Workbenches and wood expansion/shrinking

[Wend]

Hi all,

As my workbench slowly makes its way towards completion, I'm finding myself a little confused about whether I need to worry about expansion/shrinking of the wood, and if so, what I need to do about it.

At the ends I will have legs, with a piece of wood connecting them with M&T joints, then another piece across the top, and the benchtop an top of that. Something like this:

where the green lines indicate the direction of the grain.

So if I glue the cross piece to the bench top, then will I have any expansion/shrinking issues?

I've seen a number of people online who screw the cross piece on instead; would that be better? I can't see why it would: If it allows the wood to move, then presumably that means the screws wouldn't be very tight?


Thanks in advance for any advice!

 

[Sheffield Tony]

There's a lot to be learned from looking at the construction of an old wood drawing board. They had to stay flat. Wooden cleats were screwed to the back. At the centre with a round hole, the outer clearance screw holes in the cleat being slotted. The ones I dismantled had recessed (with eg. a Forstner bit) round head steel screws, with a round brass washer on the middle screw, and special oval ones for the outer, slotted holes. So the top can be screwed down fairly firmly to the cleats, yet still slide to allow for seasonal movement.

Short answer: yes to screws, best with slotted holes for the outer ones.

 

[Phil Pascoe]

Why the need for the intermediate pieces on the ends? Did you not have pieces long enough for the legs?

 

[Wend]

Why the need for the intermediate pieces on the ends? Did you not have pieces long enough for the legs?

The short answer is, because that's what I've seen other people on-line do!

It seems sensible to me, though: The legs are 4x4. The connecting M&Ted piece is a 2x4 with the 2" side up, which will be better at preventing twist. The piece on top is a 2x4 with the 4" side up, which is therefore supported by the entire top of the legs, and will provide more support to the top.

It doesn't actually make a difference for the purpose of expansion/shrinking, as without it the connection between the M&Ted piece and benchtop would have the same issue. It makes it a little easier if attaching with screws, though, as with it the screws only have to go through 2" of wood before they reach the bench top.

In any case, it's now moot for me, as I cut the legs to length earlier in the weekend

 

[Wend]

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.

 

[dzj]

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.

He's attaching 2 separate pieces (benchtop and well-board), so that each will shrink/ contract less
than a full-width benchtop.
He doesn't use glue because it would hinder seasonal movement, but rather screws set 4"-5" apart.
At this distance, there is no need (wouldn't hurt, though) for elongated holes as the amount of
expansion/contraction between them is minimal.
Gluing would not allow the benchtop to be removed at a later time, if repairs, modifications or disassembly were required.

 

[John15]

My 600mm wide bench top is screwed firmly to the subframe with no signs of movement problems since I made the bench 5 years ago. Maybe the humidity is fairly constant in my shed - 55 to 65%?

John

 

[ED65]

Wend, you don't need to worry about movement, but you do need to factor it in to be on the safe size. Just because someone somewhere else, with different wood, has gotten away with firmly screwing down their bench top is no guarantee that you will. It's particularly important to bear this in mind with stuff from American sources, since in much of the US conditions are very different to here and indoor workshops can be climate controlled the way a British home almost never is.

Where a screw has to move it can pass through a larger hole than needed or through a short slot (this can be a drilled hole just widened slightly with a round file, or by gingerly rocking the bit side to side) usually with a washer under the screw head to spread the load. Alternatively you can use a fixing like an expansion plate, so the only holes you might need to drill are pilot holes.

It should be mentioned that if you screw firmly in the centre and loosely front and back a top will expand from the centre outwards, but if you prefer you can have the expansion go in one direction only. This is helpful for any table that will be placed against the wall, where you want any expansion that occurs to be outwards only.

 

[dzj]

My 600mm wide bench top is screwed firmly to the subframe with no signs of movement problems since I made the bench 5 years ago. Maybe the humidity is fairly constant in my shed - 55 to 65%?

