screwswood
I would appreciate help so much!
I am trying to make a 'collapsible' arbor if you will. I would ideally like to saw my large birch posts in half and make them screw back together in the center, but I have no idea how the mechanics of such a thing would work. Double headed large screws!? Any suggestions would be appreciated.
You took the wind out of my sails with that last paragraph. I was going to suggest using a nut/bolt.
As far as it causing problems when the tabletop is lowered, you can recess the bolt head. Yeah, you could make the cavity in which the bolt will stay on the underside of the tabletop, but maybe you'll drill too far or crack/scratch the tabletop if you chisel it out. It could end badly... which is why you should instead house the bolt head on the top-side of the cross member.
Brown = table top
Black = underlying cross member
The one on the left is what you were talking about. The one on the right is what you want to do.
Also, use a washer under the bolt head, etc.
For trim work that will be painted, use paintable caulk instead of wood filler. It does not crack and it is very tolerant of shifts in the pieces, both due to structural movement and seasonal swelling and shrinkage.
Wood filler is better suited to holes and divots fully contained within a single piece of wood.
Best Answer
The birch posts in the original design were made a certain length to serve a particular function. I am sure they were selected on the basis of strength, stability, and durability. As a first order analysis the idea to cut them in half seems to defeat almost all the reasons that they were designed for in the first place.
It will not be possible to simply cut the posts in half and re-join them in a manner that will restore them 100% back to the original look and function. That said there may be some ways to come close to that as possible if one can look past the cut lines and some fasteners / bolts.
There is some missing information that will be needed to provide the overall solution and/or suggestions. I can provide one possible idea for the case where these posts are deployed in a vertical orientation and the girth of the post is substantial.
The idea is to go ahead and cut the post in half and then install a heavy piece of steel pipe into a hole drilled into each end of the post. The pipe would be glued into the lower hole using construction grade adhesive. Cross holes would be drilled though the sides of the upper post to accept bolts that would be threaded into prepared holes on the other end of the steel pipe. See picture below: