Domino Jointing system - what are "Extremely high pull-out and load forces"?

[sdes]

I think Festool is only suggesting that these Dominos can withstand a high pullout force relative to the standard knockdown components that are currently available. 

I understand you would like these to work in your application but that’s just not feasible.

The Domino has 3 ridges on each side that digs into the wood, consider these to be the equivalent of 3 screw threads. How much load will a 14mm screw support when only 3 threads are engaged?

Also when using a screw, a pilot hole is drilled and the screw manufactures its own internally threaded coupling. There is line-to-line contact between the wood and the fastener for 360 degrees. 
With a Domino, a clearance hole is used and the only line-to-line contact are the sides of the 3 ridges. Maybe...maybe there is 180 degrees of contact. However you’re still relying on the 3 ridges to “push themselves” into line-to-line contact with the wood as opposed to being “driven” into contact. 

So take a 14mm screw, engage only 3 threads, derate it by at least 50% for lack of peripheral contact reasons and you’ll probably be close to the pullout spec of the Domino. I don’t know what that is but it’d be interesting.

Hi Cheese, well for an inanimate dairy product you've given me a very well reasoned and useful answer ;-) thank you.

I think pretty much everything you state makes sense, and I agree with your analysis of the Festool connector profile relative to a 360º screw thread. I might give it a bit more than the 180º you give it, there is effectively a "crescent moon" shape of engagement for each barb profile, i've done a quick comparison, and assuming equivalent thread depth, I get almost exactly 2/3.

[attachimg=1]

Another advantage the Festool Connector has is that unlike the conventional insert which has both a helical thread form, so some of the pull out is tranlated to twist, and also a V form thread which slightly ramps out under load whereas the Festool connector has effectively a barb once deployed which slightly digs in under load.

Here is a manufacturer, ezlok.com, of threaded inserts who helpfulluy and not that unusually, give a range of pull-out values for various timbers for each of their inserts for furniture making. For the M8 insert (I'm assuming that 8mm is the internal machine screw thread for the mating part, and not the external one) the pull out forces seem to range from about 850lbs (385kg) for Spruce to 1900lbs (860kg) per connector for White Oak depending on wood type. 

[attachimg=2]

So, just very roughly, for an "L" lever, where the long arm is the  800mm long tread with say a short arm being 150mm distance from pivot to insert centre line.
Each insert is capable of supporting :
850lbs x (150/800) = 160 lbs (72kg) at the extremity of the tread for the worst case timber, Spruce
1900lbs x (150/800) = 360 lbs (163 kg)  at the extremity of the tread for the worst case timber White Oak.

Obviously this is only static loading and not dynamic, and no safety margin, but its quite a large person standing on the end of the tread on the White Oak staircase ;-)

So perhaps I just use some cheap threaded inserts... but dividing by 2/3 still gives a pretty good value per Domino.

All theoretical I know as far as the Dominoes as no data available, but even if with their "extremely high pull-out and load forces" they were only half as good on pull-out as a cheap threaded insert, they are in the right order of magnitude to be viable.

So, perhaps the only part I might not end up agreeing with you on Cheese, is the "I understand you would like these to work in your application but that’s just not feasible". They might not be reccomended for it, and obviously innovation with them is heavily discouraged by Festool, but if they are half as good as a threaded insert they might be feasible.

Only tests will tell.

Thanks again for your useful input anyhow. Sadly, more useful than Festool's by a factor of 10 at least.

Cheers

Stephen

 

[sdes]

Festool UK have come back to say:

“The connectors have never been marketed as a construction fixing to take high loads that may carry the load bearing potential of a human. They are designed as a KD fixing for furniture and not any other application. “

...but they do posess "extremely high pull-out force", but it is unquantified and should not be relied upon. So, nothing that supports a human. Quite limiting what with at least half of all furniture being designed to support people.

Anyhow, if we make anything during the restoration that is Knock Down, out of 30mm+ thick material, that does not need to support the weight of a human then I'll be sure to share some photos. At the moment I can't think what that would be.

