Author Topic: Domino Jointing system - what are "Extremely high pull-out and load forces"?  (Read 3217 times)

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Offline sdes

  • Posts: 21
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





« Last Edit: March 07, 2018, 06:50 PM by sdes »

Festool USA does not pre-approve the contents of this website nor endorse the application or use of any Festool product in any way other than in the manner described in the Festool Instruction Manual. To reduce the risk of serious injury and/or damage to your Festool product, always read, understand and follow all warnings and instructions in your Festool product's Instruction Manual. Although Festool strives for accuracy in the website material, the website may contain inaccuracies. Festool makes no representations about the accuracy, reliability, completeness or timeliness of the material on this website or about the results to be obtained from using the website. Festool and its affiliates cannot be responsible for improper postings or your reliance on the website's material. Your use of any material contained on this website is entirely at your own risk. The content contained on this site is provided for informational purposes only and does not constitute professional advice.


Offline Birdhunter

  • Posts: 2307
  • Woodworker, Sportsman, Retired
I built a king sized bed for some young friends. I used the Festool 700 connectors to join the bed frame corners.

They have used it for about 6 months. They have a very active 2 year old who uses the bed as a trampoline. I am assuming the bed undergoes regular dynamic stressing.

As of a couple of days ago, the couple reports the bed as totally solid.

The above is not a scientific analysis, but it is real world. I think the 700 connectors are very strong. I’m anxious to try the 500 connectors.
Birdhunter

Offline chewy

  • Posts: 85
Would you still house the treads into the strings, or literally butt the treads up with a domino to join them ? If the latter, I'd have thought shrinkage and movement would cause unsightly gaps.

Sent from my SM-G900F using Tapatalk


Offline epicxt

  • Posts: 424

I built a king sized bed for some young friends. I used the Festool 700 connectors to join the bed frame corners.

They have used it for about 6 months. They have a very active 2 year old who uses the bed as a trampoline. I am assuming the bed undergoes regular dynamic stressing.

As of a couple of days ago, the couple reports the bed as totally solid.

I thought that was going in a REALLY different direction![emoji6]
n = number of Festools I've got.  (n + 1) = Festools I want

Offline Gregor

  • Posts: 955
I'm a bit confused that Festool HQ seem to be saying that the Domino Connector is not suited to dynamic forces / loading
That IMHO is a case of CYA.
Quote
(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)
You're not, I'm not... but I'm certain that, at least in the US, lawyers could be found that would happily try to extort insane amounts of money out of Festool in case someone used the connectors in something and something comes apart with (whatever magnitude of) casualties.

Offline sdes

  • Posts: 21
Would you still house the treads into the strings, or literally butt the treads up with a domino to join them ? If the latter, I'd have thought shrinkage and movement would cause unsightly gaps.

Thanks for the response. I would anticipate trying to conceal the connector completely if possible. It is for a Larch staircase in a 300 Year old French ruin that a friend is restoring, so ideally it would still appear "traditional" even while using contemporary joining methods.



Attached is just a very quick diagram of a possible cross section. My point is that at some point if either reducing the leverage or increasing the number of connectors, it must be possible to make the treads well enough supported to give a Factor of Safety that will give a safe stair. It might be that by that point it uses so many connectors or has such a deep "bracket" (see diagram) that it becomes rediculous / unfeasible. But given that Festool claim  Domnino joints have a claimed (but seemingly totally undocumented) "extremely high pull-out force", perhaps it would not be so rediculous, but without info, and clearly Festool are, for CYA or other reasons, reluctant to give any, its tricky to guess.

Obviously the product has been through design and development cycles in Germany or elswhere, during wich they must have undertaken significant prototype testing and then testing of the production items, they must have the information, but don't wish to publish anything beyond the non specific "Extremely high" touted by their marketing blurb which might actually be of assistance to anyone doing more than decorative woodwork. Ho hum. The results of our (UK) and your (USA) blame / litigation happy society. Easier to sue someone else than do something yourself to make ends meet I suppose.


Offline sdes

  • Posts: 21
I'm a bit confused that Festool HQ seem to be saying that the Domino Connector is not suited to dynamic forces / loading
That IMHO is a case of CYA.

I think you are, how would you say, "Right on the money".


Offline grobkuschelig

  • Posts: 400
@sdes why you would want to rely on the Domino connector in the application you are showing is beyond me.

