Introduction and Questions

[Paint Chip]

Hi folks, first post here. I bought a Patriot 4X8 table from Precision Plasma with CandCNC electronics and a Hypertherm 85 machine torch 4 yrs ago but was unable to get it set up until now. I have a few more issues to resolve before putting the table to work but I am very close. One question I have is how do you hold thinner sheets of metal in place and down flat on the table? I have a piece of 1/8 steel on the table but it has a slight warp upwards in it at (0,0,0), which means setting the z home is kind of unstable. I could see where thinner gauge metal might easily get bump out of position. Also, do you position parts away from the edge of sheet metal? I'm not sure how the plasma would handle a cut line on a sheet edge. If so, how far away? Lastly, do you add any chemicals to the water in your water table for corrosion inhibiting or just use plain water?

Thanks,

Dave

[steel 35]

More info on your CNC Electronics package is needed already, DTHC II or IV
Turn the sheet over, and put the warps down when they are on the edges.
Enjoy this site lots of HELP.

[Paint Chip]

DTHC II. I thought of turning it over but I think the other side is warped the other way so it might end up the same. At least the sheet is reasonably rigid, being 1/8 inch. I think I can deal with it by manually holding it down to start. I was just wondering if anybody had figured out some way to clamp sheets in place. How do you line up a sheet so it's square to the axes? I was thinking of cutting a L shape from a 4/8 sheet of 1/8th inch and fastening it to the water table frame to use as a guide. Or is this unnecessary overkill?

[WyoGreen]

Since the torch should never touch the sheet while cutting, the sheet really doesn't move around. You can clamp it to the slats if you want to hold it in place or down. There was a thread on this site within the last few days that showed some clamps people had made from vice grips. If need be I can take a picture of the ones I made (and occasionally use).
You can use ohmic sensing to locate the torch head location (it doesn't move the material down), and the DTHC (digital torch height control) will follow the warps on your sheet, when set up right. Your mechanical touch off sequence using the floating Z torch mount will also locate the material surface, but the weight of the torch will sometimes deflect the sheet a little on thinner sheets, thus throwing off your initial torch height location and the subsequent pierce height and cut height location, which can lead to shorter consumable life. I usually just hold the sheet down on the initial height sense and subsequent pierce and starting cut, then let the DTHC take over and control the height. I just today received my ohmic sensing unit from CandCNC, so we'll see how that goes.
You can locate your 0,0 point anywhere on the sheet you want, I normally sit my zero point a 1/4" or so in from the edge to keep from cutting right on the edge of the sheet, (which can lead to a change in your cut voltage and subsequent torch up/down movement, throwing off your cut height). I mostly square up my sheet by looking at it, or measuring in from the table frame. If you are really concerned, you can set up a laser on your torch head and jog from one side of the sheet to the other to square things up. I've found that measuring from the table and starting my cut 1/4" from the edges is usually close enough.
As far as the water, there are additives marketed just for that. I just use Arm and Hammer washing soda bought at the grocery store mixed with the water to control the rust (it's cheap and works pretty good). When I'm not using the table for a while, I drain the table into a plastic drum under the table so it dries out between uses. Makes it easy to clean the table too. When it's time to cut again, a little air pressure in the drum fills the table again. There are threads on Plasma Spider about that also.

Kind of a long reply, hope some on it is of help, Steve

[Paint Chip]

Yes, that helps a lot, thanks. I like the idea of a vise grip clamp - will look for the previous posts you mentioned. Also like the baking soda idea - I have lots of that on hand and I like that it's non-toxic.

[lockeyone]

Arm and Hammer washing soda, not baking soda