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I glued up a 4' long dresser top last night, and ended up using a 1/4" spline to line up the two 12" wide sub-panels into one 24" wide one.

I was using pine quarter-sawn boards, and for the spline I just ripped a 1/4" wide strip from the end of one of the panels, since I had made them a bit oversized. So the spline had the same grain direction as the panels, except the growth rings are rotated 90°. Unfortunately I didn't really think about spline grain direction while I was gluing it up.

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Am I going to run into problems with my 3/4" wide spline expanding enough to crack the center joint on my table?

In the future, is it critical that I manufacture a long spline for an edge join with grain perpendicular, like in the image below? It seems like a huge hassle, since I'd have to have a super wide board or use dozens of little spline chunks.

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3 Answers 3

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The spline will expand at the same rate as the board it is in, if the grain is running parallel to the grain in the board, so there shouldn't be a problem. The reason splines are better when the grain is perpendicular to the join is for strength - wood will split along the grain much easier than it will break across it.

Normally, in a panel glue up like you describe, the spline is only used for alignment, not strength. The edge grain to edge grain glue bond will be plenty strong alone. I will use splines, biscuits, dominos, or dowels on this type of glue up just for alignment purposes.

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    Wood expands at different rates radially and tangentially. I'm using pine, which is almost double for tangential.
    – Doresoom
    Commented Jun 2, 2015 at 21:36
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    The dimensional coefficient for pine is about .00267. If the moisture difference is a staggering 20% between seasons, a 3/4" wide piece will expand is .00267 * 20 * .75 = 0.04" or just over 1/32".
    – LeeG
    Commented Jun 2, 2015 at 21:41
  • Good point about looking at the overall magnitude change. Plus I looked back over Matthias' test methodology and decided it was less than rigorous.
    – Doresoom
    Commented Jun 2, 2015 at 23:16
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You shouldn't have any issues. To begin you used the same wood, which they should move pretty close to the same. Second the splines are very small compared to the pieces that they are connecting. The splines should not be able to do much movement. On top of that, I think leaving the spline slightly smaller than the sum of the two grooves is standard practice, it is the faces of the spline providing most of the added strength.

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    I did leave it about 1/32" shy of the total groove width. But according to Matthias over at wood gears, pine can expand almost double the amount tangentially than it does radially. Other species seemed to be more uniform with respect to growth ring orientation.
    – Doresoom
    Commented Jun 2, 2015 at 21:41
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Am I going to run into problems with my 3/4" wide spline expanding enough to crack the center joint on my table?

Same wood, grain aligned the same way? I think you can virtually guarantee there won't be any problem.

In the future, is it critical that I manufacture a long spline for an edge join with grain perpendicular, like in the image below?

No, not for that purpose. In a splined long-grain joint like this the spline is not for strength, it is actually almost entirely an alignment aid.

Where splines are used for strength grain direction is important (if using solid wood*) as in a splined mitre, but not otherwise.

*Another option to consider for your spline material is plywood. Very stiff and very stable.

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