physics and engineering question

[llucas]

So I am experimenting with differing occasional table designs and have built a prototype in walnut and glass
The glass sits passively in a recess as seen in the pic...at least it sat passively in the recess...not so passive now as the glass is firmly wedged in the opening...the table was moved from the shop to a conditioned space a month ago and as expected the moisture content went down and changed the fit. The fit is tight in CD and AB dimensions.
We all know what they say about assumptions....but I assumed that as it dried that the change would be to make the hole slightly larger in the C-D dimension with normal contraction. I have not checked the moisture content, but would be surprised if it is not less than when the table was built.
OK wood wizards...Why is the glass firmly wedged in the hole now.

 

[JNCarr]

When you make an object larger for whatever reason (moisture in wood, heat on metal, etc.), the hole in it gets larger also. When you make the object smaller (cooling metal, drying wood) the hole gets smaller. As an example an interference fit bearing is heated to fit onto the shaft - that is the hole gets larger.
It's a bit counter-intuitive.

 

[Oka]

What Joe said ........... and moving things like the open can shift or shrink unequally which also causes binding.

When 1st fabbing, you could have added felt, cloth or a plastic equal to the edge of the glass when 1st fitting, which then allows more fit flexibility later.

 

[llucas]

Ah yes! If you draw a circle on a wet board then let it dry for a while, as the board shrinks the hole gets smaller. I was incorrectly thinking the table top was like wood plank flooring with the cracks getting bigger with drying.
My design which tied the two halves together with the cross pieces essentially turned the top into a single piece again.
Ok next time I will plan on that shrinkage....prolly more in the AB direction that the CD direction...ie the hole would likely get slightly oval as it changed size.

 

[Rwe2156]

Adding to the above, if the wood did not have those 3 sticks or the sticks were not glued in and the top was anchored at the perimeter, all the movement would be away from the glass.

 

[Willemjm]

As a rule of thumb, I use 1/8" per 12" movement from summer to winter, ACROSS the grain.

In your case that would be the CD dimension.

The tenon pieces won't move as that is along the grain.

So, across the centerline of your CD dimension, there would be a small reduction in diameter. On the edges, your tenons should prevent that, causing stresses across the center of the inner diameter.

BTW, cool design, looks great!!!

 

[CharlieGator]

It is a very nice design!
Just needed to add an allowance for the wood movement.

 

[ChemE75]

Ah yes! If you draw a circle on a wet board then let it dry for a while, as the board shrinks the hole gets smaller. I was incorrectly thinking the table top was like wood plank flooring with the cracks getting bigger with drying.
My design which tied the two halves together with the cross pieces essentially turned the top into a single piece again.
Ok next time I will plan on that shrinkage....prolly more in the AB direction that the CD direction...ie the hole would likely get slightly oval as it changed size.

Can’t see leg attachment method but maybe if the the two halves were tied together by the “undercarriage”, the solid A-B ties could be eliminated. Or maybe a semi blind sliding dovetail shape for the 3 ties would allow movement without showing gaps at each end when shrinking.

 

[Wiley's Woodworks]

Luke--You don't say if the binding in place of the glass is a problem or not. I'm just curious. If it is there's a simple solution. Move the piece back into your shop for 2-3 weeks. When the glass is removable, run a rabbet router bit along the inside edges of the top. You'll probably have to refinish the cuts, but that should be easy to match.

 

[bainin]

I would guess that those 6 exposed sticks are where the shrinkage occurred that changed things.
Sawed edges and most exposed area for water content change.

b

 

[llucas]

Wiley
Well the problem right now is removing the glass to clean it and yes I expect with spring humidity coming I will do that and cut some relief around the glass

 

[llucas]

Tom
Great suggestion re the sliding dovetails!
Now I just need to make a jig to make all 6 parallel

 

[Mark Fogleman]

These are handy: Space Balls - 100 pk

 

[CharlieGator]

Luke--You don't say if the binding in place of the glass is a problem or not. I'm just curious. If it is there's a simple solution. Move the piece back into your shop for 2-3 weeks. When the glass is removable, run a rabbet router bit along the inside edges of the top. You'll probably have to refinish the cuts, but that should be easy to match.

My thoughts exactly!

 

[Willemjm]

LOL

I have a rectangular table with a crystal glass top, which is also a somewhat snug fit. Moved house four times, and every move, the movers broke the glass.

 

[ChemE75]

LOL

I have a rectangular table with a crystal glass top, which is also a somewhat snug fit. Moved house four times, and every move, the movers broke the glass.

For our relo to FL movers did ok, mostly… but somehow managed to split the top of a drop leaf maple dining table, of course the glue joint survived. Can’t imagine a glass top surviving. Although movers did far better moving here, only some minor nicks and scratches.

 

[ChemE75]

Tom
Great suggestion re the sliding dovetails!
Now I just need to make a jig to make all 6 parallel

I can visualize making them parallel, since there is the straight edge to clamp a router jig. But the more I think about it, any sliding fit “ties” will expose a gap when each independent half contracts.