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Thread: Mortise depth vs tenon thinkness

  1. #1
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    Mortise depth vs tenon thinkness

    I am making a mahogany screen door and will be doing mortise and tenon construction of the frame.

    After milling, all of my material is 1 1/4" think and 3 1/4" wide--it is gong to be a solid door.

    In thinking about my mortises, I can either do a 1/2 wide mortise that is 1 1/2 inches deep (maybe end 2 if a max out the router bit) or do a 3/8 mortise 1 1/4 deep (or at least that is the cut depth on the bit).

    The 1/2 mortise gets me a longer, thicker tenon, but much thinner checks/sidewalls. The 3/8 mortise is more in line with the traditional rule of thirds for a mortise and tenon, but does not get me the depth that will add strength to the frame.

    Any advice or am I over thinking this?

  2. #2
    Fine Woodworking looked at tenon thickness & strength in racking; they found a 1/4” tenon broke at 717 lb, 5/16” at 988 lb & 3/8 at 1475 lb in 3/4 inch stock (non-though tenons). So from my simple perspective thicker tenons are better.
    Just a Duffer

  3. #3
    If it's good real mahogany, it's strong , so I would use the deeper mortise. I would also use a chisel to shave a little back bevel on the end grain cheeks . Would use glued mahogany pegs all the way through.

  4. #4
    Pegging does not increase strength and I believe it is unnecessary with modern glues. The practice started when glues were much less reliable. I do not do it.

    I saw the same article but could not remember where. It seems maximum strength comes from tenons 1/2 of the wood thickness instead of the normally used 1/3.

    The OP's case would potentially involve tenons bigger or less than 1/3. I would go bigger than 1/3 - the 1/2 inch.

  5. #5
    Quote Originally Posted by Jim Dwight View Post
    Pegging does not increase strength and I believe it is unnecessary with modern glues. The practice started when glues were much less reliable. I do not do it.

    I saw the same article but could not remember where. It seems maximum strength comes from tenons 1/2 of the wood thickness instead of the normally used 1/3.

    The OP's case would potentially involve tenons bigger or less than 1/3. I would go bigger than 1/3 - the 1/2 inch.
    This is true, pegging doesn't inherently increase strength of a joint, but it does add a fail-safe in case the glue does fail, and pretty much ensures the joint will last forever. Depending on the location of this screen door, if it's going to have some exterior exposure, it might add some security for the joint. It also comes in handy for "difficult-to-clamp" situations, or if your clamps are too short.

    I would also lean towards the 1/2" tenon.

  6. #6
    Keep in mind that once all four mortise and tenons are glued and assembled it will be plenty strong enough no matter if you would use 1/4" or 1/2" tennons. Same with the depth, 1 1/2" deep or 1 1/4" I don't see a great advantage.

    That being said I would do what ever is easier for me. That would be my 3/8" for me.

  7. #7
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    Quote Originally Posted by Todd Mason-Darnell View Post
    I am making a mahogany screen door and will be doing mortise and tenon construction of the frame.

    After milling, all of my material is 1 1/4" think and 3 1/4" wide--it is gong to be a solid door.

    In thinking about my mortises, I can either do a 1/2 wide mortise that is 1 1/2 inches deep (maybe end 2 if a max out the router bit) or do a 3/8 mortise 1 1/4 deep (or at least that is the cut depth on the bit).

    The 1/2 mortise gets me a longer, thicker tenon, but much thinner checks/sidewalls. The 3/8 mortise is more in line with the traditional rule of thirds for a mortise and tenon, but does not get me the depth that will add strength to the frame.

    Any advice or am I over thinking this?

    I make doors with through mortise and tenons. The stiles need the extra wood. I would use a 3/8" through tenon, cut from each side of the stile. You can get end mill bits plenty long enough, as long as they don't turn into flying steel spaghetti when you turn the router on. 3/8" x 6" seems a little out there, but Amazon has US made ones.

    https://www.amazon.com/FLUTE-EXTRA-L.../dp/B07DJ5MS7X

    For pegs I use Spanish Cedar 1/4" square pegs, tapered, glued in place, square side on the interior. Pegs want to work their way out to the exterior. The need glue. People use dowels, it looks sad in a few years when there are all these pegs sticking out.

  8. #8
    Quote Originally Posted by Jim Dwight View Post
    Pegging does not increase strength and I believe it is unnecessary with modern glues. The practice started when glues were much less reliable. I do not do it
    I think the practice started before any glue was used on most doors. I've seen old doors taken apart by removing the
    pegs. A lot of old doors had thru mortises and wedges ,in that case I agree pegs are not needed.
    Last edited by Jim Becker; 09-16-2020 at 2:53 PM. Reason: fixed quote tagging

  9. #9
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    The advice already given is very good!
    Just looking at the data Harvey posted from FWW (I don't know how the load was applied but lets assume it was supported on both ends - that's 350# per joint for the 1/4"). That's a lot! There is a minimum but if you draw the line at 1/4" then that's a lot for a screen door per joint.
    My advice - make them what matches your wood and tools with a 1/4" as the minimum. The joint will last - now if your 700# cousin Eddie hangs on it then you'll be glad to know that the screws in the hings were the first thing to pull out!

  10. #10
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    Using the finding of maximum racking strength at half the thickness of the mortised member, I would use 5/8" thick tenons, at least on the top and bottom rails (i.e. the corners).

    For mid-span rail(s), you might go with 1/2" thick tenons, to preserve more bending strength in the stiles at the mortises.

    -- Andy - Arlington TX

  11. #11
    [QUOTE=Mel Fulks;3055112]
    Quote Originally Posted by Jim Dwight View Post
    Pegging does not increase strength and I believe it is unnecessary with modern glues. The practice started when glues were much less reliable. I do not do it

    I think the practice started before any glue was used on most doors. I've seen old doors taken apart by removing the
    pegs. A lot of old doors had thru mortises and wedges ,in that case I agree pegs are not needed.
    I suspect you are right. The mortices and tenons on my old house's bay window from 1909 were pegged, which make sense, since they didn't have any good exterior glue at that time. I suppose they could have hide glued it and it dissolved by the time I removed and replaced the window 15 years ago.

    If I use pegs, it is either to aid assembly (no clamps needed) or decoratively.

  12. #12
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    I think 50 -60% of the door thickness is desirable. Why don't you play around with it. Let us know what you settle on.
    I will make test joints tomorrow.

  13. #13
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    It is normal to make with the bottom wider than the top, maybe make it 6" wide, then your tennons will be way more than needed.

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