This is how I shortened my CV shafts, cutting out a section from the middle of the axle shaft and then splicing them back together. I used a lap joint, with rounded inside corners, I think this is the perfect way to to do this job and they will never break apart.
The car project is my 1962 Mercury Comet, with a Nissan Leaf drive unit where the rear axle differential used to be. I'm using the Nissan Leaf hubs in a new rear axle and it used the De Dion axle tube design. So with Nissan Leaf hubs and a Nissan Leaf gearbox, of course I will use the Leaf's CV axles too. But they're too long for this older and smaller car! What to do? Shorten them of course!
First, I got my measurements. I put the wheels and hubs on the rear axle tube. With the axle shafts already cut in half at the center, I put their respective ends into the gearbox and hubs, and then I put the car on the ground. I overlapped the axle stubs and clamped them together. Then, I cut measure how much needed to be removed.
To make the job easier, I dissassembled the inner joints. For the Leaf CV shafts, I had to grind away at some of the housing to get the joint out. I decided not to dissassemble the outer joints as I've ready that is even less fun to do.
Now, here's my joint design. I don't know if I'm the first to do it like this but it was an original idea, to me at least.
You see, most folks just butt them up, weld around the perimeter, and slide on a sleeve and welded around both ends of the sleeve. Usually works fine but can be improved. Then somebody thought to cut the shaft at a 45 degree angle. Now, together with a sleeve, it would be much harder/impossible for one shaft to spin without the other also spinning. But the angle means that the two shafts are also pushing against each other laterally. To improve this, a lap joint can be used. But the standard lap joint has a week spot, the inside corner is a focus point for stress and crack would form there and spread. So my design is just the lap joint, but with rounded inside corners. The way I see it, this should never fail. Not at the joint at least.
To weld them together so they were perfectly aligned, I used some pipe clamps and a section of aluminum angle stock. Aluminum and not steel because the steel stock at the store was not very straight.
At first I only gave two good-size tacks and then checked for cracks.
But when I looked up close, here's what I saw:
It turns out that the 7014 welding rod I was using did not like the hardened steel of the axle shafts.
I went out and got some 7018 rods and this worked MUCH better. No cracks at all. And once I figured out that these rods needed electrode positive (my stick welding machine was set up for electrode negative previously) I was really getting some good welds.
Now for the sleeve. I calculated that a 1.375" diameter tube with .125" walls would have the an inner diameter just barely larger than the 1.11"/28.0mm diameter of the axle shafts. But I could not find such a size tube. I was about to order it from McMaster-Carr for $50/foot after shipping when it occurred to me that I can just get a 1.5" diameter tube, cut a section out down the middle, and then press it into the diameter I needed. So I did exactly that it and it worked out well. Hard to get it perfectly round but not impossible.
I also cut the ends of the sleeve at a ~30 degree angle. My reasoning is that this will both give a little more surface area for the weld, but more importantly it will spread out the heat rather than put it all in one spot along the shaft. I'm just trying to avoid any problems with HAZ because that would be the only thing that could cause the axle shaft to break.
Now I just need to paint them and reassemble the inner joints. Then put them in the car and make sure they fit.