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Thread: Need some advice on making big M&T joints.

  1. #1
    Join Date
    Dec 2018
    Washington DC

    Need some advice on making big M&T joints.

    Hi folks,

    I'm working on the biggest project I've tackled and could use some advice. It's a trestle table designed by Daniel Chaffin that I found in FWW. It involves six large M&T joints, two for foot-to-post and four total for apron-to-post.
    Chaffin big table 2.jpg

    Cutting the mortises is no problem for me. Router template plus spiral upcut bit, chiseled square.

    I'm more vexed by the tenons. I don't own a tenon jig, and the pieces are too big anyway. So I'm doing things with a dado blade. With the post-foot joint, I didn't sneak up the cuts too close (scared of a screwup), so I had more than the usual fine-tuning to do. Keeping the tenons square while fine-tuning has proven hard. I am paring with chisels and a mid-sized shoulder plane I borrowed. Here's the joint and the drawing.

    Post to foot mortise and tenon.jpgpost joint detail.png

    Couple of questions.

    What's the right way to fine-tune big tenons? With small ones, I just pare with a chisel. For these I've been using the shoulder plane, but it's narrow and doesn't keep things square. Is the proper tool a rabbet plane? I ask because I want to do it better for the aprons. Drawing below. I'm more concerned about these because they are through tenons.

    apron joint detail 1.pngapron joint finished.png

    Second, what's the best way for figuring out *where* a big tenon like this is tight? With so much tenon, it's hard to figure out where to pare.

    Third, more of a theory question. How much of the joint strength here comes from the long-grain to end-grain part of the M&T joint? I am wondering whether I can get the cheeks of the tenon right and then give myself plenty of space on the edges. Sort of like the way dominos work, with very little contact on the edges.

    Thanks! And much thanks to Dan Chaffin who answered emails to help me scale up the design.
    Attached Images Attached Images

  2. #2
    A bench rabbet plane is an appropriate tool for trimming big tenons, but kind of a pricey buy if it will be rarely used. A wide paring chisel (wide as possible) is probably the most commonly used.

    You are correct that the strength of the glue joint is in the face-grain v face-grain part.

    I'm not being flip, but if your mortise is well cut (parallel walls, even across it's length), then a dial caliper is probably the best way to find the fat spots in the tenon. Short of that, if you test fit the piece and wiggle it a little once it's in as far as it goes, you should see the burnishing on the tenon cheek where it's tightest. Or, make a go-no go gauge by routing your mortise into a thin piece of stock.



  3. #3
    Join Date
    Jul 2007
    Inkerman, Ontario, Canada
    Use a router to trim the tenons, either in a table of hand-held.

  4. #4
    Join Date
    Dec 2018
    Washington DC
    Dial caliper and go-no-go gauge! I love it. Good suggestion.

  5. #5
    A router plane will cut a surface parallel to the workpiece face. It may not reach far enough to cut the whole tenon cheek, but you can extend the initial area with a chisel or block plane and straightedge. Or go back to the dado setup and sneak up on the fit.

    Calipers and straightedge will help to find the high spots. About .004" clearance between mortise and tenon is about right.

    The tenon edges contribute little to the glue bond but a tight fit there gives mechanical strength to the joint.

  6. #6
    Join Date
    Sep 2013
    Wayland, MA
    Trim big tenons to size just like a small tenon, with a shoulder plane. You might have to take a couple passes, but no reason to buy a bigger plane for a one-time task. I do test fits and trim off the shiny spot until you can push it home. Trimming a couple thousandths off with a router or table saw is really hard and time consuming (putting them back on is even worse). A shoulder plane does it perfectly.

  7. #7
    Join Date
    Mar 2019
    Los Angeles, California
    This is essentially high quality timber framing. Hand tools rule here. Knife wall > Dovetail Saw for the shoulders. Split the tenons incrementally with a good whack of a chisel on the end grain, then a router plane to find the high spots. Set up an identical piece of stock on the end so the router plane base has two surfaces to ride on. You can also attach an extended base for the router plane.

    This is essentially the Paul Sellers method.


