Tenon length for table apron, Domino 700

[ericsink]

I’m planning a table with an apron attached at the top of the legs with dominos, so I’m bumping into the somewhat classic questions about intersecting mortises.

A few specifics about the context:  I intend to make the table legs 3.5 inches thick (call that 89 mm), and the apron 1.5 inches thick (38 mm).  The vertical size of the apron (the “width” of its boards) is currently planned at 4 inches (102 mm).  Any of these dimensions could change if I have a reason.

Strength is an issue.  This “table” is planned at 4 ft by 8 ft, and is actually a new home for my 3HP cabinet saw.  Between the saw and all its accessories and storage, there could be maaaaaybe 1,000 pounds of “payload”.  If it looks like I’m overbuilding this, that’s intentional.

I’m thinking 14 mm dominos.  The rule of thirds would say 38/3=12.667 mm, and in this kind of situation, I round up.

For the length:  It’s common to see folks recommend a depth between 1/2 and 2/3 of the width of the leg.  Using that guideline, assuming the four vertical sides of the leg are the same width, if the apron is centered on that width, the mortises will intersect. 

So I would lean toward 14x100 dominos to get a 50mm mortise into the 89 mm leg.

(1) The last time I faced a similar problem in my pre-Festool life, I mitered the tenons.  That situation was “regular” tenons on the apron boards, not loose tenons, but still, similar.  Anyway, I could do that with the dominos, but I’m kinda looking for a good way not to.

I saw one person mention (2) another way to deal with intersecting mortises:  Cut one mortise, glue in the tenon, wait for it to dry, then cut the other mortise, thus cutting into the first domino.  Interesting, I suppose.

So what’s the best way to have the mortises NOT intersect?

(3) Make the mortises shallower.  If my geometry is correct, the intersection is around 37.5 mm.  Call it 35, and the tenon would need to be a bit shorter than that.  So I would probably be giving up some joint strength.  I also note that I would be cutting tenons to custom lengths unless I go all the way down to a 10x50.

(4) Offset the apron toward the outside of the leg.  If the mortises are 50mm deep, one mortise won’t crash into its sibling mortise if that sibling is offset more than 50mm from the shared edge of the leg.  This is tricky to explain without a picture, so hopefully that makes sense.  The mortises can’t intersect if they can’t reach each other.  But again, the joint would probably be stronger if the apron is centered, right? 

In this example, there would 50mm on one side of the mortise and 89-14-50=25mm on the other side.  That might be okay.

(5) Put the two mortises at different positions vertically.  Suppose I do go with 102mm for the apron boards, and suppose I do just one 14x100 domino, which is apparently 27.9mm wide.  There’s plenty of room to have those two tenons avoid each other by putting them at different heights.  But of course, all else equal, if I have room for two dominos, I would rather have two.  Maybe I could make the apron boards even wider so I would have room for two on each side but still have them avoid each other?  Intuitively, if that were true, there would be room for three.

Is there an option I’ve missed?

Which option would you choose?

 

[ericsink]

Here’s another option I hadn’t thought of when I started this thread:

(6) The two mortises for a given domino don’t have to be the same depth, right? For example, I could use a 14x100, with 65mm of length in the apron and 35mm in the leg.

 

[smorgasbord]

#2 and #6 are effectively the same. Cutting the second mortise after the first is glued means you're cutting into the first mortise, shortening it.

• Going thicker on the tenon doesn't necessarily make the joint stronger, as the walls of the apron mortise end up thinner.

• Offsetting the apron on the legs should not reduce the joint strength. You should choose the amount of reveal that you think looks best. With a 89mm thick leg and 38mm apron, centering gives you a full inch (25.5mm) of reveal. That's a lot visually. I'd probably go for something like 10mm of reveal.

• I believe in your design that your top is resting on top of not just the aprons, but the legs as well. So the aprons are basically acting as stiffeners for the top. How thick is your top?

• At a 4' x 8' design, I assume you're including 4' long cross members between the long aprons. Maybe even an 8' long cross member across the short aprons. If you're doing both, then you have some half laps to cut, unless you piece in the cross members in one direction, using dominos there, too. These cross-members don't have to be the full 38mm thick, btw.

• You could skin the bottom of the aprons and cross-members with plywood to create a torsion box that would add a lot of rigidity to the piece.

• In a 100mm wide board, two 12mm dominos take up about 52mm, leaving 48mm to split in three ways. That could be just 12mm on each of the 3 spaces, or maybe slightly more at the top and bottom and less in the middle between them. Another reason not to use 14mm dominos, btw.

Personally, I'd use 12mm dominos and a 10mm leg/apron reveal and then figure out which tenon lengths just miss each other. OK if the mortises overlap.