John

Often times a slightly larger clearance hole will be enough to deal with seasonal movement.
55 to 65% RH (depending on temperature) will have an EMC fluctuation of ~3%.
If your bench top is made from Spruce or Pine and is quartersawn, it shouldn't move more than ~2mm annually.


edited: Replaced pilot hole with correct term- clearance hole

 

[ED65]

...screws set 4"-5" apart.
At this distance, there is no need (wouldn't hurt, though) for elongated holes as the amount of
expansion/contraction between them is minimal.

This shouldn't make any difference, what matters is the max movement between the two most widely spread, or between one centrally and the ones furthest from it.

My 600mm wide bench top is screwed firmly to the subframe with no signs of movement problems since I made the bench 5 years ago. Maybe the humidity is fairly constant in my shed - 55 to 65%?

Might be your conditions are more uniform than is common, but what size were your clearance holes? Good-sized clearance holes can sometimes be all that the screws need to be able to shift slightly. Also what the top is made from and which way the grain is oriented can be very important, with quartersawn wood moving much less than otherwise.

 

[John15]

From memory the clearance holes are just enough for the screws to pass through and are at about 75mm centres. The top is 45mm thick beech.

John

 

[dzj]

From memory the clearance holes are just enough for the screws to pass through and are at about 75mm centres. The top is 45mm thick beech.

John

A mystery then.
Sometimes you can get away with things like that, but not always.
If it was for a client it would've cracked in at least 5 places.
A while ago I posted a thread regarding similar instances and I sensed quite a high level
of unease among forumites with such and similar practices.
It does go against the grain of WW canon, nevertheless it's something we all run into from time to time.

 

[DTR]

As suggested by ED65 (but sort of opposite), my preference for a workbench is to keep the front edge flush and push any movement towards the back. One of the advantages of a tool well is that, if designed properly, it can absorb the movement of the top. When I built my workbench I had to make the top from two boards. I took advantage of that by fixing each board at the outside edge and leaving a gap in the middle for expansion (sometimes called a split-top bench). Having a loose, removable board in the middle of bench has some advantages too.

 

[DTR]

A photo of my split-top in use as a sideways planing stop. Usually it sits flush in the gap. The piece in the gap is sized to be a sliding fit in winter, and in summer the contraction of the top boards leaves a gap of a few mm.

 

[John15]

From memory the clearance holes are just enough for the screws to pass through and are at about 75mm centres. The top is 45mm thick beech.

John

A mystery then.
Sometimes you can get away with things like that, but not always.
If it was for a client it would've cracked in at least 5 places.
A while ago I posted a thread regarding similar instances and I sensed quite a high level
of unease among forumites with such and similar practices.
It does go against the grain of WW canon, nevertheless it's something we all run into from time to time.

Photos of screw holes at both ends. The ones down the sides are into the aprons at approx. 300mm centres. There's definitely no adverse distortion or cracking.

John

 

[Beau]

Yes you do need to consider it and allow for movement. Sometimes you may get away without but if you ever get any extremes of humidity something will have to give.

My workbench top is held down with coach screws with washers under the heads. The ones at the front are fixed but the owes at the rear are in slots to allow the top to expand and shrink while continuing to hold it down.

 

[dzj]

Photos of screw holes at both ends. The ones down the sides are into the aprons at approx. 300mm centres. There's definitely no adverse distortion or cracking.

John

And even if there were, functionally, it wouldn't make much difference.
You could call it an artisinal split-top bench!

 

[John15]

Cheers dzj. I guess I'm one of the lucky ones who has got away with it!

John

 

[MusicMan]

+1 for using an enlarged hole. I screwed from underneath with enlarged holes, coach bolts with washers. No cracking in 20+ years. After being abroad for 7+ years, indeed it had moved a bit; I loosened the screws, realigned the top, then tightened up again. The shed is unheated (except when I work in it) and uninsulated. The top is beech.

 

[profchris]

Cracking comes from shrinkage in low humidity, so if you attach the top when humidity is low your problem would be the top swelling. As it's fairly thick and the movement is small you may well not see any doming - but if you removed the screws at that time then the holes wouldn't line up again!