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Thread: Grain orientation wedged round M&T

  1. #1

    Grain orientation wedged round M&T

    Does the grain direction of the tenon matter in a round wedged mortise and tenon? I know that the wedge obviously needs to run perpendicular to the grain of the mortise piece or youíll bust it open. But I wonder if the grain direction of the round tenon matters. Iíve never really paid any attention in my own limited work and oriented for best visual aesthetic. But this morning I saw Curtis Buchanan say that the grain of the tenon should be perpendicular to the mortise wood (didn't hear an explanation from him on why). So I did some google image searches and looked at way too many wedged M&T and it seems fairly random with most going parallel.

    So it makes me wonder, does it make any darned bit of difference? Are there any situations where it would be better to orient the tenon one way or another other than visual aesthetic?

    Here are a couple images I snagged off google to show what I'm talking about.
    rmtparallel.jpgrmtperpendicular.jpg

  2. #2
    Your picture on the left was the way I was taught when making Windsor chairs. Of particular note is the fact that the wedge is wider than the tenon and cuts into the sides of the mortise locking things in place.
    Dave Anderson
    Chester Toolworks LLC
    Chester, NH

  3. #3
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    The wood in the mortise shrinks and expands across the width of the round hole but not in the length (parallel to the grain).
    The wood in the tenon expands in both directions albeit at different rates ( flat sawn vs quarter sawn)
    So aligning the flatsawn grain of the tenon piece to the flatsawn grain of the mortise piece means that across the grain they would both shrink and expand at the same rate and the shrinkage and expansion perpendicular would be half of that in the tenon and zero in the mortise.
    So that would be the least stressful on the joint.
    So that should be the Walnut on the right.
    If you look at the sketch it shows how the wood in the tenon shrinks and expands compared to the wood that the mortise shrinks and expands. So it would appear that the best orientation is the way they are in the sketch.
    holes in wood.jpg
    Last edited by Mark Hennebury; 04-02-2021 at 12:33 PM. Reason: correcting mistake, and explanation and adding a sketch

  4. #4
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    I don't know if it matters because none of my things have failed, but the way I learned to do it is to orient the wedge so that neither the leg or the seat can be split by the wedge getting driven home.

  5. #5
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    Iím with Dave, keeping the tenon grain aligned to the mortice. The seasonal wood movement will match, it just plain looks better as well. The wedge grain if flat sawn can also match. The wedge orientation should be across the grain of the mortice so the compression is contained by end grain. You can drive it home without worry of splitting the mortice wood, especially near an edge.
    If the wedge end is cut off proud and you plane it down it can blend in very well.
    ​You can do a lot with very little! You can do a little more with a lot!

  6. #6
    Aesthetics notwithstanding, the important thing is to look at the grain on the side of the tenon. If all faces have no run out - as they should, it won't make much difference in the strength. The important thing is driving the wedge perpendicular to the grain in the mortised piece. Also, that second pic has the tenon too close to the edge for my taste.

  7. #7
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    woodsample.jpg holes in wood.jpg

    You cannot get the wood movement to match exactly between the tenon and the mortise. Wood is three dimensional. It only moves seasonally in two dimensions. The round mortise will shrink and expand across the width but not the length, the depth of the mortise ( thickness of the wood) would also change size.
    The tenon will shrink and expand across two dimensions , but the length of the tenon won't change.
    If you look at the first photo you can see that if the wood for the tenon is cut in the same orientation as the seat, the seasonal movement will match across the width. If you look at the second sketch you can you see that the seasonal movement of the tenon across the width ( horizontally in the sketch) will match the mortise movement.
    The tenon seasonal movement in the vertical direction in the sketch would be less, the seasonal movement of the mortise would be zero. So you would have some stress in that direction, but that would be the least stress for the parts oriented this way. If you rotate the tenon 90į you will have stress in both direction, the tenon will not keep up with the movement of the mortise across the width, and would have double the movement in the direction where the mortise would have zero movement, which could crush the cells and loosen the joint. All just theoretical, the glue may have enough flexibility to cope with the actual tiny amount of movement.


    Quote Originally Posted by William Fretwell View Post
    Iím with Dave, keeping the tenon grain aligned to the mortice. The seasonal wood movement will match, it just plain looks better as well. The wedge grain if flat sawn can also match. The wedge orientation should be across the grain of the mortice so the compression is contained by end grain. You can drive it home without worry of splitting the mortice wood, especially near an edge.
    If the wedge end is cut off proud and you plane it down it can blend in very well.

