20121123

Metal fabrication basics: part 2

In our, last post, we covered the basics of fabricating things from metal. One of our visiting hackerspace guests, Travis, is building a set of hexagonal columns. This week, I thought I'd add a few more things about measuring, layout and doing accurate welding work.

Once you've cut everything out as accurately as possible (see Part 1), you need to lay out your work on a flat surface. Here Travis is using a 10mm steel plate, but anything flat (even a concrete floor) is preferable to trying to do it "by eye." 

Professional fabrication shops will often have a ground flat cast iron table. It's worth looking our for old machinery being scrapped, as an old planer or milling machine bed can make a fantastic layout table.



Note the use of magnets - you can get 30, 45 and 90 degree magnets to make fit-up easier.  For precision work, you will still need to measure the joints, but this makes things much easier.

 After the parts are laid out, we tack weld every other joint.


A good process for welding a square, hexagon, or other geometric shape is to alternate tacking the outside and inside corners, and check the work frequently.

 When we've got the basic shape tacked up, we measure all of the diagonals to really make sure it's straight. It's surprising how the initial layout can be off by 1/8" or more, even though it looks perfect to the eye. A straight edge across the diagonals doesn't lie.

 A pipe clamp makes it easy to straighten out any diagonals that are too long by squeezing them down. The errors magically equalize and disappear. You'll probably need to squeeze your joints down a little more than they need to account for spring-back of the metal.



As we get farther along, it's important to keep the whole assembly from warping or getting out of flat. Restraining the parts in at least two places prevents this. Here we are using a standard 'C' clamp on the right and a fancy welding clamp on the left.

 Some more detail of TIG welding a corner up. The TIG torch should be held at about 15 degrees from vertical and the filler rod (if used) should be fed into the shielded puddle from the side. Keep the welding time short to minimize warpage at each joint.

 As the assembly starts to take shape, you'll need to think in 3 dimensions. Here, Travis has measured out and drawn a chalk outline on the layout surface. This lets him position both the of the hexagons parallel to each other and not rotated or shifted with respect to one another. The center of each leg of the hexagon has also been marked with a scribe, as has the center of each vertical support. Lining up the marks and tacking with the hexes clamped down will assure an accurate fit-up.

This all seems like a real pain in the rear, but you will thank yourself later, when everything has been covered with hundreds of inches of weld beads and you realize that all of those little errors add up.

Coming soon: part 3.

Arclight

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