Thread: How would you cut these mortices?

Originally Posted by qwertyu

Graeme I think you have confused 2 threads. ...

Oops! I have - my bad.

Furniture And Cabinetmaking.
GMC publications, Issue 308 (on newstands now)
p120

"Araluen leg joint" - Nathan Day.

20230110_093948.jpg20230110_093954.jpg

Originally Posted by eddie the eagle

Double the hole diameter seems excessive.

Epoxy needs a minimum 0.5mm-1.0mm gluleine thickness, depending on the epoxy/manufacturer. Personally, I'd drill a 10mm hole for an 8mm threaded bar. Don't know the reason for the double diameter - might be a reason I haven't thought of.

Hi Eddie

I found some of my old notes from 40+ years ago. It was photocopied from either Gougeon Brothers book or WEST brochure - Gourgeons created WEST System. Its a bit faded except for the comments I added today. They did a lot of imperical testing which was extensively reported in the yachting and boat building media fifty years ago.

WEST Screw Testing.jpg
The Gougeon Brothers book is available on the WEST System website. Their imperical testing data is probably still available there, too, somewhere. They are Canadians, so no metric measurements!

If you do use epoxy it is best to apply release agent to all screws in case you want to remove one in the future. I just use aerosol silicon spray lubricant.

Initially, I also thought that double the screw diameter was excessive, but the epoxy is stronger than the wood, and you are spreading loads over a wider area.

Originally Posted by GraemeCook

If you do use epoxy it is best to apply release agent to all screws in case you want to remove one in the future. I just use aerosol silicon spray lubricant.

Initially, I also thought that double the screw diameter was excessive, but the epoxy is stronger than the wood, and you are spreading loads over a wider area.

Makes sense, Graeme.

Thanks for sharing the above. Shear strength definitely increases with hole dia until limit reached.

The photos in my post immediately above are the designer, Nathan Day discussing how he makes this exact joint. (Answer to the initial question)

M8 gal threaded rod, glued both ends. The thread is for keying of the epoxy, dominoes for alignment. No nuts/tension adjustment in the joint.

Plus a good photo of the design.

Thanks everyone. All very interesting.

My initial question was though, how would you cut the mortises into the concave surface another table that Nathan Day makes

All of this epoxy/threaded rod talk is secondary (but still very interesting)

I just figured that being concave the domino XL wont have enough reach

Originally Posted by qwertyu

Thanks everyone. All very interesting.

My initial question was though, how would you cut the mortises into the concave surface another table that Nathan Day makes

Hi qwertyu,

Had a look at the photos in your first post.

I'd use a slot mortiser, or make a jig/guide to hold a long-throw drill bit (auger, potentially) and use a 12mm dowel

As mentioned, Nathan Day uses a domino XL How would you cut these mortices?

I did read the original West document.
https://www.westsystem.com/wp-conten...k-061205-1.pdf
As some mentioned before in this thread, if you have an 8mm rod, drill a 10mm hole first about 70% of the depth of the threaded rod, then drill an 8mm hole approx to match the depth of hole needed, with a couple of mm extra for error.
(p379/412 on the pdf, or p365 of the book.) If you can keep the fastener centred by other means, then do so, and drill out to 100% depth.

As well, p136 gives a simplified version.

Cheers,

Oh yes - forgot about that post it was the DominoXL...

Thinking about this more, if epoxy is stronger than the wood, is there much benefit of the screw/threaded rod?

Why not just drill holes on either side and fill it with epoxy? Essentially create an epoxy dowel

What about this - 12mm hole with 10mm threaded rod vs 8mm threaded rod - which is stronger?

Or, Cut a 12mm mortice with domino and put 2x threaded rods next to each other

Originally Posted by qwertyu

Thinking about this more, if epoxy is stronger than the wood, is there much benefit of the screw/threaded rod?

Why not just drill holes on either side and fill it with epoxy? Essentially create an epoxy dowel

What about this - 12mm hole with 10mm threaded rod vs 8mm threaded rod - which is stronger?

Or, Cut a 12mm mortice with domino and put 2x threaded rods next to each other

The dominos are for alignment only, qwertyu - the strength of the joint comes from the steel rod, so I'd keep the dominos there, or use 12mm dowel in place of the dominos and drill holes with a jig if a domino XL isn't available.

The epoxy dowels would be weak in bulk - one void and they snap. Epoxy isn't anywhere near as tough or fatigue resistant as steel - I'd keep the steel in. 10mm steel threaded rod is approx 60% stronger than 8mm in tension/compression (technically it's (9^2-7^2)/7^2 before an engineer picks it up)

The epoxy gives shear strength, circumference of the hole times depth - go too deep and then tensile strength of the threaded insert becomes the limiting factor, so it's a balance

so, a 12mm diameter hole has 20% more shear strength than a 10mm dia hole, with the 10mm rod being significantly stronger and more resistant to bending.