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    Mark you contradict yourself, the two photos have 90 degree opposite tenon configurations yet you say they both match mortice movement. In the second picture the grains are at right angles, therefore they do not match with their seasonal movement.

    The second dimension (depth) movement is largely irrelevant as the wedge works at the top but to no real depth.
    ​You can do a lot with very little! You can do a little more with a lot!

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    Hi William,
    I apologize if i haven't made things clear.

    If you look at the sketch below.

    Flat cut lumber it shrinks and expands twice as much across the width A. as in the thickness B, and zero in the length C.
    So in the example below, the seat has a hole drilled in it. In theory, and for example only, lets say the hole was 2" diameter. That's 2" in width A, 2" in length C. and 2" in Thickness B..
    Now lets say the wood swells in humid weather. For example only lets say the hole in the chair now is 2.2" across the width A, it will still be 2" in length C. and 2.1" in the thickness B..

    What size will the tenon be if it is put in like i have shown it?

    It will be 2.2" across the width A. the same as the mortise.
    It will be 2.1" in direction C . in contrast to the seat mortise which will still be 2" So some stress compressing the cells.
    It will be 2" long, the mortise thickness B. will be 2.1" again some stress.

    No try rotating it 90į and see what happens.



    mortise and two tenons.jpg

    Quote Originally Posted by William Fretwell View Post
    Mark you contradict yourself, the two photos have 90 degree opposite tenon configurations yet you say they both match mortice movement. In the second picture the grains are at right angles, therefore they do not match with their seasonal movement.

    The second dimension (depth) movement is largely irrelevant as the wedge works at the top but to no real depth.

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    Yes Mark I see what you mean, unfortunately for the appearance the most stable joint for moisture change is picture 2.

    However when you add the wedge the compression in line with the rings as in picture 2 will be shorter lived compared to compression across the rings as in picture 1. Because the wedge compression must be contained by long grain I see why Dave was taught to do it like picture 1.

    The well fitted wedge and long lasting compression are probably more crucial to joint strength over time than the wood movement in an uncompressed state. Wood aligned with the grain at glue up is also far stronger a glue joint than cross grain joints, another reason for Daveís taught method.
    ​You can do a lot with very little! You can do a little more with a lot!

  11. #11
    Thanks Mark for the visuals. I guess when you rotate the tenon, it all starts to make sense. So clearly the second picture matches the wood movement.

    But this brings up another complication. No. 2 matches wood movement, but MAYBE that’s not necessarily desirable. In pic no. 1 you have contrasting wood movement which has the potential to hold the tenon even tighter. When the M&T is at it’s assembly humidity everything is neutral. But once the humidity changes and there’s expansion/contraction, they will be opposing and (in pic no. 1) you will have increased pressure wither along the X axis or the Y axis. Is that necessarily a bad thing? That’s essentially what we are doing by driving the wedge in the first place – creating excess pressure along one axis. I suppose you could argue excess pressure along the X axis (cross grain to the mortised piece) could split the mortise wood, but I think that on something so tiny the wood movement would be so small it would be nearly impossible to split the board. I’ve never seen a round M&T split the M from wood movement. I would think that would be less pressure on such a small piece that it would be considerably less than driving a wedge. Which brings me back to “does it even matter?” I tend to think no. 1 LOOKS better.

    Regardless, I just did a stool with the grain aligned (like no. 1). I’ll let you know in 50 years if it splits LOL!

  12. #12
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    Hi Chris,

    I see the wedge as somewhat irrelevant to the shrinkage and expansion discussion. The wedge serves to snug the joint at assembly, from that point forward it, for all intents and purposes, it is part of the tenon and reacts as such.

    If you rotate the tenon 90į so that it has the biggest movement in the direction of the long grain of the chair seat, you will, when the humidity increase and the wood expands, the tenon will expand in line with the long grain of the chair seat, the chair mortise will not change size in that direction. If your mortise and tenon was tight when assembled, what happens when you have the tenon expanding in a hole that won't? Something has to give. What will that be?

    Take an end grain piece of wood and stand it on a long grain piece of wood and clamp it, or put it in a vice or hydraulic press, the end grain is far stronger than the long grain. the long grain cells will collapse.

    What will happen to your tenon? the cells could be crushed and collapse. When the next season rolls around the tenon could shrink to smaller than original due to some crushing of the cells., and your tenon could be loose.

    Also the tenon would shrink at a different rate across the grain on the seat.

