I used to use biscuits a lot more, for edge glueing table tops and several other assemblies. But these days I hardly ever use them. The reason I stopped using them is because I read so much about how they don’t add any strength, that all they do is help with alignment. But I’m not sure I have ever felt like I knew the answer. I always wondered if maybe there are a lot of people out there who just don’t like new things, or whether it’s based on real knowledge, or what.
However, recently I have noticed that a lot of expert woodworkers are starting to say positive things about biscuits, including the idea that biscuits DO in fact add strength. Fine Woodwokring magazine did a review of various joints about a year ago, and biscuits made a very respectable showing.
And now, here on the current Fine Woodworking web site, I see that the article on edge joints offers a very positive statement about using biscuits to add strength. What do you know, Mr. Ragowski himself — Mr. Joint man — endorses the use of biscuits!
Here’s the quote I’m referring to:
“So why reinforce an edge joint? Reinforcements in the form of biscuits, dowels, splines, or tongues and grooves make alignment much easier. Beyond this, reinforcements provide a mechanical connection, which strengthen the joint. Without them, you must depend on the adhesive alone to hold the joint together. “
Edited 10/31/2002 12:28:45 PM ET by Matthew Schenker
Replies
I see it this way - since the glue is stronger than the wood, the wood will always fail before the glue. If the wood fails what difference does it make if there are biscuits? If the glue fails (because it's old, whatever) it fails on the biscuits too and it will still fall apart.
Perhaps part of the answer is marginally more surface area to glue, and the expansion of the biscuit forming a lock of sorts. I'm kind of on the fence on this. There's times where the situation makes the use of biscuits look appealing, and I use them. If I have total control over what's being built and how, my preference for glueing up panels and tops is tongue & groove, but that gets more complicated in thinner stock. Of course, biscuits start giving you problems when you get down to 3/8" thickness too. The place I've seen biscuits used that I'm totally against is instead of a mortise & tenon - like aprons to legs in tables and chairs. Do not see that as sturdy enough for what the piece requires. As far as alignment, if the T&G is cut accurately, that does a much better job there, and you don't worry about glueing up a large rectangle that is going to be cut into an oval top . . (gee, now that the panel is flattened, where were those biscuits?)
I have biscuits and go back and forth over their use. I live in the Mid-Atlantic where there is considerable humidity all year long so storing biscuits in their containers is really useless. My understanding is the biscuits are compressed so that when they are exposed to liquid, they expand to fill the biscuit slot space and possibly even expand slightly the space. I've never seen this happen.
I've taken brand new biscuits, measured their thickness with a micrometer, then soaked them in glue and then measured their thickness after drying - with no appreciable change in thickness. If biscuits are very susceptible to moisture, then folks along the coasts need to store them in some type of desicater - but no one does that.
While there may be some mechanical advantage to the biscuits, I agree with Kim that they are really not worth the trouble. This might be a great project for someone to investigate.
The other day I was gluing up a table top. I like to use Gorilla glue for tops to eliminate glue creep. Anyways, I droped the biscuits I was going to use in a bucket of water and begun to dampen the surfaces of the wood with a moist rag. I then spread the poly glue making sure it was in all the crevices and so on. Ran into a hitch with the clamps and couldn't find one for a couple of frantic seconds. Grabed the biscuits and started to insert them into the slots and to my surprise, they were much more snug than the dry fit -- I had to strugle with the last two, in the meantime the poly had begun to react with the water and was begining to expand. Lukily I got them in and clamped at the nick of time. These things do swell up.
I only use biscuits with polyurethane glue and water. It works for me.
First, this disclaimer. I'm a newbie. I bought a plate joiner (DeWalt) to meet the needs of my first project.. a garage workshop with wall and base cabs and a rolling work table with a flip-up top. It seemed like an extravagance because the project only called for 8 biscuits to extend the base cab tops with 3/4"X2"X4" extensions so that you could cut more than one top from a 4X8 sheet of plywood. Still, that's what the plans called for, so I bought it.
It took me a fair amount of time to familiarize myself with the machine.. and even more time to actually make good joints with it. But now I've used it many times in many different aps and I'm probably as good with it as anyone who uses one.
I really appreciate the technology involved here. The plate joiner makes glue up and clamping MUCH easier.. especially where you face complicated clamping. It holds the pieces together in good alignment while you're wrestling with clamp and square, clamp and square, clamp and square.
As for whether biscuits add strength to the joint, I'm not qualified to render an opinion.
bill
I like it for alignment on flat glue-ups and to make clamping easier on mitered corners. I agree that they probably don't add strengthSteelkilt Lives!
Biscuits and plywood (or other sheet stock) were made for each other when constructing case goods. Any other method I've tried is slower. As for strength, once a piece is installed, as long as it can take graviity loads and keep its doors on, biscuits are strong enough.
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