Need Help!- Plasma cut quality questions *PICS*

[tof1]

Made my first cuts today. Cut a 4" square with a 3" circle in the middle out of 16ga steel. I didn't clean it up and it was 98% dross free (pics below).

Here's the problems I can see:

1. There's a bit of taper.
2. The cut is jagged around the radius of the circle.
3. The corners of the square are rounded quite a bit.

Is there a common problem for any of these that I should try to fix first? I'm using Mach3, SheetCam, and I drew the file with AutoCad.

[bentecc]

A mild vibration made by both axis moving at the same time can cause those jagged cuts. Decreased your feedrate or check the gantry system if it needs some lubrication.

[WSS]

tof1,
for problem #3 you can loop the corners in sheetcam. What it does is go past the corner and come back in a loop or triangle to hit the next edge. I was watching a video in a post on CNCzone last night that showed a custom machine cut a spiral really fast and accurate! I think it was dnelso who posted it?

[tof1]

Is something mechanical the only thing that can cause the jagged cuts or can it be something with the programming?

I actually checked the loop corners box but didn't see a change on the cutting path. Do you know if there's a setting in sheetcam that I can modify? I was also wondering if you do this, is it bad to cut over the gap of the cut you just made?

[WSS]

tof1,
I am going to tag along here, I am hoping to make my first cuts early next week. I don't have a clue what to expect. I have to say those are some nice pictures. Someone should be able to give you some advice from them.

Good luck!
WSS

[jimcolt]

The jagged edges most likely are caused by some roughness in the motion...if you put your hand on the torch while it is contouring...and you notice any vibration or roughness..it will show up in the cut. You don't mention what plasma, what consumables and what power level. Are you using arc voltage height control? If the z axis is too tight it can cause oscillation that will produce edge roughness as wel...but usually not as severe as the pics indicate.

To minimize the taper...use the lowest power and the smallest consumable and the lowest speed for the material and thickness you are cutting. If it is a Hypertherm Powermax plasma..the FineCut consumables will dramatically iprove taper.

The rounding of the corners is likely due to drive tuning....it looks like your gains are pretty sluggish (you can see some heat affected shadows where the motion essentially stops)....I'm not sure of the drive tuning methods for your equipment...but you should look into this. For plasma cutting you want maximum acceleration and de-acceleration....tune the gains so to the point where the machine starts to oscilate when exiting a sharp corner (verify with a pen in the torch holder) then back off slightly until the oscilation stops.

Not bad for the first cuts!

Jim Colt

[Mongkol]

Tof1,
Could you share the design of your machine on CNCZONE? I think the experter at here can brainstorm to help you.

Mongkol

[plain ol Bill]

I had the "jaggies" on my first cuts also. After watching the tip of the plasma closely I could see some vibration on movement especially on radial cutting. I added a brace to the top of my Z and that helped tremendously. Another way I have encountered jaggies is in the cut file itself. If a curve is made of a bunch of little lines the cut is going to appear jagged. Having a cut file wherein curves are really curves smooth's these out and cuts very cleanly.

[tof1]

I'm using a Hypertherm Powermax 1000, fine cut consumables, 40A, 150IPM, CandCNC DTHC. I upped my acceleration and "looped" the corners in Sheetcam. I only had time today for one cut and this one has sharp corners but still the jagged edges. The drawing is perfect, only 4 nodes in the circle and 4 in the square. I ran it with my had on the torch and I could feel a little vibrations. I'm surprised it's shows as much as it does in the cut. I'll try slowing the cut speed down to 130IPM and see if it helps with the taper. Still not bad for a second cut in my opinion! Guess I'll have to figure out a way to brace my gantry... Thanks for everyone's help!!

[tof1]

Something I was also wondering is my Z axis tops out at 80IPM how it's set up right now. Would the Z axis not being able to move quickly enough or something to do with a setting on the DTHC have anything to do with the rough edge??

[Torchhead]

Originally Posted by tof1 Something I was also wondering is my Z axis tops out at 80IPM how it's set up right now. Would the Z axis not being able to move quickly enough or something to do with a setting on the DTHC have anything to do with the rough edge??

As long as you not getting head "bounce" (visible up and down) then the jaggies are not coming from the Z motion. There are settings in the DTHC that allow you to fine tune the gap voltage (defaulted to 1 volt "span") and the "Tip Saver" (anti-dive) Percentage in the cut profiles. The max response of the DTHC is set by the total loop speed which includes the Z axis speed. You have to normally run the Z at reduced velocity during DTHC operation because there are no accleration curves applied. So if you start with 80 IPM you may be limited to 24 IPM in THC mode. The faster you can move the Z (and still keep resolution reasonable) the better your DTHC will perform. That being said, on flat cuts the 80 IPM is probably not the cause of the ragged edges. The rounded edges are from poor acceleration in the mechanics. The control software will compensate for the inability to accelerate by rounding the corners. While you can loop the corners that won't help you when you are trying to cut detailed cuts with sharp angles you don't have scrap areas to cut on.


I have to agree that the problem looks a look like uneven motion. With MACH , which has constant velocity, and with 4 node circles motion should be smooth as butter. What is your final drive mechanics like? If it's stepper do you have belt reduction? Direct drives to pinon will run with poor acceleration and reduced resolution. Check everything for backlash.
Get the workclamp onto the workpiece and grind off a spot for the clamp to get through the rust. Make sure you air supply is dry.

Heed what Jim says. That 1000 will cut nicely if you pair up the right nozzle with the right current setting and the proper feedrate. I really like the cut quality I get with a 40A tip on material up to about 1/4 ".

Contact me on the CandCNCSupport Forum and I can give you some settings for the Cut Profile for the DTHC to check.

TOM Caudle
www.CandCNC.com

[WSS]

If you are running with belts, can you see if there is a tighter spot at one degree or another in regards to your drive gear? It almost seems like it is rhythmic, maybe the bore is off center causing the belt to get tighter and looser at (almost) consistent intervals. Maybe check to see if anything is stuck in a timing groove (like a drill curly Q or a set screw not set deep enough if it is hubless). Other than that the cuts look good and nice tolerance! The jaggies are bothersome I see. with pitched rails you have to watch for tap holes exiting through the spur side, they can leave a nice bump to wear down if you don't file it or de-burr it somehow. Those are less rhythmic but will appear at the same spot on the table where a drive or pinion will show up anywhere on the table with a regular beat. Try cutting straight cuts about a foot apart with both X and Y and see if it is more pronounced in a certain spot. At least you can narrow down a mechanical issue somewhat.

Good luck!
WSS