Too-tight biscuits

Biscuit joining tricks and techniques. March 23, 2002

My biscuit joiner is a great tool, but the biscuits are too tight. The ability to shift a panel with 3 or 4 biscuits in it when clamping should be a fairly risk-free procedure, but because they are so tight, the cumulative friction is too much. Is there such a thing as undersize biscuits or an oversize blade?

Forum Responses
I had the same problem. Buy Lamello biscuits unless you want to presort them before glue up to see if they fit.

Another thing to consider is keeping your new biscuits sealed in a zip lock bag until you need them. It doesn't take much moisture to swell them so they won't fit.

I had the same problem. Replace the cutter. If the cutter gets sharpened several times, it will decrease in thickness.

From contributor M:
Biscuits should be somewhat loose; they are not designed to align parts. They are compressed during manufacture and are designed to swell with the application of water-based adhesive. Good quality biscuits should actually rattle just a bit in the slots.

But that compression is also why excess humidity can cause them to swell and become useless. The advice to store in plastic is important to follow.

Have heard, not tried, to run the biscuits through the microwave or toaster oven to remove moisture before assembly. Glue's moisture will re-swell. Cheap solution.

From contributor M:
I can confirm that the "heat-shrinking" works with dowels. We've done it to ease assembly of house doors with many complicated joints. The dried dowels lose some girth and are easier to insert. Seems like it should work for biscuits as well.

From contributor G:
When we used dowels to fasten wood, the dowels were always heated (about 180 F) before use to dry them and shrink them. The same goes for biscuits. (Microwave is okay if short enough, so they do not overheat. If overheated, they will not adhere well to the glue. So, I prefer that you use a small hot box.) Once dry, the "Baggie" technique will work well for a week or two.

Also, if you are using a yellow glue (e.g., Titebond), this adhesive has very high instant tack. Try using a white glue instead, which can be moved easier and without affecting joint quality.

From contributor B:
We never leave biscuits out of a sealed container for more than a few minutes. We store them in plastic kitchen containers with press-on lids, and don't even leave the lid off while doing a project. A bit of extra care like this goes a long way toward avoiding problems in the middle of a project.

From the original questioner:
It appears that most users of biscuits find the tightness issue a significant problem. If a blade manufacturer were to make a biscuit joiner blade .005-.010 oversize, I think most of you would join me in buying one. If I were a blade manufacturer, I would test market a couple dozen at .165 (4mm is .158) for sure.

From contributor B:
I disagree with the idea of an oversized cutter. If you maintain your biscuits from the moment you purchase them, you will not have a problem but for an occasional biscuit.

From contributor A:
Buy a blade that makes the cuts too wide for common biscuits. Mix yellow glue with flour. It will fill the gaps easily by swelling when water in the glue sucks into the flour.

From contributor M:
I'm not sure I get it. You're saying to make the slots too wide and then thicken the glue to make up for the extra space. Why not just use a biscuit of the correct thickness and an appropriately sized cutter and original consistency glue?

In my opinion, mixing glue with wood flour is a good way to reduce its effectiveness.

From contributor A:
I didn't mean wood dust. I am talking about flour made of grain. Just common flour to bake bread. That works great and it doesn't affect durability of glue (if you mix it max 1:3). And by "too wide", I meant to make slots just a little wider so that all biscuits will fit into the slots without having trouble.

From contributor G:
You can mix "yellow glue" with flour and thereby extend the glue, using less glue in a glue joint. This is acceptable when you do not need the full strength of a joint and want to save a few pennies. But it often is not a good idea if the effort is to save money (by weakening the joint). Why not just buy the proper adhesive in the first place?

Also, if you keep the biscuits very dry, they will expand to fill the gap (which should be plenty wide if you do indeed have very dry biscuits). By expanding, they will come to within the 0.002 to 0.006 inches gap required for a strong bond. However, since the water in the glue does indeed swell the biscuit and the wood around it, you should always wait a few days before you finish sand the panel. This waiting assures that the moisture will diffuse from the localized area. If you sand too soon, the swelling will go down later and leave a small depression. Likewise, swelling pressure can be very large. So if a biscuit is used with "low adhesive content particleboard", which means the internal bond in the board is low, then it is possible that the swelling of the biscuit will pull the particleboard apart internally, leading to an observable bump (plus the board is weakened in this area).