 

[Crazyraceguy]

There are a few things at play here, tenon length being just some of it.
As [member=77266]smorgasbord[/member] said, moving the aprons outward may eliminate this problem entirely, at very least reducing the amount of overlap.
I think I would combine the idea of moving them out and alternating the spacing on the long rail and the shorter ones.
There are other factors, with a table that long, not related to this corner interference. One of those is sag, also mentioned above. The torsion box is a solid concept. The top and bottom skins don't have to be much, 1/4" plywood will do it. The stiffness comes from the skin being glued to both sides of the ribs/grid. With a 4" apron, you could get by with 2 1/2" ribs and 2 layers of 1/4" ply. You would end up with a solid structure, that would be up under the apron far enough to not be noticeable.
The next issue is racking. It wasn't mentioned in your description, but if the legs are not tied together lower than the aprons, all of the racking force is placed on that 4" joint at the top.
Racking damages more tables (and chairs) than the force of gravity alone.
In some cases, this is not practical, or visually desired. That is when the addition of metal bracketry comes up. That can take a few forms, depending on the situation.

 

[ericsink]

[member=77266]smorgasbord[/member]  Thanks for the reply!  Very helpful.

You're making me wonder if I'm overbuilding this so much I've gotten absurd.  I didn't mention this, but I actually have 6 legs, not 4.  I'm attaching a picture of the current design mockup.  (Most of the colors are not meaningful, so ignore that.)

A design driver here is to get the saw table to end up at a height of 35.5 inches while being on a really solid mobile base.  I plan to use leveling casters under each leg.  The left side of the design, where the saw will be, has 4 cross members that support the saw.  In order to get the saw height to end up correct, I am removing the black pedestal from the bottom, leaving just the gray cabinet.

The current plan for the "top" is to make it an L-shaped torsion box, 4 inches thick.  And yes, it would rest primarily on the legs.  I've had variations where the top was simply a 3/4 inch sheet, which would need more supports.

So yeah, if the top is the torsion box, the apron is mostly just there to give the whole thing rigidity and prevent racking.  Which I suppose means it doesn't need to be flush with the top of the leg everywhere.

I'm attaching a second picture with the saw itself removed.  A challenge in the design is figuring out how to get everything well supported without blocking access to the saw.  For example, there is no apron board on the upper left front, because it would be in the way of the front saw wheel or it would interfere with the fence and its rails.  That's why the front middle leg exists:  because I need a way to support a cross member to support the top, and I can't bring an apron board across the entire front.

I'm concerned about the apron board on the left side blocking access to the motor cover (which is not shown), but I need some way to support the part of the top behind the saw.  I've had variations of the plan where the front left leg is really short because nothing connects to it, and its only purpose is the caster underneath it, and then there's another leg partway back on the left side, to support a cross member for the top behind the saw.  But that design looks a bit dorky.  I might still do it that way, and if I do, I have questions about whether that "leg" is actually a leg with a caster, or if it's just a vertical piece that goes up from the lower stringer to support a cross member.

On the left side near the bottom there are two blue cross members that are taller than the ones that support the saw.  They are intended to make sure the saw doesn't fall off.  I marked them blue because I don't think I actually need them at all.

At the bottom of the front middle leg, there are two stringers (I think they're called?) that connect to the leg on opposite sides.  The one on the left is part of the support for the saw, so I plan to use the longest domino I can for that.  The one on the right doesn't need to be low, so it is elevated higher to avoid interference with the mortises in the leg.

So for the two stringers on the left side front and back:  I think I actually need them to be thick, which is why they're 38mm, because those saw-supporting cross members are dominoed into them.  All the other apron boards and stringer boards are that same thickness, but I'm not sure any of them need to be.

Thanks again!

 

[ericsink]

It wasn't mentioned in your description, but if the legs are not tied together lower than the aprons, all of the racking force is placed on that 4" joint at the top.

Yeah, I tried to keep the original post short.  Not only did I fail at that, but I omitted information that would have been helpful.  Double fail.  [big grin]

Racking is definitely a worry for a table like this.  I'm hoping the lower "stringers" are enough, but on the right side, they may also somehow end up as part of the support for storage, which I haven't designed yet.

 

[Crazyraceguy]

Yeah, that matters a lot.  [blink] You effectively have a 4' x 4' span, with a 2' x 4' wing, all potentially tied to a cabinet, along with lower stretchers. It's plenty strong/stable and with a torsion box top, even better.

Though, back to the original question. Yes, I would still move the aprons outward, to stop the intersection of the tenons.

 

[ericsink]

I'm now wondering how far into the absurd I've traveled here.