Make sense?

Originally Posted by eddie the eagle

The dominos are for alignment only, qwertyu - the strength of the joint comes from the steel rod, so I'd keep the dominos there, or use 12mm dowel in place of the dominos and drill holes with a jig if a domino XL isn't available.

You can cut 2x dominos with a domino in 1 and threaded rods in another - cutting dominos is much faster/easier than drilling holes

Originally Posted by eddie the eagle

so, a 12mm diameter hole has 20% more shear strength than a 10mm dia hole, with the 10mm rod being significantly stronger and more resistant to bending.

Im trying to compare 12mm hole with 8mm vs 10mm threaded rods. So 12mm hole are you better off with 2mm epoxy around 8mm rod or 1mm epoxy around 10mm rod. Or, even 3mm epoxy around a 6mm rod in a 12mm hole.

Originally Posted by qwertyu

You can cut 2x dominos with a domino in 1 and threaded rods in another - cutting dominos is much faster/easier than drilling holes

That'd work, but see the below.

Originally Posted by qwertyu

Im trying to compare 12mm hole with 8mm vs 10mm threaded rods. So 12mm hole are you better off with 2mm epoxy around 8mm rod or 1mm epoxy around 10mm rod. Or, even 3mm epoxy around a 6mm rod in a 12mm hole.

Working off the West research Graeme gave here https://www.woodworkforums.com/attac...hmentid=521343, it looks as though a glue line of 2mm was stronger than 1mm in pullout (shear) strength.
The tensile strength of the rod means it breaks if too small (superscript 5 in the table above.)

I'd stick with their recommendations. When I was trained it was that epoxy needs a minimum 0.5 mm glueline for strength, so 1mm epoxy may not be enough unless the fastener is centred in the hole. The West systems research seems to suggest that a thicker glueline is better for pullout strength (= shear strength,) potentially as it increases the diameter of the glue plug as my initial guess.

So, it'd be an edjumacated guess. 2mm epoxy glue line or more seems to be their recommendation. But I wouldn't go too skinny on the steel if the joint is under load.

Make sense?

Yep, thanks.

One option is to use socket cap screws to help centre the rod - put it in backwards

socket cap screws - Google Search

Just an idea.

Another thought - what has better pull out strength:
- M6 bolt in epoxy thread (ie spraying bolt down with silicon and setting it in epoxy in oversized hole) vs
- threaded insert - say 25mm insert
*** though way more work to make epoxy threads vs putting in a few threaded inserts

For an application such as this:
How To Attach Table Legs - DIY Woodworking - YouTube

Another thought - Could one cast a bunch of epoxy threads, then just epoxy them into holes when needed? Epoxy sticks to epoxy right?

I am over thinking this hehe but its just interesting

Originally Posted by qwertyu

... My initial question was though, how would you cut the mortises into the concave surface another table that Nathan Day makes ...

I have never done a Nathan Day type table so I cannot answer directly.

However, I had a similar issue when I first made a Maloof inspired chair and coffee table. The Maloof-joints have to be very accurately cut otherwise the leg would be either wobbly or askew or both. How do you hold a molded leg to cut the joint so precisely?

The solution was incredibly simple. Pre-planning. You cut the joint when the timber is still in square form, then you do the shaping.

Originally Posted by qwertyu

Thinking about this more, if epoxy is stronger than the wood, is there much benefit of the screw/threaded rod?

Why not just drill holes on either side and fill it with epoxy? Essentially create an epoxy dowel ....

Epoxy is stronger than wood, but the steel in the screws is stronger than either, much stronger.

... What about this - 12mm hole with 10mm threaded rod vs 8mm threaded rod - which is stronger? ...

Obviously, 10 mm threaded rod is substantially stronger than 8 mm rod, but do you actually need the strength of the thicker rod? In most domestic furniture, 8 mm rod is probably overkill, and 6.5 or even 5 mm is all that is required engineering-wise.

If you look at Gougeon's Table in my above post, you will see that the bigger the diameter of the epoxy filled hole then the stronger the assembly. I think this is because you are spreading the load over a wider area. If the holes are 50 mm deep, a 10 mm hole has a surface area of 1,571 mm2 while a 16 mm hole has 2,513 mm2 - a difference of 60%.

WEST Epoxy Bedding.jpg

		Hole 1	Hole 2
	Diameter	10	16
	Circumference	31	50
	Depth	50	50
	Surface Area	1,571	2,513
	Variance		60%

... Or, Cut a 12mm mortice with domino and put 2x threaded rods next to each other

Stop making things harder! My head hurts.