Rose joint install

 For the roll bar supports, the rose joint mounting is going to be crucial. It would be ideal if it was fully threaded, but I've got no way of turning a suitable insert. So I want to get as close as possible with the options I have to hand.

What I've decided on is an M12 nut, mounted on the back of a 5mm plate that will be welded just inside the tube. I will then have a second 5mm plate welded to the end of the tube. That gives me about 20mm total threaded surface, with an end plate that is double the thickness of the tube itself. The rose joint is then inserted along with a lock nut. 

The only problem with that design will be the multiple layers and how the thread will work between them. Ideally I need to weld up the three layers away from the tube, then cut the thread. That will allow me to match the thread on the nut. If I do that I'll be installing a 'top hat' into the end of the tube and only having one weld to the tube. It also means the weld between the plates will most likely need grinding away as well, again for the top hat design.

What I'd prefer to do is weld the nut to the smaller plate, cut the thread on the smaller plate, weld that into the tube, then weld the larger plate on top. But while I could cut the thread on the smaller plate, I couldn't do anything with the larger plate. 

Thinking about it, the best option would be to weld the large plate on then tap from the other side. I'll have to do a bit of work on that one to see if I can get the tap all the way through the tube, initial thoughts would be to fix the tap into a socket then use extension bars (or even a length of steel) to get the tap all the way along the tube. I might try that step first to see if it's viable. 

(later that day...)

I think I'm overcomplicating things. I screwed two spare pieces of metal on to the joint along with the bottom and top nuts. It left me with about two threads spare so I think it's perfect for the end cap. But I am going to do the whole thing off the tube first, weld all the layers together then tap straight through. As long as I get the holes in line then it'll be the strongest way of making the cap, then having only one weld to the tube will be less of an issue. 

So it'll be a fixing plate that is twice the thickness of the tube, and a total thread length twice the thickness of the joint. Then it'll need welding to the roll bar hoop, that should be pretty easy. I suspect 3mm plate will be best here, fully welded of course.