    So it would seem to me to that to consider the potential movement and design to reduce the problems that could arise, would be the best choice.

    This was just a explanation from a theoretical perspective, based on the question you asked, how much, if any, practical value it has I don't know. I just found it interesting exercise and offered my opinion and the reasons for it.

    I am sure that your chairs will be fine, but I will check back in 50 years.
    Thanks for the question, it gave me something to think about.







    Quote Originally Posted by chris carter View Post
    Thanks Mark for the visuals. I guess when you rotate the tenon, it all starts to make sense. So clearly the second picture matches the wood movement.

    But this brings up another complication. No. 2 matches wood movement, but MAYBE that¬ís not necessarily desirable. In pic no. 1 you have contrasting wood movement which has the potential to hold the tenon even tighter. When the M&T is at it¬ís assembly humidity everything is neutral. But once the humidity changes and there¬ís expansion/contraction, they will be opposing and (in pic no. 1) you will have increased pressure wither along the X axis or the Y axis. Is that necessarily a bad thing? That¬ís essentially what we are doing by driving the wedge in the first place ¬Ė creating excess pressure along one axis. I suppose you could argue excess pressure along the X axis (cross grain to the mortised piece) could split the mortise wood, but I think that on something so tiny the wood movement would be so small it would be nearly impossible to split the board. I¬íve never seen a round M&T split the M from wood movement. I would think that would be less pressure on such a small piece that it would be considerably less than driving a wedge. Which brings me back to ¬ďdoes it even matter?¬Ē I tend to think no. 1 LOOKS better.

    Regardless, I just did a stool with the grain aligned (like no. 1). I’ll let you know in 50 years if it splits LOL!

  13. #13
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    Hi William,

    Sorry, but i have no idea what you saying.

    Quote Originally Posted by William Fretwell View Post
    Yes Mark I see what you mean, unfortunately for the appearance the most stable joint for moisture change is picture 2.

    However when you add the wedge the compression in line with the rings as in picture 2 will be shorter lived compared to compression across the rings as in picture 1. Because the wedge compression must be contained by long grain I see why Dave was taught to do it like picture 1.

    The well fitted wedge and long lasting compression are probably more crucial to joint strength over time than the wood movement in an uncompressed state. Wood aligned with the grain at glue up is also far stronger a glue joint than cross grain joints, another reason for Dave’s taught method.

  14. #14
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    Once the wedge is driven you canít assume itís just part of the tenon and normal seasonal movement will continue. The wood is now in compression, at least 15%. You also canít assume the compression is in one direction, on this scale as the wood is forced out the circle compresses each half also.

    Driving the wedge against the grain of the tenon will have the greatest resistance and longevity of the joint ie: the least damage of wood fibres. To avoid possible splitting of the mortise the wedge is driven to put pressure on the end grain of the tenon. If the tenon grain aligns with the mortice grain these conditions are met for the wedge.

    As for the glue joint in picture 2 it is essentially end grain to end grain all the way round, the worst glue joint.

    In picture 1 there is far more edge grain glue joint, a better glue joint.

    On this scale the compression of the wedge dominates.
    ​You can do a lot with very little! You can do a little more with a lot!

  15. #15
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    William, that makes no sense at all!
    15% compression.??????? If you drive a wedge, you snug up the joint brining them into contact, if you drive it further , you crush the cells.

    "As for the glue joint in picture 2 it is essentially end grain to end grain all the way round, the worst glue joint." makes no sense whatsoever! Maybe you need to look at what you are saying, how about doing a sketch to explain.
    Your entire comment makes no sense at all.

    Do some sketches, show the cell structure, maybe you can help me or you understand. Because I have no clue what you are saying.




    Quote Originally Posted by William Fretwell View Post
    Once the wedge is driven you canít assume itís just part of the tenon and normal seasonal movement will continue. The wood is now in compression, at least 15%. You also canít assume the compression is in one direction, on this scale as the wood is forced out the circle compresses each half also.

    Driving the wedge against the grain of the tenon will have the greatest resistance and longevity of the joint ie: the least damage of wood fibres. To avoid possible splitting of the mortise the wedge is driven to put pressure on the end grain of the tenon. If the tenon grain aligns with the mortice grain these conditions are met for the wedge.

    As for the glue joint in picture 2 it is essentially end grain to end grain all the way round, the worst glue joint.

    In picture 1 there is far more edge grain glue joint, a better glue joint.

    On this scale the compression of the wedge dominates.

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