Problems cutting 3/4 with plasma or flame

[djreiswig]

I'm trying to cut some parts from 3/4" bar. The pieces are 3" wide and 16" long with a radius on each end. I'm cutting them from 4" wide flat bar. I have a Hypertherm 65 on a 4x8 Bulltear table with CandCNC Ethercut. I also have and oxy flame torch attachment.

I tried cutting some of these a while back with the plasma and had varying luck with the pierce and cut quality. The 65 isn't rated to pierce 3/4", but with the ramp pierce in sheetcam it is doable. As far as I recall, the cuts on the long edges would turn out angled toward the same direction like a parallelogram. I'm thinking that the pierce was wrecking the nozzle and causing this.

I switched to cutting with a flame torch. Drilling the start eliminates piercing problems. Now the issue I'm having is that the material is flexing and causing the long edges to cut with a slight bow (one side bowed in and the other side bowed out). I have the material supported 2"-3" above the slats on some angle iron just outside of the cut area. This keeps the water from splashing up. I have tried cutting out of a long bar and also using a piece about 1" longer than the finished part. You can visually see the kerf closing on the cut side when the opposite side is being cut.
I have tried to edge start with both the plasma and the flame, but cutting the waste material allows it to open up and exacerbates the problems.

Any ideas on what I should try next?

[WyoGreen]

Have you tried it with the material sitting on the slats with the water cooling it? I've cut 3/4" with just my HT45, edge starting and water splashing on it, and got a fairly nice cut.

Steve

[BTA Plasma]

If you have to sue the 65 you should be going slow. The bottom of the radius will indeed curve inward due to the loss of velocity in the beam and the trailing edge. Its inescapable with the thickness and amperage your cutting. The only thing you can do with that type of cut and that kind of power is try to minimize the trialing edge by slowing down. This will lower the life of your consumables but if you need to get it done this is a neccessary sacrifice.

[djreiswig]

WyoGreen
I only raise the material when flame cutting. With the plasma it is on the slats. I tried edge starting, and it made the flexing worse by opening up the surrounding skeleton.

BTA Plasma
I'm using the book specs for speed. I read about slowing down on the corners. With the plasma I seem to have more noticeable angling on the long edges and it is going the same direction. Like a parallelogram. I found a post on here about the o-ring on the torch leaking and causing this issue. I have replaced the o-ring before, but I will check it again.

[Bigrhamr]

Cutting from flatbar makes the flexing problem a little harder to deal with than if you were cutting from plate. With plate you could set up "bridge cutting", nest the parts side by side but leave a small tab connecting them to each other in the middle. So your cut starts near the center on one side, goes halfway around that part then turns and goes halfway around the next part, etc... When you are halfway around the last part the cut stops, you go past that tab, pierce again and go around the other half of all parts. You have to grind off the tabs, but they hold everything together and keep things from shifting during the cut, and you get a whole pile of parts with 1 or 2 pierces.
Cutting from flatbar you could still leave the tabs, but have 2 pierces per part.

[djreiswig]

I worked with it again today. Cleaned the torch and replaced the o-ring. I read that someone was having similar issues and that took care of his problem. First part I cut was almost perfect.
Second part pierced and some slag stuck to the shield. It looked like it partially covered 2 holes, but the air blew the holes back out some. This part had slight parallelogram angling. More noticeable than the first one.
I cleaned and inspected the shield. The nozzle looked fine after the cut.
I changed the drawing to edge start the next part on the one end to get rid of the pierce and fix the fouling problem. I remembered that slitting the skeleton made the flexing worse, so I put a clamp across the bar to help hold the edges together. I also shoved a shim in the kerf on the first side after it was cut to try and hold the part more centered.
I played around with some cut rules to slow down the feedrate on the pierce and finish to let the arc catch up. I had mixed results with these.
I ended up getting enough parts cut that were usable after a little grinding. I think I will get a sheet of 3/4" and see if this will take care of the flexing.
Bigrhamr
I will have to keep the tab idea in the back of my mind if I still run into the flexing. I'm not sure how much it would help on the bar.