Dowels vs. Biscuits. Speed vs. Strength

[mac davis]

I've ALWAYS used dowels and a doweling jig.. (30+ years) and, I've always hated them but saw them as a necessary evil of some types of joints..

Since getting the dewalt/craftsman b-joiner last year, I've put away my dowel stuff in a deep, dark corner somewhere... IMO, biscuits make the dowel old fashioned (not always a bad thing) and hard to align..

I seemed that no matter how precise I thought I was, (dowel jig, drill press, etc.), some joints were just a bit off.. no problem with a biscuit, but hell with a dowel..

mac

Please remove splinters before emailing

[George]

Start with a concept of load.

A biscuit installed parallel to the load has the shear strength of the glue that holds it. A dowel - loose tenon - has far greater shear strength. A biscuit installed perpendicular to the load has much greater strength, but is still only 1/8" thick, vs. 3/8 or 1/2.

Then think withdrawal. Here the initial nod also goes to the dowel for as long as the glue remains sound. Unfortunately, it's a cross-grain situation, and the dowel will work loose of the glue with humidity cycles. The biscuit suffers much less, and so over the long run will be better.

Now return to a real M/T and notice that it's also a great shear joint, but cross-grained, and subject to the same, if lesser problems than its round cousin. We pin the tenon, and there's a new shear kid in town. We have to shear the pin to withdraw the joint. This is made extremely difficult by the registry of the shoulder of the tenon with the face of the mortise.

[patrick conroy]

Chris DeHut - the guy who produces Woodworking at Home DVD did a nice 'speriment on this. His approach and explanation made sense to me. He used a dial indicator to show that a depression can appear.

Now I know what you're thinking - it doesn't take a dial indicator to know when you're depressed...

[Duane Bozarth]

:)

OK, what was the average depression he measured?

[J]

Do you have some data for this? I'm sure you could calc the shear strength of a hardwood dowel. But where is your data showing that the glue has less shear strength? And is it really the shear of the glue or the shear of the biscuit itself which is at issue? I think I saw some photos of destructive testing of biscuit joints out there. I seem to recall that they did not fail at the biscuit. Loose tenons are a different story.

And is much wider. The cross section of the typical biscuit is much larger than the cross section of the dowel. A #20 biscuit has a cross sectional area of 0.35 sq in. You would need a dowel 2/3 inches in diameter to match that cross section. Not easy to use that size of dowel in 4/4 stock. :-) This is why the loose tenon was invented.

I suggest you calculate the surface area before you make this conclusion. I'm not going to do the math for you though. It looks like it is close.

[Lawrence Wasserman]

Would that be Hamilton or Goldberg?

[George]

[J]

conclusion.

Good to see we are in complete agreement!

-j

[Mike Marlow]

What about the cross section of two dowels in the same space a biscuit would occupy?

[J]

[bridger]

shear is a direct function of section. a dowel with a given section is much thicker than a biscuit of the same section, so it weakens the board it's in more. dowel joints often fail by blowing out the wood surrounding the dowel rather than breaking the dowel itself.

[Guess who]

Do more math: The dowel has the same cross-section throughout the penetration into both pieces of wood, and the biscuit cross-section lessens away from the joint. And, with today's glues the joint will be the last to go. That makes the dowels the winners. Also there is no comparison with strength of materials; a solid hardwood dowel wins hands down over pressed paper.

We can theorise all we want until there is solid scientific [experimental results] evidence one way or another to back up "common sense". The point is that if you drop a ton weight on it, it won't matter either way, and barring that, either is "strong enough", and the important factor is ease and accuracy of assembly and cost, not strength.

[bridger]

1) shear happens at the joint. that is the measure that matters for shear. other modes of failure vary, but the section at the joint is the relevant one for most of them. 2)biscuits are made from beech, not paper. most dowels are made from birch. anybody with hard data about those species? I'm betting beech has better strength than birch....

ease of use favors biscuits, unless you're talking about using big dollar multi-spindle dowelling machines. strength data goes either way, depending who you're talking to...

[GerryG]

Ahh, I've been waiting to see if anybody raised that rather practical point! I've also seen whole dowels pulled out of a joint, but I've never seen a biscuit do so, as it breaks instead. Part of this may be the dowel having a weaker glue joint, due to the relatively large amount of end grain. Once it starts to fail, it either pulls out of the joint, or acts as a lever to blow out the side of the wood.

Now, has anybody compared the difference between dowels, from smooth to ridged to ringed? I've heard said that a ringed dowel is far stronger, but haven't tried it or seen any data. Similarly, we now have the Miller dowel system as another variable. GerryG

[Mike Marlow]

Sorry J - I wasn't asking what the cross section was. Typed out my reply in too much of a hurry. I was headed down the strength of materials road. Was it George or you that just posted the home brewed trials the other day? That proved to be a very interesting observation.

[J]

[George]

My remarks were general observations on the nature of the three joints. The ad hominem remarks made it clear that J is a convinced individual - convinced that he's correct. I elected not to play.

Now, once again, you have the two loads to consider, and the information I gave is accurate. Further, you have to consider the instantaneous blow - the one which eliminates biscuits oriented along the line of load as worthwhile alternatives.

[J]

Which ad hominem remarks? I don't think I posted anything like that.

-j