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Biscuit joining and material thickness

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Forum topic by Stewbot posted 07-27-2015 05:23 PM 985 views 0 times favorited 12 replies Add to Favorites Watch
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Stewbot

195 posts in 550 days


07-27-2015 05:23 PM

I’m sorry, I guess my biscuit joining question should live amongst the joinery board.

I’m planning on joining 5/8” cedar with biscuits and was curious about whether this thickness of material could potentially be problematic for a soft wood when utilizing biscuits. I’ve read that with the shrinking and swelling, over time the placement of the biscuits can become visible, especially after I sand everything smooth.

Just curious if anybody had any words of advice for this situation, or if there is a general rule of thumb regarding using biscuits and the thickness of material requirements for these applications.

The only material I have used biscuits on is 3/4” Ipe so my personal experience with using biscuits is fairly limited.

Thanks.

-- Hoopty scoop?


12 replies so far

View Fred Hargis's profile

Fred Hargis

3949 posts in 1960 days


#1 posted 07-27-2015 05:36 PM

What you described is exactly what can happen. As the biscuits absorb the glue, they swell. If you sand it smooth at this point, the swelling in the biscuit will go away, and you will have the futbol divot in you wood. This is mitigated somewhat on hard woods but giving the biscuit plenty of time to dry and then shrink. I think in 5/8” cedar it would be risky. You might test it and see what happens, or skip the biscuits altogether. At the very least put them in the lower 1/2 of the thickness of the board (the non face side).

-- Our village hasn't lost it's idiot, he was elected to congress.

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Stewbot

195 posts in 550 days


#2 posted 07-27-2015 06:17 PM

Thanks for the reply. I’ll do a test piece and see how it goes with time, but will also probably try and locate some thicker cedar. I also am going to try to set the cutting depth lower as you mentioned. despite looking hoopty, it would not matter in this case if this problem occurred on the underside.

-- Hoopty scoop?

View jerryminer's profile

jerryminer

528 posts in 908 days


#3 posted 07-28-2015 12:53 AM

The important thing here is to WAIT to sand until after the glue has dried and the wood has reached equilibrium. Sanding while the wood is in a swollen state from the moisture in the glue is what will cause the “telegraphing” of the football shape of the biscuit. (because the swollen wood will shrink as it dries, causing a “sunken joint”—-let it shrink back down BEFORE you sand it flush)

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Stewbot

195 posts in 550 days


#4 posted 07-28-2015 01:49 AM

Ok, thanks.

-- Hoopty scoop?

View bonesbr549's profile

bonesbr549

1176 posts in 2534 days


#5 posted 07-28-2015 01:57 AM

You might be able to get away with FF size biscuits, and I only used them a couple times on some face frames, but abandoned biscuits a long time ago. Great for aligning, but if you do it right thats not even an issue. They don’t add any strength.

good luck.

-- Sooner or later Liberals run out of other people's money.

View johnstoneb's profile

johnstoneb

2147 posts in 1639 days


#6 posted 07-28-2015 02:17 AM

Biscuits don’t really add any strength. If you don’t need them for alignment I wouldn’t use them

-- Bruce, Boise, ID

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jerryminer

528 posts in 908 days


#7 posted 07-28-2015 03:35 AM



Biscuits don t really add any strength. If you don t need them for alignment I wouldn t use them

- johnstoneb

They add a LOT of strength when joining boards end-to-end. I don’t think we know the OP’s intent yet.

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Stewbot

195 posts in 550 days


#8 posted 07-28-2015 04:04 AM

In this situation Im gluing up the edges, long grain to long grain. Based on some of my reading it appears many people suggest and agree that when glued properly edge to edge, the biscuits may not necessarily add much strength to the joint, and the glue joint will be sufficient alone. Previously I’ve done edge to edge with biscuits, but I haven’t been doing It long enough to both develop my own thoughts on the matter, or learn for myself that it may not even do much for strength.

As for butt joints, there seems to be differing opinions on how much (or if at all) the biscuits add to the structural integrity of the joint. Naturally with time I am going to try both methods and form my own opinions.

While I have everyone here, in the past while doing glue ups on edge with larger pieces, the boards start to cup as I apply clamping pressure (I use the Bessey H style bar clamps). so far all I have done to try and alleviate this (aside from just trying to set up my clamps properly etc. to minimize this in the first place) is to clamp or screw a separate board across the top to my assembly table, perpendicular to the boards I’m joining to apply downward pressure in the attempt to flatten them.

Can anyone share any techniques they do in order to alleviate the boards from wanting to cup during this stage?

Thanks.

-- Hoopty scoop?

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jerryminer

528 posts in 908 days


#9 posted 07-28-2015 04:18 AM

I solve this issue—-and the issue of alignment—-with curved cauls, clamped in pairs across the width of the panel.

It’s a common and time-proven technique. I’m sure there are youtube videos or whatever on the process

View Gopher's profile

Gopher

27 posts in 1412 days


#10 posted 07-28-2015 05:04 AM

Why not use a lap joint instead ? Clamping – alternate clamps top and bottom. They offset each other

-- Ted T. Aiken S.C.

View jerryminer's profile

jerryminer

528 posts in 908 days


#11 posted 07-28-2015 05:25 AM


Why not use a lap joint instead ?

What would be the advantage?

View Fred Hargis's profile

Fred Hargis

3949 posts in 1960 days


#12 posted 07-28-2015 12:37 PM

The cauls approach (or the one your using) is the approach I’ve always used. Clamping a piece across the assembly is often a little easier, but it doesn’t allow you to alternate your clamps (one on top, one on bottom, and so on).

-- Our village hasn't lost it's idiot, he was elected to congress.

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