Thread: Biscuit Joints

There is no reason why you should not just rout a slot from one end of the timber to the other. (Not going through the ends of course.) However, be aware that the purpose of the biscuits is for alignment only and that they are not required for adding strength to the joint.

Two clamps are not enough. You need four for best effect; two applied from each side. If you use excessive force doing up the clamps then the board will almost certainly bow. Only sufficient force to mate the two edges and just squeeze out the glue along the joint should be used. Are you also absolutely sure that your edges are exactly square?

And yes, it is usually best to clamp only one at a time, but most people do it all in one go.

To ensure that the boards remain flat whilst the glue is drying, it is a good idea to clamp a couple of pieces of straight timber to the faces of the board so that it CANNOT bend. Be very careful that these extra pieces do not become part of the whole.

There's nothing inherently wrong with the slots being slightly too long, the strength of the joint comes from the contact of the glued surfaces. If you have determined the full required width of the slot you can mark the required offsets on the work and work to those marks or create a jig to limit the router movement. If you have a router table you can using matching marks on the fence.

It's not uncommon for clamped glued boards to bow without some sort of lateral restraint. There are several ways to prevent this, clamping the ends to the support table or clamping the boards between a couple of thin boards running across the work. (use wax paper where they contact the glue joints) There a lot of descriptions on the net on how to clamp boards if you do a quick search.

It's also a good idea to alternate your sash clamps top and bottom to reduce the lateral movement, you should probably be using three sash clamps for the job (two up, one down) but you may well get a way with two if the work is also laterally restrained. There's certainly no reason why you can't glue up three boards at the same time.

The biscuit slots are meant to be slightly long. This gives you a bit of slack to move the joint slightly. This is one of the advantages over dowels which require precise alignment.

Originally Posted by Bob Willson

However, be aware that the purpose of the biscuits is for alignment only and that they are not required for adding strength to the joint.

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Sorry to disagree Bob but the biscuits definitely add to the strength of the joint. If you want a stronger join, use more biscuits.

Originally Posted by smidsy

Hei Guys,

Also, how are you meant to clamp the timber - I joined three peices and when I tightend the sash clamps to what I thought was reasonable the timber had bowed and I had to back off the clamps. Is there a trick to the clamping?

Thanks In Advance
Smidsy

Hi Smidsy,

I use panel clamps, they hold the boards flat while edge clamping.

Link to Panel Clamp

Do not over tighten your clamps, you only need a small amount of glue squeeze out. Over tightening weakens the joint.