Design question - trestle table base

[Henry W]

Short version: How would you attach the table top of a large table to 4 posts/legs inset from the corners in a trestle type design?

Much longer version:
I have a large table I am building. The top will be 10' long x 40" wide x 2" thick. Heartwood pine material - ready to glue up right now.

My question relates to the base design and attachment of the top. This link shows a similar design:
http://www.restorationhardware.com/...p?productId=prod1593082&categoryId=cat1537023
The base will be similar to what's pictured there:
- The stretcher bar will be large, flat, and low to the ground.
- The base of the legs will have the same orientation and relative size as the picture (i.e. larger than vertical pieces and be one beam perpendicular to the long side of the table).

The change from the picture is in the vertical pieces. I have 4 posts that match kitchen island posts that are already in the house (same supplier - Osborne Wood products). These turned posts have a 6"x6" square region (i.e. unturned) at both the bottom and the top of the posts. So I plan to mortise these posts into the base and then add lag screws through the bottom beam.

The base beams are 3.5" thick and 8" wide. I plan to mortise/excavate a 6" square about 1" to 1.5" deep to sink the two posts as tenons into each base. If I mortise a full 6" wide, a 1" strip of the beam is left along each side of the mortise (because base beam is 8" wide). I think that is hefty enough, but I could cut down the tenon to a square 5" on a side or less. In addition to mortising, I plan to drive lag bolts up through the beam into the posts - I know this is into end grain of the laminated posts, but I don't see how else to do this.

I am certainly open to suggestions for base construction, but my most significant question comes with attachment of the top.

I was planning to mortise each post into the top; remember the top is a full 2" thick and so I figure I can mortise 0.5 - 0.75 inches without any problem. This will register the top of the posts in the length direction, but I am concerned that I do not have enough support for the sides of the table this way. These posts are 6" square, and they will be separated by a 9-10" gap, so the outside edges of the posts will be about 22" apart. This leaves a 9" overhang from the edge of the post to the edge of the table. Is that too much? This also does not have any direct mechanical attachemtn of base and top, I need to get some screws in there somewhere.

Also, the design as described so far does not account for seasonal movement/expansion/contraction of the table top width. My sense is that I should use a 2x2 or 1x3 rail or stretcher connecting the top of the posts across the table width (out of view; there is no table skirt). The rail would be screwed into the side of the posts, and have slots for screws into the table top.

Any critique of my plans? How would you attach the table top to 4 posts inset form the corners in a trestle type design?

I am not certain that mortising the posts into the top has any value if I use the cross rails. Do you think it does?

Henry W
hwynands@ieee.org

 

[toolferone]

Wow, big table. Many large work bench tops are held in place by there mass on a few large dowel pins in the tops of the legs and let gravity do the rest. As far as lagging into end grain, a old trick is to drill into the bottom of the rail a few inches back from the end of the rail and glue in a large dowel. This way when you predrill for the lag it will be going through the dowel which will be long grain.

 

[Henry W]

Thanks Tom. Yes gravity should work, especially for a legs at the corner design. In my case the "legs" are inset a reasonable amount from the corners, so I want to be certain that the table top cannot tilt when someone like me pushes down on one side.

Henry

 

[Bill Clemmons]

Short version: How would you attach the table top of a large table to 4 posts/legs inset from the corners in a trestle type design?

Also, the design as described so far does not account for seasonal movement/expansion/contraction of the table top width. My sense is that I should use a 2x2 or 1x3 rail or stretcher connecting the top of the posts across the table width (out of view; there is no table skirt). The rail would be screwed into the side of the posts, and have slots for screws into the table top.

I think this is the answer, as long as you're leaving ample room for wood movement. W/ a 2" thick top you can use some pretty hefty screws.

I'm not sure I like the idea of screwing the cross brace into the sides of the post at the top. Since you will have a square top on the post, could you use a saddle joint (see pic below). Slip the cross brace into the saddle joint, then use pegs to secure it so it can't lift out when the table is picked up by just the top.

