Combining the Three Planes

So, after completing the first three 2-dimensional planes, it’s time to put them all together. The three sections make up the majority of the frame’s shell. As usual, we used the tube coping drill press rig and the cutoff saw to prepare the tubes that connect the two planes.

Jigging the three planes together was tricky. We started by rolling out the grid paper and screwing in supports for the lower plane. We measured out locations for the supports for the roll hoop and locked those in as well. After some checking and rechecking of critical distances and angles, we tacked the lower plane and roll hoop together, with a few connecting members.

Adding the upper plane required preparing a number of new wooden supports to hold everything at the right height. Getting everything right was difficult. We used the tack, measure, bend, tack again method to get the measurements just correct. After tacking everything in place, we rechecked everything before starting to finish weld.

Also added in this step was the first sheet metal piece. The 1/4-inch steel plate in the center of the car will mount two bearings that support the intermediate shaft and tail shaft. The shape of the plate, with round cutouts, aided in positioning the tubes. Finishing the welds took a couple hours and some contortion, as the frame is starting to get hard to work around. We placed the seats in their positions in the frame. It’s starting to look like a car!

Welding the Roll Hoop

Unlike the first two major planes, the roll hoop plane is vertically oriented in the car. Regardless, we’ll still be using the same wooden support method to jig the tubes in place. To start, there was some more bending to be done to the main hoop tube. The large two bends that form the u-shape were done with the hydraulic bender, but the gentler lower bends are to be done with the roller.

The centerlines were drawn out on the grid paper and wooden supports were screwed down. This way, as we progressively bent the tube, we would check its fit. Once we were happy with the shape, we added some clamps to hold the piece flat.

With the hoop piece in place, we added the supports for the rest of the tubes in this plane. We ran into some fit issues with these tubes. Since the hoop wasn’t curved exactly as designed, we had some gaps and interference of around 5 mm. We trimmed up the longer ones with an angle grinder. For the shorter ones, we just had to weld a couple extra beads in.

The welding was pretty easy, although some of the bigger gaps took some time. When we undid the clamps holding the pieces down, not much moved. So that’s a good sign. There was a little curvature to the roll hoop piece that made one end want to lift off the table. It looks like the welding held it all flat. The finished section looks good. A couple key measurements are correct to a few millimeters.

With the first three planes finished, were now ready to put it all together to form the front two-thirds of the car.

Welding the Second Plane

The second planar section of the frame is also horizontal, and lies directly above the first in the car. It will form the main front region of the vehicle, including the lower curved tubes of the driver compartment.

We used the same process as the first plane, drawing out the centerlines, screwing in wooden tube supports, tacking, and welding. Since this plane includes curved members and non-square intersections, it was a bit trickier to get everything lined up. For the sake of time, we stopped fully coping some of the tubes. When a 1.0 inch tube intersects a 1.5 inch tube at a tight angle, we found that it was much more time effective to just cut it flat with the miter saw. The gap formed by this shortcut was small enough to cover up with an extra weld pass. Since the coping process for angled cuts requires slow, careful cutting, we decided to use the simplified method for these cases. It saves about 15 minutes per cut.

Since this section is just a tad wider than the table, we built quick wings to support the curved tubes. After getting everything tweaked and lined up, we’re good to weld.

The welds on the dissimilar thickness material were a bit challenging. The big tubes are about 50% thicker, so the torch angle had to be bent over a good 20 degrees to keep from burning through the small tube. Everything turned out pretty well; this cheap Alpha-TIG machine is working great.

We also did some work to the first plane. We added a couple gusset pieces, using the simplified angled cut method mentioned earlier. It’s not quite a ‘plane’ anymore, since we added the front suspension mount tubes, making the section 3D.

Bending Some Tubes

The frame design includes five bent tubes. The roll hoop tube has two 6-inch radius bends to make the U-shape. The four driver compartment-surrounding tubes have gradual, single-radius bends.

We used a hydraulic tube bender for the roll hoop, starting with a over-sized piece of 1.5″ OD, 0.095″ wall tube.

We used a big sheet of paper and a protractor to measure the angle, progressively working to the desired 87 degree bend.

Here’s the completed roll hoop. The ends are each a few inches long and will be trimmed off before welding.

The other four non-straight tubes are rolled bends. We used a cheap Harbor Freight manual roller, which had a couple poor design bits. Three set screws linked the shaft to the rolling wheel. Within the first minute of rolling the chromoly tubes, all three screws sheared in half. We drilled a couple through holes in the shaft and replaced the screws with quarter-inch hardened steel pins. These held up fine.

We used a piece of chalk on a string to draw the correct radius curve on the shop floor. Once the curvature got close, we checked it against the chalk line. The first two of the driver compartment-surrounding tubes are ready to be welded.