Gap in mortise & tenon leg joints – use dowel or hex bolt as reinforcement?

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This topic contains 11 replies, has 4 voices, and was last updated by  Sven-Olof Jansson 9 months, 1 week ago.

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  • #553153

    draske
    Participant

    I’m a newbie and have been working on & off on the workbench project for over a year, and finally finished my mortise & tenon fits for the legs/crossbeams and did the joint glue-up yesterday.

    While the joints seem to hold right now, several of the joints have a large (over 1/8″?) gap on the back side, and I’m concerned that over time they will break from the adhesive.

    I was wondering if I should consider drilling holes in each joint, through the tenon, and inserting hex bolts or dowels, so everything will hold in place even if the tenon comes loose. My (completely uninformed) plan is to drill two offset holes through the 4×4 (through the tenon) on each joint with my cordless power drill and perhaps a 3/8″ bit.

    Is there a downside to doing this? If not, are dowels vs hex bolts recommended (I dont care how it looks if one is easier/better).

    Thanks!
    David

    #553163

    Ed
    Participant

    If the problem is simply that the tenon is too thin (rather than the gaps being from uneven mortise walls), then one fix is to make sure the tenon cheek is flat, cut a piece of material about 3/16 thick, and glue it to the tenon cheek. Once it is dry, use your router plane to get the correct thickness for the mortise. Sneak up on it and keep testing your tenon until you get the right thickness.

    #553173

    draske
    Participant

    Most likely the mortise hole is uneven, and wider at the back.

    The whole assembly is already glued-up.

    I’ve attached photos of the 3 worst joints.

    I’m also considering gluing shims into the holes to reduce movement, rather than going with dowels/bolts.

    Attachments:
    #553207

    Selva
    Participant

    To be blunt, with such poor fitting joints I will saw it apart and start over. Unless those gaps seen outside are only a few mm deep and the rest of the mortice snugly fits the tenon.

    You may be able to salvage the legs by turning it around top to bottom and patching the current mortice holes — depends on the location of the mortices though. You do not want the new mortices to be anywhere close to those holes. Also practice mortice and tenon on some scraps a few more times before attempting it on the leg assembly — you don’t have to get them as perfect as Paul’s but have to be fairly snug and close fitting.

    selva

    #553214

    Ed
    Participant

    Did you use a modern acrylic glue or a reversible glue like hide glue or old brown glue? If you used a reversible glue, I’d take the joints apart, get a guide block to true up the mortises if necessary, working to fresh gauge lines, then glue shims onto the tenons or cut new pieces and try again.

    If you didn’t use a reversible glue, I’m sorry to say that I don’t think these joints will be strong enough for a bench. Typical wood glues won’t fill gaps in general, so I don’t think you’ll have the strength you require for a bench. Actually, if you give it a hard whack with a mallet and a piece of scrap to protect the work, it is likely to come apart, given the photo. It may also break, though, so you need to decide how much risk you’re willing to take. Think on the positive side and realize you likely gained a lot of experience on your first M&T’s.

    The trick to these joints is to prepare the mortise first. Don’t move on to the tenons until the mortise is cut and the sides are parallel and true. Ideally, you want that to happen at your layout lines, but if you miss them, then you have no choice and you just need to get them straight, flat, and parallel. On this project, you could clamp a guide block on to guide you while you pare. Don’t second guess yourself, though, trying for perfect…usually, everything is fine right off the chisel after chopping and removing the fuzz. So, don’t tinker with it and make it wallowed and wavy. Experience will show you when it’s good enough. Sorry to sound self-contradicting.

    Once the mortise is done, do the tenon. Cut it fat. Use your router plane to get flat, parallel cheeks and progressively reduce the tenon to the required thickness. Watch the far side of the mortise when you are fitting so that you don’t break out fibers during the test fits.