Ho hum. Topic closed I think as it seems to be stone and blood out of teritory from the "official" direction at any rate.

Thanks for the responses anyhow and good luck with your non human carrying Domino XL Connector projects

Stephen

Hi
The quote above is from my series of emails today with Stephen - the connectors for the DF700 have never been designed for any  high load bearing application such as a staircase or even similar. They are a KD fixing for use on tables, frames for beds and should be used with that in mind
rg
Phil

Seems Stephen while asking for advice is only asking for justification in using these fittings for whatever application he has in mind and not interested in reasonable advice that is being given by anyone else.
My advice after seeing the sketchup drawing is ..use the DF700 as a morticer and peg the tenons of the brackets.
Rob.
[/quote]

Hi Rob,

Thanks for the response, but if you refer to my original post, you will see I was never asking for advice nor justification from anyone, sipmly some figures on pull out forces to back up the "Extremely high pull-out and load forces" claim that Festool make.

It turns out this is subjective statement with no datasheets etc available to end users to validate it or assist with projects.

Given that for any particular design problem, there are generally a variety of solutions, I was clearly hoping this connector might be part of one. I was never asking for anyone to endorse my stair design, just provide some basic info as other manufactures do for their products. As Festool had said there was no such information available I thought that perhaps the users of this site might have done some tests at some point.

Personally I enjoy finding new applications for things as opposed to always following the conventional approach, but I understand there are risks of failure and requirements for testing and evaluation - but thats called design development.

As Festool have done with these connectors.

Thanks for the advice anyhow, all useful.

Stephen

 

[Svar]

Keep in mind that the insert you are comparing to has 6 twists, while Domino connector only 3 teeth. Another weak point are aluminum bendable strips that connect the teeth with the body of the connector. The entire pullout force rests on them. Compare that to a regular steel tie bolt. Alum/Steel tensile strength depending on the alloy could be up to 1/8.

Edit: OK, you did account for three teeth in you previous post. Still, does not look like construction/stair type solution.

[attachimg=1]

 

[semenza]

Hi,

    That looks like quite a restoration project. It would be great if you wanted to start a topic and share some on going pictures and such as it progresses. It looks pretty interesting.

Seth

Yes, it is, not mine unfortunately but a good friend's who coincidentally introduced me to Festool - I've sadly become adicted.

.

Anyhow, if we make anything during the restoration that is Knock Down, out of 30mm+ thick material, that does not need to support the weight of a human then I'll be sure to share some photos. At the moment I can't think what that would be.

Stephen

Well, it would be cool to see some pictures of your friend's restoration even if it doesn't involve the Domino connectors.

Seth

 

[Cheese]

Well, it would be cool to see some pictures of your friend's restoration even if it doesn't involve the Domino connectors.

Hey...hey...hey, I’d 2nd that thought.  [big grin]

 

[sdes]

Keep in mind that the insert you are comparing to has 6 twists, while Domino connector only 3 teeth. Another weak point are aluminum bendable strips that connect the teeth with the body of the connector. The entire pullout force rests on them. Compare that to a regular steel tie bolt. Alum/Steel tensile strength depending on the alloy could be up to 1/8.

Edit: OK, you did account for three teeth in you previous post. Still, does not look like construction/stair type solution.

[attachimg=1]

You are correct, the arm elements that carry the barbs and hence the retention forces are distorted as the bolt part is inserted which must weaken them to some degree.

I'm not sure what that part is made of, there doesn't seem to be any info on it, but I have one and it seems too heavy to be an aluminium casting. I'm guessing it might be a Zinc based alloy with higher tin proportion to provide the required flex.

I would still imagine it would be the wood that fails rather than the metal fixing itself - unless used in a very exotic hardwood - Lignum Vitae would be a challenge - but for "normal" joinery timbers, I would predict the wood to be the weaker link.

Having said that, as a side note, the very old Larch timbers that the house is made of is extraordinarily hard. It is also highly resinous, which means it has an inbuilt "hotmelt".  Drive in a large screw with an impact driver and you have about 10 seconds to change your mind, once the heat generated from driving it in has dissipated and the resin re-set, trying to unscrew it will just rip the head off the screw.

