Assuming steel scrap, the correct size cutting tip, along with the proper oxygen and heat settings are important. These are all based on the metal thickness as follows:
Metal thickness Tip Size Oxygen Acetylene
1/8 000 20 to 25 3 to 5
1/4 00 20 to 25 3 to 5
3/8 0 25 to 30 3 to 5
1/2 0 30 to 35 3 to 6
3/4 1 30 to 35 4 to 7
1 2 35 to 40 4 to 8
To light the torch, open the acetylene valve 1/8 of a turn and light it. Run the acetylene flame out from the torch tip until the black carbon smoke is eliminated from the flame. Adjust to a neutral flame by opening the oxygen valve. Using the T handle on the gages, set the oxygen and acetylene according to the metal thickness. Check the "feathers" within the flame and adjust accordingly. The blue lines within the flame should be very clear, not fuzzy looking. You can sharpen these blue lines by slightly adjusting the oxygen and acetylene knobs. To start the cut, set the torch tip halfway off the edge of the metal. The torch should be held from 1/8" to 1/4" above the metal. When the metal becomes bright orange, pull the cutting valve down and begin. Move the torch in a straight line with very little motion after the cut has started. If the cut has been properly started, a shower of sparks will fall from underneath the metal. If you move too fast, the torch will most likely stop cutting all the way through the metal and you'll have to restsart the cut. When cutting, the torch tip should be held straight up and down, but when cutting light gage metal, the torch should be tilted in the direction of the cut, thereby slicing through the metal. Since you're cutting scrap, always keep a set of tip cleaners handy. You'll want to make sure in particular that the oxygen orifice in the cutting tip is clean. The flames and the oxygen should be coming straight out of the tip very sharp looking and not sideways. If not, those are signs that the cutting tip needs to be cleaned.