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I realize that every wood species expands and contracts slightly differently, but in general at what width should I consider using a floating breadboard edge detail rather than a full glue-up for perpendicular grain conditions?

crossgrain table top

I have visited sites that provide the decimal widths of expansion for various species, but do not have a feel for what levels of movement can be absorbed by a glued joint, before it begins to fail. I also understand that breadboards provide protection against cupping, but that can be addressed in a variety of ways.

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    I think your first sentence explains why this cannot have a simple answer. Your picture looks to have two different species which would be a factor as well. Thickness also plays a role. Neat to see if someone has a good take on this though. – Matt Feb 25 '16 at 13:41
  • consider that plywood is a cross grain glue joint across the entire surface of the sheet. – ratchet freak Feb 25 '16 at 13:42
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    @matt, the picture is from the internet and I chose it because it shows the cross grain directions clearly. Also, I would think the difference in species would not matter since wood does not really expand in the direction of the grain. Only the width of the top surface matters since that is where growth and shrinking will occur. – Ashlar Feb 25 '16 at 14:02
  • @rachet freak I know, and do not fully understand what is going on. I have seen plywoods that have splits in the surface, and who knows what is going on inside the panel. – Ashlar Feb 25 '16 at 14:08
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    @ratchetfreak, plywood constrains the entire width and length of the plies, unlike a breadboard, which I'm sure has a lot to do with why it doesn't split. – grfrazee Feb 25 '16 at 18:44
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I realize that every wood species expands and contracts slightly differently, but in general at what width should I consider using a floating breadboard edge detail rather than a full glue-up for perpendicular grain conditions?

Although there are some broad generalisations that can usefully be made (e.g. a post I read recently suggested that you should make a breadboard floating "for anything larger than a side table") overall I don't think this should be approached in an in general kind of way since it's all about the specifics, not just species but the cut of the board or boards is absolutely critical because radial movement is about half that of tangential movement, and can be even lower in some cases.

Say you're working with a species known for high movement (e.g. red oak) for example, if you choose quarter-sawn boards for the field of a table then expansion will be low, even compared to another species known for its stability. So, quarter-sawn red oak expands and contracts much less than flat-sawn redwood, because it's radial expansion in the former and tangential expansion in the latter.

This all assumes well-dried stock of course, if the stuff is not at equilibrium with the surroundings when building the piece movement should be expected to be higher than predicted.

Further info (also see table bottom):
Calculating for Wood Movement on Fine Woodworking
How to Calculate Wood Shrinkage and Expansion on Popular Woodworking
Wood Movement on Canadian Woodworking Magazine

Where you do decide to go with a fully glued bond this is one case where a glue noted for creep (e.g. white versions of PVA) is a superior choice to one that is very much more inflexible (e.g. hot hide glue). Where the PVA will allow a little give in the joint the hide glue might fail suddenly and startlingly with "a report like a toy pistol" as Charles Wheeler put it.

I also understand that breadboards provide protection against cupping, but that can be addressed in a variety of ways.

Yes. My feeling is that breadboard ends should only be used when you want the look (or in the rare case when you need to extend length of available material) and not as the go-to solution to helping a tabletop remain flat.

Not all glue-ups need any help to remain flat anyway, particularly if well coated with a film finish, and of course rift-sawn and quarter-sawn wood will tend to bow less than flat-sawn. But where some help is deemed needed battens on the underside I think are preferable for a number of reasons. Chief amongst them is that when the top is at full expansion or full contraction you can't see it as a noticeable step at the edge of the table, something I very bothersome myself.

Perhaps more important to a lot of woodworkers battens are far easier to install on a table. No need for the elaborate and sometimes time-consuming creation of a long tenon and deep mortise, you just make suitable holes in the batten to allow for movement, screw them to the underside of the table and you're done. So better aesthetics (IMO) and simplicity itself to fit. Win win in my book.


I think all the above links use the data from the US Forest Service, instead of just summaries the whole table might be of use:

Dimensional Change Coefficients

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