Oxy-Acetylene Cutting by nuhman10


									                               Burn Baby Burn
Once you have “mastered” the art of Oxy-Acetylene welding, next on the list of
techniques to master is Oxy-Acetylene cutting or burning. Flame cutting is often
referred to as using the “blue wrench”, a term that comes to us from those using a torch
to remove stubborn or rusted nuts and bolts by heat generated from the ”blue wrench”.
In the case of manufacturing or fabrication, torch cutting is applied to steel part

First Things First
A few simple issues to consider in the operation of the cutting torch can make the
difference between a job done quickly and neatly and one that looks hewed out with a
broad axe. Remember that flame cutting leaves rough edges with slag that must be
final trimmed later. This may be useful for arts and crafts, but is problematic when
doing precision fitting.

The primary chemical reaction in flame-cutting steel is oxidation. Because cutting is its
fundamentally oxidation, metals that do not oxidize (think rust) easily, such as aluminum
and stainless steel can not be cut with a torch. Yes it is possible to melt holes through
the material and eventually sever these non ferrous metals with a torch, think of this as
analogous to cutting wood with a campfire. It can be accomplished but the results may
be less than desired. Primarily, oxy-acetylene cutting is done on low to medium carbon
steel. These metals rust readily.

To begin, cutting tips numbers 00 through 3 will handle most typical work and an
operator with steady hands can cut steel up to 4” thick. A combination of acetylene and
oxygen form the preheat flame for a cut, and bring the material to the proper heat where
the pure oxygen jet pouring into the heated area burns (oxidizes) a path or kerf, parting
off the metal.

Follow the essential rules for lighting off an acetylene torch: Acetylene on first and off
first. Not following this rule by turning off the oxygen first allows the acetylene to burn
back in the torch creating a loud bang. This back fire can damage the torch tip and
possibly pull material back into the tip fouling the orifices. Turn off the acetylene first
and allow the oxygen to blow all combustible gas from the torch.

Figure 1: Oxy/Acetylene Flame Types

WEC 01/07 MET 357                         1
Light the torch and adjust the gas until a neutral flame is achieved. With this done
depress the oxygen lever and readjust the gas until a neutral flame is once again
acquired. A good even preheat is needed to make a good cut. If all preheat passages
are not open, shut off the torch and rub the tip in a figure eight motion against a fine
abrasive surface and relight. This will in most cases open up the preheat holes.

A pressure in excess of 5 pounds is rarely needed for acetylene. Begin with about 25
pounds of oxygen pressure which should be sufficient for most cutting. On ¼” thick or
less plate, using a 00 tip, drop down oxygen pressure to 10 or 12 pounds. Also on thin
plate, tilt the cutting tip about 45º from the cutting surface in the direction of the cut to
get a cleaner job.

With a clean tip and the correct pressures on the gas regulators, consider some fine
points of cutting. To cut a piece of old, well weathered, or heavily rusted material, sand
or scrape the cut path with a portable grinder to remove rust and prevent it flying up to
foul the tip. Rust will also tend to insulate the material from torch preheat.

                                                   Figure 2: Cutting Tip

Depending on your stance when operating the cutting torch, position the tip in the torch
head so the preheat holes are equal on each side of the cut line. This provides an even
heat and therefore, a cleaner cut.

Assume a comfortable position, steadying the torch with both hands. Hold it firmly, but
not in a death grip or muscle tension will beget the shakes and make a ragged cut.
Wear a good pair of clean goggles for two reasons; it is impossible to see the work
clearly without them and scale will fly off the plate from the heat of the torch.

If the job requires a straight line, use a piece of flat bar or angle iron as a guide,
clamping it to the work. The guide should be at least 3/8” thick to prevent the torch from
jumping on top and loosing the cut. Do not in any case use a steel square as a burning
guide—it will quickly become ruined. In most cases a higher quality job can be
accomplished by pulling the torch toward the body and holding it gently against the
guide. Rate of travel is very important in this task. Pull to slowly and overheating of the
material will result, causing “blobs” underneath. Pull to quickly and the cut will be lost.

The correct speed will become evident when sparks from the cut fan out wide
underneath and an unmistakable fluttering sound will be heard not unlike those made by
a bird (the famous acetylene raven). At all times while burning, try to keep the tip
perpendicular to the work. At tip held at an angle spells trouble and burners who can
not see straight make many poor burns.

WEC 01/07 MET 357                          2
Burning Holes
When burning odd shapes, most technicians do better work moving from right to left if
right handed because it is easier to see the layout. There are mechanical attachments
for burning circles (CWU has such an attachment). However, there are always bolt
holes to burn in plates. To burn a round hole, first burn a cross in the hole location (a
reasonably accurate measure of a cross can be done by eye). With the cross done,
simply round out the corners and show off the round hole!

Getting a hole started in a plate can threaten the welfare of the cutting tip. I pass along
a method from a very astute operator.

Staying within the confines of the hole area, preheat the spot holding the torch dead
still. Hold the torch with a thumb under the oxygen lever to keep from tripping the entire
oxygen jet at once when depressed with the opposite hand thumb.

When the preheat spot is red, very lightly press the oxygen lever and at the same time
move the tip slowly across the hole area. As the torch is moved, slowly increase the
pressure of the cutting oxygen. Slag will pile up in a neat little mound behind the
burning path. Burning will progress deeper with travel until the burn has pierced the
plate completely. Try this technique and observe how easy it really is—it can be done
all day without ruining a tip.

This technique can be use to great advantage to remove broken studs in threaded
holes. When burning in a confined space such as this keep a can of water close by and
if the torch tip overheats (sounds like a machine gun) dip the tip to regain control of
torch temperature.

Tips for Better Burning (BB)
    Excess slag at the bottom of cut indicates the preheat flame is too hot. Correct
       by reducing pressure or using a smaller tip.

         Metal does not have to be super clean for cutting.

         Most beginners force the cut by moving too fast. Slow down.

         Clean the cutting tip periodically. The cutting process tends to spatter molten
          metal back on the cutting tip, reducing efficiency.

To burn well requires taking the time to address all issues. Do not hurry the job.
Remember the metal must be hot (kindling temperature) before the oxygen jet will burn
through. When burning very thick material, take time to preheat the start of the cut.
Treat the cutting tips as an expensive tool (they actually are). Doing so will pay off in
time saved and the satisfaction of a job well done. Oh and do not pound the plate with
the tip, there is sure punishment for that sin, sooner or later.

WEC 01/07 MET 357                           3
WEC 01/07 MET 357   4

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