Before you start debur and wire brush all the metal pieces.
Once again alignment is critical to getting the frame right. Fortunately the swing rocker arm hinges are relatively straightforward, its just the main steering pivot that is tricky.
Here is the finished frame with the sacrificial bearings still in the steering pivot housing. They proved very effective by preventing warping during welding, but were totally trashed in the process.
After cutting out the all the metal, I began the frame by machining the pivot mounts, which have round fillets and holes in the ends of them. There are quite a few of these to do so you want a technique that is going to give you reliable and consistent results. This is where my vice grip adjustable spanner comes in really handy. I can grab a bunch of them and hold them perfectly square and machine them all together. Using the vice grips in this way makes it easy to transfer the work as a set between the vice, bench grinder and drill press.
I started by using a washer to mark out the area of metal I want to cut out. Notice that I have already ground the base of these mounts square, so the lengths will be identical.
Next using the cut-off wheel on the angle grinder, I cut around the outside of the fillet outline as close as possible. Here I am cutting the fillet for Items 35 as a pair (as you may have noticed, I am not wearing gloves here because they make it too difficult to operate the camera, the sparks are just for show!).
Next I transfer the work back to the bench grinder where it is finished off. Ready for the drill press.
Here are the completed Items 36 and 37.
Item 40, the frame gusset and seat mount was a bit harder to fabricate. I would suggest you drill the 8mm hole first, I didn't and it was quite tricky clamping it in the XY vice of the drill press.
Here are the finished pieces. You will notice that I rounded the bottom of them to make room for the weld bead on the frame.
I started building the frame by clamping Items 11 and 12 to the work bench and tacking and welding them together. After this I welded on Item 13, all pretty straight forward.
Next I drew a chalk line square to the edge of the work bench (313 - 40) / 2 = 136.5mm from the side. I lined the face and edge of Item