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ASSESSMENT OF OXY-FUEL CUT QUALITY

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.

1. 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.

Picture of an excellent cut quality 

Two of the most common issues found when checking the cut edge profile are, excessive rounding or beaded top edge, the possible causes are listed below.  

Rounded top edge

Possible causes:

 - Cutting speed (too slow)
 - Excessive preheat
 - Incorrect nozzle to workpiece distance (too high)
 - Incorrect nozzle size
 - Cutting oxygen flow rate (too high)

Beaded top edge

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Possible causes:

 - Mill scale, slag or rusty material surface
 - Cutting speed (too slow)
 - Incorrect nozzle to workpiece distance (too high)
 - Excessive preheat 

2. 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.  

Narrowing kerf

Possible causes:

 - Cutting speed (too fast)
 - Incorrect nozzle to workpiece distance (too high)
 - Dirty nozzle – flow of cutting oxygen interrupted
 -Incorrect nozzle size (too small for material thickness)

Widening kerf

Possible causes:

 - ​Cutting speed (too fast)

 - Cutting oxygen flow rate (too high)
 - Incorrect nozzle to workpiece distance (too high)

3. 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. There are other factors that can have a similar detrimental effect on the cut quality, they are listed below.  

Rake angle of drag lines

 Possible causes:

 - Cutting speed (too fast)
 - Incorrect nozzle size (too small for material thickness)
 - Cutting oxygen flow rate (too low)
 - Incorrect nozzle to workpiece distance (too high)

The material has not been completely severed, triangle residuum at the end

Possible causes:

 - Prematurely stopping the cutting process
 - Cutting speed (too fast)
 - Incorrect nozzle size (too small)
 - Dirty or damaged nozzle
 - Weak flame
 - Flow of cutting oxygen interrupted

4. BACKFIRING

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.  

Possible causes:

 - Cutting speed (too fast)
 - Damage, Mill scale, slag, or rusty material surface
 - Incorrect nozzle to workpiece distance (too low)
 - Weak flame
 - Gas leak on torch

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.

How to do it properly is shown in the videos. Watch Thermacut YouTube channel and enhance your professional skills!

Gas pressure settings

https://www.youtube.com/watch?v=mXNJbMhJvGE

Setting up the "right" oxy-fuel flame

https://www.youtube.com/watch?v=MCr54FVrzzY 

This is an example of excellent cut quality 

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