From contributor B:
If I understand contributor A correctly, he is saying to lengthen the biscuit slot, not thicken it. I'm sure we've all experienced annoying insertion misalignments of a biscuit or two in multiple biscuit applications. I too will often slide the biscuit jointer sideways a 1/4" or so to lengthen a biscuit slot to help avoid that situation.

As to flour, I have to wonder. I will sometimes mix sawdust with a bit of glue for a filler, but not for adding strength.

From contributor A:
Contributor B, no, contributor G was right about what I was trying to say.

I never had any problem weakening the joint. Of course, we are talking about a couple of % which the slot should be wider, not more. You can accomplish a wider slot by putting two or 3 layers of paper underneath the cutters on the sides of the router bit. (Put paper underneath and tighten screws then.) If you look at the joint like a scientist would, contributor G is 100% right.

But due to the fact that we usually have joints that fulfill durability much more than by 100% (security is often 250% or more), you won't run into trouble by widening the gap or mixing the glue with common flour. The joint will be still strong enough and won't break. By the way, using no biscuits in any joint (particle boards) may be more durable than using biscuits - just try it out. (Biscuits are often just good for easy assembling.)

Common yellow-glue in America is much more liquid than in Europe (where we call yellow-glue "white-glue", by the way). It is like thick water if you compare it to our glue in Europe. That may be because of different humidity in the air. When I was working in CA, I felt that the glue sucks into the joints much faster than in Europe, because the dry climate took the water of glue away before putting the parts together.

From contributor G:
White glues are white in color and typically have a long assembly time. The pieces can be moved without damaging the ultimate strength. There is very little instant stickiness (called "tack").

Yellow-glues are yellow in color and are thicker, have very high instant tack, cannot be moved without affecting joint strength, etc.

Neither one fills a gap well.

Some yellow-glues are also cross-linking, which gives them a degree of water resistance. But for tables, panels, etc., the water resistance is not an issue, so do not use the more expensive yellow glues. If the panel will be exposed outdoors, use an exterior glue and not the white or yellow glues.

The important points are wood swells and needs to equalize before sanding or depressions will appear later. You can order custom thickness blades made by many of the tooling manufacturers or even have a worn plate joiner blade re-tipped and made thicker by many sharpening services. Best solution is to keep your powder dry in the face of the enemy (wood movement due to moisture changes). Also check the alignment of your cut to the fence. If there is a slight non-parallel condition, it doubles when you make both slots and will make it feel like a tight fit when it is really a bind.

I'm in Florida, where it is difficult to keep biscuits from swelling. I compress my biscuits in a vise before using and this works very well. I don't know if this effects the holding power of the biscuit.

You might try putting a shaker of salt in your bag of biscuits. The salt will absorb some of the moisture in the air, and keep them dry.

I just got in the habit of checking the biscuits in a setup block first at the assembly bench. One or two pops with my 8 oz ball peen hammer usually work. Yeah, its time we shouldn't have to spend, but it sure saves time and mess at assembly.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

A good way to keep biscuits dry is to put a couple packs of silica in the container you are using. When you buy a new pair of shoes, there are little packets of silica in the box. They draw the moisture.

Comment from contributor C:
Why not just use one of the closet water absorbers (or break one into smaller coffee filter envelopes) to absorb moisture in your biscuit box? Cheap, reliable and simple.

Maybe even hot glue a film canister into the bottom of the box, punch some holes in the canister lid, pour in some water absorbing 'stuff' and job's done. Just be careful to keep an eye on the stuff, as it keeps working long after it is submerged.

Comment from contributor I:
Nobody mentioned using a piece of sandpaper to make the biscuits a little thinner. It may take a little extra time, but if you aren't going to take proper care of your biscuits, you're gonna have to pay the price one way or another.

Comment from contributor H:
The problem with tight slots for biscuits can be solved by taking the blade from your jointer and putting a piece of a one thou feeler blade folded in half (two thou) between the blade stiffener and the blade. This makes the blade wobble, slightly. Not enough to be noticeable, but it cuts a wider slot, and your biscuits go in easier.

Comment from contributor T:
See the "Biscuit Press" in the WOODWEB classifieds. This tool solves the problem and also allows more open time with your adhesives when gluing up large assemblies. All major brands of biscuits swell. This tool is slick and really works.

Comment from contributor R:
Wood biscuits do swell if they pick up moisture. Lamello manufactures a fiber #20 biscuit that relieves this problem. They will swell only slightly over time, but if kept enclosed have a much longer shelf life than the compressed wood version.