Q. I recently cut out some hubs. They were supposed to be completely circular, but only half the part turned out that way. The other half of the hub was somewhat straight. Do you know what could cause this and how to fix it?
A. While parts that are not properly shaped are almost always blamed on the cutting process, often flat spots on circular parts or holes are related to the height or motion control.
I suggest you start by mounting a pen with a felt tip and fine point in place of your torch. Go through a test run, then carefully inspect the part to see if the same anomaly (flat spot) occurs. If the drawing on the part looks right, then the machine would appear to be okay, and you might be looking at a mechanical issue such as backlash.
To determine is this is the case, cut the part with the height control in manual mode and the torch at a fixed height. If the flat spot is minimized in this situation, then your likely problem is your height control. It's probably not moving from pierce height to cut height quickly enough, and may be cutting too high at the beginning of the cut. There are two ways to fix this. (1) Adjust your height control system; or (2) increase the length of your lead-in so the height control has more time to reach the proper position.
The third thing that could cause you to get a non-circular part are worn or damaged consumables. Inspect the nozzle and shield carefully following the instructions found in the Consumables Inspection section of your operator's manual. Look for small nicks, burn marks or plugged bleed holes. Also, make sure the center orifice is perfectly round and has sharp edges. If not, it is time to replace your nozzle and / or shield. The swirl ring and retaining cap can also affect cut quality in a directional manner. Replace them one at a time with new parts and try the cut again.
If the problem still isn't fixed, then my final piece of advice is to rotate the torch anywhere from 90 to 180 degrees. Try performing your cut again and then check to see if your flat spot has moved. If it did indeed move, the issue could be with your gas flow. It could be restricted or a leak could be occuring somewhere along the line.
Hypertherm's Jim Colt is an avid fabricator who has worked with industrial cutting systems for more than 35 years. He is the author of many articles and
white papers on plasma, laser, waterjet, and oxyfuel cutting and a frequent speaker at key industry conferences. If you have a question you'd like Jim to answer please send it to firstname.lastname@example.org.