Loose/floating tenon in tabletop - Woodworking Talk - Woodworkers Forum
 
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post #1 of 17 Old 03-14-2015, 05:35 PM Thread Starter
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Loose/floating tenon in tabletop

Hello. I need a bit of advice. I'm designing a coffee table based on some loose plans I found online. The table looks like this:



The original plans had the tabletop screwed in from underneath with pocket holes. I was trying to think of a way to replace the pocket holes with a different joint and thought a floating tenon might do. So I designed in the floating tenon like this:



I'm worried that the different grain directions will cause a problem, though. The grain will line up with the rail on the bottom of the joint, but if the top has the grain going lengthwise then the tenon will be perpendicular to the top grain. I'm not sure if that will be a serious problem, though. Can anyone give me some advice? Should I maybe shape the joint differently, or use a different kind of joint?

Thanks for any help you can give.
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post #2 of 17 Old 03-14-2015, 05:59 PM
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You are correct, you can't glue together wood in perpendicular directions. As shown in the top picture you could screw the board underneath provided you elongated the screw holes to allow for shrinkage. Then the screws should only be tightened barely snug.
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post #3 of 17 Old 03-15-2015, 03:52 AM Thread Starter
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The thing that really has my scratching my head is that I don't see how a regular mortise and tenon joint isn't an example of wood being glued together perpendicularly. Isn't it? And if so, why is that different than this and how does that allow you to not worry about wood expansion and contraction? Or do you still account for it in a normal M+T joint?
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post #4 of 17 Old 03-15-2015, 08:49 AM
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The board running under the top you don't want to mortise and tenon it to the top and you especially don't want to glue it on. This will certainly prevent the top from moving and the top will split to releave the pressure. The rest of the joints are fine using a mortise and tenon. On a large surface like the top each board is only going to shrink maybe 1/32" and the accumulated shrinkage of maybe 5 boards on the top would be closer to 5/32. This is a lot of movement but on the base each board is going to shrink so little it doesn't hurt.
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post #5 of 17 Old 03-15-2015, 07:34 PM Thread Starter
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Okay, I think I understand the problem, but I'd like to share my thoughts so someone can confirm for me.

This is what I see. The top surface and the bottom rails have grain going in the same direction. That means that the middle of the table should shrink the same amount on the bottom and top:



So even if the shrinkage on the top is pulling the sides of the table in, the same is happening at the bottom, so it shouldn't cause stress. But there would be stress at the joint itself. I think the bottom rail is a more usual joint:



I have it set up here as a wedge mortise and tenon, but I don't know if I actually designed it right so let's pretend it's a through mortise and I glue the tenon in. The glue contact surface looks like this:



In the rail, this 1" will shrink however amount inside the mortise, while the mortise's grain is perpendicular so it won't shrink in the same direction. But from what I understand, a joint like this is fine and can just be glued and there's no problem.

The top, meanwhile, has a contact surface that looks like this:



(Sorry about the crappy view, but I wasn't sure how to visualize it well). The grain of the top is going in the same direction as the 1/2" axis of the tenon, so if 1" is fine from a shrinkage perspective in the bottom rail, I would suspect that 1/2" is fine in the top. But the rail supporting the top has its grain going in the same direction as the 4" axis of the tenon, which is the same grain direction as the tenon. That's great for the tenon and rail, but that means the tenon is shrinking perpendicular to the top, along a 4" contact surface. I can see why 4" might be a lot, but I've seen really large glue-ups of joints this size. For example, you can see in this episode of New Yankee Workshop (https://youtu.be/g2j9Of6Hbrg?t=4m43s) Norm builds a door using floating tenons roughly this big.

But even if one of those 4" tenons is okay in the top (which I'd still like confirmation of because I'm not sure) there's more to the story, because the upper rail supporting the top connects the 2 tenons:



So while each 4" tenon might be fine (which needs confirmation), there is actually a 13" section undergoing shrinkage, and 13" might be too much? Is that an accurate assessment?

Thank you so much for helping. I know this is a lot of pretty basic stuff I'm asking.
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post #6 of 17 Old 03-15-2015, 07:45 PM
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It's not the length of the table top that shrinks, it's the width. Any time you build anything with a large piece of solid wood you have to allow for the wood to shrink in width. On another thread someone made this table and glued boards across the ends and screwed boards on the underside preventing it from shrinking. In order for the wood to releave the stress the individual boards shrank breaking the glue joints and cracking. For this reason I wouldn't mortise and tenon and especially not glue a board under your top.
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post #7 of 17 Old 03-15-2015, 09:23 PM Thread Starter
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Quote:
Originally Posted by Steve Neul View Post
It's not the length of the table top that shrinks, it's the width..
Ohhhhh, wow, I don't know why I thought it shrunk along the grain. I'm sorry about that. I'm going to go do some reading about how and why wood shrinks so I can better design this. It really didn't occur to me that that was what I got wrong.

Thank you! I'll make a new post when I have a new design.
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post #8 of 17 Old 03-15-2015, 09:52 PM
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It's the moisture content of the wood. Wood shrinks a lot after it is first cut from the tree but as it ages the amount of shrinkage is less the more it dries but it never quits. Unless you have a moisture content meter you also don't know if the wood you buy is properly seasoned. Also you can take a piece of furniture from one part of the country that is damp to an arid part of the country and it will start shrinking again. I inherited an oak table that was at least 100 years old that the wood was in perfect condition. I moved the table from Illinois to Texas and within a few months the table split in more than a half dozen places.
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post #9 of 17 Old 03-16-2015, 12:03 PM Thread Starter
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That makes sense. Yeah I knew it had to do with moisture but just not why. I just did some reading and figured out how moisture in the wood results in shrinkage as its lost.

