When building a frame is there a way to join the sections together without having to fish plate?
You have a couple of options:
- Open butt joint
- Butt joint with backing
- Lap Joint
- Offset joint
Open Butt Joint
I would not recommend this for joining frame rails, even though some manufactures (Toyota for example) recommend this type joint for sectioning procedures.
Butt joint with a backing
Can be made from a piece of one of the frame rails, or other scrap metal.
This type of joint will add some strength to the weld and is a little more forgiving to less experience welders
A flanged lap joint uses one piece to great a flange for the other to be slid over and welded.
Tapered lap joint
You cut the corners out on the end of the rail, bend the flange down giving you a backing to weld too.
Involves making a Z cut in each frame rail giving you more of a surface area to weld too. Can't find a good picture.
If you want to add strength to the joint without using a fishplate I would recommend making an insert 4 - 6 inches long. Plug weld it on both frame rails as well as a butt joint where they touch. After dressing the weld properly it will be very strong and look like one piece of steal. Again can't find a good picture.
I was watching the show called Stacey David's Gearz ... it's one of those Saturday morning vehicle shows (at least that's the time I see it). He's been around the block a couple of times and his shows are a little cheezy, but he seems very knowledgeable. This one episode I saw, he was shortening the frame of a C10 to take it from a long bed to a short bed truck. He notched a section out of the C-channel frame to shorten it the length he needed. The picture below shows how he did it:
If you notice, the cut the section sort of "stepped" (best way I can describe it). The idea is to configure it in such a way as there isn't a single weld straight across, but rather, spread the load of the weld through a long portion of frame, making it much stronger.
I don't know if this is what you had in mind, but he didn't use fish plates to strengthen the weld after he had put the two halves together. He just relied on the weld itself to create the strength. Personally, I think I would have put a smaller section of C-channel inside the frame to provide stability, but that's just me.