Normally we MIG weld two items that a clamped together but has anyone used two earth clamps, one on each item?
This will remove the need to firmly clamp the items against one another for a good path to ground since the earth clamps will do that.
Good or bad idea?
I'd say a good idea!
You need to secure both parts to stop weld distortion pulling the items askew, unless of course you dont care about distortion!
I think, you think too much 
distortion correction tool = big hammer
one of two ways - either have a second clamp bolted to the main clamp, or have two clamps on a piece of fat cable to attach to each piece.
Yep - I did something similar when I had to weld some large plates together that were too cumbersome to try and clamp. I propped them in place with
loads of bits of wood etc, and then connected them together with jump leads. Earth clip on one side, and weld away.
Cant see you would need it, as soon as you start welding the current will flow along the weld itself
normal to tack first measure / check this tack will conduct for the rest of the weld ?
I've found that good solid earthing on both sides can help the first tack welds on larger stuff.
Ed.
How big a gap are you leaving between the 2 pieces, sounds like too much to me.
I use a 12mm plate as a welding table connected to the earth clamp and place the items to be welded on it.
Standard practice in welding shops - at least it used to be
quote:
Originally posted by v8kid
I use a 12mm plate as a welding table connected to the earth clamp and place the items to be welded on it.
Standard practice in welding shops - at least it used to be
althought the thickness doesn't really matter, i use a piecee of 3mm works fine and I can lift it onto table and put it away when I dont need it.
quote:
Originally posted by l0rd
quote:
Originally posted by v8kid
I use a 12mm plate as a welding table connected to the earth clamp and place the items to be welded on it.
Standard practice in welding shops - at least it used to be
Still is i think. Up to the point, that you get morrons welding the chassis with the jig table.
(Heard it happened a few times in the past in Morgan cars)
[Edited on 28/7/10 by l0rd]
Slightly off topic, but a good tip. Most hobby MIGs have very inadequate earth claps - get a cheap set of mole grips and weld an M8 bolt to it, and fasten the earth cable to this. VERY stable, excellent connection and wont keep falling off.
^^^ ooh, you are so cunning, but itsn't that a chicken and egg situation?
If you don't have a good clamp, how do you clamp to your clamp for a good connection? 
but about the gap, somtimes you have a piece that is not quite flag/square so there are gaps, enough to casue problem.
waving the torch between the two pieces and starting with the earthed piece sorts that but this is usually in cramped situations where it is hard to
reach and hard to see - hence the 2 clamp (TM) propostition.