Table Top Cracking - What Did I do Wrong

[JohnM Martin]

Would like some opinions on what I might have done wrong on this dining table top. It is constructed of solid walnut with bread board ends. The picture below shows a crack that has started (no doubt due to moisture changes) since the weather has started to change. It happens to be on the very edge of the table. I did my best to account for wood movement by gluing only the center tenon in the breadboard end construction. Since this is on the edge, there is no glue on this tenon and based on the location, it almost looks like it starts at the edge of the tenon underneath? Maybe I did not allow for enough side to side movement with the drawbore dowel?

Secondarily, is there anything I can/should do at this point to repair it and/or prevent it from getting worse? Note, it is finished with Arm-R-Seal.

397747

[brian zawatsky]

Perhaps the slotted hole thru the tenon for the draw bore pin was too tight, and as the wood shrunk it became bound up on the pin, thereby not allowing it to move laterally when it tried to shrink. That’s my best guess, anyway.

[Ken Fitzgerald]

John,

I don't know what to say about what caused the crack. It could be the wood naturally releasing tension or seasonal changes in humidity.

Personally, I'd wait until the crack has stabilized a bit and then make an "epoxy cocktail" used by turners to fill the crack. An epoxy cocktail is made by mixing epoxy and thinning it with DNA (denatured alcohol). When thinned it can be poured and will run into the crack. When the alcohol dries, it will harden although it's somewhat weaker I suspect. The hardened mixture can be sanded and buffed. I've used it on turnings.

Sorry for your dilemma!

[JohnM Martin]

Thanks, Ken, I'll give it some more time and see what happens. Thank you for this advice.

[JohnM Martin]

That's what I'm thinking as well. I did purposely widen those holes, but maybe I didn't get it wide enough. Sigh, what a bummer.

[Bill McDermott]

John, It's still solid. That's a minor cosmetic thing that only bothers you. I understand your quest to learn from the event. But I want to encourage you to enjoy that your solid joinery still serves and the crack only bothers you. Fill it with epoxy and enjoy the holidays without thinking about the crack. It's wood. Beautiful table. :)

[Prashun Patel]

That may not be a seasonal expansion crack. it may be a drying crack. That board looks like it goes straight through the pith where the crack is. If yes, then it might have been stressed even before you glued up.

[Charles Guest]

Drawbores can crack a piece as readily as gluing the tenons will. The drawbore pin needs to be tapered. When the wood shrinks and swells the pin moves up and down and will usually end up proud of the surface, but this movement prevents cracks. Never tap a proud, but tapered, pin back in flush and of course never glue it. Never use plain dowels as drawbore pins (caveat, see below), as they can lock in and make the cross grain situation worse than if it had been glued.

I'd drill it out, let it sit for a few months, and then put a tapered pin in.

A lot of auger bits were/are purposely made 1/64th over their nominal size. You can get away with plain dowels, maybe, if you've used one of these, and maybe even get away if you used a little bit of white glue on it. That said, the tapered pin is your best bet. As somewhat of an aside, Elmer's White Glue advertises that it's been made 'stronger' and I fear the flexibility a lot of woodworkers have counted on over the years may not be a reality now.

[JohnM Martin]

perhaps this is where i went wrong. I did use a standard dowel with an auger bit. Then i used a file to elongate the drawbore holes horizontally. Potentially stupid question... I'm very new to this, how do you make a tapered pin?

[Charles Guest]

perhaps this is where i went wrong. I did use a standard dowel with an auger bit. Then i used a file to elongate the drawbore holes horizontally. Potentially stupid question... I'm very new to this, how do you make a tapered pin?

Whittle them with your pocket knife. Or use a chisel and bench hook. S/B pretty much a straight taper from a relatively short straight section for apperance. You leave the top rectangular and untapered for a short distance, somewhat less than the distance from the top surface of the table to the thickness of the top mortise wall. If you make it untapered for the whole distance from top of table to the top of the tenon, you risk locking it in. It should still pull up tight. Drawboring requires more precision than a lot of people give it credit for needing.

