Need help with joint selection

[chet jamio]

I'm going to build a floor stand for my flat screen TV which is 135 pounds. Obviously, I'm a little concerned about joint strength. Here are some photos of my current design. This is going to be my first project, so I'm not sure about joint design and strength.

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JOINT A - material is 2" x 2". I plan on using a 3/4" square floating tenon that extends 3" into each part

JOINT B - material is 4" x 4". I plan on using a 2" square floating tenon that extends 4" into each part

JOINT C & D - these shelves will have a cable box and DVD player on them. There's nothing real heavy, but the support at the legs isn't very wide. My only thought at this time is to dado them into the legs. I would prefer a mechanical lock to stop the legs from splaying outwards, but I don't want exposed dovetails on the shelves to be visible on the back side of the legs. I've also considered using some 1/2" stainless steel rods (2 per side) to attach the shelves to the leg which would create a floating look. However, I fear the rods would be too close together to stop the shelf from tipping.

JOINT C - 1" thick shelf into 2.75" x 3" leg, shelf is solid
JOINT D - 2.5" thick shelf into 4" x 4" leg, shelf is hollow shell (torsion box)

Also, if anyone has any comments about the look of this project, please let me know. I plan to use cocobolo for the legs and feet, but haven't decided on what to use for the shleves. I think I want something a little darker and with less grain than the cocobolo, but about the same color. The 2 arms that attach to the TV wont be seen.

[Prashun Patel]

Clever.

My instinct is that it might not be stable. I'd make the legs out of a single piece. I think Joint B will receive the greatest stress and probably wouldn't trust the joint unless it was reinforced with some kind of bracket.

If I were doing this, I'd plan on bolting the unit to the wall. With the wall bearing the majority of the weight, the the joints would be largely aesthetic and I'd probably pocket screw or dowel them. Purists like M&T for strength and longevity, but my guess is that your TV will obsolesce before the stand, which you'd probably want to change at that point anyway.

[glenn bradley]

+1 on single piece legs. The spoil can be used for another project and the extra $$$ you spend to get the boards will be a lot cheaper than your new TV taking a dive. JMHO.

[chet jamio]

I can't attach anything to a wall. If I could, I would just buy a wall mount.

It would take a large block to make the legs out of one piece (10 x 20 x 40). It would be an elaborate glue-up of several boards and I think it would be difficult to work. It would be mostly hand sculpture at that point.
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The way I see the joints failing is that the weight of the TV being not directly over the joints. The TV would try to gap the back of the joints, not the sides. I could add some pins for reinforcement. They wouldn't be seen. If the pins were too big, it would degrade the strength of the tenon. It would probably be best to use small strong pins, likely brass or stainless steel.
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[chet jamio]

How do you guys see the joint failing? In what direction? I'm locked into seeing it a certain way. If I could see from your point of view, maybe I could alter the joint without making it from one piece. For example, I could move the joint between the leg and foot to a different location to allow the leg to rest on the gound and bear the weight directly. Or maybe keep the joint where it is and use a sliding dovetail. Or maybe...

Back to the point, how do you the original design failing?

[Lee Schierer]

Several areas bother me. JOint A looks weak. I would use a metal dowel with screws in from teh back (holes can be plugged with wood s they don't show. SImilar to what Chet shows.

Next the feet have teh grain shown as runing vertical. This may proove to be too weak. Run the grain horizontally in the feet.

Finally the shelf concept look weak if you cantelever all teh weight out the front. Move them toward the back slightly so the center of gravity of the component is centered directly in line with the legs when viewed from the side. Then the shelf will not be torqued. A sliding dovetail in the legs to attach the shelf would also keep the legs from splaying ( you could also install pocjket screws from the bottom of the shelf into the legs and plug the holes). Make the shelf from two pieces, slide the front half with the dovetail on the back portion the same width as the legs and once the shlf is in place glue on a narrow board to the back side of the shelf that hides the dove tail and also locks it in place.

[Prashun Patel]

If you can't bolt /strap it to a wall, how are you going to prevent it from tipping backwards?

If you really can't attach it to a stable structure, then can you add a third leg?

I don't really see the JOINTS as being the issue; it's the stability of the base that worries me.

[chet jamio]

If you can't bolt /strap it to a wall, how are you going to prevent it from tipping backwards?

It will be placed against a wall, so it won't be able to tip backwards.

[chet jamio]

Next the feet have teh grain shown as runing vertical. This may proove to be too weak. Run the grain horizontally in the feet.

