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Making strong "Z legs" for a table (lemmy.world)

submitted 9 months ago by [email protected] to c/[email protected]

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Hey WW community. Exactly as the title says - I am making some work tables for my kids. They'll be used for homework, laptops, etc. And they're kids, so one can guarantee a little bit of abuse.

They're designing them, and the "z leg" is all the rage apparently - I am in the process of designing these in sketchup, and wanted to get some input/ideas on how to make these really strong. I obviously cannot make them out of a single piece of wood, so there will be some joins, probably on the apex points. Oh and the designs currently require the apex points of the Z to be rounded ....

I'm considering using half-lap bridle joints, making the Z's angular, then routing the curves into them.

Input, advice, links to designs welcome!

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[–] [email protected] 3 points 9 months ago (2 children)

Thanks for the input. To simplify we're going with straight edges (I think it'll look classier too) and I am going to put some dowels in for a bit of additional joinery strength. See pics attached for some drawings. Also I think technically this is just an angled bridle joint, not a half-lap bridle joint.

Now I have a further Q. And that is "how does one cut the inside of the outer(!!) bridle, on an angle to accurately match the taper?" (see the triangle with text labels on the drawing). The offset from horizontal is about 2.5 degrees due to the tapered nature of the horizontal legs.

[–] [email protected] 3 points 9 months ago (1 children)

You don't actually need to continue your taper through the bridle joint; just start your taper behind the joint.

[–] [email protected] 1 points 9 months ago

Yes, I reached a similar conclusion as well. Not sure if that makes it simpler or more complex. Time will tell!

[–] [email protected] 1 points 9 months ago (1 children)

I’m not sure about the dowels

The bridle joint will want to pivot and the desk acts as a big lever exerting a lot of force onto that joint with a large force multiplier.

It might be worth upgrading the dowels to steel bolts, but I don’t know a lot about the strength of wood under these forces.

If the dowel is 1” In and the Z is 48” long (no idea if that’s close), it exerts 48x the desk weight on to the dowels. You look at oak, a desk top of 50lb, a 1” dowel would be at over twice its breaking point. Two dowels and glue in the joint would will help, but steel bolts will have a much higher rating against shear here.

All of this is napkin math, and I’m not an expert at all here.

[–] [email protected] 1 points 9 months ago

Your napkin math is good though. Excellent points thank you!

[–] [email protected] 2 points 9 months ago

I think your current plan makes the most sense and will be the easiest to execute.

[–] [email protected] 2 points 9 months ago (1 children)

Have you thought about reinforcing the joints with steel? Lots of bending moment on a Z.

[–] [email protected] 2 points 9 months ago

Yes, that's what I was most worried about - movement that would lead to breaking. I think a healthy sized bridle joint coupled with some dowels will do the job.

[–] [email protected] 1 points 9 months ago

You just pound /glue a small wedge into the gap, since no one will notice discontinuities in end-grain. However, I can also imagine a bandsaw jig that clamps the bridle piece to an inclined plane till you get the angle you want. (Contrasting dowels will look nice! Dowel-makers are stupid easy to make.)