    These legs have plenty of meat on them. You have room to make them wider, so if you can get the current tenons out, you can pare the mortises wider to get good surfaces and then go from there. So, you may be able to save the legs. I cannot tell from the photos whether the mortise walls are wallowed out all the way through or if you just have some bruising near the surface. If it is just bruising near the surface and most of the wall is good, you can keep going. You could also saw the tenons off, then drill out the sawn off tenons and pare the mortise. That’s another way to save the legs, but you’ll need to cut new stretchers/aprons.

    Hope that helps.

    #553215

    draske
    Participant

    Thanks everyone.

    I certainly improved a lot from the start of this project until now, and it was a bit embarassing to post these photos. But I was at the point where I said it was just time to move forward with what I had…perhaps that was hasty.

    I used Titebond glue.

    I followed Ed’s recommended method and used a router plane to trim down my tenons to fit in the mortises. I believe the face of the mortise that touches the shoulders of the tenon is a snug fit… but that my mortise holes in general are off-center due to 2 factors: Poor mortising technique (creating ‘wavy’ walls) as well as lack of square measurement as the wood is rounded and I may have been off by as much as 1/4″ when marking the mortise hole on the front compared to the back.

    Ed – Do I understand you correctly: your suggestion is to break down the piece, cut new crossbeams with fatter tenons (e.g. 3/4″ instead of 1/2″) and expand the current mortise hole width to that fatter tenon size (3/4″)?

    Also – is the dowel / bolt idea totally misguided? If we’re essentially in the “Start over” phase, then it would seem any risk with the method isn’t really a factor…but I’m curious if there’s even any potential benefit or not. If so, it certainly would be easier to drill a few holes than redo M&T.

    • This reply was modified 9 months, 1 week ago by  draske.
    #553216

    Sven-Olof Jansson
    Participant

    Brackets – unorthodox and ugly – are holding my workbench together since a couple of years; and having a workbench really facilitates making a new one with improved joinery (double tenons and mortices, and bolts to further pull the assembly together).

    /Sven-Olof

    Sven-Olof Jansson
    London, UK; Cambridge, MA

    Attachments:
    #553219

    draske
    Participant

    Thanks Sven-Olof – that’s inspiring 😉

    Do you mind posting a photo of one of the brackets on your bench, so I can see how you’ve attached them to the legs?

    I agree I’d like to make a new, more stable bench using my existing bench once this is done, likely after trying a few small projects.

    #553220

    Sven-Olof Jansson
    Participant

    Thanks!
    Here are three photos. Hopefully, they will provide some information.

    /Sven-Olof

    Sven-Olof Jansson
    London, UK; Cambridge, MA

    Attachments:
    #553224

    draske
    Participant

    Very helpful, thank you

    #553225

    Ed
    Participant

    Yes, I was suggesting to break it down and move to fatter tenons so that you can have another go at the mortises, but I agree with Sven-Olof: If you find another way to make the bench rigid, you’ll have a bench to work on and, some day, you can build another. You may find the bench lasts longer than you think. Screwing a piece of ply that spans from leg-set to leg-set is a quick way to add rigidity. I put a piece of 1/2″ ply that was roughly 2′ x 5′ across the back of a wobbly bench that was abandoned by the previous owner of our house and it made the thing quite rigid side to side. What I don’t think will work is pounding shims into the existing joints or putting bolts / dowels in them, but I could be wrong.

    #553230

    Sven-Olof Jansson
    Participant

    If you are wrong Ed, then I failed in getting both dowels and bolts right, and in hindsight (the retrospectoscope – world’s best examining instrument, it never fails) the reason is obvious: racking occurred because the tenon and mortices did not keep the shoulders of the tenons sufficiently tight to the surface of the legs. Once the shoulders were off, no amount of closing the faces of the tenons and mortices would reduce the racking.

    Far as I can remember, the importance the tenon shoulders has within WWMCs not been emphasised to the same level as by some other experts: e.g. Terrie Noll (The Joint Book, The complete Book Guide to Wood Joinery; Quatro Inc 2002).

    /Sven-Olof

    Sven-Olof Jansson
    London, UK; Cambridge, MA

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