Butt-Welding Tubes And Shafts

Naturally, adequate weld penetration is imperative to safety, but there’s more to it than amperage and speed. In classes that I took at a welding school, instructor Carl Occhialini revealed a few techniques that bear repeating. He recommended grinding the tubes in a joint at a 45-degree angle to half the tube’s thickness. This chamfer, as it’s called, increases surface area and encourages penetration.

He also cautioned against tightly butting together critical joints even when they’ve been chamfered. Welded metal shrinks ever so slightly as it cools. The material in a joint will shrink consistently if penetrated fully by the weld; however, it’s very difficult to ensure perfectly consistent penetration, especially on a small-diameter, heavy-wall tube. Any material not fully penetrated in a tightly butted joint will resist the welded area as it shrinks. The stress can take the weakest path of resistance, usually through the bead as a crack.

The solution is easy enough: leave a slight gap in the joint. This open root as it’s called not only encourages penetration but it lets the welded area shrink on its own terms even if that penetration isn’t absolutely complete. Occhialini suggested 1/16 inch for most automotive applications.

Woolery went another step. He machined the parts so one slips into the other. It makes the joint self align and uses the core of the stub as a backer for the weld. In other words, it becomes part of the joint. When using such a backer—whether a reduced part of a solid stub or a smaller rod or tube slipped into a hollow joint—Occhialini recommended increasing the root gap to about 1/8 inch to encourage the bead to penetrate the backer.

The third benefit of Woolery’s way is a fail safe, sort of like wearing a belt and suspenders. Though he’s a very proficient fabricator, Woolery admitted that he won’t trust the integrity of such a critical part to a single joint. In this case he created a redundant joint by pinning the parts together through the reduced diameter of the stub and welding the pin in place.

There really is no such thing as too much caution when it comes to safety. Wellbeing is too great to trust to indifferent workmanship. If you aren’t fully confident in your skills then prep the job and have a skilled fabricator weld it. Even if it doubles the cost of the job, assure yourself it’s cheaper than the alternative—a failed steering shaft and a crash.

SOURCE
Buffalo Enterprises
Arlington
WA
360-652-7684
http://www.inliners.org/buffalo
Thun Field Rod & Custom
253-677-9526