  8. #8
    The method I've been using recently for things like this:

    - Cut the shoulders with table saw. Exact depth isn't important, since it will be fine-tuned later. Just make sure the cut isn't too deep.
    - Split off most of the waste with a chisel.
    - Use a router with an extended base and a top-bearing template bit to remove the rest of the waste. The base rides on the face of the workpiece, and the bit sticks down to trim the tenon. The bearing will prevent the router from removing any material past the shoulder. The router should have fine adjustment so that you can get the right depth. This may take a few passes until you get the right depth. Using calipers and/or a router height gauge will help to get the correct setting more quickly.
    - A router plane can be used instead of the router, or in addition to the router for even finer control of depth.

    This is similar to the Paul Sellers method, but with more power tools.

    I'm in the middle of making some tenons this way, so I have the router setup handy for a picture.

    Last edited by Winston Chang; 05-18-2020 at 6:40 PM.

  9. #9
    These days I would use my domino and loose tenons. Before getting it, I made tenons in wide pieces using my RAS and dado blades. I deliberately left a little to be trimmed with my shoulder plane which is 1 inch wide.

    You could use a plunge router to make a mortise in these long pieces to use loose tenons but I probably would not. I cut recesses for bed rail hardware this way but those recesses are only 3/16 deep. It seems it would be much more difficult to guide the router with the workpiece at an angle if doing a 2 inch deep mortise..

  10. #10
    Join Date
    Dec 2010
    Newtown, ct
    If you have a bandsaw. Cut the tenon shoulders on the table saw and use the bandsaw with a fence to cut the cheeks. You can get very close this way. If the cheeks still need paring, a router plane makes the job very easy. my two cents.

  11. #11
    Join Date
    Jun 2008
    So Cal
    I also cut my tenons on my bandsaw. I like going over slightly and trimming them down with skew angle block plane. If it warm and sunny they should be fairly tight. If I make them fairly tight on a rainy day they most likely will be too loose on a sunny day.
    A good woodworker keeps one eye on the weather

    Good Luck

  12. #12
    Join Date
    Dec 2018
    Washington DC
    Harold - I have a little 10" Jet bandsaw. Love it, but the fence isn't accurate, so no good for tenons.
    Andrew - I worry about that. Yesterday was warm. I think to myself that plenty of antique furniture holds together and they probably didn't sweat it like I do or have high-tech PVA glues. So I'm probably over-worrying. Maybe we all over-worry.

  13. #13
    Consider something like a pantorouter. I made a solid Ash table with some very large bridle joints of a similar size, and the pantorouter worked out great to make very accurate joints on that scale. I did have to find a very long router bit, which took a while, but it ended up great. A wooden pantorouter can be homemade, or a prefabricated aluminum/steel kit can be purchased. I would not build one just for one project, but I use mine all the time for plenty of other things...finger joints, M&T joints of any size, cutting hinge mortises, etc...can also be used as a horizontal router table.

  14. #14
    Join Date
    Sep 2014
    Swampscott, MA
    I've used the router/fence method suggested for long pieces (too big for bandsaw or tablesaw) with large tenons a number of times with good results. I first wrap the shoulder lines around the piece with a knife and the cheek lines with a marking gauge, route to <1/16" then finish with a chisel and rabbet or shoulder plane. The knife lines give a well defined "end point" to work to which helps keep things clean and square

  15. #15
    Join Date
    Aug 2013
    Princeton, NJ
    Blog Entries
    I make joinery like this on the regular. I use a router table to make the tenons, the shoulders are only as accurate as the square cut on the end of the tenon, so make that very very square because you trim your tenon cheeks.

    I make all around with a knife gauge, this I do mainly to prevent chip outs.

    IMO that design for the batten support and how it connects to the upright is odd to me, I don't like that section of short grain between two stretchers. Personally, I would opt for a single heavy stretcher, set it slightly lower and have the top battens mounted on the top of the uprights.

    I typically also tenon the uprights directly into the table top and build the battens to expand out from center, this makes a table that is extremely strong against racking.

    The first thing every single person does upon checking the quality of your work is go and try to rack the table, so I like them to be impressed in that regard.
    Last edited by Brian Holcombe; 05-19-2020 at 9:32 AM.
    Bumbling forward into the unknown.

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