Beginning Ox/Ac questions

"mhorowit" wrote: (clip)Welding around the outside isn't a problem, but now, how do you get to the inside to finish the weld? ^^^^^^^^^^^^^^^^^ How should I put this? You should have thought of that when you were designing the piece.

I once had a professor who designed a vacuum chamber, made of rectangular steel plates, welded at all the edges. In order to minimize trapped gasses inside the chamber, he specified that ALL the seams had to be welded on the inside.

that actually can be done... provided that the weldor remains to live inside the chamber.

Welding both sides of a joint you describe is greatly overkill. Properly done the melting of the edge of the plate to the outside of the tube is more than enough. When you weld up the inside of the joint between sheet and tube you end up with a fillet weld that is many times larger than the joining section of sheet. All you do by welding both sides is distort the joint and move the flex point further into the sheet. The reason that the back side has more scaling is that you have heated the metal without the benefit of any shielding. The oxygen can reach the underside while the area you are heating is protected by the oxidised gasses in the flame envelope. Randy

I've gotten past the 'make a puddle' stage, moved to fusing two pieces together in a butt weld and have moved to making a 'T' weld (standing on piece of metal at 90^ to the other and welding along the common edge). Couple of questions have arisen:

1. We are told to tack the work together, then include the tacked section in the final weld. There is no mention of cleaning 'scale' after tacking, but it hasn't been a problem; HOWEVER, when I turn the work over to weld the other side, there is a lot of scale present. How come there wasn't scale in the first case, but there was in the second?
2. Visualize a piece of ½" (or smaller) tubing (.037") to which you need to weld .037" sheet tangent to the tubing (assume the sheet metal is ½" square). The edge of the sheet metal piece is the only part touching the tubing. Not a problem welding along the outside and only a minor problem welding on the inside. Now, put another piece of sheet parallel to the first and on the other side of the tube. Welding around the outside isn't a problem, but now, how do you get to the inside to finish the weld?

- Thanks in Advance : Mike

I don't think he will live long, given that it's a vacuum chamber and will eventually have to be used for its intended purpose...

-- Ilya

maybe he could be given a space suit like those on the space shuttle...

Thanks for both answers and the explaination - Mike

This is normal practice for high vacuum work. For high vaccum you have so much trouble with outgassing that you might even build heaters into the chamber walls so that you can bake out any adsorbed gases during pumping down.

Rather than trained mice as welders, it's not unusual to put a large bolted cover into one or more sides of the vessel, then work through that for final welding. Usually though a vacuum chamber is made in two bolted halves, simply because you also need access to get the experiment into it.