If the pin ends up too proud during after a few years, you can trim its head a little with a chisel but as mentioned never tap it back flush.

To state the obvious, a crack almost always means something was too tight and/or constricted, whether glued, pinned, or both. The other causes could be wood too green, not allowed to acclimate to the shop, high humidity in the shop during the construction of the piece, breadboard too wide, lots of things.

[Flamone LaChaud]

Take a normal dowel and a small block plane, sharp chisel, or whittlin' knife (whichever you have or are most comfortable with - even sandpaper wrapped around a block of wood) and start at one end and slowly peel material away to make a taper. Doesn't have to be a lot - usually 1/16" taper will be sufficient - just enough that the wood will have a direction to push/escape. Think the end of a pencil, just not so abrupt.

[Simon MacGowen]

Would like some opinions on what I might have done wrong on this dining table top. It is constructed of solid walnut with bread board ends. The picture below shows a crack that has started (no doubt due to moisture changes) since the weather has started to change. It happens to be on the very edge of the table. I did my best to account for wood movement by gluing only the center tenon in the breadboard end construction. Since this is on the edge, there is no glue on this tenon and based on the location, it almost looks like it starts at the edge of the tenon underneath? Maybe I did not allow for enough side to side movement with the drawbore dowel?

Secondarily, is there anything I can/should do at this point to repair it and/or prevent it from getting worse? Note, it is finished with Arm-R-Seal.

397747

I am not buying any of this tapered dowel/pin as a cure at all. I've never used tapered pins; I use regular dowels just like any other furniture makers do, and I have not experienced any cracks or splits (Japanese woodworkers use tapered nails (bamboo), but not for draw boring purposes).

Gluing only the center tenon is not enough to deal with wood movement. Did you do the following?

Did you elongate the holes on the tenons?
Did you make the other mortises a little wider than the tenons?
Did you have sub tenons? If so, how wide and long were they?
How wide was the center tenon? If it was over 4" wide, you should not glue the whole center tenon.

If you did not take care of the above properly, probably that's why you're in trouble with the crack. There is no guarantee that this is the only crack you will experience. Time will tell.


Edit: I would not use epoxy to fix a crack. Inlay is my preferred method. But if you are going to use epoxy, use the flexible type (google to find it). But as others have suggested, don't do any fix until your table has undergone acclimation in its new environment (and if there are new cracks, you can fix them all at once). If it were my table, I would wait after a couple seasonal changes before fixing anything.

Simon

[Charles Guest]

The wooden pin is tapered just like the steel drift pin you use to snug the joint up in the first place. Snug up with a tapered drift pin. Drive a tapered wooden pin into the hole.

A drawbore will hold as tightly as glue, and produce the same cracking as a glued joint, unless the pin can wiggle just a little bit. A rived pin will also bend a little. The bend and the wiggle keep things from cracking. Otherwise, it's locked in as if it were glued. If you haven't experienced a failure, consider it a gift from the woodworking gods.

[JohnM Martin]

I am not buying any of this tapered dowel/pin as a cure at all. I've never used tapered pins; I use regular dowels just like any other furniture makers do, and I have not experienced any cracks or splits (Japanese woodworkers use tapered nails (bamboo), but not for draw boring purposes).

Gluing only the center tenon is not enough to deal with wood movement. Did you do the following?

Did you elongate the holes on the tenons?
Did you make the other mortises a little wider than the tenons?
Did you have sub tenons? If so, how wide and long were they?
How wide was the center tenon? If it was over 4" wide, you should not glue the whole center tenon.

If you did not take care of the above properly, probably that's why you're in trouble with the crack. There is no guarantee that this is the only crack you will experience. Time will tell.


Edit: I would not use epoxy to fix a crack. Inlay is my preferred method. But if you are going to use epoxy, use the flexible type (google to find it). But as others have suggested, don't do any fix until your table has undergone acclimation in its new environment (and if there are new cracks, you can fix them all at once). If it were my table, I would wait after a couple seasonal changes before fixing anything.