I also read about tangential vs radial vs longitudinal shrinkage. That was the big "wow I didn't realize how ignorant I was" moment. So I understand it all way better now.

And I think I understand my concerns better now too. My design doesn't have an apron/skirt. That means the tabletop has to provide the stability against the table collapsing. What I've read about methods of attaching a tabletop sound loose, which is great for shrinkage but not stability. If someone could confirm for me that elongated screws still provide stability then I'll use those at this point, since I like how it looks without an apron, but otherwise I guess I'll add an apron.
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post #10 of 17 Old 03-16-2015, 02:00 PM
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Quote:
Originally Posted by ditzy View Post
That makes sense. Yeah I knew it had to do with moisture but just not why. I just did some reading and figured out how moisture in the wood results in shrinkage as its lost.

I also read about tangential vs radial vs longitudinal shrinkage. That was the big "wow I didn't realize how ignorant I was" moment. So I understand it all way better now.

And I think I understand my concerns better now too. My design doesn't have an apron/skirt. That means the tabletop has to provide the stability against the table collapsing. What I've read about methods of attaching a tabletop sound loose, which is great for shrinkage but not stability. If someone could confirm for me that elongated screws still provide stability then I'll use those at this point, since I like how it looks without an apron, but otherwise I guess I'll add an apron.
I don't know the overall dimensions of the table but the top appears to be about 1 1/2" thick or more. This should be sufficient for a table up to about 5' long to not have a skirt.

In order to make it more stable you might make these boards a little wider and put mounting screws into the top on each side of the leg. You could even countersink the area where the screws go so they don't show from the end unless you got down on your hands and knees and looked up at it. If you elongated the holes through these boards and used a pan head screw with a washer and not tighten them up super tight it should allow the top to shrink and still be tight enough the table would be stable.
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post #11 of 17 Old 03-16-2015, 04:03 PM Thread Starter
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Thank you, Steve Neul! This is a coffee table, so it's not quite 5'. I'm at work so I don't have the SketchUp with me, but I think it's 4' long and 1 1/2" thick. I'm glad I won't need a skirt ^_^. I'll play with the thickness of those rails and see how I can fit mounting screws on both sides of the rails. Having a screw on the inside and outside of each rail is a much more comforting thought, stability-wise, than just a single screw.

Thank you!
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post #12 of 17 Old 03-16-2015, 06:49 PM
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Quote:
Originally Posted by ditzy View Post
Thank you, Steve Neul! This is a coffee table, so it's not quite 5'. I'm at work so I don't have the SketchUp with me, but I think it's 4' long and 1 1/2" thick. I'm glad I won't need a skirt ^_^. I'll play with the thickness of those rails and see how I can fit mounting screws on both sides of the rails. Having a screw on the inside and outside of each rail is a much more comforting thought, stability-wise, than just a single screw.

Thank you!
This is what I was thinking.
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post #13 of 17 Old 03-16-2015, 07:26 PM Thread Starter
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One thing I was thinking is that I might not like the step that that creates from the top down to the legs, but I can play around with tapering them or trying some other things. Thanks! I'll post the finished design idea when I come up with something I like.
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post #14 of 17 Old 03-16-2015, 11:58 PM Thread Starter
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Okay, so I tried a few approaches to creating a larger mounting width, but none of the ways I tried were particularly visually appealing. I still found myself really liking the original width of the rail, visually. But I did add the mounting holes as you suggested. I didn't elongate them in the design, but I will in the rail for the actual table. Also, you may want to open the full sized image for these images so you can actually read the dimensional markings.

Here are the dimensions of the table:


And here are the mounting holes with dimensions, viewing from underneath:


And just for good measure, here's a wider shot of the mounting holes looking from on top so you get a sense of the spacing:


The pilot holes in the design are 1/8" dia. and 1 1/2" long for #8 wood screws with, hopefully, a 3/4" shank. I need to confirm that I can get a screw like that but I can adjust the design for a shorter shank as needed.

What do you think. If I use elongated screw holes on the rail and include a screw on each side of the rail, will it be enough for the table to be sturdy?
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post #15 of 17 Old 03-17-2015, 12:59 AM
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You should be fine with the new design. Honestly, a single screw in the middle of each stretcher is enough to keep the table sturdy, so long as you dont try and lift the table by the top. Personally, id go with 2 screws per stretcher instead of 4. The more screws you have in an area the more likely you are to split the wood

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post #16 of 17 Old 03-17-2015, 10:15 AM Thread Starter
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Damn, I hadn't considered that. But I do find it very likely that the table will be lifted by the top. It's a coffee table, so it's probably going to be moved around more often than a bigger table might, over the years. With 4 screws in each stretcher do you think it would be safe to lift by the top? I think I'd risk the splitting for that; while I don't want this table to break it any way, I recognize that I have a lot to learn about woodworking and am willing to learn some lessons the hard way.
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post #17 of 17 Old 03-17-2015, 10:52 AM
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If it were me I would put six screws in each leg. It's not so much you need it for lifting or moving the table since there is no skirt on the table the top will be prone to cup warp and the additional screws will help prevent this. Also when you finish the top you should plan on finishing the underside of the top. Left unfinished the underside would be able to absorb moisture from the air and cause it to warp.
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