About a year ago I asked for suggestions on how to cut joinery in round timbers. I want to thank everyone who contributed to that discussion as it generated several interesting approaches and much food for thought. That discussion is here:http://www.tfguild.org/forums/ubbthreads.php?ubb=showflat&Number=23375&page=1
As It turned out, I ended up cutting the joinery in 20 peeled hemlock logs up to 24' long and 24" in diameter. Far to many to be messing around snapping lines, "hovering" stuff and hoping it all worked out in the end. What I needed was a giant jig to make this a reasonably fast and more importantly - precise process. More research lead me to Jeff Arvins article "Round post, square beam joinery" in the September 1996 issue of Timber Framing and also reprinted in the Guilds publication "Timber Frame Joinery and design" Vol. 2.
Squinting at Jeffs photos in the article lead me to build a set of rails 34' long and about 26" wide using a pair of 2x12 LVLs. Suffice it to say that these rails were laboriously straightened, plumbed, leveled and sworn at daily.
The peeled log post to be was cut about a foot longer then finished length and 8" plywood squares were screwed onto each end using a level. The post was then centered on a cross piece between the rails so that it was supported on each end by the plywood squares - These squares now accurately identify the four sides of the post and the center.
I made a series of jigs out of birch plywood that slide on the LVL rails. This one is for using 10" and 16" saws to crosscut the timber. With this first cut at the base of the post I'm establishing the finished bottom - the point where all measurements start. As I rotate the post to cut joinery on all the sides, I always lined up with a square and made this cut first. With all the joinery cut and the post moved on to saw horses I could finish cutting through the log with a handsaw and have a nice square base.
On this project all the connecting timbers were 6x8s and the braces were 3x6s - So I built birch plywood jigs to center and rout the housings for these members. Basically I'm routing windows into the log.
I used two routers with mid size and long top mounted bearing bits to make the housings. As the plywood jig holes elongated with use - I glued in strips of metal strapping to return them to size and prevent further erosion of the template.
Moving the plywood jigs to either end of the log and measuring down to the center of the 8" plywood square gives the height of the jig above the post center. Measure down from the top of the jig to the housing and subtract from the height and you have the distance of the housing from the center of the post. Center to center post dimension minus your housing to center offsets on each end equals your shoulder to shoulder connector length.
I then screwed a simple plywood template on to the new flat spots and the Mafell made quick work of the mortises.
A little clean up with a chisel and it's time to roll the post 90 degrees and repeat the process.
This is a view of the top end of the post - I've used another plywood jig to rout flat spots to define a shoulder. I'll finish cutting a tenon here with the 8" square removed and the post on sawhorses.
A couple pictures of the frame raising--