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Thread: Mafell DDF40 Duo Doweller - Domino now has some competition

  1. #31
    Quote Originally Posted by Steve Rozmiarek View Post
    Keegan and Mike, note this part of my post "An appropriate sized dowel that cannot be subjected to twisting force, is just as strong as any other loose tenon. In fact a dowel made of the same wood as a domino would actually have higher shear strength than a domino if you chose size based on equal surface areas."

    That's the key, do the math. A dowel of equal surface area as a domino will have more cross sectional area then a somewhat oval shaped domino. A bigger cross sectional area equals more shear strength, as I said all other things being equal.

    Mike, you're doing the math wrong. The correlation of the space it takes to fit the dowels and empty space between as to equaling a domino that occupies the same total area is the wrong way to look at it. Like I said, use surface area. To take your analysis to the absurd, why not use 1/8" dowels in the theoretical situation? It'd still justify using the same enormous domino following your logic.

    Dominos are fine, so are dowels. Fixed tenons are better. That is the art of good craftsmanship, knowing when and how much.
    The analysis that I did was for the real world situation. I didn't use 1/8" dowels because chair manufacturers use 3/8" dowels. And if you want to use dowels that would have approximately the same long-grain-to-long-grain surface area as the loose tenons that I calculated, you'd have to use about 9/16" dowels. The problem with using dowels that large is that it would take up a lot of a 3/4" thick piece of wood. 3/4 (or 12/16) minus 9/16 gives 3/16. That has to be divided in two because you're going to center the dowel. That leaves 3/32" on each side of the dowel which is pretty weak.

    No, dowels are not as strong as a good mortise and tenon joint, even if it's a loose tenon. No matter how you look at it - from the mathematics or from experience with failed chair joints - it always comes out with the same answer.

    Mike

    [And, of course, my analysis is all about surface area.]

    [One more thing: A dowel with the same surface area as a tenon does not have greater shear strength. It may have greater torsion (twisting) strength.]
    Last edited by Mike Henderson; 07-27-2021 at 1:30 AM.
    Go into the world and do well. But more importantly, go into the world and do good.

  2. #32
    Quote Originally Posted by johnny means View Post
    I doubt chairs made in a factory somewhere in Asia would fair any better if the were made with Dominos. If your describing failures in which the glue surface was the point of failure, you're describing a failure of workmanship and not a failure of dowels. All the glue surface in the world is useless if all you do is squeeze in a drop of glue and bang in your dowel/tenon. Also, just measuring the glue surface doesn't tell nearly the entire story. Two dowels spaced apart will often provide more resistance to twisting or racking forces than a single tenon placed in the middle of a joint. Then, of course, there is the problem of comparing those cheesy pine dowels to beech dominos.
    The failures we see in chair joints are not glue failures. The dowels are well glued. The problem is that the wood that the dowel is glued to fails in shear.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  3. #33
    I think Mafell had the patent on those locating pins. The first generation domino had those, but Festool was forced (so the story goes) to come up with a different solution, hence the little tabs on the later domino 500. I have the XL which again uses locating pins. Mafell enjoys a great reputation in Europe, so the quality is there. Dowels? Nothing says “IKEA” quite as well.

  4. #34
    One interesting question is why do the chair manufacturers only put two dowels in that joint at the back of the chair seat. It's clear that two dowels do not provide sufficient long-grain-to-long-grain glue surface area to stand up to normal use for many years. An additional dowel might provide enough additional glue surface area to provide a much longer life for that joint.

    My belief is that there's no value to them in adding that additional dowel. Two dowels will allow the joint to survive for perhaps five or more years and by that time the customer probably forgot where they purchased the chairs. People seem to just accept these joint failures on chairs.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  5. #35
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    3/8" dowels were fine in chairs until people began to swell up to pachydermatic proportions (like me!).
    Now a fat-boy-tubby-gut like me can wreck a chair PDQ just be sitting on it...
    My granddad always said, :As one door closes, another opens".
    Wonderful man, terrible cabinet maker...

