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Forum topic by Erik07 posted 02-17-2021 01:33 PM 363 views 0 times favorited 12 replies Add to Favorites Watch
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Erik07

14 posts in 82 days


02-17-2021 01:33 PM

Hello Everyone,

This is my first post here, but I’ve lurked for some time after google results frequently take me here. I recently completed a table base made from walnut with half-lap joints at each corner and what I believe are halved crossing joints for where the 2 rectangular pieces fit together. So that the table is collapsible, there are 2 bolts holding everything together where the pieces cross (top and bottom). I had left the halved crossing joints somewhat loose so that assembly wasn’t too difficult and to allow for any wood expansion.

Everything seemed fine until the glass tabletop was placed on top at which point it was obvious the whole thing is somewhat wobbly and twists a fair amount. I think it’s possible I left the middle joints too loose and the whole thing might be a bit too thin for the weight supported… pieces are ~1.5”x3”. Does anyone have any thoughts on where I went wrong?

Separately, I’m a bit confused about wood expansion. I have half-lap joints which should be very strong (so I’ve read), but have opposing grain directions… do these joints work out since one side of the wood is not restrained? Could the halved crossing joints have been left fairly tight without concern for wood expansion? Or is this joint different than the half-lap since the crossing pieces are restrained by the opposing grain direction?

I appreciate any input!


12 replies so far

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bilyo

1363 posts in 2181 days


#1 posted 02-17-2021 09:13 PM

The individual members of the legs are thin enough in both directions that wood movement should not be a issue. The half lapped joints should work just fine. The two “squares” that form the legs should be rigid enough to provide the support needed. I believe the problem is in the two center joints. When it is all assembled with the glass top in place, you should be able to observe where the “flex” or “give” is occurring. I think it is likely that all of it is happening at the two crossing joints. You must design that joint with two important features: I must fit tightly and the two halves must have enough surface contact to resist movement and breakage.

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SMP

3945 posts in 984 days


#2 posted 02-17-2021 10:45 PM

What I would probably do is get some of the same wood, 8/4. Cut a perfect circle, maybe 8-10” diameter. Then cut this into quarters. Attache these quarters to the top center section in a way that you can still knock it down, but so that these wedges prevent the legs from racking.

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Lazyman

6979 posts in 2466 days


#3 posted 02-17-2021 11:17 PM

Half laps are only strong when tight and locked together. If there is any room at all, it is not going to be solid. I would try to take some thin veneer strips or some other thin stock and wedge it in there to see if that helps. Even playing cards might work as a temporary shim just to see if that helps. if not, you may have to come up with some other way to lock everything together. With this design you may still get a little wobble even with tight joints.

As bliyo said, you don’t have to worry about wood movement on pieces only 1.5” wide.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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Erik07

14 posts in 82 days


#4 posted 02-18-2021 06:04 PM

Thanks for the responses! It seems the general consensus is that the middle joint is just too loose, which makes sense. The table is pretty sturdy except for when you try to sort-of rotate the tabletop- this causes the whole base to twist and flex. Next time I’ll just have to make that center joint tighter.

This leaves me somewhat confused about wood movement though. At what point, if ever, would that middle joint need room for wood expansion? It would seem to me that the wood would be constrained if it had a very tight fit and the wood expanded a little. Is there no problem since both pieces have the same dimensions? Whereas if 1 piece were 10x as big as the other, it would expand 10x as much, but the other piece would not be 10x as strong?

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bilyo

1363 posts in 2181 days


#5 posted 02-18-2021 06:27 PM



Thanks for the responses! It seems the general consensus is that the middle joint is just too loose, which makes sense. The table is pretty sturdy except for when you try to sort-of rotate the tabletop- this causes the whole base to twist and flex. Next time I ll just have to make that center joint tighter.

This leaves me somewhat confused about wood movement though. At what point, if ever, would that middle joint need room for wood expansion? It would seem to me that the wood would be constrained if it had a very tight fit and the wood expanded a little. Is there no problem since both pieces have the same dimensions? Whereas if 1 piece were 10x as big as the other, it would expand 10x as much, but the other piece would not be 10x as strong?

