M/T leg into tabletop question

[James Biddle]

I'm building a coffee table where the top is a 3/4" thick piece of freeform curving zebrawood and the wenge tapering legs are 1-3/4" x 4" at the top. The design calls for the legs to attach directly to the top and I'm thinking M/T would give me the strongest joint. A couple of questions:

What size would you make them?

I'm planning on using a Festool plunge router on a guide rail to cut the mortices. How would you make the tenons?

Is epoxy overkill for glue or would you recommend something else?

Thanks for your help. James

[Greg Sznajdruk]

James:

Not quite sure what you are asking. The top is 3/4" are you intending to mortise the top? Do you intend for the tenon to show through the top?

Greg

[James Biddle]

Yes, the mortise will be in the top and I do not want the leg tenons to show through.

[Greg Sznajdruk]

James:

That leaves the mortice less than 3/4" even at 1/2" the tenon has very little mechanical support. How big is your table top?

If this is what you want to do would adding stretchers between the legs be in keeping with your design? Stretchers would add additional bracing to the legs.

Greg

[James Biddle]

Aprons or stretchers aren't an option on this design. If additional bracing is necessary, I would need to add it inboard of the legs (at the top) so that it will not be seen in the finished product. The problem is that since the top is a freeform design, the legs do not line up and lend themselves to easily adding aprons or stretchers between the legs for additional support.

[Greg Sznajdruk]

James:

You have a real challenge on your hands. Do you have any photo?

Greg

[James Biddle]

Here's what I'm up against. Sorry about the crude design.

http://home.comcast.net/~jbiddle/pics/zebrawoodtable.jpg

[Dan Oliphant]

James, to use the concept that is shown in the drawing my suggestion would be as follows;
Leave the tennon as long (in length) and as wide as possible without making it visable, Minimal shoulders.
Pre drill (and counter bore) two pilot holes per leg in the top (don't panic yet!!)
After the legs are glued, install the wood screws then plug the pre drilled holes with bungs. The bungs must be the correct grain pattern and size to properly match the hole.

[James Biddle]

James, to use the concept that is shown in the drawing my suggestion would be as follows;
Leave the tennon as long (in length) and as wide as possible without making it visable, Minimal shoulders.
Pre drill (and counter bore) two pilot holes per leg in the top (don't panic yet!!)
After the legs are glued, install the wood screws then plug the pre drilled holes with bungs. The bungs must be the correct grain pattern and size to properly match the hole.

Dan, that was my first thought. My question is: what are the odds that I can hide the sharply contrasting grain of zebrawood? I can pull a plug from the hole and reinsert it (if it comes out cleanly), but the filler in the drill bit kerf (is this a real term?) would also have to be matched. I'm thinking this would be difficult.

[Richard Wolf]

I think you should attach plates to the top of each leg and then screw the leg plates into the table bottom. This would give you alot more surface area to glue and screw.

Richard

[Scott Vigder]

First, I do think epoxy is overkill. Fresh yellow glue, properly applied, works great.
Second, can you add one inch "wings" to the legs where they attach to the table. Each wing would surroung the leg, in effect creating a mortise. I would mortise into the table top no more than 1/2 inch. Glue/screw the wing to the tabletop, then to the leg (with proper clamping). The wings would help create surface area to absorb the lateral forces.
Good luck with the Zebrawood.

[Dan Oliphant]

What Richard and Scott suggest would also work well. My comments are based on using a streight or tapered plug cutter that is matched to a given drill size. Matching the grain should not be that big an issue, that is based on the experience I have had with zebrawood.

[Don Baer]

James,
I'd use floating tenions about 2/3 of the size of the legs. I'd also use the yellow glue of your choice. Thats just the way I'd do, it doesn't mean it's right.

[glenn bradley]

I think MR. Wolf is onto something. The engineering challenge is one of joint strength to lateral stress (I'm a network engineer, not a P.E. so large grain of salt required here). The amount of area for the top of the leg just doesn't seem like it will provide enough lateral strength for this table to last very long. Good luck with your challenge.

[Chris Mann]

I've made tables similar to that. I epoxied threaded rod into the legs and used a haefle expanding nut in the top which locks in place when the leg is screwed in. Everything is holding just fine (but my table has 9 legs).

For your situation I'd probably epoxy a metal plate into a recess in the table and tap it, then screw the leg into that. If you decide to go with regular mortise and tenon, I'd split the tenon in two and use epoxy to glue it in. Something like each tenon 1"x1 1/2"

[James Biddle]

I think you should attach plates to the top of each leg and then screw the leg plates into the table bottom. This would give you alot more surface area to glue and screw.

Richard

Richard, got a pic or source so I can better understand your concept?

[James Biddle]

Here's my favorite suggestions so far:

Increase the thickness of the tabletop at the legs.
Make an inside apron, an 'X' between the legs.
Make a metal plate of some sort to screw down into the legs and then up into the table.Any of these options look more appealing than the others?

[John Fry]

Zebra is readily available in thick and wide boards. I'd probably make the whole top thicker, (6/4?), do a gentle bevel of the underside of the edges for the desired fineness of the top's edge, and then you could go with much deeper mortises.

It might also help keep an unsupported top of this size, flatter in the long run.

My 2 cents!

[James Biddle]

What if I change the leg design to something like this (didn't take the time to round over the edges)? I could M/T the legs to the braces and screw the braces to the top through some slots.
http://home.comcast.net/~jbiddle/pics/zebrawoodtableV2.jpg

[John Fry]

I think this is a much better design. It will not only improve the structural integrity of the legs, but will help to keep your 3/4" table top flat.

And yes! "Screws through slots", or some method of allowing the top to expand and contract, is extremely important here.

[tod evans]

What if I change the leg design to something like this (didn't take the time to round over the edges)? I could M/T the legs to the braces and screw the braces to the top through some slots.
http://home.comcast.net/~jbiddle/pics/zebrawoodtableV2.jpg


james, i agree that this would be a much more stable design. when i read your first post all i could think was that it would be an exersize in futility. this design will add structural integrety to the legs and will help hold the top in plane. ...02 tod

[Rob Russell]

I may be swimming against the tide here, but I don't think that the extra "bracing" under the table is going to do much for the legs, unless the legs are locked to the bracing with sliding dovetails.

With all that cross-grain joinery, the bracing would have to be screwed to the top - it can't be glued or I'd think the top will split over time. Given that the bracing has to be screwed, if you mechanically join the legs to the bracing, the legs can't be hard glued to the top. If you glue the legs in to the mortices, they will effectively need to move with the tabletop as it expands and contracts. If the table top is expanding/contracting, the legs are trying to move in or out but the bracing (which I'm assuming is connected to the legs) won't expand or contract at the same rate - and that spells trouble.

I don't have an answer, but it seems to me that you need to come up with a way to support the legs that doesn't involve lots of cross-grain joinery.

Perhaps another 2 layers of zebrawood right around the legs to give you a significantly deeper hole to put the wenge legs into. You'd want to orient the extra blocks the same way as the table top. You could bevel them away from the legs for a lighter-looking appearance.

Just my 2 cents.

Rob