  6. #36
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    This popped up in my IG feed this morning. You could ask his thoughts on the mafell vs the others.
    Screenshot_20210727-083910_Instagram.jpg

  7. #37
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    Most all the joint failures I have seen over the years involved dowels. There are places they make sense but not many. I don’t think the Domino has any competition from Mafell. Loose tenon and tenon joinery is just better. Especially since the Domino became available. That said, the Domino could use better adjustments. They could learn from my Dewalt biscuit joiner which has a rack and pinion fence and marks on the side to indicate the center of the cutter. I really like the machine but it is not perfect.
    Last edited by Charlie Jones; 07-27-2021 at 9:56 AM.
    Charlie Jones

  8. #38
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    If I put my engineering hat on (which I haven't for a while) then here's my take.

    A broken dowel is the result of a failure, but is not the failure. The failure in these cheap chairs (in my experience) is when the leg and the rail are no longer one solid piece. Whatever happens after that is a result of that failure. That's where we have to look.

    So, the question then is what causes a chair to get wobbly (i.e. the rail and the leg to come apart) and I can see only one culprit: the glue didn't hold the two pieces together under stress. BUT, we have to dig deeper because that can driven down into more large categories:
    1. The glue failed because it wasn't strong enough
    2. The surface area the glue needed wasn't enough
    3. The design of the chair didn't distribute the torque correctly, etc.
    4. The glue wasn't applied properly

    I don't immediately blame the dowel just because it broke. A domino may have survived, but you would have still been given a chair to fix. Also, to be clear, unless someone is jumping on the seat of these chairs and pin is broken directly in the middle of the joint, these are not failing in shear. My guess is they are failing in tensile or bending. But again, by the time the pin finally gives out, it's been trying to take on the a lot more work than it was designed to do. If you're interested, you can look up pieces of wood materials failing in different scenarios and then look at the pins you get that are broken and compare to verify how the pin broke.

    Example:
    We once had a client that had terrible fatigue cracking problems in their shafts (compressors). So the shafts would eventually vibrate as the crack propagated deeper and deeper. The failure wasn't the crack, the failure was the corrosion on the outside of the shaft that created high stress zones that allowed the crack to start. Once the crack started, the shaft already failed. So we had to fix the corrosion (shaft material, customer changes their process, etc).
    Last edited by andrew whicker; 07-27-2021 at 10:36 AM.

  9. #39
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    Last edited by lowell holmes; 07-27-2021 at 10:53 AM.

  10. #40
    Quote Originally Posted by andrew whicker View Post
    If I put my engineering hat on (which I haven't for a while) then here's my take.

    A broken dowel is the result of a failure, but is not the failure. The failure in these cheap chairs (in my experience) is when the leg and the rail are no longer one solid piece. Whatever happens after that is a result of that failure. That's where we have to look.

    So, the question then is what causes a chair to get wobbly (i.e. the rail and the leg to come apart) and I can see only one culprit: the glue didn't hold the two pieces together under stress. BUT, we have to dig deeper because that can driven down into more large categories:
    1. The glue failed because it wasn't strong enough
    2. The surface area the glue needed wasn't enough
    3. The design of the chair didn't distribute the torque correctly, etc.
    4. The glue wasn't applied properly

    I don't immediately blame the dowel just because it broke. A domino may have survived, but you would have still been given a chair to fix. Also, to be clear, unless someone is jumping on the seat of these chairs and pin is broken directly in the middle of the joint, these are not failing in shear. My guess is they are failing in tensile or bending. But again, by the time the pin finally gives out, it's been trying to take on the a lot more work than it was designed to do. If you're interested, you can look up pieces of wood materials failing in different scenarios and then look at the pins you get that are broken and compare to verify how the pin broke.

    Example:
    We once had a client that had terrible fatigue cracking problems in their shafts (compressors). So the shafts would eventually vibrate as the crack propagated deeper and deeper. The failure wasn't the crack, the failure was the corrosion on the outside of the shaft that created high stress zones that allowed the crack to start. Once the crack started, the shaft already failed. So we had to fix the corrosion (shaft material, customer changes their process, etc).
    The failures we see in chair joints are not broken dowels. The dowel pulls out of the mortise in the back of the chair. The failure is not a glue failure - there is wood still attached to the dowel. The failure is that the stress on the dowel (outward) exceeds the shear strength of the wood that the dowel is inserted into. The remaining hole, after taking the joint apart is larger than the original hole, another indication that it was not a glue failure.

