmon,

Check to see if you can control the initial pierce height. If the torch is too close while piercing, it would also cause slag to go back on the tip.

Depending on the quantity of parts and size, you could set them up to edge pierce. That way the slag is blown down instead of back up into the tip.

The parts could also be gang cut, reducing the quantity of pierces. This will only help with your external cuts.


Regards

 

[EdDanzer]

Preheat should be about 1 minute per inch of thickness on material over 1” thick. Preheat an area than move about .020-.050” then pierce. Start with the tip close and raise it as the material starts to build up. Move at ½ cutting speed for until pieced through. You will have to experiment with each thickness of plate to optimize the operation. It may need to adjust the pressure for piercing different from cutting.
Some companies have tips just for piercing plate over 6” thick.

 

[marmon]

Yes Darken99 there is a timer to set for "torch up" time before peirce. I have gone as far as programming 3 seconds of up travel before the peirce (say 3 inches above the plate) at that time. berkshire, your idea works (its what we do know occasionally) but requires layout time when multiple parts are done. What i was hoping for was to keep the process as automated as possible. Actually i should have first asked if anybody knows if its even possible to peirce heavy plate efficiently.
EdDanzer i can honestly say i have never pre-heated that long, usually after say 20 seconds on 1" plate the material starts to turn molten on the surface, then thats when we pierce, i will try a slower longer pre-heat and see if that helps.
Thanks for everybodies suggestions so far

 

[Darken99]

Do you change the tip to material height for different thicknesses?

 

[Ataloss]

I'm not that familiar with automated oxy-acet cutting, so if this is not possible, just ignore me, but could you possibly set up 2 torches on your machine? Have one torch angled (and further optimized) for piercing, and the other straight for the rest of the cut pattern?

Bob

 

[marmon]

Darken 99 no i don't generally change the peirce height. What i have found what works best on the heavier plates, still generally will work on the lighter stuff just as well. We also have some really neat stainless lined tips, peirceing is a real problem with these as they have a very small orfice, say .050 for 1/2- 5/8" plate. So any dirt ruins the cut. A real neat feature with these is they run 100psi, so you can actually continue a straight line cut 8-10 inches above the plate. Its hard to believe but it works.

 

[Darken99]

I am talking about operating height. We have 2 different plasma cutting tables that use different fuels. Both require different tip to material distances for cutting different thickness materials. One table requires the tip to be .150" from the material at 1/2" and .300" at 2". I am not sure if this is the case for oxy-acet, but it might be something to look at.

 

[marmon]

Oh ok, with our plasma, we have a control box that you set different voltages for different thicknesses of plate and tip amperages. This voltage is read by the computer and the z-axis adjusts accordingly. Its great for warped plate as it follows the plate. As fo oxy-acet once running there seems to be little problem with a wide range of operating heights. Generally we run just above the blue cone for oxy-acet.