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Thread: Depth When Cutting a Mortise with Router

  1. #1
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    Depth When Cutting a Mortise with Router

    I just bought a whole range of Floating Tennons from Festool. Hardwood. They are part of Festool's Domino system.

    I'll be using my own router to cut the Mortise slots. I know from reading that the mortise should be fairly tight for the sake of strength. I've been told that to prevent 'hydraulic lock' there's need to provide room for excess glue.

    But how much end play should I allow on the depth?

    Gary Curtis

  2. #2
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    I have only just started using floating tenons and I'm sure someone who has done a bit of it will chime in and help us both. I try to leave about 1/64" extra depth in each hole. I wouldn't count on that being real accurate but 1/32" in each hole would be all I would want to add unless I hear different from our experienced panel of experts ;-)
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  3. #3
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    I don't do much floating tenon work but when I make a regular mortise and tenon, I don't worry if the mortise is a bit deeper than the tenon is long. I never measured it exactly but I'm sure I sometimes leave as much as 1/8" to 3/16". Sometimes less, of course. But I want to make *sure* my tenon doesn't bottom out. I don't think a bit of space at the bottom of a mortise affects the joint strength to any degree.

    I often drill out my mortises and with a brad point bit, you leave a small hole in the center of the bottom of the drilled hole. That makes some room for excess glue, also.

    Mike
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  4. #4
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    I'm with Mike. It doesn't hurt anything to go deeper. I usually go for 1/16-1/8" but don't really measure it. It's not like the end grain of the tenon is going to have much to do with joint strength even if you tried to make it perfectly mate with the mortise bottom.
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  5. #5
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    Wats a 'floating tennon'?

  6. #6
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    Quote Originally Posted by gary curtis View Post
    I just bought a whole range of Floating Tennons from Festool. Hardwood. They are part of Festool's Domino system.

    I'll be using my own router to cut the Mortise slots. I know from reading that the mortise should be fairly tight for the sake of strength. I've been told that to prevent 'hydraulic lock' there's need to provide room for excess glue.

    But how much end play should I allow on the depth?

    Gary Curtis
    Gary,

    I think end play is a good thing. Would it help to make a narrow channel in the face of the tenon to help the excess glue escape?

    Thanks,

    Bill

  7. #7
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    Frank, you see the concept in the illustration provided. It is not quite as strong in preventing 'racking' as a full tenon. But because the M/T joint is almost all long side-grain, it is at least 12 times stronger than a dowel. Dowels and the mating hole are (except for a narrow patch along the side) almost all end grain.

    The glue adhesion in any kind of a tenon bonds the tenon to the mortise as a sort of laminate. Like plywood, it is super strong. I just placed a large order with Festool, so I thought I would throw in a variety of their Domino tenons in various sizes while I was at it. I will use these a lot, cutting the mortise(s) with a router machine I have called a WoodRat. It acts kind of like a CNC milling machine, hand powered.

    Bill, the Domino plugs come from the factory grooved with channels lengthwise on the narrow dimension, and embosed with chevron glue slots on the wide sides. Dominos, though made of hardwood, are compressed much like Bisquit plates, so when coated with water-content glue, they swell in the joint, locking them in. And they're cheap, so why not Dominos?

    Gary Curtis

  8. #8
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    Here is a picture of a Domino. Made of compressed/solid beech.

    Gary
    Attached Thumbnails Attached Thumbnails Picture 1.png  

  9. #9
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    Gary,

    I've read and watched "Pros" using the floating tenons in fine and not so fine woodworking. The only critical aspect seems to be in the cheek dimensions as this is the long grain to long grain connection providing the strength. I was astonished the first time I saw the amount of excess room left on the curved sides of the mortise (what might make more sense as the ends of the mortise) which actually looked like an extra 1/4" on each end. (The thickness of the floating tenons for this particlular piece where 1/2" thick so this might be a scale issue as to how much extra could be left.) It was described as allowing alignment room . If one was using the actual Festool Domino for this type work, there is a setting on the tool which sets the first mortise at one end of the board to fit precisely with the domino size (both dimensions, not including depth of cut) while I recall another two settings that provide two different amounts of "alignment room" so even if the mating mortises are minutely off in the mating pieces it should still go together fine.

    I know this doesn't address your original question about the actual depth, just hoped it might add to the overall theme for you or anyone else ready this thread.

    Regards,
    Lee Laird
    Austin TX

  10. #10
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    Smile

    You are absolutely right Lee. When cutting a mortise with a router bit, you get the oval shaped ends. If left alone, and your tenons are cut from square stock, you will have slop on the sides that weakens the mechanical strength of the M/T joint. It doesn't change the glue strength, but reduces the resistance to torque forces.

    Two things can be done. You can square off the ends of the mortise with a chisel. Or, you can round off the tenons to conform the profile of the mortise. Either way, gives a tight fit in the lateral dimension.

    The square M/T looks nicer during construction. But, since nobody sees the inside of an M/T on a piece of furniture, I opted to buy the ready-made Dominos which already have oval-profiled sides (not the cheeks). They mate up with a Mortise cut using a straight router bit. It simply is not good to have side-to-side slop in an M/T joint. The Domino machine sold by Festool allows the user to 'dial in' the fit according to application. For a large plywood butt joint using Dominos, you would want some end-play in the mortises for assembly during the glue up.

    Gary Curtis

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