    Glue is stronger than wood and we see that in the failure mechanism of a rear chair joint.

    The solution to this problem is to increase the long-grain-to-long-grain glue surface area. A tenon has more surface area than two dowels. Perhaps 3 dowels would provide better holding power.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  11. #41
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    I think they fail because they are not designed to include enough splay, the joinery is often installed perpendicular for ease of manufacture, the structure isn’t well supported and the joinery is not well chosen or sized.

    The bridge didn’t fall over because of the tack welds, it fell over because there was only tack welds...
    Bumbling forward into the unknown.

  12. #42
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    I misread the shear. I thought you meant the pin sheared.

    I wonder how many other problems you are fixing at one time... better glue, tighter / smarter clamping, bigger tenon, better wood for tenon, etc.

    Quote Originally Posted by Mike Henderson View Post
    The failures we see in chair joints are not broken dowels. The dowel pulls out of the mortise in the back of the chair. The failure is not a glue failure - there is wood still attached to the dowel. The failure is that the stress on the dowel (outward) exceeds the shear strength of the wood that the dowel is inserted into. The remaining hole, after taking the joint apart is larger than the original hole, another indication that it was not a glue failure.

    Glue is stronger than wood and we see that in the failure mechanism of a rear chair joint.

    The solution to this problem is to increase the long-grain-to-long-grain glue surface area. A tenon has more surface area than two dowels. Perhaps 3 dowels would provide better holding power.

    Mike

  13. #43
    Quote Originally Posted by johnny means View Post
    I doubt chairs made in a factory somewhere in Asia would fair any better if the were made with Dominos. If your describing failures in which the glue surface was the point of failure, you're describing a failure of workmanship and not a failure of dowels. All the glue surface in the world is useless if all you do is squeeze in a drop of glue and bang in your dowel/tenon. Also, just measuring the glue surface doesn't tell nearly the entire story. Two dowels spaced apart will often provide more resistance to twisting or racking forces than a single tenon placed in the middle of a joint. Then, of course, there is the problem of comparing those cheesy pine dowels to beech dominos.
    This has been my experience as well. When I make a dowel joint, I am doing so with care and taking the time to paint the dowel completely with glue,and use a long swab to paint the inside of the hole completely with glue. The failed factory chairs I have encountered have looked like a machine was squirting glue into the hole before the dowel was inserted with the expectation that the glue would wick its way up the flutes or spirals. Some of them were made with "pre-glued" dowels which are a case study in compromises.

    The most recent chair repair I did was a failed mortise and tenon joint. That's right. What happened was the customer's teenage daughter was leaning back in the chair while sitting at the dining table doing her homework. So the chair seat levered against the rear legs and the failure was in the surrounding wood above the joint that was insufficient to stand up to the torque so it gave way to the load.

    This points to the larger issue.
    It is not reasonable to declare one type of joinery superior to another in a vacuum or based on a singular criteria like surface area calculations. Every furniture design is unique in terms of the load situation. It is tempting to think bigger is always better but like the recent repair I did, (that particular) M&T joint simply removed too much native wood.
    There are times when a well made and properly sized mortise and tenon is the best choice, and there are times when a well made dowel joint is a better choice. It's up to the craftsman to consider the pros and cons of different joinery approaches and make a rational selection.

    For anyone interested enough in this subject, here is a link to a detailed test done by Mattias Wandel on the two types of joints being discussed here: https://woodgears.ca/joint_strength/dowel.html

    I think his conclusion supports what I am saying above, which is that neither type of joinery is "better" than the other. It really depends on the situation, design and execution of the joint.
    Last edited by Edwin Santos; 07-27-2021 at 9:19 PM.

  14. #44
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    Great post Edwin and hopefully this might get people back on the main topic of comparing the duo dowler and the domino
    Last edited by Johnny Barr; 07-27-2021 at 7:17 PM.

  15. #45
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    Edwin had me on board until he mentions Mattias Wandel the YouTuber.
    The dominos makes a mortise with round corners. It. Doesn’t take much effort to square them up with a chisel and cut a integral tenon on the other half.
    A mortise that’s square to the face of work piece isn’t as appreciated as it should be. Floating or integral it’s very strong.
    Aj

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