Cross half lap joint with a mortise and tenon

NOTW

Notw
Senior User
So my project ADD kicked in and the workbench is on a slight hold as I build a new base for my dining room table. I never really liked the old base so I decided to come up with a new design. The legs are an X pattern with cross half lap joints connecting the two together then from one X to the other I was going to mortise a stretcher to connect the two together. Now I am curious if adding the stretcher in a mortise will weaken the half laps?

The X's are already made and are made of poplar 3' thick x 5" wide
Table.jpg
 

McRabbet

Rob
Corporate Member
Your SketchUp drawing presents itself as an intriguing optical illusion (but it is a good drawing!). To answer your question, Yes, I believe the through tenon will weaken the half lap joint significantly. I would reduce the size of the tenon along the vertical axis and make it a wedged tenon to insure that the outer leg in the pair is held tight.
 

NOTW

Notw
Senior User
Your SketchUp drawing presents itself as an intriguing optical illusion (but it is a good drawing!). To answer your question, Yes, I believe the through tenon will weaken the half lap joint significantly. I would reduce the size of the tenon along the vertical axis and make it a wedged tenon to insure that the outer leg in the pair is held tight.
Since the base is ultimately going to be painted white anyway would I be better off cutting the stretcher to the dimension of the inside of the base and screwing it into place and then plugging the holes?

Never had the patience to learn SketchUp, but for some reason AutoCAD, Inventor, Revit, TwinMotion, etc came easy....go figure
 

Graywolf

Board of Directors, Vice President
Richard
Corporate Member
You should not have any problem with what you plan
 

Inarticulate

Matt
User
It seems to me that your mortise size and position effectively severs each leg piece into two separate sections. There might be a very small sliver of wood along each side but that's all. I would not expect it to be very strong at all. Imagine if you built each "X" with 4 separate pieces of wood, rather than two, glued together at the center. That's about the level of strength I think you'd achieve. A small reduction in mortise size, giving back a little more thickness as its thinnest point, would go a long way to adding some strength back. Again, all these conclusions are based on just what I can visually tell from your picture. I'm a mechanical engineer but not a very experience woodworker so take the feedback with a grain of salt! :)

-Matt
 

Gotcha6

Dennis
Corporate Member
My dad built one like this 60+ years ago using no nails. The tenon and mortise do need to be smaller and a draw bore wedge placed into the tenon to keep everything tight.
 

NOTW

Notw
Senior User
I agree, I decided since I like the proportion of the stretcher and didn't want to make is smaller I would use long screws to attach it to the X's and then plug the hole with a dowel. Since its going to be painted anyway I can do any filling around the dowel I need to and make them disappear. Thanks for all the input.
 

NOTW

Notw
Senior User
So here is where i am, drilled two 3/4" holes with the forstner bit then drill through for the 1/4" lag screws. Going to screw the lag screws into the stretcher (after pre drilling) and then plug the hole with a 3/4" dowel, fill any voids with wood filler sand and get ready to paint white. PS the more extension cords you have out the better you work....right?

IMG_3763.JPG
 

Lhloy

Larry
User
Too late now, but in general I don't like putting screw threads into end grain. Your lag screws are certainly better here than wood screws, and the longer the better.
 

NOTW

Notw
Senior User
I did a trial run with the screws last night and after putting 2 screws in each end I was able to stand on the stretcher with no issues. I doubt my table top weighs as much as I do and it wouldn't be supported by the stretcher anyway. Tonight i will glue the stretcher in place, screw it in and add the plugs.
 

RickR

Rick
Senior User
The stretcher is there to resist racking rather that taking weight. As such the lags are under tension and might tend to pull out of the end grain over time. You could insert a dowel into the stretcher from the bottom so the lags have something more (cross grain) to bite into.
 

Jeff

Jeff
Corporate Member
I did a trial run with the screws last night and after putting 2 screws in each end I was able to stand on the stretcher with no issues. I doubt my table top weighs as much as I do and it wouldn't be supported by the stretcher anyway. Tonight i will glue the stretcher in place, screw it in and add the plugs.
I don't think that the issue is whether it will support the weight of you or your table top but rather the side-to-side racking that can occur with lots of hand planing from end to end.
 

Gotcha6

Dennis
Corporate Member
I don't think that the issue is whether it will support the weight of you or your table top but rather the side-to-side racking that can occur with lots of hand planing from end to end.
That's why my second suggestion was bed rail bolt sets. No danger of stripping on them, and they can be concealed to a degree, but under a table like that, I doubt it will matter. IN any case, they can be retrofitted into the holes you made for the lag screws if you have problems down the road. Good luck with the project.
 

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