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Thread: Question on "Structural Miter" Joints

  1. #1
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    Question on "Structural Miter" Joints

    Hi, I got a question regarding a leg joinery for my ongoing desk build (my first standalone non-shop furniture).

    I came up with an overall design then decided to work out the details as I build along. (This I'm quickly realizing was a risky decision...) The leg design is heavily inspired by Aled Lewis' Wish Desk (which was on the backcover of a fairly recent FWW issue). Being a FWW backcover story, the details of how to build the desk was not fully elaborated in the magazine (only how to attach the legs to the top). So, I took a stab at it and came up with the leg joinery:

    LauraDesk 7.jpg

    Ideally, I should've made these long loose tenons integral; but the designing-as-building bit me hard in this case as I already cut the leg pieces to have only 1"-long tenon. Literally within a second after the saw separated the last offcut, I realized 1" is probably too short for what it is (legs are milled from 6/4 stock). That is how the mixture of a long (3-1/2" - 4") loose tenon next to 3/4" integral tenon came about.

    Am I overcomplicating this joinery? Any alternate ideas?

    BTW, the front and rear legs on each side will be joined together by some yet-to-be-determined method, and there is a horizontal stretcher right behind where the legs are crossing.

    Thanks for your inputs!
    Kesh

  2. #2
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    That's an engineering challenge. Your approach is fairly complex. If you have already machined everything and don't want to do it again, use a high strength glue and hope for the best. If you are prepared to do it again, consider a half lap joint. You only see one face so make the face side with the mitre line and the bulk of the joint behind. Good luck. Cheers

  3. #3
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    I did consider half-laps during initial design, but I wanted to stay true to the look of the Lewis' desk. The outer leg joint is exposed from above.

    furniture_image03-2.png

    Also, I wanted to mill the two parts as closely together as I could for the best grain continuity. These are the reasons why I chucked the half-lap idea.

    I guess my question boils down to this. Do the integral tenons in my design do much? If The long loose tenon and splines can provide enough strength, it would simplify the construction substantially.

  4. #4
    pleasedon't think I'm raining on your parade, but boy, have you bitten off a big chunk. You say you are a beginner, basically. That desk is complicated and an engineering marvel. I have been wwkng 30 yrs and make windsor chairs, and would find this table a huge challenge. the legs are the simple part. It is the ability to get those legs attached to the tabletop, considering the movement of the top getting all the mortises to line up would be daunting for most of us. I sure wish you luck, and post pictures of your building experience.
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  5. #5
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    Since my question was on the leg joinery, I left out my biggest cheat The top will be made of appleply, which I purchased 4 years ago when I promised my wife of her desk (surely you know how that goes, right? lol). Ply top will simplify top-leg joinery greatly, and I'll use M-T joint (you can see the tenon outlined in my drawing) with a couple screws to join top and front legs.

    Being an engineer by trade, I'm drawn to well-engineered piece like Lewis' desk. His sliding dovetail to join the top and front leg is a genius design, and the most trickiest part of the build, had I copied his design entirely. Eliminating this and breaking down the project into parts, I decided this build is manageable with my current skill and tool sets. We'll see. I'll try to snap a few pics along the way (it's hard though)

  6. #6
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    That is a beautiful piece of furniture, delicate with strength. What are the O/A dimensions? Your wife must really be pleased with the design.

    Some questions that will display my ignorance of this design:
    1. Will the stretcher be attached to the rear of the drawers?
    2. What is the extension behind the drawers for?
    3. And whate is the rectangular hole in the top for?


    Keep the pictures coming - interesting project!
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  7. #7
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    Thanks, Al. But let's not get ahead of ourselves here. There is still a possibility of her ending up with a pile of scrap wood instead of a desk. haha

    Quote Originally Posted by Al Launier View Post
    What are the O/A dimensions?
    Pretty standard desk dimension: 5' x 2' x 30" The depth is governed by that of the ply in hand.

    Quote Originally Posted by Al Launier View Post
    1. Will the stretcher be attached to the rear of the drawers?
    No, the stretcher and the drawers will be completely detached. The drawers are suspended from the top with a threaded rod and its side is attached to the front leg with screws. Here is the shot from under the desk:

    Drawers.jpg

    Quote Originally Posted by Al Launier View Post
    2. What is the extension behind the drawers for?
    It is intended to host the subwoofer of her computer speaker system.

    Quote Originally Posted by Al Launier View Post
    3. And whate is the rectangular hole in the top for?
    That is the recess to hold a power strip and for cable management.

