What to look for during oxy-fuel cutting and assessment of the cut quality? Thermacut® can reveal some of the typical cut quality errors that are commonly experienced, their cause and a few tips on how to eliminate them. Although, the oxy-fuel cutting process is considered to be quite a simple one, sometimes operators can be faced with cut quality issues. There are 4 main points to focus on when assessing cut quality.
Poor cut edge profile
The primary check should be to examine the cut edge of the workpiece. When correctly cut the workpiece should have a clean sharp to very slightly rounded top edge, with a small amount of easy to remove slag on the bottom edge. Two of the most common issues found when checking the cut edge profile are, excessive rounding or beaded top edge.
Incorrect Kerf form
Kerf width is critical to the dimensional accuracy of the workpiece. The volume of material that has been removed should be approximately the same on the top as on the bottom of the workpiece, resulting in parallel kerf walls and dimensional accuracy. Possible causes of a non-parallel kerf are listed below.
Cut edge indications
Last but not least, it is important to check the rake angle of the drag lines. When all parameters have been correctly adjusted, the drag lines are almost vertical. A cutting speed that is too fast is often found to be the main cause of drag line angle change.
Operators can experience a “Popping” sound when cutting, this is known as a Backfire, the effect of multiple backfires within a short distance is shown in the image below. Possible causes are listed below.
The above article clearly indicates that cut quality is adversely affected by incorrect cutting speeds. Too low cutting speed leads to a rounded or beaded top edge, deep drag lines and the formation of heavy slag on the bottom edge. On the other hand, too fast cutting speed increases the angle of the drag lines on the cut face, high speed slag is formed on the bottom edge. CAUTION! High speed slag is a thin beard like formation, it is extremely sharp, handle with care.
An equally common factor that affects cut quality is nozzle to workpiece distance. Too high distance leads to a rounded or beaded top edge, uneven and slightly bevelled cut face. Too low nozzle distance results in deep drag lines, damage to the material surface and kerf caused by the flame’s preheat cone, bevelled cut face that is in reverse to the nozzle being too high.
The worse cut quality can be expected when using a damaged, dirty or scaled up nozzle, dirt can be from many sources, the scale is Mill Scale from the hot rolling process used in steel production, scale adheres to the nozzle and deflects the oxygen cutting stream, this produces a cut with excessive slag, undercut, and uneven cut face.
Incorrect gas pressures or flame setting alone can be the cause of numerous cut quality issues, where possible, follow the guidance given in cut charts, check for gas leaks in the torch fit up, hoses and gas regulators, check for sticking pressure gauge indicator needles.
When assessing cut quality, it is essential that all variables are considered and their effects through interaction.