Tom Porter's Recumbent Bike Plans - SWB versions
Tom Porter's Recumbent Bike Plans 2005

Dual 700C Z-Frame Build 2005-       Tubes    Frame    Dropouts    Stays    Final
Fitting and Welding the tubes
Brazing/ Welding Fixture
This fixture is used to clamp the bb shell to the tube while joining. It consists of a wood block screwed together to clamp tubing, a 1” OD tube that fits inside bb shell and drilled for 3/8” threaded rod to pass through. Pressure is needed to hold shell square because if not done when heat is applied to one side of shell and brazed it will pull the other side away from tubing losing square and causing all sorts of problems. 
Threaded rods are about 10” long and nuts are used for tension. After fixture is installed just clamp tube to anything solid for brazing.

There is a spot braze on either side of tubing/bb shell. Oh, and make sure the shell is installed with the threads on the proper side!

Here is a drawing to help make the holding fixture. It shows design improvement, as the prototype wasn’t quite right. This will work better.

Aligning BB And Downtube
Okay, now take the tube with the BB spotted, reset in the frame blocks and using the square face the BB shell, level it.
Take the next set of blocks (No.3 & 4) and align and square on layout lines, then screw them down. Place the downtube in the blocks and check the miter angles for match.
As you see we’re ever so slightly off, just a fraction of a degree. So we’ll have an adjustment to do.
A belt sander makes this much easier. I set the table at 65 degrees and the miter guide at 90 degrees. The spring clamp holds the guide in place. 
The tube is held against the guide a CAREFULLY sanded to a better angle. The clamp on the tube is for the photo only.
Work ends of both tubes to make the miters match almost perfectly, a very small gap is okay. The protractor in this photo shows we have a perfect 130 degrees.
Now go through the same rigmarole with the seat tube joint, and there you have it! You are ready to weld the joints.
Welding & Gussets For Mitered Tubes
This section shows the BB/Downtube joint and also applies the to the downtube/seattube joint.
First, spot weld the upper and lower joint (shown by arrows). Weld about ½”, I did this one with my MAPP/Oxygen setup. Some say using MAPP gas versus Acetylene is more difficult but I find it just the same. The MAPP is set at 14psi and Oxygen is 20psi. I used 1/16” mild steel rod as the frame will not be heat-treated. The rod is ER 70 S-2. There must be a metal shield under the weld area so you don’t start the table on fire. Let joint cool before removing it from the frame blocks. A good tack weld will be strong enough to place the joint in a fixture and complete welding the entire joint.
Be VERY careful when welding the outside (closest in photo) as it is easy to blow holes here, the rest of the joint is much easier and forgiving. 

When all is welded (and cooled) you may want to take a file and pretty up the weld a little or smooth it out. When you got it lookin’ good the next step is to add the gusset. 

This is a paper template which will make it a piece a cake.

The idea here is to download this image and cut it out.
Just checking to be sure, it looks good.
Now take the template and tape to an at least 4” piece of 1.75” OD tube and in this example I outlined it with my Sharpie.
Pull the template off and voila.
Makes some cuts to remove waste material, I did these on my table saw with cutoff wheel (or use hacksaw).
The next step is to turn into a U shape. This a Vise-Grip sheet metal tool, I use this and a pliers to pry it into shape.
Take an aviation snips (Wiss is great) and rough out close to layout line. This is easiest if you take multiple 1/8” wide cuts.

Lastly, trim down right to the layout line. The piece on right has machinist’s dye applied and scribed, just to show another way of doing this.

This shows the trickier bit; rough out the inside relief joint and then remove the metal down to the line with a bench grinder (you could also bench grind the whole thing).
After you get a pretty good fit its ready to join. This is a pipe clamp,  because you don’t want the edges to be too tight. There should be a .008” gap (like a sheet of paper) for the brass to flow under the gusset properly. This is IMPORTANT. 

If you’re welding the edges it should be tighter. Spot join at both ends at bottom.

I now take the whole thing and put it in a bench vise and then carefully apply pressure locally to get the proper brazing gap. I then finish joining all around.
Now for some final cleanup!
Installing the Headtube
The assembly has now to have the 1.25” hole cut in it for the headtube. So it is marked and goes into the miter jig. Check the jig to see where the bubble level is and then position the level on the BB face to match.

In the picture you can see the slight nick to make sure I’m hitting the mark (kinda faint on photo).

If you look carefully you’ll see the table of the drill press is not level, but not to worry just make sure the bubble is in the same spots at both locations (I’ll get around to making the drill press table square eventually).
Hole cutting is very simple with a jig. Clean up the burrs and test fit the headtube. You can also see the inside of the weld and it seems good.
Just some spot brazing for now.
Welding Downtube To Seat Tube
Mount the first welded assembly back in frame blocks, level and clamp. Then mount seat tube and clamp. Tack weld just like the first joint.
Place in holding fixture and weld completely around. This joint looks pretty good; this is what we’re striving for. Although this weld is good, a simple butt weld of such thin wall tubing is not going to hold up at all. The gusseting of both joints is vital to getting this frame to work. 

I don’t think I discussed this before, but the reason the design is built this way is that getting this thin wall tubing bent properly is difficult and expensive.

The gusset is brazed on exactly the same way as the previous joint.
Here it is. Clean up the brazing flux and work on the fillets. The next steps will be to make dropouts.

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