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View Bodgers's profile

Joint two dowels - square or round tenon?

by Bodgers
posted 09-03-2018 02:03 PM


19 replies so far

View eflanders's profile

eflanders

339 posts in 2909 days


#1 posted 09-03-2018 03:24 PM

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.

View Bodgers's profile

Bodgers

6 posts in 960 days


#2 posted 09-03-2018 03:35 PM



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…

View clin's profile

clin

1128 posts in 2055 days


#3 posted 09-03-2018 04:33 PM

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.

-- Clin

View AlaskaGuy's profile

AlaskaGuy

6484 posts in 3368 days


#4 posted 09-03-2018 04:42 PM

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…

- Bodgers


But why????? What was their reasoning

-- Alaskan's for Global warming!

View 000's profile

000

2859 posts in 1958 days


#5 posted 09-03-2018 04:47 PM

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

View Bodgers's profile

Bodgers

6 posts in 960 days


#6 posted 09-03-2018 04:57 PM

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.

View Bodgers's profile

Bodgers

6 posts in 960 days


#7 posted 09-05-2018 07:32 AM

View TravisH's profile

TravisH

764 posts in 2994 days


#8 posted 09-05-2018 11:14 AM

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.

View EarlS's profile

EarlS

4423 posts in 3407 days


#9 posted 09-05-2018 11:24 AM

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.

-- Earl "I'm a pessamist - generally that increases the chance that things will turn out better than expected"

View jerkylips's profile

jerkylips

495 posts in 3629 days


#10 posted 09-05-2018 02:16 PM

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.

View ScottM's profile

ScottM

756 posts in 3206 days


#11 posted 09-05-2018 02:25 PM

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.

View therealSteveN's profile

therealSteveN

7523 posts in 1633 days


#12 posted 09-05-2018 02:40 PM


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.

-- Think safe, be safe

View DS's profile

DS

3727 posts in 3479 days


#13 posted 09-05-2018 02:59 PM

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.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View Bodgers's profile

Bodgers

6 posts in 960 days


#14 posted 09-05-2018 03:19 PM

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.

View jmos's profile

jmos

918 posts in 3429 days


#15 posted 09-05-2018 04:39 PM

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?

-- John

View eflanders's profile

eflanders

339 posts in 2909 days


#16 posted 09-05-2018 10:05 PM

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.

View TheFridge's profile

TheFridge

10863 posts in 2545 days


#17 posted 09-06-2018 12:09 AM

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.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View therealSteveN's profile

therealSteveN

7523 posts in 1633 days


#18 posted 09-07-2018 07:31 AM

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.

-- Think safe, be safe

View WoodenDreams's profile

WoodenDreams

1293 posts in 970 days


#19 posted 09-07-2018 07:48 AM

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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