Just the plain thought of a stair tread being only supported by a couple of dominos is ridiculous in my opinion. There is not really any physical support for the loads you might encounter. Thinking past pure „weight of people“ what about possible loads being carried up the stairs?

There is a reason why we have been using interlocking (dovetail etc) and through-bolt constructions for decades and even centuries.

I would not want to trust my life with the small Domino connect and would strongly advise against doing so yourself.

Be safe in whatever you do! [emoji4]

There are ways to hide bolts etc, if it is just an optical issue...

Edit:
One additional thought:
The “extremely high” might also just be in comparison to other similar systems like what IKEA uses...
« Last Edit: March 08, 2018, 06:33 AM by grobkuschelig »

Offline Rob-GB

  • Posts: 1078
Having built many staircases including a geometric windered stair (spiral) the forces that are imposed on the stair is multi directional and loading can be quite high at times.
For the domino fittings to be asked to work under all the stresses that can be imposed in such an application is just asking too much.

Just for the exercise of it the main points are:-

Passive load: The stair needs to support its entire weight

Active load: The stair could have many heavy adults on it at one time or heavy items being moved up or down it.

Shear: The load effect on the components in a downward direction, this would include the strength of the timber above the domino fixing mortice.

Twist: Action between components when weight is applied to one corner of a tread

Spread (between strings): The treads will flex, in use dependant on loading and can act against the strings pushing them apart, which seems easier with a "spiral" stair as they seem to want to oppose each other anyway [eek]. An Inny and Outy conflict perhaps  [big grin]

Not surprised that Festool don't want to give a nod to using them in this application but maybe they could have given a decent reason as to why not.

Rob.




Problem? No such thing! Only a solution waiting to be found:- RJ

"A $2 guppy swims......" Deke

Offline sdes

  • Posts: 21
@sdes why you would want to rely on the Domino connector in the application you are showing is beyond me.

Just the plain thought of a stair tread being only supported by a couple of dominos is ridiculous in my opinion.

There is a reason why we have been using interlocking (dovetail etc) and through-bolt constructions for decades and even centuries.

Thanks for the response. You are correct that things are done in a traditional way for a reason, but that is also often because they didn't have a particular technology or materials available to them at that time. The Festool Domino is supposed to offer a revolution in joining for timber - I was hoping to use that potential.

I totally agree that "the plain thought of a stair tread being only supported by a couple of dominos is ridiculous in my opinion", but that is not what I suggested, you will see I said the joint "might become rediculous before there are enough connectors". I'm asking to quantify what force / loading this particular fastener can carry, I would then, as stated, see if, allowing for a significant Factor of Safety, as previously noted, it would be feasible to use them in this situation, as many as would be needed to get to that Factor of Safety.

I wouldn't rely on one thread of Spectra line to hang from, but put enough together and they are used for parachute manufature all over the world, but if the manufaturers of the Spectra line geave no clue as to breaking strain, the parachute maker would have to do their own tests.

Why use Dominoes for construction and not traditional Mortice and tennon / dovetail joints etc, well, time, simplicity, carry the tool to the job etc, basically all the reasons in the Festool Domino marketing literature I suppose.

I agree I'm not proposing a traditional approach to joint fabrication, but isn't making robust connections in a new, quick and elegant way the whole point of the Festool Domino Machine?

I suppose I should never have mentioend the staircase, all I really wanted is some figures for pull out forces for the
 connectors! Don't worry, I shant kill myself, or more importantly anyone else. I'm really just trying to establish feasibility of different approaches. Given I have the 700XL machine, it seemed logical to at least see if using it was a viable option.

I would also add I'm not a complete numpty. I built a steel spiral stair with separate cantilevered treads 15 years ago that has had many heavy people use it and carry stuff up and down without issue. But in that case I did have the ability to calculate the forces, because I knew the specifications of the four stainless steel bolts that carry each tread, the thickness of the wall of the centre mild steel tube and hence the strip out force of those threads in that steel. A factor of safety of 3x for a 250kg loading, and each trad was capable of supporting something over 750kg before failure. But I did need a bit of information about the connectors!

The claim of "Extremly high pull-out and load forces" seemed to indicate to me they might be suitable, but if as you suggest Festool should have added a : "compared to a much smaller IKEA connector" or similar, then yes, I should give up this path immediately.