    Mind you, the plans for the drawer assembly and the recess are nowhere near final. I figured I will wait to make the final say until I have a dry-fitted desk (top + legs) in front of me. It will give me much better sense of their proportion and function.

  8. #8
    I'd rout a seat for some aluminum, a mending plate, maybe on the back side if you don't like hardware.

  9. #9
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    @Pat - That's an intriguing idea. Although having a mending plate on a show side quickly got shot down by my dear customer, I'm contemplating to use it along the arched underside (say 1" wide plate on 1-1/2" thick wood). A couple questions.

    Do you think there is enough strength if I replace the loose tenon with the mending plate? The short tenon would be extended in place of the loose tenon in this scenario.

    What thickness Al stock do you recommend?

    Thanks!

  10. #10
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    Kesh

    I agree with Pat about using a blind mending plate. That would put a lot of strength into the joint(s)
    I would use both the mending plate and the tenon, but nothing says that tenon can't be replaced with 1/4" drill stock epoxied in place. I think you'll find that the 6/4 legs are going to actually be stronger than you think. Of course people tend to lean on desks, so you will need some extra "beef".
    I made a piece for my wife, a library shelf system, and used blind, and recessed, drill rod and all thread. The piece has an "airy" look to it, but it's pretty stout.

    It's a nice desk
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  11. #11
    "What thickness Al stock do you recommend?"
    ***********************************************
    1/4 or 3/8 is plenty. Its strength will depend on your fasteners, how many & where they're placed.
    I tap and use 1/4-20 cap screws.
    Its nest should be sweet and without much play.

  12. #12
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    It's a bit hard to describe but the half lap still works. To me it is intuitive but that's the way I design. The best I can say is that you stop the inner half lap just below the top edge.

    I have doubts about the longevity of your proposed design as there seems to be too much end grain gluing which is not so good.

    If you use the embedded steel or aluminium plates, make sure the back is roughened preferably blasted and epoxy glue in place. Metal to timber will fail over time if not fully bonded as they are such dissimilar materials.

    Composite construction like this is good. I am currently working on a dining suite for a client where this is the only way to do it. The table is 3 metres long on a single pedestal. Steel inside plywood core is the construction method so that it is slender and strong which is what you are trying to achieve. But remember, the reinforcing plate must be bonded to work. This is where flat pack furniture really fails among other reasons of course. Cheers

  13. #13
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    Thanks all for the great suggestions.

    Quote Originally Posted by Mike Cutler View Post
    tenon can't be replaced with 1/4" drill stock epoxied in place.
    This might work even better than mending plate for me! I've been thinking about how to jig up to flush mount the mending plate on the arched bottom surface (and how to bend the plate to the right radius). I came up with an idea but more complicated than I like (essentially a rocking router base). Metal rod, on the other hand, means I just need a drilling/doweling jig, which is a whole lot simpler proposition.

    Because I'm a paranoid, I'd like to use a thicker aluminum rod (3/8" maybe?). Knowing myself, I'll hit the rod with a router bit when I create mortises on these legs for the stretcher.

    Quote Originally Posted by Wayne Lomman View Post
    If you use the embedded steel or aluminium plates, make sure the back is roughened preferably blasted and epoxy glue in place.
    Thanks for this tip. I never thought of this. Mind elaborating a bit on "roughened"? Is sanding the rod surface to 120 grit sufficient?

  14. #14
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    It should be rough enough to file your finger nails on the metal surface. If you burnish it, you will have wasted your effort. Use sharp paper and change it often. Abrasive blasting is best as you get myriad peaks and valleys about 60-70 microns in height and the resulting increase in surface area is astronomical. Spent far too of my life in the abrasive blasting and industrial painting industry where this is a crucial starting point for any job.

    By the way, can you not put a flat plate over the joint on the inside face instead of inside the radius or drilling pins in place? Cheers

  15. #15
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    Quote Originally Posted by Kesh Ikuma View Post
    @Pat - That's an intriguing idea. Although having a mending plate on a show side quickly got shot down by my dear customer, I'm contemplating to use it along the arched underside (say 1" wide plate on 1-1/2" thick wood). ...
    Kush, If the boss is against metal/wood, and if you are a member of FWW, show her some of the pics from this recent article. http://www.finewoodworking.com/how-t...-and-wood.aspx. It changed my life (and got me researching milling machines).
    Mark McFarlane

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