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Is this going to be strong enough?

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Forum topic by CrankAddict posted 01-15-2019 06:22 PM 758 views 0 times favorited 24 replies Add to Favorites Watch
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CrankAddict

58 posts in 311 days


01-15-2019 06:22 PM

Topic tags/keywords: joinery table mortise tenon dowel

Hi Everyone,

Very new to woodworking and still ensure of how to design things that require actual strength. I’m looking to build a table like this:

30” high, 42” diameter glass top. The glass itself will probably be 70lbs, so the legs definitely need to be able to hold some weight. I’m thinking of making the legs out of 1 5/8” thick stock, perhaps walnut. I’ve seen a commercial version of a similar table that appears to use dowels to secure the upper and lower leg parts:

It also has a metal brace attached to the bottom of the X. I’d like something that is strong enough using all wood, no metal. I was thinking about a tenon that went into a mortise in each leg piece kinda like this:

Would that be as strong or stronger than the dowels? Or is there some completely different technique I should be considering?

Thanks!
Jeff


24 replies so far

View MPython's profile

MPython

166 posts in 322 days


#1 posted 01-15-2019 06:46 PM

That looks plenty strong enough to me, much stronger than dowels. If it were me, I might pin each section of the uprights to the tenon just to be on the safe side.

View Richard Lee's profile

Richard Lee

254 posts in 1284 days


#2 posted 01-15-2019 06:48 PM

I think the way you have drawn it will be fine, 17 1/2 lbs per leg and add a Turkey and fixens I doubt each leg will have to support 30 lbs each which is minimal for your design. Until someone sits on it then !!!!!

View Kazooman's profile

Kazooman

1359 posts in 2462 days


#3 posted 01-15-2019 07:04 PM

One thought is to be certain to make the mortise and tenons as deep as you can (within reason, of course). The bulk of the strength in the joint will be be from the faces of the tenons. The end grain joint of the legs and the leg assembly to the cross pieces will be weaker. Any weight on the table will be trying to spread the joint apart, pivoting on the inside corner of the joint.

I like the table design. I would recommend spending the money to get a top made with low iron glass. That eliminates the dark green color regular glass would have at the edge replacing it with a nice light blue.

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SMP

1392 posts in 415 days


#4 posted 01-15-2019 07:23 PM

Maybe draw bore them, rather than the way they have them, to keep them nice and tight.

View Phil32's profile

Phil32

623 posts in 413 days


#5 posted 01-15-2019 07:45 PM

Mortise & tenon joints are strongest where the loads are at a right angle to the tenon. The angle of the legs, both to the floor and the top, would tend to separate the joints. The area of glue surface may be greater than dowels, but I question joining three pieces with a single mortise & tenon as with each leg.

-- Phil Allin - There are mountain climbers and people who talk about climbing mountains. The climbers have "selfies" at the summit!

View pintodeluxe's profile

pintodeluxe

5986 posts in 3322 days


#6 posted 01-15-2019 08:08 PM

I’ll bet those are screws with tapered plugs to cover the hole.

I would be tempted to build it that way too. My reasoning is this… mortises are going to be difficult to cut on those angled legs. However, with a handheld router and edge guide it could be done.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View BlueRidgeDog's profile

BlueRidgeDog

499 posts in 288 days


#7 posted 01-15-2019 08:50 PM

You are not using all the long grain to long grain glue up area you could get out of the joint. I think if you made the tenon taller, near stub level, you would get a stronger joint…that and four pegs and you would be good to go.

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Aj2

2488 posts in 2307 days


#8 posted 01-15-2019 08:54 PM

I agree with Pinto those are plugs covering screws.

-- Aj

View CrankAddict's profile

CrankAddict

58 posts in 311 days


#9 posted 01-15-2019 09:58 PM

Thank you to everyone for the helpful feedback. Based on what I think I’m hearing, I have modified the tenons to extend out further, to be full height, and to have pins. Is this roughly what all of you were describing?

View BlueRidgeDog's profile

BlueRidgeDog

499 posts in 288 days


#10 posted 01-15-2019 09:59 PM



Thank you to everyone for the helpful feedback. Based on what I think I m hearing, I have modified the tenons to extend out further, to be full height, and to have pins. Is this roughly what all of you were describing?

- CrankAddict

With a tight joint, that should be solid. IMHO!

View JohnMcClure's profile

JohnMcClure

706 posts in 1150 days


#11 posted 01-15-2019 10:25 PM

The experts here have weighed in and know much better than i do, but I dont understand why each leg is made from 2 pieces. If you have wide enough stock available, the top and bottom leg parts could be one piece, thus stronger.
If you don’t have wide stock, you could glue some up. More waste this way, but less chance of failure?
Either way post it as a project when finished, and if it ever breaks post that too so we all learn!

-- I'd rather be a hammer than a nail

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CrankAddict

58 posts in 311 days


#12 posted 01-15-2019 10:52 PM



The experts here have weighed in and know much better than i do, but I dont understand why each leg is made from 2 pieces. If you have wide enough stock available, the top and bottom leg parts could be one piece, thus stronger.

I’d be curious to hear more about this as well. Somewhere I read that with a 2 part leg you want the grain to run straight in each leg piece for maximum strength, which is what you’d get here. It would also waste less and allow for narrower stock to be used as you said. With legs this long that is a pretty dramatic difference. By my measurements it would require 10.25” wide stock to cut them out of a single piece, but only 3.25” wide stock for the 2-piece version.

View bilyo's profile

bilyo

850 posts in 1612 days


#13 posted 01-15-2019 11:20 PM

I understand and agree with the need for the largest tenon possible. However, if I was the person doing this, I would want to allow for a bit of error in cutting the mortise. With the tenon the same height as the rest of the horizontal member (no top or bottom shoulder), if the mortise is cut slightly taller than needed for the tenon you will end up with a small visible gap. I don’t think a small shoulder at the top and bottom will affect the strength of the joint by much and will ensure that a small gap will be covered.

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CrankAddict

58 posts in 311 days


#14 posted 01-15-2019 11:26 PM

With the tenon the same height as the rest of the horizontal member (no top or bottom shoulder), if the mortise is cut slightly taller than needed for the tenon you will end up with a small visible gap. I don t think a small shoulder at the top and bottom will affect the strength of the joint by much and will ensure that a small gap will be covered.

- bilyo

That actually makes a lot of sense. Especially at this point in my woodworking journey, I shouldn’t be opting for any techniques that require “perfect” cuts. Even a 1/8” shoulder on top and bottom should provide a safety net for me.

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clin

1067 posts in 1505 days


#15 posted 01-15-2019 11:53 PM

I think you about have it. I agree that using a single piece of wood for the legs would weaken them. I think the angle is a bit too much in this case.

I would consider a few other things. One is thicker stock. 1 5/8” seems a bit narrow to me. If that photo is 1 5/8”, it does look right.

Another idea is to add two pegs. As you have drawn it, the peg looks like a pivot point. Obviously the shoulders would prevent it from rotating, as should the glue. But two pegs would create a very positive joint that would remain strong should the wood shrink and the joint loosen, even without any glue. Though I would still glue it.

The middle looks like a half lap. It looks like the piece, with the notch in the top, would try to spread and open up the joint. While I know you don’t want this, a metal strap across that on top would provide that support. Just not sure how you could get some tensile strength across the top of that piece. Maybe embed a spline across the top.

My guess is there is something you could do that would be a bit stronger. But, just because something can be stronger, doesn’t mean it’s not strong enough.

I like the design and look forward to see your finished project.

-- Clin

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