Thread: Table Top Attachment

Hopefully the dodgy paint drawing makes sense. Orange and yellow are holes larger than the screw you intend to use. Use a screw and washer so the washer sits against the counterbore and allows the top to be secured, just don't screw it to heck. The yellow hole being larger than screw allows for wood movement. My dimensions and proportions are a bit off, but I hope you get the meaning. Probably should've used circles for the bottom view...

Edit: Also meant to include that the movement direction will the at 90 degrees to the grain direction, so you could go for elongated oval holes in the leg. Might be a challenge on two of the legs where the oval would be along the short width of the leg.

table top.png

Originally Posted by Alkahestic

Hopefully the dodgy paint drawing makes sense. Orange and yellow are holes larger than the screw you intend to use. Use a screw and washer so the washer sits against the counterbore and allows the top to be secured, just don't screw it to heck. The yellow hole being larger than screw allows for wood movement. My dimensions and proportions are a bit off, but I hope you get the meaning. Probably should've used circles for the bottom view...

Edit: Also meant to include that the movement direction will the at 90 degrees to the grain direction, so you could go for elongated oval holes in the leg. Might be a challenge on two of the legs where the oval would be along the short width of the leg.

table top.png

Thanks Alkahestic,

Appreciate the effort to put the diagram together.

I had read about this technique and in reading your post I can see that I can drill an oversized hole rather then try and work out how to elongate the hole and then try and plug. Is there a formula/rule for calculating how much bigger the oversized hole needs to be?

One thought/question I did have about this technique is the need to oversize the holes for the two screws in the top of the cross that runs down the middle of the table in the direction of the grain? As expansion is only across the grain and the connections to the other cross will utilise the oversized hole technique, I do not see any due force on the two tight screws.

I will probably just oversize all to be sure however I would still like to understand the science behind it.

Cheers
Damien

My understanding is that wood will move in all directions but generally any movement that is not tangential to the grain is small enough to be negligible in comparison. That said, if the top is fixed 'hard' at two points, issues can arise if the panel expands and contracts unevenly. From what I've read (and done), panels should only be rigidly fixed at one point.

This Fine Woodworking article goes over how to calculate movement but requires membership Stop Guessing at Wood Movement - FineWoodworking to read.

Originally Posted by Alkahestic

My understanding is that wood will move in all directions but generally any movement that is not tangential to the grain is small enough to be negligible in comparison. That said, if the top is fixed 'hard' at two points, issues can arise if the panel expands and contracts unevenly. From what I've read (and done), panels should only be rigidly fixed at one point.

This Fine Woodworking article goes over how to calculate movement but requires membership Stop Guessing at Wood Movement - FineWoodworking to read.

Thanks again.

I reviewed the detail in that edition and came up with a whopping 40mm of movement.

0.0037 (movement value) x 10 (assumed fluctuation in MC) x 1150mm = 42.55mm

My legs are only 35mm wide.


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I think that if you were to use fixed screws along the two legs that run with the grain and then over size the holes on the other pair of legs you will be fine. 4 screws in total.
I would not bother plugging the holes either. If the table is to be moved to another location or needs refinishing down the track, the plugs will be a pain to remove and no one is going to look under the table while it’s in use.

Originally Posted by Cal

I think that if you were to use fixed screws along the two legs that run with the grain and then over size the holes on the other pair of legs you will be fine. 4 screws in total.
I would not bother plugging the holes either. If the table is to be moved to another location or needs refinishing down the track, the plugs will be a pain to remove and no one is going to look under the table while it’s in use.

The maths suggests I need 40mm holes which is not an option

I always find this a useful article to reference. The section on elongating the holes might be useful.

https://www.finewoodworking.com/media/TabletopsFlat.pdf

One of your “legs” runs down the centre of the table inline with the grain. Your could actually hard fix to that member because the table will expand to each side away from that centre line. I would still have small slots on these as there will be a small amount of expansion lengthwise.
The other “leg”, which runs at 90 degrees to the grain is where the larger expansion holes need to run and that’s OK because the slots will run along the member so the width is not a problem.
Heres a quick picture.

6DF9B763-8C57-4743-BD5D-AD9C3B8F3BAC.jpeg

I also firmly believe that once the table is within a stable environment, there will be nowhere near 42mm movement. My outside table under the roofed pegola is 1050mm wide and goes from hot to cold over the year and maybe changes 6mm but it’s jarrah.

I bought some of these from Lee Valley and I have used them on two table projects. I like them a lot.

Originally Posted by Damienol

Thanks again.

I reviewed the detail in that edition and came up with a whopping 40mm of movement.

0.0037 (movement value) x 10 (assumed fluctuation in MC) x 1150mm = 42.55mm

My legs are only 35mm wide.


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What about using another piece of timber that sits on top of the legs. Screw the intermediary piece into the leg and then screw the top on the intermediary piece, with holes 43mm wide. In the paint special below, the grey dots are your screws into the top and the brown bits are the intermediary timber boards. They wouldn't have to be too thick, just enough to hide the washer and screw head in a counterbore and a few mm of 'meat'.

Edit: My betters have spoken. Also, it occurred to me after posting below, you could if using an intermediary piece, make it quite narrow, like 50mm or so, have the hole centered and hide the screw head washer with the leg.
table top 2.png

Originally Posted by Damienol

The maths suggests I need 40mm holes which is not an option

40mm sounds very excessive, you will get sideways movement but for the size of the table I cannot imagine it ever being 40mm within a home. Even from green milled timber to dry you would not see that amount of shrinkage over that width. And length wise it will be lucky to be more than a mm or two. With the amount of movement you have calculated the table will turn from round to oval and back again, the overhang over the legs will change and the customer will question that too.
And yes I mill timber too.

Originally Posted by Cal

40mm sounds very excessive, you will get sideways movement but for the size of the table I cannot imagine it ever being 40mm within a home. Even from green milled timber to dry you would not see that amount of shrinkage over that width. And length wise it will be lucky to be more than a mm or two. With the amount of movement you have calculated the table will turn from round to oval and back again, the overhang over the legs will change and the customer will question that too.
And yes I mill timber too.

Thanks Cal,

I find it hard to believe it could move 40mm however that is what the maths from the fine wood magazine suggest it could be up to. I have made 10 tables in the last couple of years, all hard screwed and I have had no issues.

Originally Posted by markharrison

I bought some of these from Lee Valley and I have used them on two table projects. I like them a lot.

They look pretty cool Mark and provide me with a clear picture on what I need to do.

I know you dont have to plug the holes however I like the finish. As such I need to work out how to replicate with round holes

Originally Posted by Lappa

One of your “legs” runs down the centre of the table inline with the grain. Your could actually hard fix to that member because the table will expand to each side away from that centre line. I would still have small slots on these as there will be a small amount of expansion lengthwise.
The other “leg”, which runs at 90 degrees to the grain is where the larger expansion holes need to run and that’s OK because the slots will run along the member so the width is not a problem.
Heres a quick picture.

6DF9B763-8C57-4743-BD5D-AD9C3B8F3BAC.jpeg

I also firmly believe that once the table is within a stable environment, there will be nowhere near 42mm movement. My outside table under the roofed pegola is 1050mm wide and goes from hot to cold over the year and maybe changes 6mm but it’s jarrah.

Thanks Lappa, this is what I am thinking too


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