Using Biscuits on Plywood Edges

[crybdr]

Hi all,

Has anyone here used biscuits to 'edge' glue high grade veneered plywood (3/4") successfully? I'm concerned that the expansion of the biscuit will eventually drive the plywood layers apart - creating localized ripples. I'd be gluing the plywood to regular timber.

I'm confident biscuits would work when 'face' gluing the plywood - I'm just not sure about 'edge' gluing....

Any advice or experience would be appreciated.

Best,

crybdr

[JR]

Yes, I've done it on this project. It seems to be holding up nicely.

JR

[cabinetman]

I'm not a fan of biscuits. For most edge to edge, or edge to face butt type joints, just glue and clamps works better, IMO. In some instances, the use of biscuits limits the glued surface area. They can expand and create visible raised areas. They don't add much torsional strength. They don't insure alignment.

In cases that I would determine a biscuit type connection would be advantageous, I prefer to use a wood or plywood spline.

.

[Cochese]

I have and they worked fine. My experience was opposite of the above.

[JimD]

Biscuits have their place and can cause problems like C'man describes but in my experience they rarely do. I am not a professional but it's been quite awhile ago that I put my 1000th biscuit into something so I have some experience. I do not remember if I am on my 2nd or 3rd order of 1000 - I think it's my third but then I've purchased them in smaller quantities too.

The concept of edge and face grain doesn't really work on plywood with the grain of each layer alternating. Plywood is a good application of biscuits IMHO but depending somewhat on the grade of the plywood, the issue C'man mentions about telegraphing may be more of a risk. Lower grade plywood has voids that may telegraph anyway. If you put a biscuit into such a void area, you probably increase the risk that the area will get thinner as the glue dries and show on the surface. If you're lucky, it is inside the piece and you don't see it. If you're really lucky the biscuit fills a void and you decrease the risk. If you are unlucky, you see it. I do not remember ever having this happen in solid hardwood but I have seen it a time or two in plywood and I think this is what happened.

I also agree that the biscuit does not guarantee alignment but it certainly helps. I've made a few boxes for speakers out of plywood edged with 3/4 hardwood strips. The strips permit me to put a 1/2 or larger radius on the edges of the boxes which I like. When installing these strips onto the plywood, I like using biscuits. I could just use glue and it would be strong enough but the biscuits add some strength but more importantly, they help keep the pieces together as I put the clamps on and ensure I have at least rough alignment. Glueup is a somewhat tense time for me so the little bit of extra help at this time is important to me. I do not get flush surfaces fromt the biscuits but I avoid an alignment issue big enough to ruin the piece. I leave the edging thicker than the plywood and trim after glueup. That's the only way I've been successfull. Trimming to flush is another somewhat tricky task. But you have that no matter how you put the edging on.

Wood magazine has tested joints several times and illustrated the limited strength of biscuits. I don't think their tests were entirely fair but their results still are somewhat useful as a caution about depending on a major strength increase. The other useful thing their tests provided me was a greater appreciation for pocket screws - they tested surprisingly well. This may again be partially the way the tests were done but still it is something to consider when choosing joints.

Jim