Anyhow, I've just purchased a 300kg / 600 lb "crane guage" from Amazon, so if there are no users with specific info, and Festool don't reply further, which perhaps I was being optimistic about (It has now been escalated aparently, so perhaps some limited info might be forthcoming), I will do my own pull out tests on some different timber varieties and see what evaluation I can do.

I will then obviously keep those results secret - as I won't want to be sued!

Cheers

Stephen

Offline Michael Kellough

  • Posts: 3475
I’d use the tie bolt with a deep bracket.  Large Domino’s would be okay for the lower connection of the deep bracket if the tread wraps around the column to negate twisting loads. Just sink the tie bolt a little deeper and cover with a plug.

I’d also set the tie bolt and especially nut a bit lower (than in you drawing reply #5) so you keep some continuous long grain wood above the nut so the whole corner of the strut doesn’t split out.
« Last Edit: March 08, 2018, 07:59 AM by Michael Kellough »

Offline Phil Beckley

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Hi Stephen
 I have replied to your enquiry via email.
rg
Phil
Festool U.K Employee | Festool UK Website


Offline HarveyWildes

  • Posts: 781
I'm with @grobkuschelig and @Rob-GB on this one.  You're not getting much mechanical advantage from the domino as you would with through bolts and washers, and you're not even getting the advantage of increased glue area from using wood dominos as loose tenons.  People have done strength tests on dominos for rail and stile cabinet doors (lightweight compared to stairs), and they are not as strong as traditional mortise and tenons, although still plenty strong for the application - just do a search on "domino strength test".  Of course, a lot depends on what size domino is being used, and the tests are for wood dominos, not the new connectors, so you must take the tests with a grain of salt in terms of absolute results.  Still, the results are consistent enough that I would want tests that proved the strength of the connection before I would use dominos on stairs per your diagrams.

If you do more testing, I'd also test for the repetitive "shock" forces that stairs are subject to, not just a single joint failure test.  I could see the repetitive shocks loosening the fit of the connector in the wood very gradually over time, until it suddenly fails.  Think of teenage boys running up or down the stairs two steps at a time.
« Last Edit: March 08, 2018, 10:02 AM by HarveyWildes »

Offline sdes

  • Posts: 21
Yes, I have seen some of the videos on the wooden dowels, but as per the link in the original post, the elements I'm intersted in are the Connnector system ones they make the "extremely high pull-out force" claims for.

They are only available in one size currently, for the 14mm diameter cutter. These ones:



Spax for example have a 40 page document about their screws with differnt multipliers to be used when used in various different natural or composite timbers and applications. I'm not seeking that level of detail, one page or even a few lines of info would do it, but just something empirical / objective to back up the subjective "extremely high"

This is the sort of info Spax give:





Offline SRSemenza

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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
« Last Edit: March 08, 2018, 10:56 AM by SRSemenza »

Offline sdes

  • Posts: 21
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.

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

Offline Svar

  • Posts: 1423
I would not use domino connectors for stairs, not in the design in your #13 post. The anchor part is too shallow, does not even compare to a large (8-10 mm diameter) wood screw in terms of pull out force. If you think tie bolts are ugly use wooden plugs or screw studs.
« Last Edit: March 08, 2018, 12:15 PM by Svar »

Offline Cheese

  • Posts: 4836
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.

Offline Phil Beckley

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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
[/quote]

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
Festool U.K Employee | Festool UK Website


Offline Rob-GB

  • Posts: 1078

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
[/quote]

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.
Problem? No such thing! Only a solution waiting to be found:- RJ

"A $2 guppy swims......" Deke

Offline sdes

  • Posts: 21
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.



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. 



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

Offline sdes

  • Posts: 21

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

Offline Svar

  • Posts: 1423
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.

« Last Edit: March 08, 2018, 04:00 PM by Svar »

Offline SRSemenza

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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

Offline Cheese

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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]

Offline sdes

  • Posts: 21
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.

(Attachment Link)

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


Offline Birdhunter

  • Posts: 2307
  • Woodworker, Sportsman, Retired
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.

Birdhunter

Offline Svar

  • Posts: 1423
@sdes , 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.

Offline Michael Kellough

  • Posts: 3475
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.

Offline sdes

  • Posts: 21
@sdes , 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.

Offline sdes

  • Posts: 21
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

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Offline Cheese

  • Posts: 4836
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.
(Attachment Link)

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.