What changes are needed to cut steel thicker than one inch?
Because cutting thick steel requires more oxygen than thin steel, a special oxygen regulator with the capacity of delivering more oxygen volume at higher than welding pressures may be needed. Larger diameter hoses may also be required. The welding acetylene regulator is fine for cutting. Also since there is much higher oxygen consumption and more rapid cylinder depletion than in welding operations, the typical cutting regulator is a two-stage regulator to maintain a constant working pressure as the cylinder gas dwindles. Oxygen regulators specifically for cutting usually have low-pressure gauges (on the output or torch side of the regulators) with higher pressure calibrations than welding regulators. OFC operations on extremely thick metals can require 100 to 150 psi (6.8 to 10 bar) oxygen pressures.
Table 2–1 Optimum pressure and gas flow settings for cutting various metal thicknesses
What important facts should be remembered regarding cutting tips?
•They made are of copper and can easily be damaged if dropped. Tips from one torch maker cannot, in general, be used in another manufacturer’s torch.
•If you have removed the tip nut that retains the torch tip, and the torch tip is stuck in the torch body, a gentle tap on the back of the torch head with a plastic hammer will release the tip.
•Care should be taken when cleaning the tip to avoid breaking off the tip cleaner inside the torch tip.
How do high-speed cutting tips differ from standard ones?
Regular cutting tips have a straight-bore oxygen channel and operate from 30 to 60 psi (2 to 4 bar). High-speed tips have a diverging taper and permit operation at oxygen pressures from 60 to 100 psi (4 to 7 bar). This permits a 20% increase in cutting speed. They are used only on cutting machines. See Figure 2–3.
Figure 2–3Standard cutting tip (left) and high-speed cutting tip (right)
What fuels other than acetylene are used for preheating in the OFC process? These are often called alternative fuels.
•Propane
•Natural gas
•Propylene
•Methyl acetylene-propadiene stabilized (known as MPS or MAPP® )
What changes must be made to OAC equipment to properly utilize alternative fuels?
Torch tip designs are frequently different because alternative fuels may be supplied at lower pressures, have different ratios of fuel to oxygen, and different flame and burn rate characteristics. Several manufacturers offer alternative fuel torches. Figures 2-4 and 2-5 shows alternative fuels cutting tips.
Figure 2–4Alternative fuel gas cutting tip
Figure 2–5Alternative fuel gas tips are two piece tips consisting of [1] an outer shell, [2] an inner member, [3] grooves for preheating flames, [4] extremities of grooves and [5] a cutting oxygen bore
Why are these alternative fuels used when acetylene always produces a higher pre-heat temperature?
Alternative fuels are far more stable than acetylene therefore much safer to handle. An alternative fuel may also offer significant cost savings. Fuel selection is a complex matter involving material thickness, cutting speeds, preheat time, fuel performance on straight lines, curves and bevels, and their impact on the total cost. The availability of fuel, labor, the cost of preheat oxygen, and the suitability of the alternative fuel to perform related processes like welding, heating, and brazing also influence total production cost. While fuels other than acetylene do not produce the high flame temperature of acetylene, some can produce a greater volume of heat output throughout the outer flame envelope. This gives an advantage to some alternative fuels in cutting thick steels.
What other differences are there between cutting with OAC tips and OFC tips?
The main difference is the distance the cutting tip is held above the metal. The acetylene cutting tip is held so the pre-heat inner- cones are just above the metal; the alternative fuel cutting tip should be held outside the skirt which further away from the metal. See Figure 2–5A.
Why is the alternative fuel cutting tip held further away from the metal?
The heat characteristics of acetylene are different from other fuels; in Chapter 1, Figure 1–3 shows the hottest part of an acetylene flame is at the inner-cone. Alternative fuel gases characteristics are different and the hottest part of the flame with these gases is outside the skirt. See Figure 2–5A.
Is there a difference in the way alternative fuel gas tip is lit and adjusted?
The acetylene tips are adjusted by opening the fuel valve until no soot, at the end of the carburizing flame, is visible at the end of the acetylene flame then adjust the flame to neutral by adding oxygen. Once the heat cone flames are at neutral depress the oxygen lever and look at the flame if it appears to have a feather or carburizing flame adjust the oxygen with the oxygen stream lever depressed until the flame again looks neutral; now cutting may proceed.
Alternative fuel tips are adjusted by opening the fuel valve enough so the gas can be ignited followed by adding a small amount of oxygen reducing the flame enough to see the inner-cones; then alternate between opening the fuel valve a small portion at a time followed by opening the oxygen until you see the heat cones sticking out of the end of the tip approximately
Figure 2–5AAppearance of the OFC flame inner-cones and skirt
What other OAC tip designs are available?
A wide variety of tips are available. See Figure 2–6.