Properties of Wood

[kmealy]

Good resource and I use it often.

Someone posted a project on Facebook yesterday. A 6'x6' top dining table made from 2x6s and had "breadboard" ends without any movement allowance. Then was planning to pocket screw onto the base with 4 or 5 cross beams. A lesson in wood movement when that thing explodes. Going to be an expensive lesson in how to allow for wood movement.

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[Nickp]

Good resource and I use it often.

Someone posted a project on Facebook yesterday. A 6'x6' top dining table made from 2x6s and had "breadboard" ends without any movement allowance. Then was planning to pocket screw onto the base with 4 or 5 cross beams. A lesson in wood movement when that thing explodes. Going to be an expensive lesson in how to allow for wood movement.

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Using this table as the example, how would someone allow for movement...what should they have done...? Assuming they wanted the boards oriented as pictured...or is that part of the problem...?

[kmealy]

Using this table as the example, how would someone allow for movement...what should they have done...? Assuming they wanted the boards oriented as pictured...or is that part of the problem...?

These are called "breadboard ends." The usual way to accomplish this is a series of tenons in the long boards and over-width mortises in the ends. You glue only the center M&T and peg the rest in oval holes. Now, seasonally the end boards are either going to stick out too far or be a bit recessed. The other choice is to do tongue and groove or shiplap across the long boards. But that creates cracks that accumulate crumbs on a dining table.

I once had a customer ask me why their table had this step in the ends like that and I had to explain wood movement.

https://www.woodmagazine.com/woodworkin ... breadboard

\https://www.popularwoodworking.com/tech ... rd-ends-2/

[Cherryville Chuck]

Besides what you said Keith when you mark the holes for the dowels you should offset them 1/32" so that when the dowels are driven in it pulls the end tight to the table. One of the cheap tools that is handy for marking the center of holes is a set of transfer punches which come in a set like drill bits do. I bought my set for under $20 from Lee Valley but Harbor Freight also sells them for around $13 I think.

[Alex]

Wood moves seasonally in width, but not so much in length. It can also cup widthwise, which can lead to a table surface that could have a gentle curve, or maybe ripples - depending on the orientation of the growth rings in the final glue-up. The point of breadboard ends is to do away with that.

But, since the ends have a grain direction perpendicular to the rest of the table, you have to allow for movement. As kmealy noted above, sometimes the table sides will stick out, and sometimes the end will. The table where I am sitting now is made of southern yellow pine. (I made it a couple of years ago.) Right now, in December with its dry air, the breadboard end is sticking slightly out. In the middle of a humid summer, the center boards will be wider.

IMG_1640.jpg

Here is a construction detail from the table. It is 36" wide, and I put 5 tenons across its width. They all got pegged, but only the center tenon is glued. The other tenons are held in their mortises just by the pegs (these do have glue to hold them in place), and again, the holes in the tenons are oval-shaped to allow the tenons to move seasonally across the pegs. As noted by Chuck and kmealy, the holes can be offset a fraction, to pull the end in to the table. You can also make a 'sprung' joint by planing off a tiny amount, increasing toward the center of the breadboard end. This is to keep tension on it, to keep it pulled tight to the rest of the table.

breadboard end tenons.jpg

adjusting tenons.jpg

IMG_1641.jpg

done!.jpg

Alex

[kmealy]

good photo essay, Alex

BTW, the rule of thumb is that wood moves negligibly in length. It moves roughly twice as much around the annual rings (known as tangentially) as it does from the center out (known as radially). That explains why when you see a firewood log that's dried, it opens a crack like a piece of pie was removed. The actual amount and difference varies by species.

The original reference I think was https://www.fpl.fs.fed.us/documnts/fplg ... gtr190.pdf Just about anything you ever wanted to know about wood

[kmealy]

good photo essay, Alex

BTW, the rule of thumb is that wood moves negligibly in length. It moves roughly twice as much around the annual rings (known as tangentially) as it does from the center out (known as radially). That explains why when you see a firewood log that's dried, it opens a crack like a piece of pie was removed. The actual amount and difference varies by species.

That's also why quarter- and rift-sawn wood is more stable. It's not going to cup and the most variance is in thickness, where it seldom matters. And why dowel joints loose strength over time. I took a "Make a Chair from a Tree" class from Drew Langsner and every piece in that chair was grain oriented in a way to minimize negative effects of wood movement.

The original reference (since deleted) I think was https://www.fpl.fs.fed.us/documnts/fplg ... gtr190.pdf Just about anything you ever wanted to know about wood

[Alex]

Thank you.

As for your original post about “an expensive lesson in how to allow for wood movement”: there are standard ways to do so many things in carpentry, like build tables. Some of these things have been around for centuries. A little bit of research would have helped your Facebook table maker avoid problems down the line.

I had my own problem – mentioned in another thread here – about a chessboard table top warping because I only glued the pieces to one side. I should have done my research!

Alex

[Cherryville Chuck]

One thing I found that helps control warpage is to groove the backside of the wood where no one will see. I like to go about 1 to 1 1/4" apart and about 1/4 to 1/3 of the thickness. That breaks up some of the tension in the wood. I learned that from a local mill that makes solid wood siding. They used a molder that put in rounded grooves like a round nose bit would (and not nearly that deep). I just make grooves on the table saw.

Besides taking wood movement into account for breadboard ends it should also be taken into account for attaching the top to the support frame. I usually use Z clips for that and usually get them from Lee Valley but I think I've gotten them from Windsor Plywood too. The LV ones: https://www.leevalley.com/en-ca/shop/ha ... em=13K0101 You cut a groove in the apron pieces with your TS before assembly. I like to go slightly wider than the instructions call for to make sure I snug the top to the apron. You can always leave a slight gap under them if the screw gets really tight before the clip bottoms out.

They also make an elongated washer which will work too but I've never used them: https://www.leevalley.com/en-ca/shop/ha ... on-washers Drive a round head screw in the center and it can expand or contract.

[Alex]

For attaching table tops, I like to go 'old school': blocks attached to the aprons, with an elongated hole in them to allow for top movement.

I readily admit that Z-clips would probably be easier and faster. But they add an element (modernity? mass-production?) to the work that I don't like. I would rather go for the more timeless blocks and screws.

IMG_1656.jpg

Alex

[Cherryville Chuck]

A friend of mine agrees with you. He mills a strip on his table saw and then cuts pieces off at the chop saw. You need a wider groove to fit them into on the apron so a single saw cut isn't wide enough for wooden ones. I would change one thing on yours though and mill them so that you are screwing through flat grain instead of edge grain.

[kmealy]

... I would change one thing on yours though and mill them so that you are screwing through flat grain instead of edge grain.

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[Alex]

It took me a minute to get that. I didn't realize that they were for a groove in the apron. I just attached mine to the apron with two screws, and left an oversized hole through which I screwed them to the top.

But yeah - flat grain.

Alex

[Cherryville Chuck]

Sorry Alex, I couldn't tell that there wasn't a groove in the apron. I'm not sure exactly how my friend does it but I would just take a srip of wood about 3/4" thick and 1 1/2" wide and rabbet one side so that it leaves a lip about 3/8 to 1/2" wide by 1/4" or slightly less thick. Then you can chop strips off that and drill for the screw holes. You want the groove in the apron wide enough for a slip fit with the cleat.