What causes this?

[NorthridgeFab]

What causes these cut marks on the bottom of the cut? I only have this issue with 3/8 and thicker.

I'm thinking the water table maybe causing the arc to bounce around. But I do not see other people with this issue that have water tables.

[travmc1]

What machine, cut speed, amps, are you using? are you using THC? what is the height of the water? maybe try lowering it and check your tip and electrode.

[travmc1]

Forgot to ask what direction are you cutting in?

[NorthridgeFab]

Powermax 85
85 Amps
New tip/electrode
35-40 ipm
CandCNC THC (Also tried it with thc off)

Water was right at the bottom of the plate. Can't lower it because we need to keep the plate as cool as possible to keep the rings from warping. I'm starting to think it's not just the water, there are shops that cut with the plate submerged and do not get these results.

You know I think I may have had the reverse cut direction clicked in mach! If thats all it was I'm going to feel stupid.

[NorthridgeFab]

Yep it was cutting CCW. I do not understand why sometime SC puts the cut path in different directions. I've always checked the "reverse" box when cutting holes, etc. For this piece I had to check "reverse for the outside cut and un-check it for the holes.

[travmc1]

Thats cool you figured it out. When I looked at the lines on the side of your cut it look like it was going CCW. Good to hear it wasn't somthing major wrong.

[WSS]

Check the drawing for stray nodes or lines. Sometimes a very short line placed just right will make the cam think the part has more than one item, it may not put a path to it depending on your tolerance settings but on import it will see it. AutoCad has a purge feature that will get rid of those nodes and lines, I assume most other cads do too. Try a select all and see if some odd part of the drawing turns pink or whatever your selection color is. Just a thought. I had a ghosting problem recently from a drawing that was drawn on a later version than I had, it would show on selections items that had been deleted previously on my "old" version. Goofy and hard to figure out.

WSS

[NorthridgeFab]

Check the drawing for stray nodes or lines. Sometimes a very short line placed just right will make the cam think the part has more than one item, it may not put a path to it depending on your tolerance settings but on import it will see it. AutoCad has a purge feature that will get rid of those nodes and lines, I assume most other cads do too. Try a select all and see if some odd part of the drawing turns pink or whatever your selection color is. Just a thought. I had a ghosting problem recently from a drawing that was drawn on a later version than I had, it would show on selections items that had been deleted previously on my "old" version. Goofy and hard to figure out.

WSS

Thanks for the info, I will check it out. Currently it's cutting perfect once I got the directions right.

[jimcolt]

Water does have a rather dramatic edge roughening effect. Cutting dry (either a downdraft table...or a water table with water low enough so it does not even splash the plate) will always produce the smoothest cut.

The plasma process (the temperature and the DC power) cause the water, which is H2O (hydrogen and oxygen) to break down through a process called Dissociation....which separates the oxygen and hydrogen molecules. The hydrogen molecules burn rapidly...and tend to deflect the arc, which produces more pronounced lag lines.

Take a look at your "wet" cuts....and compare to the attached cut done on my downdraft table, dry. This is cut with a Powermax at 45 amps...it is 3/8" steel.

Jim Colt

[WSS]

Thanks for the info, I will check it out. Currently it's cutting perfect once I got the directions right.

You mentioned sometimes sheetcam will reverse the path directions. That may be why.

WSS

[carbidecraters]

In my years of laser cutting and plasma cutting I have only seen low air pressure cause breakout on the bottom of a cut.

[xavier2nd]

I don't understand how cutting in CW vs CCW can make a difference??