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.



Metal fabrication basics

Dake Arbor Press, Vise and other basic stuff

This weekend was busy. In addition to visiting a new hacker space here, we helped out a Nullspace Labs guy from downtown L.A. with his fabrication project.

Fabrication generally refers to cutting and joining metal pieces to make a 3D object from basic pieces of stock that come in sticks or flat sheets. This is a bit different than machining, which is more about precision stock removal.

Making something out of square, tubular, or angle stock isn't necessarily hard, but having it end up with tightly-fitted pieces, square corners and a flat base requires more than just access to a MIG welder and a hack saw.

Here are some tips:

1. Take the time to come up with an accurate representation of what you need to build on paper. This can be anything from a simple draing on graph paper, to a full 3D model in Solidworks or similar.

For one-off projects (i.e. an adapter bracket for a car engine, the centerpiece of your modified geodesic dome, etc), an easy way to model it is with cardboard and masking tape. Trace out your design, cut it out and tape it together. Use scissors and more tape/cardboard to revise the design until it fits perfectly with all of your existing pieces.

2. Buy extra material. If your project is estimated to take 28' of stock, buy at least one extra 10' stick and go with 40'. There's an excellent chance that you will screw up on one or more cuts or welds.  The leftover stock can always be racked up for the next project.

3. Measure 4 times, cut once. Tolerances stack up, so it's wise to treat every cut you make as if it were a precision-machined part. If your finished item needs to be +/- 1/8" over 24 inches, you'll be wise to make all of your cuts better than 1/32" . Check all of your angles with a a protractor, take care when marking and clamping, and check each finished part with an appropriate caliper, measuring tape, etc.

Pieces for a series of hexagons. The stacked ones are all cut to 1/32" using the dry cut saw above.

4. Use the right cutting tools. For cutting sticks of metal, a horizontal bandsaw makes clean cuts and is quiet. It can also run attended, but it is slow for cutting through thick sections. For precise cuts, a cold saw is fantastic, as it generates very little heat and leaves an excellent, straight cut edge

It's little brother is the dry cut saw, which is noisier and not quite as precise, but cheaper and portable.  An abrasive saw works too, but requires more finishing of the cut edges and generates abrasive dust, so it's best used outside. Good discussion is here.

5. Clean and debur your pieces. Use a file, deburring tool, or a whire wheel to clean up any cut edges.  Remeasure everything that has been cut and reject or re-cut pieces that don't meet tolerance.  New metal often comes from the supplier with a thin film of oil on all surfaces. Remove this with a degreaser (water-based detergent) or a cloth with a small amount of Acetone.

NEVER use a chlorinated solvent (i.e. some types of brake cleaner) as these can turn into toxic Phosgene gas when exposed to welding heat.

Cleaning an inside piece with a full-round file.

6. If you're going to weld something, cut some extra pieces from the same material and practice welding them first. You want to get penetration of your workpiece, but you don't want to burn through or put excessive heat into the material (this is a source of warpage).  Practice any out-of-position (i.e. not flat on the table) welds that will need to be done as well.

7. Think about how you're going to assemble the item. If it's possible to rotate the article or assemble it from smaller pieces, see if you can do all of your welding in the "flat" position. This is much easier than vertical, horizontal or overhead. Don't start out fighting gravity if you don't have to.

8. You NEED a flat surface if you expect your item to come out flat. Use the floor, a granite tile, a flat steel table or anything flat you can get hold of to do your initial layout.
A small dog. Not suitable for laying out your project on.

9. Lay out the pieces, measure, take-weld, and then measure again.  If you weld a very small amount on each piece, you won't warp it and you can usually bend and adjust the piece to fit. For example, a square frame should be laid out, clamped down as needed (a collection of C-clamps, Vise Grips and magnets helps here!), tacked up and then re-measured. For a square, measure the diagonals and squeeze them into position with your hands or a bar clamp if they do not match. Welding the inside corner of one and the outside corner of the next makes this easier.

10. When you have the pieces tacked up, take your time with the welding and don't overdo it. For a typical project made from 1" or 25mm square steel tube, it's usually not necessary to weld all four sides of every joint completely. Instead, make lots of small welds, alternating between parts of the project, and stop frequently to let the pieces cool. Metal that got hot will shrink as it cools, so keep distortion down by keeping the heat down, clamping/restraining the pieces, and welding both sides of a joint alternatingly. Also make the small welds first and the biggest welds last. Break the big welds into "stitches" and don't put down more metal than is needed.

Hope this helps!