# Joint two dowels - square or round tenon?



## Bodgers (Sep 3, 2018)

I posted this question on a UK forum I am a member of, and there was a clear response over which was better.

My (bad) sketch shows the two options. Key point is that the two dowels are subject to rotational force. Beech is the material.










What would be stronger?


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## eflanders (May 2, 2013)

If there is rotational force between connected parts, it's a no brainer, the square tenon. It provides the most surface area for glue and a keyed connection.


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## Bodgers (Sep 3, 2018)

> If there is rotational force between connected parts, it s a no brainer, the square tenon. It provides the most surface area for glue and a keyed connection.
> 
> - eflanders


I agree. But, the replies I got on UK workshop firmly came down against the square tenon and in favour of the round.

Interesting…


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## clin (Sep 3, 2015)

There's no question the square one will transmit the rotational forces even with no glue. So in that sense it is better. But if it isn't a tight fit, the glue will be holding it. And if the glue fails it might not matter if it's square if the pieces would separate.

Glue is also strongest in shear, and the round one the entire glue surface is in shear. The square also applies tension to the glue join, so maybe it would tend to fail more. But of course it would have more glue area.

Concerning the U.K. choice of round over square, was that because of function or perhaps related to how you would make the joint or even how you might assemble the finished item? Or maybe they know something about the application. For example that square versions tend to crack because the corners of the mortise to create stress risers.

If the rotational forces are relatively low, I think round is fine. But if it gets serious torque, square would be mechanically secure. So without knowing more, I go with square.


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## AlaskaGuy (Jan 29, 2012)

> If there is rotational force between connected parts, it s a no brainer, the square tenon. It provides the most surface area for glue and a keyed connection.
> 
> - eflanders
> 
> ...


But why?


> ?


?


> ?? What was their reasoning


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## 000 (Dec 9, 2015)

Furniture dowel and recess the handle into the head with glue.


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## Bodgers (Sep 3, 2018)

The reasoning was related to what clin refers to.

Weakness at the corners and the fact that cutting the square on the end of the dowel and the mortise in the other weakens the whole thing.

One poster refers to grain direction. But I don't see that as being any different. Unless this was the shear thing.

The application is a tail vise screw. The thicker dowel is the head end.

Nothing special about the assembly. It will just be glued (possibly with epoxy) and then clamped (if needed).

I will post the thread later.


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## Bodgers (Sep 3, 2018)

https://www.ukworkshop.co.uk/forums/viewtopic.php?t=113688


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## TravisH (Feb 6, 2013)

I have thought about this one before. We can measure all the forces at play and determine the scientific answer but if both methods work for the intended application and we don't truly know the numbers equates to basically a bunch of woodworkers playing arm chair physicists. Interesting discussion and something I could see Mathias Waddell or other online video woodworker putting to the test. Historical use likely can be used as "best method" of construction.

Historically the pieces I have looked at, old work benches, grape presses, wheels, old wood gears, and other "vintage" wood screw applications always appear to use round mortise. I figured most of that was a function of method of making and ease of repair. Doesn't get much quicker than drilling a hole and rounding a spoke. Now many of these pieces are hundreds of years old and have been used a few times over that time period and appear no worse from wear. I have observed several that have been pegged but very few.


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## EarlS (Dec 21, 2011)

I think I would go with a loose tenon, that is, a rectangular dowel that has had the edged rounded off. The accompanying mortise would be made using a router (see the Leigh mortise and tenon jig). The problem of sharp corners that would be week points where the stress/load would cause the wood to break would be reduced/eliminated.


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## jerkylips (May 13, 2011)

I would tend to think that if you're gluing it, the shape would be somewhat irrelevant. Modern glues like Titebond are so strong, the wood will likely fail before the glue joint. In your application, I can't imagine you'll be stressing it that much.


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## ScottM (Jul 10, 2012)

To me, the question is, how much rotational force are we talking about? In other words, will the wood stand up to that force regardless of the joinery method.


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## therealSteveN (Oct 29, 2016)

> If the rotational forces are relatively low, I think round is fine. But if it gets serious torque, square would be mechanically secure. So without knowing more, I go with square.
> 
> - clin


I agree with this reasoning, and if making this joint I'd would proceed that way, but it would be nice to know what you were making. I would assume the Brits knew, and it had a low rotational force. Otherwise their answer makes no sense.


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## DS (Oct 10, 2011)

Unless you are making them by the thousands for resale, it doesn't really matter.

Heck, build one of each, try them out, then let US know which one was better.

My 2 cents…
Have fun.


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## Bodgers (Sep 3, 2018)

Hard to quantify the exact forces.

This is a tail vice screw. So, if you imagine the 'Head'/thicker end with a wooden bar going through it (which is the screw handle), this is about 250mm long, the hub being 60mm in diameter, so you have a force being applied on the end of that by my hand and arm (I don't work out, and my arm isn't that strong) to turn the screw that clamps a work piece.

It is possible that the wooden handle passing through the head will break first, I suppose.


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## jmos (Nov 30, 2011)

The square end would certainly transfer the rotational forces better, and not rely entirely on the glue for strength. The round would be easier to cut.

If it were me, I think I would go with a round mortise and tenon, and peg it with a wooden or metal pin. That would also prevent relying entirely on the glue to transfer the forces.

In this application, I don't think I would rely just on glue. How many glued parts have we each seen fail on chairs?


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## eflanders (May 2, 2013)

I was the first to say square here but now that there's more information… Rotational torque in this application could be quite substantial. Substantial enough to break a square or rectangular tenon without glue. This is because a sharp edge will always fail before a radius edge will. This may be the reason why others voted round. If I was making this, I'd use a round tenon, pegged with a round dowel. That would distribute the forces the most equally with or without glue.


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## TheFridge (May 1, 2014)

Well without glue and applying the same torque the round will just spin. Making it useless.

The square has much much more surface contact area than a pegged joint.


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## therealSteveN (Oct 29, 2016)

Plus if breakage is even a consideration, usually drilling a hole through the center of anything starts it along the path toward weakness.

Eflanders talking about glue failure, were these chairs made in this century, or are you talking about hide glue? If it's hide glue, you have to know much of it was brewed up by a guy with absolutely no knowledge of chemistry, and using products with absolutely zero quality control. Sometimes I think it's incredible any old furniture isn't just a pile of loose boards.

Any failures of modern lab made glues is most always because of product that was open too long before use, frozen. or simply applied out of it's working temperature range, or other environmental problems before application.

Fresh PVA glue, with surfaces properly prepped, and the glue applied within it's usage range, is just going to be at least twice as strong as the wood itself. It's almost immeasurable because the wood breaks before any real data can be formed.


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## WoodenDreams (Aug 23, 2018)

I think this is a matter of personal preference. Whether you choose square or round, I would use Epoxy instead of wood glue. the advantage is over time. With the round it doesn't matter on direction of force. It's the same in each direction. With the square the dowel only has equal force on 4 sides. When the square is not square with the force, the joint will loosen quicker over time….. I myself would use the round with epoxy.


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