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  1. #1
    Join Date
    May 2006
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    Default Tenon length of table rails into 100x100?

    Hi there,

    I'm in the process of designing a table with M&T joints. I have top and bottom rails on a 4 sided table, so I will be having to make 16 joints. The rails will be 75 x 19, and the legs 100x100, with the rails having a slight (5mm) offset from the front of the leg. Rule of thumb suggests the tenon depth to be 2/3 of the mortised piece - i.e 66mm. I know this is subjective, but this seems like a fair sized tenon - do you think you need this depth for strength? Timber is spotted gum. Method for cutting mortises is a hammer and a sharp chisel...

    Cheers,

    Tom

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  3. #2
    Join Date
    Jan 2004
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    686

    Default

    I'd go with the rule of thumb, Tom, using secret haunch tenons.

    This will make your work a little less and give you mortices instead of bridle joints.

    They're not difficult to cut by hand. Don't believe everything you read, just keep the chisel sharp, this being the key difference between doing the job slowly and getting it out of the way quickly. You should be able to clear about 3mm per stroke into the mortise, on a mortice about 50mm wide, this would be about 10-15 strokes, making rate of progress about 10mm/min, with about 1 minute fiddling time per mortise and about 3min sharpening and setup time.

    Cutting time for the mortises should therefore be about 4 per hour.

    Overall, if this was me quoting, I'd be looking at allowing 12 hours from machined stock to finish mortices on all 16, including cutting time and fitting/fiddling/trimming time.

    Cheers,

    eddie

    edit: I'd probably cut barefaced tenons about 3/8" thick or so -> makes fitting easier

  4. #3
    Join Date
    Oct 2005
    Location
    Yass
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    1,196

    Default

    IMHO, a 66mm tenon sounds excessive. You've got some substantial legs there as part of the design, I presume, rather than because that strength is required. With proper glueing, I'd be surprised if there were much difference in strength between, say 25mm and 66mm once the piece is built. And that difference is unlikely to come into play unless you're planning for all your (and my) overweight rellies to dance on the table at once. The stretchers certainly would not need that kind of strength.

    Over the last several years I've been going smaller and smaller on tenons, finally moving to floating tenons which in your situation I would make about 20mm deep. So far everything is holding together very well. Perhaps others could comment on their experience. I believe we have all been over-tenoning.

    Regards,

    Tex

  5. #4
    Join Date
    Jun 2006
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    Macedon, Victoria.
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    Default

    The biggest stress on the tenons is turning the brute over (if you work alone). If it survives that, it'll be more than enough for routine stresses.

    Bill
    Chipslinger

  6. #5
    Join Date
    Jun 2005
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    Default

    Hi Tom,

    I'd agree with both Tex and Billrule. I've made a couple of benches with 100x100mm legs using about 75mm or less rails. and Since you have both top and bottom rails, 20-30mm should be more than adequate. When cutting the tenons, I leave a 10mm shoulder and cheek and this seems to give a very secure joint.
    As Bill says, if you can turn the beast over, you're not likely to have problems.

    Regards,

    Rob

  7. #6
    Join Date
    Mar 2007
    Location
    Michigan
    Posts
    4

    Default

    That's why they call it a general rule.......it's general.

    One of the problems that I have encountered is that I like to center my stretchers on the leg which automatically rules out the general rule. A tenon can only extend 2/3 in one side. When placing a stretcher 90 degrees in the same leg, that tenon can only extend 1/2 - 50% of tenon thickness.

    Most of the tables that I build are mission style and I build a four sided leg using a locking 45 router bit. Because my legs are hollow, the material thickness dictates how long the tenons need to be (usually 3/4"). I've built plenty and not had an issue yet.

    As long as you start with a joint that has a good line to line fit you shouldn't have a problem.

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