Finally the shelf concept look weak if you cantelever all teh weight out the front. Move them toward the back slightly so the center of gravity of the component is centered directly in line with the legs when viewed from the side. Then the shelf will not be torqued. A sliding dovetail in the legs to attach the shelf would also keep the legs from splaying ( you could also install pocjket screws from the bottom of the shelf into the legs and plug the holes). Make the shelf from two pieces, slide the front half with the dovetail on the back portion the same width as the legs and once the shlf is in place glue on a narrow board to the back side of the shelf that hides the dove tail and also locks it in place.

Regarding grain direction, the CAD tool doesn't allow me to rotate the grain in the pictures. What's shown isn't the direction I was planning.

The shelves are a real pain right now. I don't want to center them on the legs as this would prevent me from pushing the stand tight against a wall (as stated previously, I can't attach anything to the wall).

If I want to make the legs out of one piece, I could change the design so the legs lie more in a single plane. I don't like the design as much.

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[chet jamio]

I would use a metal dowel with screws in from the back (holes can be plugged with wood s they don't show.


Great idea. My original thought of wooden floating tenons was what I figured to be the weak point. I hadn't considered using metal dowels. That would make thinks much better.

[Matt Day]

Personally, I think you're plan should work. Maybe consider beefing up the 3/4" tenon to 1"? But a 3/4" x 3/4" tenon that is 6" long (18in2 surface), and a 2" x 2" tenon that is 8" long (64in2 surface) creates a lot of glue surface IMO.

How about instead of one hunk of wood for each leg, you laminate smaller pieces?

I recently read an older article on FWW online that said a floating tenon is weaker than a non-floating tenon. So that may add a bit of strenght rather than doing it floating.

P.S. Here's an article to look at on some interesting joints, that hold a lot of load:
http://www.festoolusa.com/Web_files/Domino_WCM_zigzag.pdf (http://www.festoolusa.com/Web_files/Domino_WCM_zigzag.pdf)

[Jim Becker]

Have you considered doing a bent lamination for the structure? You could veneer the exposed laminated surfaces after the fact using a vacuum bag.

[chet jamio]

Maybe consider beefing up the 3/4" tenon to 1"?

Based on the comments so far, that's exactly what I'm thinking. I also like the suggestion of using a metal tenon. I will definately beef up that joint.


How about instead of one hunk of wood for each leg, you laminate smaller pieces?


My plan was to laminate a 4 pieces of 5/4 x 8 stock for each leg, 5 pieces for the foot. Is that what you meant?


Have you considered doing a bent lamination for the structure? You could veneer the exposed laminated surfaces after the fact using a vacuum bag.

One idea I had early on was to make the legs out of laminated MDF and then veneer them. I threw out that idea once I decided I wanted to add a 3/4" radius to the legs. I hadn't considered a bent lamination. As I understand, I would have to do the lamination with the cocobolo to allow for the radius. If so, why add the veneer? This isn't an area I'm real familiar with.

I appreciate everyone's input. What I haven't heard yet is anything about the aesthetics. I'm not set on anything. I just whipped the pictures together based on some loose ideas. So please critique or offer total redesign. I'm quick with the CAD, so I'll look at everything.

[Jim Becker]

You would only veneer the edges if you didn't want the "look" of a bent lamination. With a species like cocobolo, which generally has a bit of figure, the laminations are going to be pretty obvious just due to the inability to grain match the edges of the layers. Very subjective thing, of course, 'cause it's totally about the "look", instead of functionality.

[Mike Wilkins]

Looking at the side view would give me concerns about stability, like tipping backwards. You stated it will be against a wall. But why not consider adding a couple of inches to the backside of the feet(part B I think) to give a larger footpring for the base. It would resist tipping backwards and provide more stability. Good design by the way.

[Bill Keehn]

I second that. Put a heel on the feet.

[Bill Keehn]

I think I would put a shallow notch in the legs and a large notch in shelf, so that it wraps around the leg a bit. I'd countersink a hole on each leg and put a threaded steel rod through them and the shelves. Cap each end good an tight with a lock nut and cap them with plugs.

Consider torsion box construction for the shelves. Make them a little thick and then not only will they be strong, but you will also have more of a surface inside the notch at the leg to help support the shelf.

I wouldn't use a single piece of wood for the legs since the feet would be weak. I'd attach the feet with a half lap or saddle joint, which the latest FWW indicated were the strongest against wracking forces.

If you do bent laminations, I don't think it would be bad if you didn't veneer the edge, as long as you keep the layers of the lamination in order. That way the grain match should be pretty good. Use a bandsaw since it has a small kerf.