- Erik07


Wood expansion/contraction is a function of changes in temperature and humidity, species, and size. And, it occurs perpendicular to the grain. There are calculators on the web that help in making calculations based on those factors. In your case, the 1 1/2” dimension will simply not expand or contract enough to be concerned about. If you were cross lapping 6 or 8” boards, then there might be some concern.

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Erik07

14 posts in 82 days


#6 posted 02-18-2021 07:05 PM

Understood, do all woodworkers use these calculations every time they have a joint which has opposing grain directions? Is there any general rule of thumb I can use? I’m still quite new at woodworking and don’t yet have a moisture meter either.

I just want to make sure I don’t do the opposite and not allow for expansion on some other project.

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HokieKen

17541 posts in 2217 days


#7 posted 02-18-2021 07:51 PM

You have to look at the scale of your parts when considering expansion as well as paying attention to grain direction. If you have an 12” wide board and it will expand/contract by 10% across it’s width, then you have 1/8” movement and you have to be sure your base structure is such that it can accommodate that. But, if you rip that 12 board down into 2” strips, now you have pieces that will only move about 1/50” which is pretty negligible. And, like bilyo pointed out, the wood only expands in one direction for all intents and purposes.

In the case of your table, I would have made the bridle joints as tight as possible.

-- I collect hobbies. There is no sense in limiting yourself (Don W) - - - - - - - - Kenny in SW VA

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Lazyman

6979 posts in 2466 days


#8 posted 02-18-2021 08:16 PM

...and 10% expansion or contraction would be more in the range of a freshly cut tree to zero moisture content (aka as green to oven dry). Walnut is in the range of about 5-8% green to ovendry. Kiln dried wood would probably be more in the range 1-2% under normal indoor circumstances so mostly comes into play when working with wide panels such as for a table top. When attaching a 36” table top, a 1% movement would obviously be .36” and if you didn’t attach it so that it could move, it might be enough to buckle or even break apart.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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bilyo

1363 posts in 2181 days


#9 posted 02-18-2021 09:43 PM



Understood, do all woodworkers use these calculations every time they have a joint which has opposing grain directions? Is there any general rule of thumb I can use? I m still quite new at woodworking and don t yet have a moisture meter either.

I just want to make sure I don t do the opposite and not allow for expansion on some other project.

- Erik07


The answer is yes and no. The calculators and tables are always there to use when in doubt. However, after going through the process several times, you get experienced and develop a “feel” (as with most anything – practice makes perfect) and you just know where there is a likely problem. Also, studying joinery books by the masters will help immensely in developing experience and “feel”.
Another aspect of this is to remember that most furniture these days spend there whole lives in climate controlled conditions and are rarely subjected to conditions (extreme temp/humidity changes) that will cause problems. However, it is still a good plan to follow the “rules” just in case the piece has to spend some time in a non-climate controlled storage unit some day. Otherwise, your granddaughter may end up with a piece that has destroyed itself.

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Erik07

14 posts in 82 days


#10 posted 02-21-2021 06:25 PM

Thank you for everyone’s responses! I think this clears things up a bit.

Here’s a link of the twisting. It looks like I’ll just need to remake the base unless it looks like it’s not the loose middle joint causing the problem.

https://youtu.be/lJeuBVaM2K8

View bilyo's profile

bilyo

1363 posts in 2181 days


#11 posted 02-23-2021 02:20 AM

I think that even if the center joint was tight there is enough flex in the entire assembly that you will get that kind of “wiggle”. Frankly, I think that the whole assembly is physically and aesthetically just too light. If you want to keep that design and have it be more rigid, I would remake it using larger components. Even with that, I think that some lateral support between legs needs to be worked into the design.

Another option might be to make a pair of laminated circles with radii equal to the distance from the center to half way to the outer leg. Mortise these into the horizontal members and screw them from underneath. These will stiffen up the whole assembly considerably. You can test this by making some plywood circles and clamping them in place temporarily.

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Aj2

3827 posts in 2876 days


#12 posted 02-23-2021 04:34 AM

I looked at the video and I think some Newtons law might be in place.
Obviously your glass top out weighs the base.
My gut tells me your base need to approach the same density and weight if you want that design.
Good Luck

-- Aj

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