If I stay with 6 legs, and if I make the top a torsion box, and if I still keep both an upper and lower connection between the legs (apron and stringers), then maybe the strength of those joints from apron to leg isn't much of an issue at all.  Maybe the aprons don't need to be 4 inches wide.  Maybe I just keep the mortises 25mm deep where they won't intersect.  And probably the apron boards can be just 1 inch thick instead of 1.5 inches.

 

[rmhinden]

In my last project, making a base for cabinet, I centered the mortices on the legs and mitered the dominos.  Worked fine.  Minimal loss of surface area on the dominos, resulting in a strong joint. 

This had two dominos (vertical) per stringer. 

Bob

 

[smorgasbord]

I should know not to respond to questions like this without diagrams. This isn't a table supporting a heavy saw, it's a base to raise the tablesaw up a small amount, and then a side wing with storage underneath.

The design makes no sense to me at all. The table is deeper than your saw's cabinet, but not by enough to really add any significant outfeed table area. The rear legs above the saw's base won't hold anything heavy. The side table needs to be kept mostly clear so you can cut wide things, so you're not going to have anything heavy on it. The saw's base is so close to the floor that you could just build a low platform that rests on the aprons, or just put some blocks/wheels at the corners of the saw to raise it to the desired height.

 

[ericsink]

The design makes no sense to me at all. The table is deeper than your saw's cabinet, but not by enough to really add any significant outfeed table area. The rear legs above the saw's base won't hold anything heavy. The side table needs to be kept mostly clear so you can cut wide things, so you're not going to have anything heavy on it. The saw's base is so close to the floor that you could just build a low platform that rests on the aprons, or just put some blocks/wheels at the corners of the saw to raise it to the desired height.

LOL.  I’m not sure it makes any sense to me either.

I plan to make a second 4x8 table to go behind this one.  When they’re pushed together, the result would be a full 8x8 with plenty of outfeed behind the saw.  Maybe the first table shouldn’t be so deep, but the area right behind the saw will need grooves for the miter slots, so figured if I do that in the table that holds the saw, then the other table will be more general-purpose.

I’ve considered making the full 8x8 table right from the beginning, but no doorway of that room would accommodate it.  I’ve considered using domino connectors to allow it to be taken apart.  I just figured two separate tables would offer more flexibility.

I could simplify things by using a side mounted workbench caster, but I’ve used those for other things in my shop, and I’m not sure they’re up to the task here.  The leveling casters seem way better.  And the height of that caster plus the height of the saw cabinet itself dictates a lot about the design.

I started this thread thinking I could keep it focused on the specific issue about the domino mortises, and as I said, that failed.  But I do really appreciate the comments.  [big grin]

 

[Packard]

I worried myself sick over how to attach a bathroom vanity to the wall so it could “float” with no legs.

I took the easy way out.  I added two narrow legs about 10” in from the front.  It took most of the load.

So, what is the reason you cannot add a couple of extra legs?  If you tuck them in from the perimeter they will be nearly unseen.  And they can arguably carry more weight than any tenon could.

There are so many factors involved in calculating tenon size that it make me dizzy. 

The length bears on pull out strength.

The width and thickness bear on shear strength.

Wood can handle huge loads in the axial compressive (straight loading downward), so that will never be a real factor as long as the tenon does not bend. 

Basically, if you have a board joined by tenons, the bottom of the joint is in compression and the top of the joint is in tension.  Only the tension is worrisome. 

I don’t know where you will find the published data, but pull-out strength is the key.  You have to first establish the amount of pull out strength that is required, and then calculate how you can reach that number (size and number of tenons).

Or you can add a couple of inconspicuous legs.

 

[ericsink]

So, what is the reason you cannot add a couple of extra legs?

In this design, every leg is another leveling caster.  I worry about how things will roll if I have too many of them.  And if the caster is in the middle of the table, it'll be hard to adjust its height.

But yeah, with no casters involved, adding legs does seem like an easy way to stop worrying about tenon strength.  [smile]

 

[Packard]

My studio lights were always on 5 casters.  No noticeable difference in rolling resistance, but greatly enhanced stability.

Here is a spring loaded caster.  You select the amount of load it should carry:


Plenty of options in this search:
Ohttps://www.google.com/search?q=spring loaded caster wheels&ie=utf-8&oe=utf-8&client=firefox-b-1-m

Lots written on rolling resistance of casters, primarily in reference to wheel chair design, but the data might be of interest.  Basically, larger diameter urethane casters have the least rolling resistance.
https://www.google.com/search?q=do ...tance?&ie=utf-8&oe=utf-8&client=firefox-b-1-m

This manufacturer goes into detail.  (But read with a critical ear.  They have something to sell.  But pretty clear and concise.)
https://www.bulldogcastors.co.uk/blog/castor-wheels-roll-resistance/