seat frame: dado instead of mortise/tenon?

[sluggo]

I'm thinking about how I would like to make my seat frames. Ideally I would like to do a mortise and tenon but I'm not so sure that I can do a very good mortise. I don't have a drill press and I don't have a plunge router. I'm pretty sure that if I tried to do a mortise by hand that it would be pretty sloppy. So I'm thinking about using a dado blade on my table saw. I can cut a groove down the center of the wood that runs fore/aft and then cut a mating joint on the crossmembers. Once glued I don't think I'd have to worry about the crossmembers moving fore or aft. I could maybe even add 1/4" dowel through the joint from the bottom but that might compromise the overall joint strength. One concern I have is that with a continuous groove I won't want to cut the dado very deep, maybe only 1/4" or 5/16". One possible solution would be to cut the dado groove deeper, maybe 5/8" deep by 1/4" wide and then glue a filler piece of wood to fill in the gap between the two crossmembers. Another concern is that water may hold in the groove and accelerate rot, but I'd think that a good coat of epoxy would minimize that risk.

I'd like to hear some thoughts on this. Good idea, bad idea, ideas on how to cut a mortise, etc? btw, another idea is to buy a cheap used plunge router for $75 and try the mortise and tenon route...

thanks


cheers

[Glen Smith]

Some thoughts:

You could just butt the pieces together and insert two dowels per joint. Use thickened epoxy to glue the dowels and the joints.

Or, you could make a half-lap joint since you have a dado blade. Again, use thickened epoxy for the gluing.

[sluggo]

One thing I was wondering is if it's common to have the sides of the seats angled. ie wider towards the middle and narrower towards the end.

And yes, maybe the lap joint is a great way to go. The crossmember would go beneath the side pieces, right? since the crossmembers transmit the load to the hull and the side pieces would sit on top?

cheers!

[Glen Smith]

One thing I was wondering is if it's common to have the sides of the seats angled. ie wider towards the middle and narrower towards the end.

I am not too clear on what you mean here.

I think I would make the half lap joints so the side pieces are beneath the crossmembers so you don't break the look of the continuity of the crossmembers. Being epoxied together, it will be plenty strong.

[Bryan Hansel]

Don't be afraid of a mortise. Mark out the square on the end of the board, use a drill to roughly drive out the hole. Go a bit deeper than you need. Then use a sharp chisel to clean up the mortise. This method works just fine. I've used it for every mortise on a skin-on-frame kayak and in timber framing.

[Rick]

Another way to make a mortise is to split the crossmember stock down the center with the table saw, then mark out the locations of the mortise on both inner sides on the halves. Rout out each half of the mortise by making progressive cuts with the saw blade adjusted precisely for height. Then clamp and glue the crossmember halves together with thickened epoxy and clean out the excess epoxy from the inside of the mortise.

After the epoxy cures, sand off the roughness on the inside of the mortise with a sanding stick, this helps ensure the thickened epoxy used to glue in the tenons later on will bond strongly.

The tenons can be made the same way, by using the table saw blade adjusted to the proper height and making progressive routing cuts so that the tenon fits precisely into the mortise.

There will be a glue line down the center of the smoothed-off crossmember, and the end of the tenon will be visible, IMO, it looks good, and the close fit of the precisely-cut epoxied joints adds strength.

[edmontonian]

Hi Sluggo,

I successfully built my first set of canoe seats from Martin Step's Green Valley plans using floating tenons and modest tools. After varnish and caning, they are simply beautiful.
I used a router and made 5/16" mortises at 1" deep x 1 & 1/8" wide on each side of the long members and the cross members. After that, I cut to length four 1 & 7/8" tenons x 1 & 1/8" wide to precisely fit the widths of the mortises. I chose to round over the tenons to make things fit snug, but not so tight that I needed pliers to remove the tenons. I also used thickened epoxy to glue all surfaces to each other, clamped the seat frames until dry, and everything worked out fine. The key for me was patience and thoughtfulness between each step in order to get it right the first time! ;)
edmontonian
:eyebrows

[KenC]

You don't need a drill-press or a plunge router ... a hand-cut mortise is actually not that difficult ... especially a small one as you'd use for a canoe seat ... do a google search for some pointers, grab some scrap wood, and make up some practice joints ... you could drill out (freehand) most of the waste, but I'm not sure that saves much time ... once you get a good rhythm going, it doesn't take long to hammer out a decent mortise ... the trickiest part is actually getting the tenon shoulder to fit nicely with no gaps ... that's the only part that's visible.

[sluggo]

I think I'll try a practice mortise and see how it goes.

thanks

[AlanWS]

A half lap should be fine, but I would definitely make the bottom of the crosswise load bearing parts continuous, as the other way would severely compromise strength. Glue with epoxy.

That said, if you want mortise and tenon joints, do it. You can chop them by hand, or getting a router may be a fine idea. Even a little router like the Bosch colt should cut 1/4" mortises without any problem. If you do use a router, the easiest way is to cut mortises in both parts, and use an inserted tenon (sometimes also called a loose tenon.)

You can certainly make the seat sides slant so as to follow the hull, wider toward the center of the canoe. I made my seats both wider toward the bow, and narrower toward the stern. I also made my seats substantially bigger than most. It adds very little weight, and significant comfort.

If you make trapezoidal (slanted side) seats with mortises and tenons, I would seriously consider the router method with inserted tenons. It's easy and strong just to cut the pieces to fit, then mortise each perpendicular to its mating face. The tenon stock can have its grain run straight across the joint. I used Frid's jig for this. http://www.highlandwoodworking.com/inde ... stom&ID=29

[sluggo]

I tried a mortise and tenon. First I made a mortise with a chisel, then I cut a tenon using the table saw, just nibbling away. I didn't even bother to put on my dado blade. It didn't seem overly difficult. I can't say that my mortise was pretty or perfect, but I got a tight fit. Since I'd only have to do 8 pairs, I figure I cut cut each tenon to match my somewhat poor mortise. I figure I'll try it and if it doesn't work, I'll just try something else.

cheers

[ealger]

Sounds like you're having fun.
You probably already know this, but it's suggested that the mortise be about 1/8" deeper than the tenon. This is to give some space for the extra glue/epoxy to settle. I also like to cut a small groove (1/32") in the length of the tenon to help relieve any glue pressure in a tight tenon. The strength of the tenon comes at the shoulders so they should fit nicely.
Ed...

[KenC]

I can't say that my mortise was pretty or perfect,

Doesn't need to be, since nobody will ever see it.

[Karl Huebner]

Why can't you use cedar - pine - ash strips? You could glue a rough seat together and then shape it any way - any design you wanted? You could even do built in in-lays... and match whatever design on the yoke - deck>