Domino 500 + Mortise Size

[designcutbuild]

Hi,

I'm building a pretty heavy coffee table which will support a decent amount of weight -- it's essentially our dining table/foot stool etc.

I'm planning to use the Festool Domino 500 to attach the aprons to the legs and repeat the process for a shelf on the lower part of the table.

My question was around the mortise sizes -- if I put it in the middle setting, which gives me a bit more space during assembly, is that a horrible thing structurally? I figured that the glue will fill in that space, but i'm not entirely sure how that will affect the structural integrity.

I looked into a few woodworking forums and there is conflicting info. I came here for the final say

Two specific questions:
1) Does having additional space affect the structural integrity despite the glue filling the space? How much so?
2) Should I be adding the space or should i try to make it an exact fit? (Context: I'm pretty new to woodworking)

Thanks!

 

[Midnight Man]

From what I understand, you should make your first mortise in a row the exact width needed.  For additional motises, you dial up to the next width, which allows a little "play" room when fitting the pieces together.

You should aim to use a domino 1/3rd the thickness of the material being joined (e.g. if you're working with 18mm timber, use a 6mm domino).

With apologies, I don't feel qualified to answer the question on mortise strength when making them wider than the tenon (domino) - I will leave that to those wiser on these forums

 

[Birdhunter]

I use all narrow mortises on one side and all medium on the other side. This gives me left to right alignment "slop". Narrow to narrow requires perfect mortising.

The tenons provide sheer strength and tensile strength. Neither are affected by mortise width. The glue that squishes into the wider mortise areas does not really provide any structural strength. The glue on the tenon bonds the tenon to the board's wood fibers and swells the tenon somewhat. This bonding provides left to right strength.

A light coating of good glue all over the tenon will be adequate. Too much glue can cause the wood to blow out as you tap the tenon into the mortise. I like Titebond III.

Follow the "thirds" rule on tenon selection and you should be fine.

Doing a dry fit is worthwhile. You verify that everything fits and you know what your clamping procedure will be.

 

[ear3]

Welcome to the FOG.

Short answer is no -- the dominoes fit so snugly in the mortises, that when the expansion takes place from soaking up the glue, they will lock in place really no matter if its the narrow setting or the widest setting (the only way the joint would be slightly compromised is if you use the widest setting on both sides, and the domino sat diagonally -- but even then it would still be fairly strong).  The real reason to use the narrow setting, at least on one of the mating mortises, is for exact alignment purposes.  You will find, especially if you are doing a run of several dominoes (like on edge joining a two boards), that it's actually better to use the wide setting on some of them, because that allows you to fit the boards.     

 

[RussellS]

Two specific questions:
1) Does having additional space affect the structural integrity despite the glue filling the space? How much so?
2) Should I be adding the space or should i try to make it an exact fit? (Context: I'm pretty new to woodworking)

Wood glue, yellow and white glue, does not fill space.  It forms a microscopic bond between surfaces.  It has no filling ability.  The fill amount has no strength.  Squeeze out some glue into a bottle cap and let it dry.  Then try to break it.  It will break easily.  Blobs of glue have no strength.  All the strength comes from the surface connection between the wood.

Structurally, the mortise and tenon connection is not as strong if its not an exact fit.  Any space is less strong.  BUT, it may not matter much.  The tenon sides will glue to the sides of the mortise even if the top and bottom do not touch each other.  With the strength of modern glues, that may be more than strong enough and once it dries, its good.  Never come apart.  The chemical connection of the glue holds everything together.

By adding space, you are missing and eliminating the mechanical connection.  With space, the bottom of the tenon is not resting on the bottom of the mortise.  There is no gravity, mechanical, structural connection between the mortise and tenon.  You are relying solely on the chemical connection of the sides of the tenon and the sides of the mortise derived from the glue.

 

[Lou in DE]

When I got the 500 last year, I was a little surprised by how wide the 2nd and 3rd setting were - I seldom if ever use the widest setting and sometimes use the middle one. On some projects I use only the tight setting for all and if I need a little wiggle room I will shave off a little from the sides of one or more dominos - I think that is a better option than having all that space filled with glue.

A little test I did when I first got it - with some probably slightly off measurements but you get the idea  [big grin]

Lou

 

[Birdhunter]

My guess is that in the real world the structural strength between using only the narrowest or only the widest would be very much the same.

 

[HarveyWildes]

I've never had trouble locating the holes side-to-side so that the fit is nearly exact.  Almost never.  When I do misalign the holes, I use a block plane to narrow the Domino that's going into the misaligned holes just enough to make the fit work.  If one side is already glued, you could use a chisel or rasp.

That said, I've used the wide setting a lot because I make my own wide dominos, and I make them so that they fit in a full depth hole as well (unless that ends up cutting through the work piece).  You get nearly double the gluing surface compared to a stock domino.

 

[designcutbuild]

I use all narrow mortises on one side and all medium on the other side. This gives me left to right alignment "slop". Narrow to narrow requires perfect mortising.

The tenons provide sheer strength and tensile strength. Neither are affected by mortise width. The glue that squishes into the wider mortise areas does not really provide any structural strength. The glue on the tenon bonds the tenon to the board's wood fibers and swells the tenon somewhat. This bonding provides left to right strength.

A light coating of good glue all over the tenon will be adequate. Too much glue can cause the wood to blow out as you tap the tenon into the mortise. I like Titebond III.

Follow the "thirds" rule on tenon selection and you should be fine.

Doing a dry fit is worthwhile. You verify that everything fits and you know what your clamping procedure will be.

I have Titebond III, so that's perfect.

Can you maybe share a bit about the difference between shear and tensile strength? I'm using this to connect the coffee table legs to my apron if that makes any difference!

 

[Birdhunter]

Sheer strength would be trying to break the tenon by pushing up on one side of the joint and pushing down on the other side. Not going to happen. Tensile strength would be trying to pull the tenon apart by pulling on each end. Not going to happen.

I've done exactly what you are trying to do with the apron and leg joint. I used two tenons, one above the other. No problem.

 

[Jesse Cloud]

If you expect to put a load on the piece, as you will with a table or footstool, it is better to use a tight fit if possible, at least in one of the two pieces you are joining.

If you are building something you hope to pass on to other generations, the glue will definitely dry out at some time and a loose fitting tenon will be a problem - witness all those cheaply made kitchen chairs with legs coming loose.

Buy some low cost hardwood lumber and make a few test joints to see if you feel you need to use wide settings.

Now, having said that, I'll take back a little bit of it.  The problem with tables and shelves on tables is that the wood in the tabletop and in the shelf will grow and shrink as humidity changes.  The wood in the aprons will grow/shrink in a different direction.  You need to design around that.

For the top, an easy fix is to attach the top to the aprons with "Z clips" - google 'tabletop fasteners".  These clips have a slot that you put a screw in the middle of.  As the top grows and shrinks, the screw can move in the slot and your problem is solved.

When you attach the shelf to the legs, it would be ok to use tight mortises on the back legs, and wide mortises on the front.*

*The wood will move at 90 degrees to the direction of the grain in the board, so if the grain runs from left side of the to the right, the top will grow toward the front and back.  If the boards in your top run from front to back, then the top will grow toward the right and left.  This will affect where you put your z clips or loose mortises.  PM me if this doesn't make sense.

 

[Naildrivingman]

I agree with Jessie. I have built several tables and fasten apron to the table with Kreg screws. I am not trying to hijack your original post, but I feel using a domino exclusively is not always the best course of action.

 

[davetink]

Paul Marcel say  undefined  Not scientific but....