More on Oxy cutting

[Grahame Collins]

Journeyman Mick wrote

<Perhaps gas pressures would be helpful, plus a trouble shooting guide, ie if it looks like this you're doing this wrong etc>.

Hi Mick

I try and teach my students that like most processes its not pick up and use it.
No its pick it up and check it!
What for?

Essential: The basic theory of how it all works

Steel plate comes in various thicknesses and naturally the differing thicknesses take varying degrees of energy to start the exothermic reaction for cutting.
What happens?
The steel is set on fire by oxy acetylene /or lpg nozzle flame.

The pre heating flame heats the plate edge up to a temperature that is conducive to oxidising (burning) when injected with a clean high stream of pure oxygen.
Note! that I said a clean stream of pure high pressure oxygen.

Most cutting heads that one picks up, the cutting nozzle will be dirty. The end of the nozzle is covered in tiny little spatters. Some of these spatters can impinge on the central nozzle orifice and encroach into the jet stream as it is being used and distort the shape of the jet stream (the jet stream being the long narrow flame shape)produced when the lever is pressed.

The nozzle openings can be cleaned with wires or special drills available from sellers of oxy cutting equipment. A point to note is that the wires –are just that-WIRES- not files or high tensile probes- they are meant to dislodge particles that find their way back into the jet stream orifice- if the wires are not successful try the drill- each drill and wire is suited to the particular diameter orifice. Use drills and wires held singly with finger and thumb. Match the wire or drill to the correct diameter.

Another point to note is that normally only the orifice needs to be cleaned out. If you clean the orifice and don’t get the parallel jet stream the nozzle is still dirty or buggered.


Different thickness sheet needs different nozzle diameters.

I am referring to Comet 3 type nozzle (sizes) here so if yours is different you will need to look it up.

No.6 3mm plate Oxy 200Kpa acetylene 100Kpa
No.8 ….6mm Oxy 200Kpa acetylene 100Kpa
No12….10 to 20mm plate Oxy 250Kpa acetylene 100Kpa
No 15 20 to 75mm Oxy 300Kpa to 400Kpa acetylene 100Kpa

Any freehand cut above 25mm thick needs an extremely steady hand and a clear head.
Don’t try these thicknesses after a night on the turps.

Back soon
With more on set up and pressures

Grahame with arthritic hands.

[Grahame Collins]

Oxy cutting like anything else has some recognized standards against which performance is measured.
If assessing a cut ,I am looking for

• A straight cut
• A clean square top and bottom edge
• Drag lines within 10% or so of vertical
• Minimum slag adherence

How to achieve the above

A straight cut
depends on a perpendicular presentation of the torch to the job, not holding it too rigidly and smooth movement as well a following the line .

Clean and square edges
are a product of correct nozzle selection and keeping the nozzle clean and set at optimum pressure.The nozzle preheat flame must be within 1mm of the plate surface. The preheat flame must be set to neutral.
a way of achieving optimal pressure is to set the reg pressure while holding down the cut lever.Set the pressure to get the longest cutting jet stream.

Drag lines
within 10% of vertical come about by a correct cutting speed -listen for the tearing paper sound and watch for slag coming of the base of the plate at 10-15 degrees in big volumes.
I am still unable to load pics or graphics as the software says I exceed a limit.

Grahame

[BobL]

How do you get nice square sharp corners? mine looked a bit ragged, hole blown open, is it best to overcut and then start on a face?

[journeyman Mick]

Thanks Grahame,
More on Oxy cutting for an oxy moron (me)
I've saved it and will print it out for next time I need to do some cutting.

Mick

[Grahame Collins]

How do you get nice square sharp corners? mine looked a bit ragged, hole blown open,

A fault I observe often in many flame settings is in that the flame is not set at true neutral. That or the nozzle size is one too large for the plate thickness.

Neutral preheat flame is best found by lighting the torch and adjusting the acetylene back until the oily smoke is almost gone and the flame is at the point of just lifting off the nozzle end.

When you add the oxy the flame is adjusted so that it does not "ROAR".
You should be able to talk to some one there next to you with out having to raise your voice thats when you know the flame is set neutral.

Normally one cuts from an edge.Its a matter of preheating until temperature is raised enough for cutting to begin.

It is excess oxy in the preheat flame and excessive cutting pressure that gives the raggedy blown open look (or the too large for the plate nozzle) I bet this happens on the lighter plate?

The flame heat needs to be tailored to be hot enough to just raise the plate temperature to ignition point (cutting) and no more.

The jet stream does the cutting, as mentioned in the earlier text the .Oops nearly forgot.The jet stream lever is depressed all of the way and held there during the cutting procedure.Its not variable like an accelerator.

It takes a while to get in the groove and keep the preheat flames about a mm off the plate.Thats because it is difficult to see that from from where your head is when you cut.

If your a right hander put a glove on your left hand and keeping the hand flat but palm vertical,rest the torch on top half way down your left index finger.

Roll the top of the hand forward to adjust for height of the preheat flames. Relax the grip on the torch and slide your hand along the cut line to support your torch.

I hid the trainer wheels ,my students have never used them and yet the interested ones can cut very well.
Does that help?

Grahame

[BobL]

So what about the corners? Should you be able to do it in one smoothaction or is it best to overcut and then go back and start the cut from a flat face? My corners are where I paus too long and I end up blowing out a big hole.

[Grahame Collins]

So what about the corners?

My corners are where I paus too long and I end up blowing out a big hole.

As if you are cutting a square or a rectangle?

Pausing, just adds to a temperature increase . Particularly if the metal is small in thickness and in area, it quickly gets too hot.

Consider what the process is here!. Above a certain temperature ( 915 degrees C ) the metal just oxidizes (catches fire )

When the area of the plate is too small it cannot dissipate the heat build up in the way that a piece of plate of bigger volume can.

This preheating is intended to be in the precise local area of the flame cut - within a couple of mms- once that area of high temperature is extended from there is a loss of control preventing a precision placement cut. An oxidizing flame adds to the overheating situation.



At the same time the symptoms here indicates that the flame is still set oxidizing.The flame appearance be soft and ends rounded as opposed to being sharp and pin pointed.

Stop completely and let it it cool,or cool it with water and come back to it and check your flame setting.

Grahame

[BobL]

As if you are cutting a square or a rectangle?

Yep !

The rest makes sense. Next time I'm cutting I will take a picture.

Thanks very much for all the info.

Cheers

[Grahame Collins]

Guys ,
i seem to have a problem uploading any more pics or diagrams,

Here a link with some extra info that may be worth reading.

http://images.google.com.au/imgres?i...icial%26sa%3DN

Grahame