Propane torch cutting problem

I have just switched over from acetylene to propane for my cnc torch table.
I am getting very good quality cuts when doing flat surfaces. Even better quality than the acet, which is surprising because the acet was very good.

The problem occurs when I am doing an angled cut, the oxy cutting jet seems to sputter or miss for a split second creating a very rough cut and every four to six inches or so I actually lose the cut, which is really annoying.

I am cutting 3/8" plate. First I roll the plate to a gentle radius and then cut a window out of the inside, I cut it all in one setting so the top and bottom cuts are square to the plate and the sides are angled ( 15-20 degrees), I didnt measure the angle it is just a guess and I do different parts which are rolled to different diameters.

So I tried to increase the temp, I tried to slow down the feed speed, I increased the oxy pressure, I tried different heights above the work and nothing I did made any difference. I replaced the tip with a brand new tip, no difference, I tried the next larger tip size, no difference. Is there a pulling my hair out emoji?

Eventually when I finish the angled section and start cutting square again (on the same part) it cuts fine. This problem takes a lot of grinding to remedy and is a real time waster.

So any tips for angled cutting would be very welcome. It seems like a preheat problem but slowing down the feed should solve that issue, it did not.
Hopefully I have included enough information.

Thanks in advance for the help,
Michael

There aren't that many variables. Oxygen pressure, fuel gas pressure, speed, cutting height, cutting tip.

Of course, when you angle the torch the effective thickness of the metal increases. And if you don't do something about the height of your tip above the work, it may also increase on the slanted cuts.

You have checked the tip chart from the manufacturer, right?

Finally, you should think about the amount of preheat. You could go to a tip with more preheat or less preheat.

Beyond that you're into regulator and/or torch repair.

Not a solution, but some stuff to think about. Hope it helps!

metalmagpie

Michael,

Dumb question...did you switch to a propane cutting tip when you switched gases? The propane tip has different dynamics than its acetylene counterpart.

Stuart

http://www.cousesteel.com/AndysPlace/PropaneAcetylene.html

I am using propane tips. I am using the right tip for 3/8" material, I went one tip larger as a test on the angled cuts. It made no difference, still spit spit and quit unexpectedly.
Whenever I have had cutting problems with acet I just change the tip and all is well.
My propane tips are all brand new and have done only a few feet of cutting if any.
I was also getting huge bubbles of slag on the bottom side, cutting square the cuts were near perfect with almost no slag.
I had no trouble making these cuts with the acetylene, although they did cut a bit rougher for some reason, almost like the jet of oxygen was being deflected back and forth as the cut progressed, in increments equal to the jet width.
A minor amount of grinding would clean up these cuts from the acetylene.
Keep in mind I make furniture and the torch cutting and grinding process is the finished process, no welding or other machining is being done. I need to get to perfectly smooth quickly.

Michael

Could it be a gas flow pattern / swirl or whatever issue.

On the flat the gas is pretty much in clear air all-round as it mixes into the flame. On the angle you have a limited shrouding effect one side and fully free air the other. Maybe the difference can upset the flow pattern enough to upset the cut. Maybe try a sort of temporary air dam on the other side of the angle so conditions are very similar each side of the cut. Torch would be working in a Vee so to speak.

Clive

I run propane pretty much for everything, and I have about 30 tips.

I would suggest looking for a tip with central
cutting hole sized for your 3/8" thick material,
but with more (or "heavy") preheat.

I Have some of these, I run airco/concoa torches.

Clive,
That is worth a try as it does seem like there is some interference which would not be affected by slowing down the travel speed.

Doug,
I will look for those special tips, I run Airco/Koike torches. Have you done much bevel cutting?
What pressure are you running your propane for a number 2 tip?

Thanks
Michael

M. Moore said:

Clive,
That is worth a try as it does seem like there is some interference which would not be affected by slowing down the travel speed.

Doug,
I will look for those special tips, I run Airco/Koike torches. Have you done much bevel cutting?
What pressure are you running your propane for a number 2 tip?

Thanks
Michael

Click to expand...

I haven't beveled with the machine torch, but do vary the tips
for the job (heavy plate, heavy scale etc)

I was in a shop that did production 3 torch beveling of approx.
3" plate. They run propane, but I didn't get a look at which tip. Didn't seem to have problems, however it was a dedicated
machine (straight line).

Starting and stopping sounds like the problem I had on 1" thick,
sort of a "hogging"....found I was running too much oxygen.

We played years ago with oxy/propane machine cutting.
Propane volume takes something like 4 to 4.5 parts to 1 part oxygen to target a natural flame,around 5,500 BTU useable.
Where as acetylene takes about 1 part to 1 part oxygen to target a natural flame, around 12.700 BTU useable.
That said we had a similar problem as you do, when we were beveling on heavy plate, we had to nail the area of cut as well as just beyond the cut area with a grinder as mill scale was popping off at the very edge of the cut and effecting cut quality.
We also slowed down travel speed with propane.

We do a similar type of cut. We cut large windows in 9-1/2" OD x 1/4" Wall DOM and use a Koike Oxy/Propane torch. I think we use a #1 tip and do not have any issues. The cut is always very clean regardless. When cutting the slanted portion the operator only slows down slightly and uses the height control to maintain tip to material distance. We use a Koike 106-D7 tip. I have found that the D7 tip has a much cleaner cut over the standard 106 tip. https://www.koike.com/documents/Product-Brochures/Accessories/CUTTINGTIPS_PGA_ENG_WEB.pdf

Walter