Deep, wide mortise and tenon versus multiple Dominos?

[LLesniak]

I have a question about a joint that is used in the structure of one of my looms.  This mortise and tenon joint is used to connect a large upright (88" tall, 6" wide, 1 1/2" thick) to a foot (40" long, 5" tall, 1 1/2" thick).  Think of an inverted T.  The existing design uses a mortise 3 1/8" deep, 4" long, and 3/4 inch wide with a matching tenon to join the two pieces making for a strong, stable joint.  The tenon is currently cut on a radial arm saw and the mortise on a hollow chisel mortiser.  I'd prefer to make this joint using my Domino but I'm concerned that the resulting joint will not be as strong as the existing M&T since the Dominos themselves penetrate less deeply into each part.  Would a double row of Dominos hold as well as the M&T?  Has anyone in the group used the Domino for joinery of this scale?  I've used my Domino extensively but have been working more toward the small end of our loom assemblies than the largest structural joints we create.  The attached photo shows one of these looms fully completed with the joint in question at the base of the upright on both sides of the loom.

Any thoughts or input would be greatly appreciated!

Larry

 

[Jesse Cloud]

Hey Larry,
First of all, let me say that is one cool loom.  My wife is a weaver and was fascinated by the design.  Do you have a website?

On to the nitty gritty... I'm a big fan of the Domino and use it a lot, but I would be wary of using it for this joint.  Although it looks like most of the motion in your loom is vertical, that post is still one heck of a lever that will multiply any horizontal stress on the foot.  I would stay with deep wide mortises. 

Although you might try to alter the design, say with corner blocks to allow the use of dominoes.

Might be a perfect application for the bigger domino that is rumored to be in the works. [cool]

 

[Notorious T.O.D.]

Are you thinking a 2x2 pattern of 10x50mm dominos as the replacement of the mortise and tenon?
Can you fit 3 dominos wide? To me it looks like the load on the joint is pretty balanced. So, I am thinking the dominos might be sufficient. I guess I would tight fit the dominos in the end of the upright and then use a bit wider mortises in the foot.

Best,
Todd

 

[chrisrosenb]

Very nice loom.

I would make them as you are.
As much as I like the Domino, I would not trust it in the application. You have a lot of leverage on the upright against the feet when the loom is in use.
With the mortise & tenon, the parts are locked together. The wood has to break for there to be a failure of the joint.
If you use Dominos, you will be depending only on glue joints to keep the connection from failing.   

 

[Tim Sproul]

I wouldn't use the domino but for a different reason than previous posts.  You have a stretcher very close to the foot/leg joint.  The stretcher is connected via a through tenon that requires you to excavate a mortise in the leg rather close to the leg tenon that fits into the foot mortise.  If you were to use loose tenon joinery at the leg to foot joint, I think you risk significant loss in strength in the leg because you would have multiple mortises very close to each other.  You would have the mortises of the loose tenons at the end of the leg and then the mortise to accommodate the stretcher.

Without that stretcher so close to the bottom of the leg, I think Dominos would work just fine.

 

[Notorious T.O.D.]

But Tim, do the mortises not get filled by the loose tenons which basically makes them solid again?  So, in the end would it really matter especially if the Domino is only going 25-28mm deep?

Best,
Todd

 

[Tim Sproul]

Perhaps in theory...but in practice, no.  The bottom of a mortise won't match the end of the loose tenon.  Close, and perhaps even close enough to make no practical difference.  But this ignores grain.  If the grain in the leg isn't parallel with the mortise for the loose tenon, you've severed a bunch of the long fibers and filling the void with a loose tenon won't get you back to the same strength as before the mortise.

 

[LLesniak]

Thanks for the feedback!  I've decided to leave the design as-is for many of the reasons listed above.  The primary concern is that the upright does generate leverage against the foot and I don't think the Dominos would provide a strong enough joint to resist the forces involved in weaving on this loom.  The deep M&T locks the upright and foot together in such a way that only a catastrophic failure of the wood could break the joint.  So M&T it is at least until some other method of joining these parts comes along (i.e. Super-Domino?).

Thanks again for your posts.

Larry

 

[fshanno]

Well if you don't try it how will you ever know?  Nobody has shown me any numbers proving it won't work.  I say go for it.

 

[Notorious T.O.D.]

And if it does fail what is the rate of failure vs any assumed gain in build efficiency using the Domino?

Best,
Todd

 

[woodguy7]

Larry

I think you have made the correct choice.

Woodguy.

 

[fshanno]

Somebody has got to take it to the failure point in real life.  I don't recall ever seeing a photo of a real life domino joint that has failed.  Has an M&T joint at the tee ever failed in one of your looms?  Do they ever get loose and start wiggling?  If so what do you do?

 

[Chrisem]

This article always impressed me.

http://www.festool.co.nz/artikel/artikel_weiterleiten.cfm?id=487