The 28' tall larch newell that we have put in for the spiral stair is new (about 6 year old) larch though, so quite a bit softer owing to the faster way the trees are grown in modern French forrestry, and should be significantly softer than whatever fasteners we eventually choose.

Thanks for your thoughts & observations on the Domino connector.

Stephen

[attachimg=1]

 

[Birdhunter]

In looking at Svar’s picture of the jam connectors, it struck me that one could fill the cavity with something like JB Weld before and after seating the jam connector. Hat would give the joint a lot more pull out resistance.

My bed project using the corner connectors didn’t impose any or much pull out stresses.

I’d love to see how others are using the connectors.

 

[Svar]

[member=64706]sdes[/member] , are the treads tied to each other along the outer spiral? Directly or through handrail? This will redistribute the load to multiple treads and change the picture.

 

[Michael Kellough]

Whatever figures you find for strength of fasteners in wood have to be carefully considered.

For example the EZ-Lok insert pull-out figures are for installation in the side of boards not end grain in which case they have very low pull-out resistance due to the fact that the knife edge external threads neatly slice through the end grain severing it so the fastener pulls right out.

For end grain inserts the more common blunt thread style holds better but not well enough for a cantilevered load.

 

[sdes]

[member=64706]sdes[/member] , are the treads tied to each other along the outer spiral? Directly or through handrail? This will redistribute the load to multiple treads and change the picture.

Ideally, from a "wow" perspective / objective, each tread would be a separate element, but yes, you are quite right to say that linking them would significantly change (reduce) the requirements placed on the retention connector.

 

[sdes]

Whatever figures you find for strength of fasteners in wood have to be carefully considered.

For example the EZ-Lok insert pull-out figures are for installation in the side of boards not end grain in which case they have very low pull-out resistance due to the fact that the knife edge external threads neatly slice through the end grain severing it so the fastener pulls right out.

For end grain inserts the more common blunt thread style holds better but not well enough for a cantilevered load.

Very true, I imagine the extremely high pull-out force might drop to just very high or worse on the end grain.

Thanks for the input.

Stephen

 

[Cheese]

Having said that, as a side note, the very old Larch timbers that the house is made of is extraordinarily hard. It is also highly resinous, which means it has an inbuilt "hotmelt".  Drive in a large screw with an impact driver and you have about 10 seconds to change your mind, once the heat generated from driving it in has dissipated and the resin re-set, trying to unscrew it will just rip the head off the screw.

The 28' tall larch newell that we have put in for the spiral stair is new (about 6 year old) larch though, so quite a bit softer owing to the faster way the trees are grown in modern French forrestry, and should be significantly softer than whatever fasteners we eventually choose.
[attachimg=1]

Interesting photo...the renovating process will be even more interesting. We’re eager... [smile]

As a rule of thumb, the pullout value in endgrain is about 25%-30% less than sidegrain.

 

[Alt]

Hi,
I am trying to determine if the 700 KD connectors can hold for a bed frame application, as follows.

[attachimg=1]

The material is solid maple, 1 3/4", weighing almost 150 lbs.
Each long side, uses 2 connectors and one 100 mm domino.

Is there a way to figure out if the middle connection will hold?
Or, should I add additional support in the middle, which I'd rather not?

Thank you,

Alex

 

[Cheese]

Hi,
I am trying to determine if the 700 KD connectors can hold for a bed frame application, as follows.

[attachimg=1]

The material is solid maple, 1 3/4", weighing almost 150 lbs.
Each long side, uses 2 connectors and one 100 mm domino.

Is there a way to figure out if the middle connection will hold?
Or, should I add additional support in the middle, which I'd rather not?

Thank you,

Alex

Festool recommends at least 2 Dominos and 1 KD connector like this...

[attachimg=1]

 

[Alt]

Thank you [member=44099]Cheese[/member] !

