For a bit of fun Here are some silly and not so silly joints I designed for one of the carpenters fellowships joint busting extravaganza at Frame 2011 run by the Carpenters Fellowship at St Fagans National History Museum
The testing rig was designed to load the joints up to 10 tonnes force in tension, however a fault caused the rig to be limited to 5 tonnes on the day. For a full write up of all the experimet by the Carpenters Fellowship please contact me or them and I will see if I can have copy's made or sent.
I am concentrating on the joints I made for the test and their results.
The stopped splayed scarf with wedges and 4 turned pegs
A reasonably traditional joint often used where a timber is in tension or bending.
It is made by cutting a diagonal (splay) across a timber to be lengthened and another continuing timber. The splays are then tabled and cut back to leave a square gap where the folding wedges are inserted. Squints are then cut in at a steeper angle than perpendicular to the splay to restrain the joint. Finally four peg holes are drilled through the joint perpendicular to the splay.
The joint started to show some slippage at approximately 4 tonnes load. An important point to note is that since the pegs where drilled at approximately 90 degrees to the splay, the slippage was in the direction along the pegs. It strikes me that if the pegs had been oriented at 90 degrees to the axis of the timber then perhaps the slippage would be reduced.
At 5 tonnes the table of the splay gave way with a crack. Note the split along the grain, and also further displacement in the direction of the pegs.
Axially Loaded Butt Joint with Threaded Rod
Traditional joints designed to be in tension generally ovelap and rely on interference to oppose the tensile forces being applied. The idea for this joint was to start with two timbers joined with simple 90 degree butt.
Glue was banned so my first tought was to insert a threaded rod up the centre of the two timbers. A 35mm hole was bored up the centre of the timbers. Green timber doesn't generally drill very well along the grain like this but after some sweat and percerverance we achieved approximately 5" of depth in each peice.
The whole was tapped with a threading kit available from woodworking shops. A 1.5" rod was turned on a pole lathe and threaded with the same kit.
The rod was screwed into one of the timbers.
The fit was tight so a oil was applied to the thread before the two timbers were "wound" together.
A couple of clamps came in handy for "tightening".
Some additionall goose necks and squewiff dovetails where also added to the joint. The dovetails where made from purpleheart, for a bit of variety.
Since the rig was broken we only could test up to 4 tonnes. The joint showed a few millimetres slip at 4 tonnes.
The dovetail is commonly used in joinery to hold timbers together. On a large scale it tends to perform rather worse than expected. Especially since green timber dries and shrinks resulting in the surfaces no longer make the same contact as they once did.
I decided to take the dovetail to a ludicrous extreme.
It ended up a bit frankenstein monster in the end.
Just to add to the fun I decided to make the intersecting plane of the two beds of the dovetail off centre...Which was probably not the best idea I've ever had.
Note how the dovetail is made of many dovetails.
Since I had a 1/2" thread / tapping kit I used this as well. Why? Why not? Each semi dovetail was tapped and a threaded rod inserted once assembled.
Inserting the threaded rods. Again Oil is used to lube.
Extra bracing was attached to the outsides of the joint to prevent the somewhat thin dovetail walls from bowing outwards under load.
Unexpectedly the joint was pretty rubbish. Although it achieved 4 tonnes it deflected massively. The bracing was showing signs of slip and about to fly off, so it was ruled out on safety grounds. Boo hiss....
Maybe a little was learned but not much...
The Carpenters Fellowship at Frame 2015 is doing it again with the bending of a scarfed timber.
So watch this space...