Workbenches and wood expansion/shrinking

[Wend]

OK, so it sounds like what I should be doing is getting some 100mm coach bolts and washers, drilling some holes through the cross piece that are comfortably bigger than the diameter of the bolts but comfortably smaller than the washers, and then screwing up through the bench top with them. So the coach bolts are gripping the bench top firmly but not the cross piece, but the washers squeeze the two together and I guess friction stops them sliding around.

I'm still a bit confused about how wood movement is allowed to happen as it expands/shrinks, but the top doesn't rock back and forth while I am planing etc, though.

 

[MusicMan]

Wend, the force exerted by the screw is much less than the force exerted by wood swelling on absorbing moisture. Soyou can do the screw up pretty tight and it will still slide.

 

[ED65]

So the coach bolts are gripping the bench top firmly but not the cross piece, but the washers squeeze the two together...

Yep, that's it.

...and I guess friction stops them sliding around.

No they slide as needed, that's how they allow for expansion or contraction; expansion and contraction to be more precise, since both will occur as the seasons progress.

Depending on the material of your benchtop and its depth you may need to allow for as much as 5-6mm of total movement from driest to wettest, so drill the holes accordingly or lengthen them into slots. This clearance can be spread across two holes or just one hole can be enlarged so that the bolt that passes through it is the one that does all the moving (the other bolt that can't move will remain fixed and the wood will push out from it).

 

[Beau]

So the coach bolts are gripping the bench top firmly but not the cross piece, but the washers squeeze the two together and I guess friction stops them sliding around.

Yes friction is doing the work. Do them up tight and it wont slide undesirably but will be able to move under the high forces expansion and shrinkage apply.

 

[Tasky]

https://youtu.be/iPOZYiTBCrU?t=2147 at 35:47 is Paul Sellers doing something similar, incidentally, but I don't think he ever explains why he's using screws rather than glue.

Sellers's benches are designed to be knocked down and transported, which is also one reason for the leg wedges recessed into the aprons.