The long piece is comprised of 2 smaller pieces joined by 2 KDs at the edges and one domino in the middle.

A KD connector, I would believe, is, at least, as strong as the domino; thus a connector and 2 dominoes would be the same as 2 connectors and a domino, strength-wise.

My concern is that, given the weight of the frame and the weight of a human, the middle KD connection may buckle (or fail).

Thoughts?

 

[Cheese]

Actually the strength is coming from the Dominos. The KD only allow for disassembly.

 

[Alt]

Good point, now that I think of it!

It would then seem that having another couple of dominoes would make the connection strong enough
to withstand the frame weight & the human w/o buckling. Is that what you mean?

Thank you [member=44099]Cheese[/member] !

Alex

 

[Cheese]

Good point, now that I think of it!

It would then seem that having another couple of dominoes would make the connection strong enough
to withstand the frame weight & the human w/o buckling. Is that what you mean?

Thank you [member=44099]Cheese[/member] !

Alex

Is there a reason why you're not making the sides in one piece? You'd then be assured of not having a buckling issue. I'd expect that once this is setup it'll be a long time before it's torn back down.

 

[Alt]

That would indeed have been ideal.
I had to use connectors because I have no way of moving the sides (if they were whole) to the destination (3rd floor, narrow stairs, etc.).
Thank you [member=44099]Cheese[/member]!

 

[Mini Me]

I'm interested in the possibility of using the Domino Jointing System to hold in treads on a wooden spiral staircase.

I cannot find any data on their strength or range of pull out forces. The Festool website simply says "Extremely stable with dowel and clamping element for strong connections. High pulling and tightening distance. Extremely high pull-out and load forces"

I'm trying to find out what do Festool rate as an "extremely high pull-out force", is that 100N or 10,000N?  I understand there is probably considerable variance depending on the variety of timber, but even a general range for a few conevntional joinery timber types would provide a valuable indication.

The website page I am looking at is this one:
"https://www.festool.com/Products/Accessories/Pages/Detail.aspx?pid=201827&name=Connector-for-DOMINO-joining-machine-EV-32-Set"

Having contacted Festool UK, who then contacted HQ (Germany I assume), they say

"We have deliberately not listed strength data,  because they are always dependent on various factors such as the type of wood, construction or design of the connection.

For such statically loaded connections as in staircase construction one needs an approval. Unfortunately, we do not have them.

To hold the treads on a wooden spiral staircase, the DOMINO connector is exposed to dynamic forces.

For this application the Domino is not to use"

I'm a bit confused that Festool HQ seem to be saying that the Domino Connector is not suited to dynamic forces / loading, apart from a wooden sculpture, there are not many applications where such connectors are not dynamically loaded in some way or another in use.

I'm also not expecting them to in any way guarantee my design, just provide some figures to illustrate their "Extremely high pull out force" claims they make for the connector itself. (The method of use for each connector type is pretty prescriptive) I could then use that info with a significany Factor of Safety to see if the kind of loading I'm anticipating would even be feasible. (I'm not expecting Festool to provide any approval for my design as they seem to suggest, that woud be equivalent to expecting the maker of a rivet to approve the bridge it might be used in)

So, I'm wondering has anyone tested the pull out force that Festool describe as "Extremely high"? There is someone doing, by his own admission, a very unscientific demo of the strength here :

Many Thanks in advance for any info.

Stephen

[attachimg=1]

I am totally puzzled, stairs as a rule never get or need to be pulled apart so why use a connector designed to do just that. Use the Domino to make a big floating mortise & tenon or a conventional mortise can be cut using the Domino and tenons are easy to cut using a table saw or a bandsaw and once the saw is set up correctly they will fit without further handwork. I can understand the wish to minimise handwork because of the number of pieces involved. If the back of the tread cannot be seen then a loose tenon could be fitted to the stringer and the tread simply pushed on to the if the tenon was open at one end and a filler piece put in place if the back was not hidden. I also understand it is human nature to decide on a system and then not want to change our ideas because we are convinced we are right, been there and done that myself.