This is a saddle joint I used on a single post table I recently built. If you want to look at other pics, I think this link will get you there. The table is very different from yours, but the saddle joint concept may be close enough.

View image in gallery​

This pic shows the cross brace attached to the top of the post using a saddle joint.

View image in gallery​

HTH

Bill

 

[mlzettl]

Henry,

Several thoughts come to mind:

• The width of the base relative to the top will determine how much force is required to get the table to tip. If the width of the base is the same or greater as the top, the table cannot tip, no matter the force, assuming the force is perpendicular to the top. So, I would make the base 34-36" wide. Unless you are standing on the top, the table shouldn't tip.
• I would not make mortises in the base the same size as the square section of the leg pieces, simply because getting a good fit is difficult. Make a regular mortise and tenon joint, and I would make it a through mortise to increase the surface area for gluing. With a properly made joint, I don't think that any screws would be needed. The mechanical stability of the joint and the glue should be sufficient. If you wanted to make it even stronger, a wedged through tenon would be an option.
• A top this wide glued up from solid material should have at least a couple of battens to help control and cupping that may want to occur. Two of the battens could also double as the aprons in between the tops of the legs. Since your table is so long, I would probably add and additional two battens in between the two leg assemblies. I would use a mortise and tenon joint or a saddle joint, as Bill so nicely illustrated, to attach the apron to the legs. One dowel in each apron that has a conical tip can help to register the top in the correct position. Then, you can attach the apron to the top with screws or lag bolts that go through slotted holes in the apron to allow for wood movement. Another method would be to attach the top using wood cleats that fit into slots in the apron, and the cleats are screwed to the top. This is a fairly standard method of attaching the top of a table to its base. This is demonstrated by Gary Rogowski in a recent issue of FWW in an article about making a small trestle table, a good article to review for your project.

Spending your time planning will pay off in the long run. Don't cut any wood until you are sure that you have done the best you can in designing the project for appearance, strength, and function.

I hope this helps.

Matt

 

[Henry W]

Bill, Matt:
Thanks for the replies and insights.

I had cross-posted this thread on WoodCentral, and fairly similar ideas came up there. I agree (now) that I do not like the idea of screwing the cross pieces into the posts, nor am I likely to pursue the idea of mortising the top of the posts into the table top. I just don't see the value of that (now).

I believe that I will use the saddle joint with cross pieces idea that Bill was describing; the picture you showed does illustrate perfectly what I need to do.

Matt - do you know which issue of FWW that article is in? I'll check the current issue, but I am not a subscriber (yet?). I am familiar with the idea of wood cleats for table top attachment; I have used them on much smaller end tables and a Hall table, but these had an apron. I has not considered wood cleats for use on a trestle design with cross pieces.

Much appreciated help guys. Here is the real value of a great WW board - design help before I make mistakes!

Side note: I also was able to borrow Chris' long pipe clamps yesterday. :thumbs_up:eusa_danc
I need these to glue up that 40" wide top. See this post to understand more:
http://ncwoodworker.net/forums/showthread.php?t=35796
That's another even more tangible demonstration of the great generosity of NCWW members! Thanks Chris!

Henry W

 

[mlzettl]

Henry,

The article is in the October, 2010 issue, no. 214, page 28, "The Versatile Trestle Table," by Gary Rogowski. Gary is a practical guy, and a good teacher.

We are anxious to see some photos during and after the build.:thumbs_up

Matt

 

[Henry W]

Matt:

Thanks for the article info, I am guessing that issue and article are long gone from the newsstands.

As this piece dwarfs everything else I have made - all together, all least in terms of BF - I have remembered to take pictures in process.

So far I have glued up three sections of table top and used up more glue than I use in a typical year. Joining 2" boards along 10" (8 boards total) takes a lot more than a dab of glue! Pictures taken but look pretty much like you'd expect for a long board glue up. Next is to join these three into one table top - and get them into one plane. We'll see how that goes.

Also starting on trestle base soon - that will be more intricate, if anything on this massive table can be called intricate.

You'll see some pics in the next week.

Henry