Special techniques to cutting a non-90 degree mortise?

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  • #637361
    GfB
    Participant

    I’m in planning stages of building a small sitting bench. The legs will be M/T into the top, and I’d like to splay out sideways at about 5-7 degrees from 90.

    The methods I’ve seen are:
    1. Mark the top, then follow around the edges at angle to mark the bottom, and carefully chop until both sides meet.
    2. Make an angle block to guide my chisels.
    3. Set a bevel gauge to x degrees, and use that to check progress inside the mortise.

    Are there other methods?

    #637646
    YrHenSaer
    Participant

    Are you asking about angled tenons into the inclined section of the back stile? Using the full available width of the tenon gives the most strength which means that the angled cut is in the stile. (You can cut a 90 degree mortise and do the angles on the tenon section, but this sacrifices some of the tenon width).

    If the angles are in the stiles, here’s how I’ve done it in the past, assuming that the meeting of the mortise and tenon on the arm is in a straight line. (It gets a little more complex for compounded angles).

    1 – Decide on the angle you need, then mark it onto a straight bit of scrap that can be laid on or clamped to the surface to be mortised. Cut it straight across – this is your reference. Use the acute angle for one side of the slot and the obtuse one for the other.
    2 – Mark it all out on the surface of the stile and the angles on the sides in pencil.
    3 – In the centre portion of the mortise cut a 90 degree slot to the required depth. It now remains to cut the side angles as needed.
    4 – Pare back the angled edges to the limits you have set, starting from the edges of the mortise, using your reference guide to check your chisel angle. Acute one side, obtuse the other.

    Hope that this helps. The alternative is to use a mortise machine with an adjustable head, which few of us have.

    #637719
    GfB
    Participant

    @YrHenSaer,

    I’m talking about angling the mortises. The tenons would be square. As you said, angling the tenons could decrease strength. If anyone has a good reason to angle the tenons, I’m interested.

    In your second part, it looks like you’re describing building and using an angle block (my method #2). This is the method I will most likely be going with.

    #637979
    Ed
    Participant

    We angled the tenons in the rocking chair. Paul taught that it was far easier than trying to angle the mortise. Some say that angling the tenon leads to short grain in the tenon, but I think, realistically, this effect is negligible. If you think about how most grain wanders around anyway, I doubt there’s much effect at all. The point of this is that you can layout an angled tenon with a bevel gauge, then work to your lines. Meanwhile, you just cut a normal mortise. If I had to cut an angled mortise, which I’ve not had to do, I think I’d chop it narrow and use a paring block to get my angle, assuming it was a small angle.

    #638168
    Sven-Olof Jansson
    Participant

    But wouldn’t all types of joints based on the legs protruding into the benchtop be of long grain – short grain, as long as the faces of the top are its top and bottom surfaces? If so, would doweling the legs and then joining them as in the Shaker-Style Bench Seat be an alternative?

    The legs wouldn’t have to be cylindrical, only the ends joining with the top. In all three WWMC projects using these joints, wedging is also included. I suppose that’s to assure the joints will be tight, also with changing humidity.

    The attached photos on benches and leg joinery, hopefully provide some additional information.

    Sven-Olof Jansson
    London, UK; Boston, MA

    Attachments:
    #648932
    keithm
    Participant

    I think Jeff Miller wrote an article in Fine Woodworking several years ago about a Rube Goldberg jig to do this. IIRC, Paul poo-pooed the whole idea and said that the loss of strength on an angled tenon was negligible. If it wasn’t Paul, it was someone else I respect.

    #648960
    Larry Geib
    Participant

    I think poo-pooing is putting Paul’s reaction a bit mildly.

    He used the term “horrendous blunder”

    https://paulsellers.com/2017/05/re-easy-angled-tenons-article-fine-woodworking/

    Follow the link on those words for more.

    • This reply was modified 2 years, 9 months ago by Larry Geib.
    #637937
    Larry Geib
    Participant

    Paul discusses an article in a 2012 FW magazine article where jeff Miller recommends straight tenons twice and generally doesn’t agree for a few reasons

    https://paulsellers.com/2012/05/more-flawed-concepts-from-fw/

    https://paulsellers.com/2012/12/fine-woodworking-qa-section-answer/

    #637938
    Larry Geib
    Participant

    If anyone has a good reason to angle the tenons, I’m interested.

    Paul has good reasons.

    https://paulsellers.com/2012/05/more-flawed-concepts-from-fw/

    And:

    https://paulsellers.com/2012/12/fine-woodworking-qa-section-answer/

    Five years later the same author show how to cut angled tendons. Paul critiqued that, also.

    https://paulsellers.com/2017/05/re-easy-angled-tenons-article-fine-woodworking/

    #649023
    Benoît Van Noten
    Participant

    deleted
    double comment

    #649389
    Benoît Van Noten
    Participant

    about non-90° tenon, chair makers used a “boîte à recaler”.

    It is visible in this 1912 film of the “école Boulle” in Paris:
    https://www.ina.fr/video/VDD10045545

    At about 2’48” you can see one at each apprentice workbench.
    At about 5’40” you can see it in use.

    how to make one: https://www.furnituremaking.com/chairs-and-more/difficult-tenon-shoulders-made-easier
    As one can see, it needs a dedicated saw mounted in a plane-looking body.

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