Stainless backside purge

Should work, but you will need to keep your amperage down to the minimum to prevent distortion. Do short stiches with spaces between, then come back and stitch the gaps. Break up the heat. If you want get real fancy add a water line to the backing bar to make it into a chill bar.

Ernie is the guy with the experience, and you have his reply above.

I've messed around with a bit of TIG'ing of stainless welding.

I found "feed argon directly into a channel" arrangements leaky even at high argon backpurge flows.

I've used arrangements where there is a back-fitting with a channel, but the purpose of this is to contain a drilled copper "purge tube"

See

The copper "purge tube" is obtained by centre-popping along a piece of copper tube at 8mm to 10mm (5/16th to 3/8th-inch) intervals then drilling through with a 1mm (40thou of inch) drill in "dremel" tool or a bench/pedestal drill. Close the "outboard" end - crimping it flat in a vice being just fine.

I think theere are good physical reasons why this arrangement using a "purge tube" works very well. The holes are slightly restrictive so there is a very small but never-the-less there pressure along the tube, making the argon flow equal from distant as near holes with respect to the argon supply. That pressure is very low and the argon flow out of the holes is slow and lazy, meaning it will not suck-in air, if there is any around. That slow argon flow out of the holes is never-the-less directional and plays on the underbead, maximising the outcome of a silver underbead.

To test the fitting (no sheet metal in place yet), you can use a small gas-fuelled lighter - a "cigarette lighter". You can't hear, see or feel the argon, but the gas-flame won't burn in it - it will go out immediately the end of the lighter gets over the purge tube, if it is purging. Go along the arrangement, re-striking the lighter and spot-checking at various places along the purge tube. That's how you can end up at very low backpurge argon flows and know that the arrangement is doing its job. As I mentioned in "physical reasons", I think a low flow speed, hence flow rate, is actually desirable.

Cover-up the ends of the channel as they emerge at the edges of the job. This is so that argon can leak put but air cannot find its way in against this flow. Wire wool has been suggested, though I never did more than stack-up little bits of offcuts.

Maybe in advocating the "purge tube" arrangement I am missing some reality of the job when you move beyond bench-welding small pieces of stainless?

A month or so ago there was discussion of arrangements for argon supply

- such as having two flowmeters, one for the torch and one for the backpurge. Also the suggestion of having a big obvious on/off valve for the backpurge argon near the job, so you can stop/start easily without wasting gas, and so you can see at a glance whether the argon is on or off.

Richard Smith

Nice simple back purge tube.

I've seen sintered metal back purge block used for turbulent free flow. Thin sheet metal is welded to back/sides of sintered metal directing flow out front.

I recall Ernie writing that he uses a variety of objects, including cheap aluminum pie plates for purge dams. So for something like that, what do you attach the pie plate to the weldment with? Tape?

Ciao, David Todtman

Aluminum foil tape, as used for ductwork. It sticks very well, and if the glue burns you can still get it off. I also use it to hold purge tubes behind welds. If your welding amps are low enough the tape just gets warm.

Thanks for the information everybody. I got my backup Argon cylinder filled today and plumbed a regulator and flowmeter up to my purge bar and tried it tonight. Yes, it leaks. I had to use a wire brush to clean the oxidation off the underside of the weld, but it works well enough that there was no sugaring and the backside had flowed correctly.

Thanks again, Bob

Try helium next time, it is far more efficient at carrying away heat.-Jitney