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Would Baltic Birch fall into the "Softwood and Plywood" (coarse thread) or "Hardwood" (fine thread) category when it comes to screw threads?

The application is putting threaded inserts to a Baltic Birch table top so I can attach removable legs. The manufacturer of the threaded inserts (EZ-Lok) divides their threaded inserts into two categories: "Softwood and Plywood" (coarse thread) and "Hardwood" (fine thread) and I'm not sure which to use since Baltic Birch is a plywood made of hardwood plies?

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    Not to take anything away from @MattDMo's Answer, but good to hear you were planning on doing some tests on scrap. This is something more woodworkers would benefit from for a wide range of range of things, from this and general screw-holding through all forms of staining and finishing.
    – Graphus
    Nov 13, 2022 at 2:03
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    To your specific query, how thick is your top? Threaded inserts can add a lot of strength but how exactly are you using them to attach your legs? A table's top/leg attachment can be subject to quite a bit of strain in normal day-to-day use, but beyond that are many possible interactions that can stress the attachment far beyond 'normal' but still being fairly commonplace, from simple bumps with a hip to being dragged over the floor (esp. carpet) all the way to someone perching on an edge or outright sitting on it..... or heaven forbid, standing on it.
    – Graphus
    Nov 13, 2022 at 2:06
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    Simply screwing legs with a threaded rod installed in their ends into the bottom of a table does not yield a strong or particularly stable table. However it could be strong enough; only you can judge what will be suitable for the size of the table, the length of the legs, where you have it positioned in your home, the maximum load, and not least how careful you/family members/friends can be with it through its expected lifespan. It's perfectly OK to build something that has to be babied a little bit if you want to, plenty of 'art furniture' must be treated this way and that stuff costs $$$$!
    – Graphus
    Nov 13, 2022 at 19:16
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    But, if you do want to make something more stable and secure the way many MCM tables and consoles had their legs attached — one example being the metal plate you first linked to — is still a very viable method. Google "MCM leg mounting" for more variations if you like. One step up from this, especially suited for legs with any splay, is the wooden cleats or mounting blocks that have a shallow mortise for the top of the leg to fit into (adds tons of resistance to flexing). These can still be bought commercially, but are actually relatively easy to user-make if you're careful.
    – Graphus
    Nov 13, 2022 at 19:26
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    No, the type I was mentioning would have a mortise (a round or oval recess) in them. I know they exist, but I'm struggling to find any to link to now! [All the ones I am finding photos of, both wood and metal, just use threaded inserts.] Anyway the mortise isn't vital, just desirable for max strength.
    – Graphus
    Nov 19, 2022 at 22:49

1 Answer 1

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"Standard" plywood in the US is generally made of softwood plies, and may or may not have a thin hardwood veneer on one or both sides - so-called "maple", "oak", etc. plywood you see in many hardware stores and home centers. I believe that is the kind of plywood EZ-Lok is referring to. The veneer is simply for appearance, and doesn't do much of anything for the strength of the product.

On the other hand, as you mention, Baltic Birch and the similar American Birch plywood products are made with birch through and through, and so would classify as a hardwood. Their fine thread inserts should work well in these sheets. That being said, unless your inserts are going to be subject to extraordinary loads, the coarse threaded inserts should be fine as well. I have been known to use coarse-thread wood screws in hardwood when fine-thread ones weren't available, and I've never had an issue with them. If you are putting the inserts into thinner sheets of plywood, the higher thread count of the finely-threaded inserts may be desirable, but in many cases coarse will be sufficient. Without more details on the dimensions, it's difficult to give more specific advice, but my gut feeling is that either will work here. For utmost peace of mind, though, go with fine. It certainly won't detract from the strength of the connection.

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  • Thanks Matt! I already ordered some coarse ones and was going to test it out first in some scrap. Great to hear it may work :-)
    – Max
    Nov 12, 2022 at 23:05
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    The coarse thread worked just fine!
    – Max
    Nov 22, 2022 at 13:02

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