I want to build a couple of Conicle fermenters about 13 gallon (UK) these fermenters are usually made using SS Hoppers made by Toledo Metal Spinning . The problem is getting them to Australia is prohibitive , so I want to fabricate myself , I have worked out how to plot the conical from the Snips website , what I need now is to find out how to protect the under side of the weld , shield it with gas obviously ......how do the Pro's do this ?
You make back-purging widgets to flow argon behind the weld.
Few people buy them Most build their own. I have made scads of them.
A simple one is a line of gas lens collet bodies soldered together. Have a partner follow the weld on the other side with the jet, or just position it, weld a few inches, reposition and repeat.
Don't get in a hurry. SS loves to distort from heat. You could try capping off the whole thing and filling the interior with argon, but it would take a while to purge all the air out.
Very important to spread the heat out and use the lowest amperage possible.
Do some tests to see exactly how much amperage it takes to get a clean weld on the backside with your purge jets in place.
You will have to pickle the welds after welding to remove stray iron particles from the surface. This is called Passivation. Commercial pickling pastes are available, but be very careful, they are dangerous.
I do a lot of welding for Breweries around Seattle.
In addition to Ernie's recommendations, aluminum stock can easily be machined to do the job. A router can be used with a carbide flute to machine a trench in some bar stock. This trench will channel the argon behind the weld, and maintain an inert atmosphere behind the weld. Since you are welding a conical surface, the area in contact with the curved surface of the cone can be profiled to match, a grinder will do the job. This will allow you to match the changing radius of the conical section. You can make a template to help you profile the aluminum for a real good match. It isn't that hard from what I've experienced, and depends on how critical the part is, and how much argon you can waste. In your case, you can probably live with the little bit of argon waste for a couple of parts.
In a sectional view of the alu stock, here is what the trench should look like: ___ ___ | |__| | |__________|
A top view of the trench: ______________________________________________________ | ________________________________________________ | | (________________________________________________) | |______________________________________________________|
You can easily make a piece to match the length of the weld, and the pointed end of the cone as well. The reality is that you don't need a milling machine to make this type of tooling, a high-powered router will do the job.
Next, you add a barbed fitting to feed the inert gas to the trench, on the back side: ___ ___ | |__| | |__________| \_/ \_/ \_/
Since the aluminum stock is easy to work with, you can also drill the part to feed from any point. Sometimes, feeding from the ends is useful. Instead of going to the trouble of putting a barbed fitting, I now drill a 1/4" hole, stick a piece of 1/4" ss tube, and strike the aluminum with a hammer to lock the ss tube in place. Some tygon tubing easily seals onto the 1/4"ss tube. This technique will allow you to make very small, or very thin jigs.
Your sheet stock is bent into the conical shape, and you are left with a straight line butt weld. The aluminum stock can provide several functional features, including back-purging of the weld, mechanical support for the butt weld, and it can act as a heat sink to help minimize the distortion from welding. One of the ends of the aluminum stock can be longer to allow holding in a vise. This may facilitate making a better cone.
One method to avoid distortion in ss welding is to weld short stitches. You start by tack welding. The tacks should be very close together for thinner stock. I think the rule of thumb is that the spacing between tacks should be about 4x to 8x the thickness for ss. You then make very small stitches linking the tacks together, and you join pairs in different parts of the weld. Avoid joining juxtaposed pairs of tacks. The parameter of import is to NEVER weld continuously for long periods. This is what causes distortion in thin sheet stock. If you run some test pieces, you will be able to gauge how much time the torch can stay on continuously before things start to deform. You then try to stay well below half this time. Usually, the 4x rule spacing, and joining non-juxtaposed pairs works fine.
The aluminum is easy to machine to match complex surfaces and joints, it doesn't distort, and doesn't get welded to the ss. You can even run some cooling water through the aluminum, but machining the extra conduits is a bigger job. If you were making these all day, some water cooling would be a real benefit. These jigs can get hot.