advice on ohm connector over the shield?

[i_r_machinist]

I've been running a Samson table with Plasmacam on a random basis for two years now. When the machine was first set up, we built a brass collar that mounted over the shield. The collar was the ohm connector, and would touch the plate before the shield would. We ended up taking the collar off and bought a retaining cap that had a steel clip that would make contact with the shield and make the ohm connection. This works fine for us as we mostly cut 1/2"-3/8" steel plate, and our consumables budget is pretty much unlimited.
My son's school bought the same table and I'm setting it up for them. Needless to say, their budget is VERY limited, (I paid for the FineCut nozzels). I made a brass ohm connector to mount on the shield, but it doesn't touch the plate, just makes a connection to the shield.
I've been watching youtube videos, and many have the collar that touches the plate. What is the benefit of this, except to maybe keep the shield from being marred? Do I need to make one for the school?
http://www.youtube.com/watch?v=GQU24eX8Nlc
thanks
i_r_machinist

[mdwalker]

Here is one I posted a while back for powermax 1000.

http://www.plasmaspider.com/viewtopic.php?f=61&t=1203

Danny

[Francisco]

The idea of the ohmic sensing system is that, by detecting the surface of the material, your machine's DHC has a reference point from which to move the torch up to piercing height, let the arc be established, then bring it down to the cutting height, at which point the voltage feedback loop takes over. If your ohmic sensor is set to touch the material before the shield does, this will introduce an offset you'll have to compensate for, yet it won't make any difference in shield life (sensing is done before the plasma arc is present).

Some consummable setups (Powermax 30, for example) don't include a shield; in those cases, you'll need to make some sort of sensing probe set to touch the material when the nozzle is a few thou shy of the material.