Simon

I think my error might have been in not elongating the dowel holes enough on the tenons. I did make the mortises wider for the sub tenons. I don't remember exact measurements.

[Andrew Hughes]

That may not be a seasonal expansion crack. it may be a drying crack. That board looks like it goes straight through the pith where the crack is. If yes, then it might have been stressed even before you glued up.

This is what I’m thinking you had a crack there from beginning. It’s still a good table that’s just building character

[brian zawatsky]

When I built my dining table a couple years ago, I drawbored the breadboard ends using dowels that were only very slightly tapered, if at all. I made the hole in the tenon 2x as wide as the diameter of the dowel. The top moves very noticeably in width from summer to winter and the drawbore does not restrict its movement at all.

[Pat Barry]

Do you happen to have a picture of the breadboard and tenons before final assembly and glueup? I dont buy either the theory of the dowel pin cause or the pith crack.

[Warren Mickley]

Your board is plain sawn. It cracked right at the most vulnerable spot, where the radial surface is perpendicular to the face of the board. The radial surface is the easiest to split, which is also why quartersawn walnut boards are prone to tear out. So the board was restricted from shrinking somehow as the humidity dropped and it split at a weak spot, not necessarily any relation to the tenon. This particular split is not near the pith, which for walnut is about 1/8 inch wide.

If the humidity drops more, the crack could get worse, but otherwise will not get worse on its own. I would not worry about it.

This is an example where work that is easily disassembled would be advantageous. If it has been made with repair in mind you can easily take it apart and see where the problem might have occurred. Tapered pins pop out somewhat easier when hit from the back side and actually in this case you would not have needed glue.

[Simon MacGowen]

If you haven't experienced a failure, consider it a gift from the woodworking gods.

I have not experienced any breadboard end failures because of the attention I paid to the holes, width of mortises relative to the tenons, and gluing surface in the center tenon. My continued success has nothing to do with the dowels themselves. I do not taper any dowels/pins; occasionally, I may chamfer very slightly one end of the dowels to ease the entry.

If the tenon holes are not properly elongated, tapered pins may reduce but not eliminate the risk of failure.

Simon

[JohnM Martin]

Do you happen to have a picture of the breadboard and tenons before final assembly and glueup? I dont buy either the theory of the dowel pin cause or the pith crack.

I'm not sure, but I will check my phone and see. Probably would be good to post here for learning purposes.

[Brian Holcombe]

I think this is due to not enough room for expansion.

[Andrew Seemann]

I think my error might have been in not elongating the dowel holes enough on the tenons. I did make the mortises wider for the sub tenons. I don't remember exact measurements.

My guess is that this is the most likely explanation. If I am visually scaling your picture right, you have about 12 inches between the two tenons in the picture. For flat sawn walnut, you could be looking at somewhere between 1/4" and 3/8" shrinkage between summer and winter, possibly more if the lumber was a little on the damp side when you built it.

My rough guide for wood expansion is to figure about 1/4 inch per foot from summer to winter for quarter sawn wood, and 1/2 inch per foot for flat sawn for summer to winter. I up this for less stable species like maple and birch, and sometimes cheat it down for more stable like oak and white pine. I'm also in a place with warm humid summers and cold bone dry winters, so seasonal movement can be extreme here.

When I do a joint like that, I drill it through, and then pull the pieces apart and then drill another hole (or two) adjacent to it in the likely direction of movement and chisel it out to a slot (spring and fall I go both ways). For summer and winter, I also remove some wood in the opposite direction of likely wood movement, in case the wood is drier or wetter than anticipated.

Wood movement is just one of those things you seem to have to learn the hard way. I know I have several pieces in my past that exhibited spectacular failures from not anticipating and accommodating wood movement.

[Charles Guest]

It's possible the tabletop would have cracked completely on its own even if set aside in your house and not mounted on the undercarriage.

That said, I'd still rive and taper pins in the future. Aesthetically I don't care for the perfectly round pin look, it screams "I'm a dowel," and you can avoid this easily if you make your own pins. You can whittle dowel stock to a rectangular head, but if you're going to the effort to do that you might as well make your own pins.