Before you start debur and wire brush all the metal pieces.
13 Jan 2007
For this part of the project I needed 40 x 10 x 1.6mm Rectangular Hollow Section (RHS), however profiles like this arn't available in my part of the world, so I had to cut down some 40 x 40 x 1.6mm RHS and weld it back together to make it up. This was not an easy job, so if you can get 40 x 10 x 1.6mm RHS -- even if it requires a little extra expense or effort -- its definitely worth it.
So I started by clamping the RHS to my work bench and sliced off about 7mm off the side using a metal cut-off saw blade and a ripping guide. The cut was reasonably straight, but the steel bows as soon as you cut it so it gets in the way as you work. I decided to make it 7mm rather than 5mm because the 5mm test piece I cut, bowed something severe. Obviously when I reached the table edge, I moved the piece back, re-clamped it and continued. Once you are done, it is a good idea to blow out the saw with some compressed air, that grit looks pretty nasty.
With the two sides cut, I grabbed a length of heavy gauge 40 x 40 x 5mm RHS and clamped the two sides together with once side running against it. This pulled the bowed metal pretty straight, but you need to use quite a few clamps to remove all the gaps. To weld it together I began by tack welding both sides at 100mm intervals. Then I began welding doing 25mm welds off each tack, alternating sides with each set of welds -- so you end up doing 4 sets of welds on each side. This approach is intended to minimize warping. While the new length of 40 x 10 RHS is mostly straight along the horizontal axis it was clamped to, it warped in a few places along the vertical axis which I hadn't expected. Next time I do this, I will clamp it in both directions against some heavy 50 x 6mm angle.
Here is the final piece welded together. It is not dead straight, but for the most part I am pretty happy with it. I will make sure that when I cut out the pieces, that the cuts are at the warps so as to minimize their impact.
With items 9, 10 and 11 cut out, I first welded 9 and the two items 10 together. Here I am clamping the welded items 9 and 10 piece to items 11. First I marked out some lines in chalk on my table then positioned the pieces. I clamped them to two blocks of 1.5" plate that I use for just this type of job. They are perfectly square edged and very heavy, so you can place them where you want, clamp stuff to them, and they don't tend to move. I use this set up to do my tack welds, then properly clamp the tack welded pieces so that can't warp and then perform the welds.
Here is the finished piece with items 9, 10 and 11 all welded together.
Next I cut out item 18 and set it on a piece of 3mm plate to raise it to the correct height for clamping. Once again I clamp, tack weld on all four sides, then weld starting on the top, then the bottom then alternate sides.
Here we see the finished welds. Notice that dispite my best efforts, the front is slightly warped (away from the piece of plate at the front) by approximately 1mm. I will be able to fix this later when I come to weld on the bottom brackets, so its not a major problem at this time.
Next we cut out the endcap (item 41) from the 1.5mm plate. Because this is so small, I don't even try to clamp it, instead I hold it down with one had using a length of 14mm square bar, then with the other hand I tack weld it. Once tacked, weld alternate sides. Then grind the weld bead back, flat against the sides so the BB Mount sleeve will fit over it.
Copyright © 2007 Henry Thomas