Blind wedged mortise and tenon - how???

[stuckinthemud]

I am slowly building a Celtic type lap harp and find I may need to construct it using blind wedged mortice and tenons. Never actually made such a joint. Any tips?

 

[Droogs]

Once you have made the tenon and are about to cut the slit for the wedge to go into, drill a 3mm hole at the spot you want the slit to stop. This will help prevent the chances of a split happening as you hit the tenon home and the wedge works its way in. Also the wedge should be around 7-10 degress and 5mm shorter than the depth of the tenon.

hth

oh and remember to have a little space for the tenon to move into in the side of the mortice

 

[MikeK]

And most importantly, in case it needs to be stated, you only get one chance to assemble the joint with the wedge(s) in place. If the shoulder of the tenon doesn't fully seat (because someone didn't pay attention to the guidance about the length or thickness of the wedge), the joint will likely not come apart for a second attempt.

 

[Ozi]

Make a couple of practice pieces you don't mind laughing about

 

[hlvd]

 

[sawtooth-9]

Once you have made the tenon and are about to cut the slit for the wedge to go into, drill a 3mm hole at the spot you want the slit to stop. This will help prevent the chances of a split happening as you hit the tenon home and the wedge works its way in. Also the wedge should be around 7-10 degress and 5mm shorter than the depth of the tenon.

hth

oh and remember to have a little space for the tenon to move into in the side of the mortice

Thank you for that, I will need to do 120 of these "joints" soon.
So, I assume the cut for the wedge is 3 mm wide ?
My issue is how to make the wedge. Any ( helpful ) suggestions would be highly valued. The wedge around 7 - 10 degrees seems a lot ? I assume you are looking at 3 1/2 - 5 degrees per side ?

 

[clogs]

someone else will know but pretty sure the Japanese will also have a joint for that job...
They are the master of joints in wood.....
I'm always amazed at what they turn out...

 

[rogxwhit]

drill a 3mm hole at the spot you want the slit to stop. This will help prevent the chances of a split happening as you hit the tenon home and the wedge works its way in

I don't subscribe to this - what difference could it make? If the wood wants to split it's going to split, surely? You may be thinking of other materials with a different structure to wood, which has a directional grain?

My issue is how to make the wedge.

Ha! Well you can prepare strips of the root wedge section, part them off to length (crosscut on bandsaw), and hand chisel them against a stop. That'll be fun for 240 wedges! Or you can prepare material to be as long as a wedge, as thick as a wedge is wide, and of any width. Then make a small jig to get the angle, and part off wedges from your prepared blanks (ripcut on bandsaw). The bandsaw is safe for small parts where a circular saw isn't.

 

[Droogs]

@sawtooth-9
The slot for the wedge is only as wide as the saw kerf, otherwise the wedge wont push the sides of the tenon out and just slide into the gap. Yes 3 1/2 - 5 deg either side of the wedge centreline.

@rogxwhit
It makes a massive difference and no the wood wont split. The hole is positioned as in the cut away below and does 2 things, first acts as a stress relief and 2nd provides the tip of the wedge with somewhere to go that is much easier to move into than between the fibres of the endgrain that it will meet, just in case you have made the wedge too long. The tip will follow the curve of the hole and crush itself rather than split the wood

This is the way I was taught to make them by a traditional ebenieste atelier master and how they have been made for over 500 years

 

[rogxwhit]

I still don't hold that the holes act as 'stress relief' as labelled in that image - I think that terminology belongs to other materials that are not wood, that don't have a 'linear' grain structure. What the holes have done there is reduce the thickness of wood that has to flex as the wings of the tenon are forced apart to fill the mortice - but the kerfs could've been placed more outboard to begin with which would have the same effect, and that's what I'd have done. And surely one wouldn't have cut a wedge too long? I think we're going to have to differ on some aspects here. ;-)

At least I agree about the wedge angle!

 

[Yojevol]

I don't think the wedge taper angle is important. All the wedge is doing is forcing the tenon side strips into mortise gap. The wedge length is not critical either but it needs be short enough to avoid hitting the end of the saw cut. The most important dimension is that of the thick end of the wedge. Ideally it should be the saw kerf + mortise taper gap. I would add a bit more on to allow some crushing of wedge and tenon fibres.
I would certainly recommend a few tests using the proposed timbers. It might be worth setting up an open-sided test so that you can actually see what's going on as the joint is closed up.
W.r.t. the 'strain relief hole' I'm with @rogxwhit and Paul Sellars rather than an ebenieste atelier. A better term would be 'strain control hole' as that is what it is doing. It certainly is not relieving strain or stress.
Brian

 

[Jacob]

Deleted. Sorry - crossed posts!
It was stuckinthemud talking about celtic harps but sawtooth-9 talking about 120 M&Ts. I got them conflated.
sawtooth-9 presumably thinking of his "koji" boxes mentioned before.

 

[Peter Sefton]

Do a couple of samples from dimensioned drawings with the same timber, assemble and cut in half both to see it clamps up and then that it holds the joint as planned. You can make an angled piece of timber to assist you getting the correct internal slope and matching wedge. If the wedge is too long it won't pull up, if it tool thin it's almost useless.

Fun and very strong joint.

Cheers

Peter