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post #1 of 15 (permalink) Old 01-22-2019, 03:11 PM Thread Starter
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Excuse the grade school drawing but hopefully will address my point. Iím new to mortise and tenon jointing so I bought the Trend MT JIG and I'm going to attempt to make a stool. This jig apparently has the capability for complex MT jointing but Iím wondering what the ďstandard ď or maybe correct way to make this MT. These pieces join at a 10 degree slant. Should tenon be parallel to ground or at the same 10 degree slant or does it make a difference.



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post #2 of 15 (permalink) Old 01-22-2019, 03:32 PM
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Problems like this are why we strongly encourage members to fill out their profiles. I don't really know what I should suggest to you because I don't know what you have to work with. I'm not sure how easy it is to match angles on both pieces but the website says you can. The wood above the tenon is fairly short which will make it fragile so I would either make a narrow tenon (up and down) to fit at the bottom of the cross member or just do a saddle type joint joint and maybe pin through the tenon with a few small dowels. Do you plan on doing lots of MT joints? That's an expensive jig.

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post #3 of 15 (permalink) Old 01-23-2019, 12:05 PM
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Hi and welcome. Don't know what tools you have yet, so here goes.

I think it is far easier to cut a mortise at 90 degrees to the surface of the leg. Easier to hog out and shape the recess that way. The tennon, then, has to be cut as shown, at the same angle as the end pieces, but still 90 to the angled end cut.

I would make a pattern out of card stock first, with the angles laid out. Draw the tennon first, then use it to mark the location of the mortise on the leg pattern piece.

The challenge will be cutting the long grain piece to the correct angle. There are several ways to do that, most easily on a band saw, then hand work with a chisel to get it just right. If you don't have a band saw to work with, then I suggest getting a Japanese Dozuki hand saw. It is wicked sharp, but because it cuts on the pull stroke,it is easily controlled and very precise. To increase accuracy, while holding the handle, place your index finger on the side of the blade. It will reduce side movement of the blade--same as controlling a chef's knife.

To remove the shoulders, if you have a table saw, you will have to use your miter gauge and a flat top tooth blade set to a precise height (or a Dado set). The miter gauge will have to be set to the exact angle of the stool's splay. Take multiple passes to shave the tennon to the size you want, but it is almost always a good idea to have the tennon 3/8ths or so to make it easier to drill out the mortise. Leave just a little extra thickness (1/16th or so) so you can use some sandpaper or a chisel or hand plane to fit it to the mortise.

Mark the location of the mortise using the pattern, then carry the mark around to the narrow dimension of the board. Now you can use a drill press if you have one, to hog out the length and width of the mortise. If you don't have a drill press, you might try it with a drill guide of some sort. You could also use a plunge router, by sandwiching the leg in a number of thicknesses of scrap wood, to give you a flat surface for the router to ride on. Drill 3/8ths holes a little deeper than the tennon is long, then use a chisel to carve away until the sides are straight. You chisel must be ultra sharp. If you can't shave hair off your arm easily, it is not sharp enough.

The inside walls of the mortise should be flat and as smooth as possible.

Next, you will shave the sides of the tennon to make a snug fit in the tennon. Once it fits snugly (without forcing it in place), apply glue to both mortise and tennon and press them together. I would do this last, and I'd number both mortise and tennon so during glue up, you don't get confused about what goes where. Remember, each joint is custom fitted.

I am not familiar with the setup you have, but these are some options. If the jig will do any of these tasks, use it. But for me, the basic answer to your question is to allow the tennon to look like the drawing. Easier to cut the tennon angle than the mortise. And all the strength will be from the glue. I'd use some extended open glue. and be careful that the clamps don't throw the stool's structure out of square.

That's my opinion anyhow.
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post #4 of 15 (permalink) Old 01-23-2019, 12:23 PM Thread Starter
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Wow Tom,
I can't thank you enough for the detailed answer. Totally understand what you're saying. Much more useful than the canadian shame I received.

Eric
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post #5 of 15 (permalink) Old 01-23-2019, 12:25 PM
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You are likely to get different opinions on which tenon is stronger...inline or angled. One side will opt for the inline tenon as the cut will not cross the grain and the entire tenon will be along the grain lines. The other side will opt for the angled as it will require the angle cut on the shoulder of the tenon versus cutting the mortise at an angle.

On your Trend jig it looks like the angled mortise can be accommodated but you will need to match the angle to the angle of the shoulder on the inline tenon. You will also need to cut the end of the tenon to the same angle to accommodate the angled mortise (at the bottom).

Don't know the Trend jig but you might need to cut the angled mortise in a separate operation than the inline tenon as opposed to the Trend advantage of cutting the mortise and tenon with one setup when the M&T is square.

Good luck...hoping there will be lots of discussion on this as I've grown an interest in the Trend M&T jig versus a dedicated mortising machine.

Good luck...let us know what you decide and what you needed to do to accommodate...
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post #6 of 15 (permalink) Old 01-23-2019, 01:04 PM
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for legs and aprons inline M&T's are stronger...
they more shear and pull out resistant...
the compound angled M&T shown slide apart when the joint is stressed because of the up angle of the tenon...
inline (full horizontal) will loosen but resist sliding apart...
to improve even further, make the tenon a down angle...

for the M&T glue up...
Bob Van Dyke Shop Tip: Glue Squeeze Out...
and see the PDF...
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post #7 of 15 (permalink) Old 01-23-2019, 08:05 PM
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I wouldn't be too concerned about the strength of the tenon, there is more than enough straight through grain on it, but the reason I suggested to keep it horizontal is what Stick said. Eric what I said wasn't meant to shame you, it was meant to encourage you to fill out your profile so we know what you have to work with. Otherwise it's like being expected to hit a bullseye using a gun with no sights.
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post #8 of 15 (permalink) Old 01-23-2019, 08:47 PM
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I think the glue will prevent the problem Stick suggested, just don't let a gogo dancer work out on top of that stool.

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post #9 of 15 (permalink) Old 01-23-2019, 09:00 PM
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The problem Tom is that it stresses the glue joint more than a horizontal tenon does making it more likely to fail.

Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.
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post #10 of 15 (permalink) Old 01-24-2019, 12:47 AM
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Actually, I like Charles' idea of in his first post "just do a saddle type joint". Lots of glue area and the cross member ends will be longer.
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