Glue Myths: 3. Biscuits

Part 3 in a series on how PVA glue interacts with wood grain. This video measures the strength of biscuits.
Links to previous videos in this series:

[SwitchAndLever]

I never realized people used biscuits like that to add strength, I've only ever used them to help with alignment when gluing long edges together, as the depth from the top surface with the biscuit cutter can be easily set.

[BLenz-]

Interestingly, we found similar results when adding reinforcing fibers to concrete. I work for the CA DOT testing cement and concrete. A number of fiber reinforcing products have come around over the years. I'm talking like 1/2" to 2" fibers, not reinforcing steel. There have been plastic fibers, glass fibers, and metal fibers. Seems like it would make the concrete stronger, right? Well, it never seems to increase the maximum strength, because the fibers don't really get involved until there are significant displacements inside the concrete, and that doesn't happen until after the concrete breaks at maximum strength. BUT, what the fibers do well is holding concrete together after it breaks, much like the residual strength Patrick found here.

[kylehazachode]

I use biscuits as an easy helper to keep wood aligned, especially on heavy long boards. Paired with pinch dog clamps, biscuits really help when your panel clamps just aren’t long enough or you just don’t want to use them. I’ve never viewed biscuits as a reinforcement but rather as an aid for aligning.

[answeris]

I think the main reason why the biscuits are made in an angle is because you don't know what direction you are going to need the biscuit. Most cuts in woodworking are rips and crosscuts. So having the biscuit in a diagonal covers both options

[shaunsaggers]

Hi Patrick, I'm not a great woodworker, but I do know a little about stress and strain. the way you tested the biscuit joint creates bending through the piece, which means axial tension is generated at the top surface, and compression at the bottom surface. because the biscuit is in the middle of the piece, at the neutral axis, it actually doesn't see much of the bending stress, so it's not contributing much to the strength of the joint.

aside from helping with positioning the pieces, what I'd expect the biscuit to help with is transfering the stress from one side of the glued joint to the other when you're putting the bending across the long aide of the wood (similar to the prying apart of the joint which you showed in the images at time 7:30 in your video). I'm not sure how great the effect is, but if more of the force is transfered into the fibres in the biscuit, then from there into the fibres on the other side, it would probably do more for increasing the bending strength of the joint... the top half of the biscuit would be transfering axial.

hmmm. didn't explain that well but the summary is that for your tests, the piece was bending around the biscuit, with the biscuit being in the neutral plane for that load orientation

[vladtepes]

mitre joints are not designed to resist or withstand the direction of the forces applied in these tests. they're meant to go around corners, so the forces they must resist are those that try to deform them in the (x, y) axes by closing or opening the angle at the joint. this press test applied force only in the z axis, which is not what mitres are designed for. biscuits and splines strengthen the joints in the (x, y) axes, which you proved when you could not pull the pieces apart by hand. i bet if you tested mitres with and without the reinforcement of biscuits you'll find a huge difference.

[Michael-Makes-Stuff]

This series is fantastic! Thank you very much for your research and for sharing it with these videos. Keep ‘em coming! 👍🏼

[csimet]

My understanding was always that biscuits add no real strength, rather are used for part alignment only. Love to see you test dowels and dominos as a comparison.

[bobfugazy]

Thanks Michael. Like Switch & Lever below, I always used biscuits for alignment and with my coffee. Nice work.

[PatJHeffernan]

Unfortunately you’ve tested these in bending with the biscuits at/near the neutral axis (middle) where stresses would be near zero until after failure of the top/bottom of the specimen. Failure of the top/bottom would largely not be affected by the biscuit at the neutral axis. You’ve indicated the limitation of your force application, but you didn’t limit your conclusions to reflect them. To use your bending setup effectively, maybe consider putting a pair of biscuits so they are located away from the centre of the wood specimen to see if they add strength when actually being stressed before failure. It is nice to see a more deliberate effort to pull apart some of these glue myths, keep it up! 👍🏻

[Alkz]

One point to consider: the biscuit was inserted to the neutral axis formed by the bending moment. I think this location plays a bigger role then it's mechanical properties or the gule.

[jacobgrover]

I have really been enjoying the conversations you have brought up to the woodworking community. Thank you

[richardflorence]

As others have said, alignment is the issue when I pull out the biscuit joiner. Almost always in situations where I have side grain to side grain - not end-to-end grain.

The myth seems to be that people use biscuits for strength since most comments state they have never relied on them for strength. The series does help us all in that it is consistent in method and either reaffirms what we suspect or blows up the folklore of others.

Great series with a lot of lessons.

[Lakoda]

Great series but as you pointed out in the end, you didn't test the most common problem forces.

[mboyle]

I just watched your glue series and was impressed by the experimental setup that allowed head to head comparisons among the different woods and joint orientations. Avoiding differential calculus in favor of a solid "4X stronger" evaluation was a great choice enabled the design of your samples. Very informative, great presentation, fantastic attention to detail.

[nickpelov]

I watched a video about what makes the material strong when forces are applied like this. One surface is under compression, the other is under tension. Most of the material in the middle does not contribute too much. So the only way to increase strength is to put something on the surface. even thin veneer will do better than biscuit. However ff you are making a frame a spline will add to the strength, because as you said there are other directions that forces can be applied from.

[SkylersRants]

Those other force directions that you leave for others are the primary way that miter joints are used!

[mikkelkirketerp]

Hello Patrick Sullivan.
As a mechanical engineer I can tell you why a biscuit wouldn't add strength in your test, even if the biscuit was made of something much stronger.
Whenever a given object is experiencing bending, the actual stress in the material is NOT uniform across the thickness of the object.
It is the highest the further away from the center (oversimplified) you are.
And as such, a biscuit put in the center of the boards won't ever get to help reinforce the joint when it comes to bending.

Perhaps a test with a board thick enough to have two biscuits in the height plane could prove that biscuit either add or subtract strength.
The biscuits would need to be close to either surface instead of bundled together in the center.
So perhaps a 20mm thick piece with either biscuit being only 4mm below the surface would prove to be stronger or weaker than without.

[kylejacobs]

Your 3 point bending test illustrates a simple engineering concept for beams. Loaded beams have one side in compression and one side in tension. In between is a thing called the neutral axis, where the stress is zero. In your case, because the biscuit is located in the middle (which happens to be the neutral axis for this cross section), the bending does not even put stress on the biscuit until after the butt glue joint has failed.

[collinmerrill]

Great videos Patrick. I know some others have suggested it, but it is "common knowledge" that domino joinery is just as strong as a traditional mortise and tenon. I've always doubted this, but I would love to see a test done. Keep the videos coming! Woodworkers have been doing it right for millennia, but technology has changed and anecdotal truisms can be tested!