Biscuit joining vs. tie-bolts
Is it possible to join post-form tops and shop-built substrate with biscuits? June 24, 2001
What are the pros and cons of using biscuits to join shop-built substrate, and to join post-form tops? I've heard that they may swell and cause problems.
For joining postform tops, how will you hold the surface flush until the glue dries, keeping in mind that you have quite a few surfaces to line up, cove, front roll, etc? Also, how will you clamp a miter? A double 22º miter? And although you probably wouldn't do this in your shop, but rather in the field, how would you clamp a top 20' or 30' long? For postformed tops, miter bolts/tight joint fasteners/dog bones and glue are the only solution.
The whole spline/biscuit system is totally overrated. It is a cumbersome process that has little, if any, value to the manufacturer. The problems the system creates outweigh any positives.
From the original questioner:
My experience when assembling miter joints in the field (post formed tops with the recessed draw bolts) always ends up being a tedious "tighten and adjust, tighten and adjust" affair when trying to flush up the top surface.
The idea of using biscuits was born with the hope of reducing *some* (not all) of the fuss factor when assembling in the field. I think this approach would work, but I've heard that glue and biscuits can cause swelling - creating more problems then the technique would solve.
I understand where you are coming from on the tedium with the draw bolts. We have two guys put miters together all day. I did it myself back about a hundred years ago. With as many as I have put together, it becomes pretty much second nature. I have put my fair share together in the field--a little more challenging, but the only way to do it.
As far as the biscuit joints and expansion problems, I have not heard of any, but again I don't know of anyone in our area assembling joints in this fashion.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
I have put hundreds of miters together and though it sometimes is difficult when you are doing a u-shape with lazy susans in the corners, it would be impossible to keep the joint tight with biscuits, whereas with draw bolts you bolt it, get your check and leave... biscuits are not the way.
Comment from contributor B:
I have put many many post form tops together with biscuits, but I always use the bolts too. I have never had a swelling problem, but for me the biscits are only there to line the top surface up faster. The key to lining the top edge up is to snug the bolt up and use a rubber mallet to bump the high side of the miter down. You can hit it pretty hard but when you get to the bolt at the backsplash, hit the top edge of the backsplash to drive the high side down. Do not bang on the deck, or else you can crack the joint where it rolls up. I swear by the biscuit jointer to line it up!
Comment from contributor C:
Use biscuits to line up the job, and then use the bolts for the actual strength of the joint, and to hold the biscuits during the glue setting period. Commercially, for installations where this would eat into your profit margin, not so great, but not a bad idea if money is a little less important, and finish and speed is of the essence. Perhaps a little over the top, but gets the best out of both systems (biscuit for ease of lining up the surfaces, and the bolts for getting around the whole 30" clamp issue).
Comment from contributor D:
We use the biscuit/drawbolt method in our shop. It does ease the alinging problems somewhat, but does not eliminate all problems. I have recentley dispensed with the biscuits because, occasionally, I have an alignment problem and the biscuits negate any further adjustments (only one chance to get it right!). But I do like the added strength the biscuits give in resisting joint separation over time.
Comment from contributor E:
According to an article on biscuits, the swelling is most common if the cut is < 1/4" from the surface.
The solution is to not plane or heavily sand the surface for a couple of days unitl the join/wood equalizes.