How Tightly Should Biscuits Fit?

[Wiley's Woodworks]

No, I'm not writing about eating too much for breakfast, so save those jokes. I just put together a 2 shelf bookshelf carcass with 45* miter corners. I put 3 biscuits per corner for a total of 24 slots cut. I dry fit the carcass and had to use clamps to get the corners to fit together properly. Some biscuits had to be hammered in, some wouldn't stay in the slot. I blame an inconsistency in the biscuit thickness; a few .000 can make the difference. After the first corner I plunged the cutter into each slot 3 times to make sure the slots were cleared, but it didn't make a difference. After dry fit some biscuits had ridges forced into them by the harder maple shelf wood pushing the softer fibers of the beech biscuits, which I sanded down before reinserting and glueing.

Previously I learned the hard way about putting wood glue on the biscuits at glue up. The biscuits immediately absorb the water in the glue, swell up, and become impossible to press down all the way into the slots. How tightly should the biscuits fit into the cut slots? Short of pre-sanding every biscuit to thin them down, what is a solution for getting a light-touch press fit?

 

[wsrhue]

My biscuit joiner is over ten years old and I've never replaced or sharpened the blade and no biscuit fits the slots. I obviously need to get a new blade but I don't use it very often. When I do need to use it I use a hammer to compress the biscuits.

 

[dancam]

Your biscuits might be swollen. I hammer mine a few times before inserting or you can squeeze them in a vise.

 

[tvrgeek]

You need to keep biscuits in a dry location. My shop stays at 50% and I have no problem. I have a DeWalt tool. Never had any problem with fit. I have on occasion had to do as Dan suggests when my shop was not air conditioned. Maybe differences in biscuit brands?

 

[Wiley's Woodworks]

Beat 'em with a hammer!! A simple, elegant solution. Works like a champ. I already learned you can beat them too thin, so in the future I'm going for a light touch press fit. I want the biscuits to hold pieces together during dry fit.

 

[tvrgeek]

Just looked at my cans. Bosch brand. If anything, they could be snugger. Using them right now to help align trim pieces in my bath project. Not as flush as I had hoped. Never actually been very happy with them. I consider them a finish carpentry technique, not good furniture. Maybe it is technique, but I don't see much variability there.

 

[Rwe2156]

I wouldn’t recommend multiple plunges, that’s a recipe for a loose slot.

I’ve got a plastic bag of Porter Cable biscuits laying unsealed in a drawer (80-90% humidity) that have to be 20 years old. Maybe one out if 20 is tight, I just toss it.

You don‘t really want to be hammering them in, b/c if they are that tight, when the swell from glue moisture, it could create a bulge in the board, particularly plywood, especially 1/2”.

Reminds me, kigoroshi is a technique in Japanese ww’ing that uses this to create extremely tight joinery. I’ve also heard it called “killing the wood”.

 

[ssmith]

My "shop" has no temperature/humidity control, so to minimize that problem I try to keep the containers closed securely during storage. It seems to work fairly well - about 80% of the ones out of the (Bosch) container fit snugly with a few pounds of hand pressure. The remaining 5% are too loose and perhaps 15% too tight - any that are go in the trash.

+1 on "don't hammer them in", since they're intended to swell slightly with glue application. Someone (here?) mentioned glued up joints will swell anyway so it's bad practice to sand until you give it time to fully dry. That seems sensible, so the last thing you'd want to do is aggravate that problem with too-tight biscuits.

 

[Leatherneck]

I have a biscuit cutter that I bought 22 years ago and it is still as sharp and as good a cutter as new. Also, my tools are in my garage shop all year round and I have never had biscuits swell up so that they would not slip right into the cut slot. The problem discussed here seems strange to me.

 

[junquecol]

For biscuits that are too large, try microwaving them for about 30 seconds. This get rid of excess moisture. I keep my biscuits inside climate controled house, only taking out what I need right then. One thing I learned the hard way, is any joint secured with biscuits, wait at least a week for swelling to go down before sanding.Otherwise you will have this neat football shaped divot in your finish. DAMHIK!

 

[tvrgeek]

Thinking I should just get a nice 18 gauge pin nailer to hold alignment while the glue dries.

 

[pop-pop]

Thinking I should just get a nice 18 gauge pin nailer to hold alignment while the glue dries.

Scott, view a pin nailer as a new learning opportunity. You get to figure out how to deal with pins sticking out 1/16” and long ones that follow the grain and curl out. YMMW.

Good quality pins (say Hitachi, PC, Bostitch) help.

 

[tvrgeek]

I use my brad nailer all the time for things that get painted. Seems the pin nailers leave such a tiny surface hole they work on natural finishes as the "fill" is not very visible. The pressure and "jolt" from a full size brad does make things occasionally move and of course, not paying attention to grain can get the brad to exit the side. Sounds like a pin nailer is all the same problems, but worse. Maybe I should save the bucks. 23 gauge is darn thin. I have just used hidden internal cut-off brads to help alignment, but they too can follow the grain even if only 1/8 inch up.

I can see why some have jumped from biscuits to the Festool thingie, but it does not work on thin stock. The built up crown I am doing in my bath is only 1/2 inch and a domino is way too big, and expensive. For furniture, I guess proper joinery is the answer, maybe followed by dowels. A few millennia of experience can still trump new gizmos. A really good dowel jig is a lot cheaper than a Domino!

 

[Wiley's Woodworks]

I wouldn’t recommend multiple plunges, that’s a recipe for a loose slot.

I’ve got a plastic bag of Porter Cable biscuits laying unsealed in a drawer (80-90% humidity) that have to be 20 years old. Maybe one out if 20 is tight, I just toss it.

You don‘t really want to be hammering them in, b/c if they are that tight, when the swell from glue moisture, it could create a bulge in the board, particularly plywood, especially 1/2”.

Reminds me, kigoroshi is a technique in Japanese ww’ing that uses this to create extremely tight joinery. I’ve also heard it called “killing the wood”.

 

[Wiley's Woodworks]

Dr. Bob--You don't hammer the biscuits into the slots. You hammer the biscuits flat until you get the light touch press fit you're after. I flatten my biscuits on the anvil of my mechanics vise, but a hardwood bench top would probably work well.

 

[BKHam]

Dr. Bob--You don't hammer the biscuits into the slots. You hammer the biscuits flat until you get the light touch press fit you're after. I flatten my biscuits on the anvil of my mechanics vise, but a hardwood bench top would probably work well.

several people have mentioned hammering, vise-ing, or compressing the biscuits in some way pre glue up. I would only do this on the ones that are tight. the ones that fit well, I would leave as is so you get that positive registration during glue up. I keep my biscuits sealed and have taken great care in cutting my slots and have had the same issue with variable tightness.

note, those compressed biscuits will expand to tight pretty quick with glue on them.

 

[Oka]

I was taught the hole should be appox 2-3 thousandths larger than the biscuit to allow space for the glue and wood imperfection. But then I was told that 40-50 years ago, so maybe there is some other recommendation. What I do know is if the joint is too tight then, the stress tension can cause issues long term (warping or separation)