Knockdown Bench: Good News and Bad

The knockdown bench right before I drilled the holdfast holes in the benchtop.

The knockdown bench right before I drilled the holdfast holes in the benchtop.

Here’s the good news: The bench is assembled and works well. I’ll explain the construction details in the coming days.

KD_broken_fastener_IMG_0197

And the bad: I destroyed one of the knockdown fasteners tonight. I tightened one of the 3/8” hex-head bolts that fastens the top, and the head of the bolt began to spin freely. Nuts on the tee-nut. The collar of the tee-nut had ripped free from its mounting plate. The broken metal looks porous and weak. I am not happy.

I am going to torture-test a few of these tee-nut fasteners from McMaster-Carr and see if they all break or if that one is an outlier.

Stay tuned.

— Christopher Schwarz

About Chris Schwarz

Publisher of woodworking books and DVDs specializing in hand tool techniques.
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23 Responses to Knockdown Bench: Good News and Bad

  1. Jon Quinn says:

    They are probably made from a casting. You need to find a friend with some metal cutting machine tools and mill some out of bar stock

  2. sawmillman says:

    Yes a good machinist friend such as myself. Could make some dandy little fasteners that would last a lifetime.

  3. I would use square nuts (hex nuts are too small and would sink into the wood too soon) mounted in a mortise. You could epoxy it in or cover with a strip of wood to hold it in place. In this way the nut will sink into the wood before it would ever turn!

  4. smbarnha says:

    It looks like those inserts are cast or sintered zinc and since they aren’t rated for load, they can’t be relied on even if you find that a few of them work. The next batch might be stronger or weaker. If you want to make it work and can make it fit, you could probably turn the insert around so the threaded collar is pushing against the flange. Not low profile, but it would be stronger.

    I’m sure you’ve seen things like the EZ-Lok, too. I haven’t used them but they look good. http://www.ezlok.com/InsertsWood/hardWood.html

    • smbarnha says:

      If I used the EZ-Lok, I would drill a through hole and insert the screw from the other side, so it’s pulling the insert into the bulk of the wood. I’m not sure if that works with the joints you’ve set up.

      I like the idea of a square nut, too.

      • steveschafer says:

        One note regarding brass threaded inserts: Some of them (I can’t tell if it’s true of the EZ-Lok ones that you’ve referenced) have an asymmetrical thread profile and are meant to be installed so that the tensile load from the bolt/screw comes from the end of the insert that has the cross slot. You won’t get the full rated load strength if the tensile load comes from the other end.

  5. Bummer. That really sucks.

    >

  6. sjschmidtky says:

    Chris, can you comment on how you attached the top to the sides? Also, how did you determine the size of the holes for the holdfasts?

  7. Joe McGlynn says:

    I’d bet that the nuts are resistance welded to the flange. I’d also bet that a decent fabricator could make absolutely bulletproof nuts like that. Laser cut some 1/8″ discs for the base and TIG weld a heavy hex nut to the flange. If I still had the CNC in the garage I’d say to just machine them out of solid steel stock, but that’s overkill probably.

    • Joe McGlynn says:

      I take that back about the nuts being resistance welded on, I just read on mcmaster.com that those are “zinc alloy”, not steel. So they are almost certainly cast. Maybe I’ll make up a trial set this weekend for fun.

  8. jenohdit says:

    Am I wrong to laugh at everyone who rushed in to buy hardware in the past few days and is now considering returns or cancelled orders? Not laughing at Chris though, I’m proud of him in fact. That’s how the process of discovery goes.

  9. Great looking bench! Looks solid as a rock

  10. I hate those kind of t-nuts. Most are not made like that, but rather by stamping and forming, and are much stronger. The one you show is cast from pot-metal and they ALWAYS break like you show. Don’t bother to test them, just get different style ones. I usually get mine from Outwater Plastics.
    Richard Oedel, Chair, NH Furniture Masters

  11. pathdoc70 says:

    CHRIS, Not good, hopefully it is a bad “egg”. Did you make my mistake of “just a quarter turn more and it will be really tight”. and SNAP! Quote (not mine) for you: “Don’t let perfect be the enemy of good.” Then of course, there is always a torque wrench use possibility. Mike O’Brien Valley Head, AL

    Sent from my iPhone 5s

    >

  12. heidtwd says:

    Please try the #90611A450 weld nut from McMaster. It’s stamped from mild steel and has 3 holes for fasteners just like the cast one you tried.

  13. gburbank says:

    good old barrel nuts won’t let you down. It just takes a little planning (and careful drilling) to use them.

    • I have built many, many things with barrel nuts. The goal here is not to build one bench that I can build. But to build dozens of them in a classroom environment with people with little or no woodworking skill.

      Installing barrel nuts with a brace and bit is not something I’d want beginners doing on their first day.

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