Dross on 18g HRS

[thefabguy]

Hello everyone....

Here's another "nubie" question for all of you. Last night I was cutting some "artsie carftsie" stuff. The material I was using was .048 (18g) HRS. Lots of slag/dross especially were the small curved details were. I tried a few things, but I think because many of the line segments on the DFX file were so small, the machine was pausing (ever so slightly) at the start and stop point of the lines....Here are the settings I got the best results with....Still nowhere near optimum. Hyperthem 45 set at 40 amps, air pressure at 70 psi. Feedrate, 175 ipm. I have THC, and I keep the torch roughly .06 over the material. It was piecing about .093 off the material. Because the dross, was worst were the small contours on the DXF are located, so Im thinking that slight pause got the material too hot?

[abmagrum]

hi you need 90 psi minimum you idealy want to start out at about 120-135 psi.

this will probably fix youre problem

[muzza]

Although a little low at 70 psi I doubt this is the cause of your dross but I would pull the air up to about 90, check your air lights on the front of your PM45 for optimum pressure at the cutter. By what you've told us I'd suggest it is more speed and smoothness related.

The two main causes of bottom dross are cut speed and cut height.

Not sure what table or programs you are running but if you have lots of segments and small ones at that I start with the drawing and join curves or nodes to make the file run smoother and even reduce nodes or smooth the drawing. This will take out a lot of the slowing down and bring you back closer to your set cut speed. Although you have your speed set at 175ipm (a lot slower than I cut 18 g) if it is a small detail type drawing with lots of tight sections I doubt you are even reaching half of the set speed (if your running Mack it will show you actual speed) Also as your machine is slowing down your set cut amps will be affecting your cut height as your THC will try to compensate so you wont be running at set torch height again causing dross.

So basically try cleaning up your drawing and increase your cut speed.
Hope this helps
Murray

[jimcolt]

My guess would be that the machine is not maintaining the correct speed in the "small contours" that you mention. Dross is most often caused by low speed with plasma....although height will affect it as well.

Not sure what machine you are running....often it is just that the drive motors are not properly sized to maintain speed while cornering. Or....if there are tuning adjustments increase corner speeds or accleration rates.


Jim Colt Hypertherm

[thefabguy]

Thank you for the reply gentlemen! I think both of you are correct. Ill bump the air back up to 90. It was suggested to run with less air, but that didn't help at all. As for the file, yes it had many little line segments, and I have cleaned it up a lot. I created the file in solidworks using arc's and splines, then exported the DXF to Fabriwin so I could do a bit more detailing. Fabriwin converted the arc's and spline to the little line segments. There are some settings I need to adjust, so it doesn't do that. I have cleaned up the file, so now it should cut better. I also agree with the motor's on the machine being too small, at least from an appearance stand point...Its a Burntable, and that was one of the first things I noticed after I purchased the machine....the drive motors look small. Nothing I can do about that, so Ill just have to make sure the files have longer segments. Also, would you please suggest the cutting speed? Per the book, I think it should be somewhere around 240 ipm?

[muzza]

Book gives it at about 365 ipm @ 45 amp or about 270 ipm @ 30 amp, (that's midway between 16 and 20 gauge as 18 isn't listed). I usually run it at 6800 mm per minute (about 272 ipm) at about 35 amps, I've just found this the sweet spot on 18 gauge which I usually cut at least 10 sheets of a week. As the consumables wear I drop the feed rate back to 90% in Mach, sometimes if I start getting a bit of dross on more detailed type stuff I'll even bump the feed rate up to 110% even 120%. If I have small internal holes I pull the feedrate back to 60%.
Murray