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Loose Tenon Question

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Forum topic by 47phord posted 10-11-2012 01:11 AM 1172 views 0 times favorited 15 replies Add to Favorites Watch
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47phord

175 posts in 985 days


10-11-2012 01:11 AM

I’m building a Mission-style sofa and have run into a problem. For the front rail, the plans call for cutting a 1/2” tenon by milling off 1/4” each face of the board to form the cheeks of the tenon. The problem is the stock you make all this from is 3/4”-meaning if you follow the plan, you end up with a 1/4” tenon not 1/2”. So my question is this, I know I can glue new cheeks onto the tenon and remill, but will that be strong enough? I’ve also considered just ripping the tenons off and routing a mortise for a loose tenon in the rail (I’ve already cut the mortises in the legs). Or, should I just chuck the rail onto the scrap pile and go buy another board? My biggest concern is strength, this is one of the rails that supports any posteriors that will sit on the sofa so failure is not an option. Thanks in advance for your help!


15 replies so far

View HorizontalMike's profile

HorizontalMike

6968 posts in 1662 days


#1 posted 10-11-2012 01:25 AM

A well glued 1/4” tenon should be strong enough, though 1/2” would be better. If you have no problem creating the 1/2” deep mortise for the floating tenon then I would recommend doing so. Personally, I would NOT spend extra to do so, but that is a decision for you to make. If the stock is 4in wide then absolutely no problem. If it is only 3in wide then maybe, but that is a personal choice. You have NOT communicated the width of the rail, and that is important.

-- HorizontalMike -- "Woodpeckers understand..."

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patron

13171 posts in 2089 days


#2 posted 10-11-2012 01:37 AM

how thick is the rail piece

sounds like you haven’t cut the tenons yet
skip the 1/4” tenons
and go for the 1/2” ones

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View derosa's profile

derosa

1557 posts in 1583 days


#3 posted 10-11-2012 01:43 AM

Sounds like he has cut it already David. Mike has a large part of it, width here is going to matter, personally I would remill and have no problems going with floating tenons because I know my furniture is going to be jumped on by large dogs and growing children neither of which is gentle. If you don’t have those concerns then 1/4” is liable to be fine.

-- --Rev. Russ in NY-- A posse ad esse

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mcase

438 posts in 1877 days


#4 posted 10-11-2012 02:16 AM

What width are the mortises you cut? Are they 1/2” of 1/4”? If you have already cut 1/2” I would chuck the rail. Re-gluing cheeks sounds like asking for trouble and a 1/2” mortise in the end of 3/4’ rail would leave only 1/8” walls. I agree with horizontal you can get away with the 1/4” especially if its a 4” rail. Almost all the strength is in the glued surfaces anyway and a 4” provided a lot of glue ares plus a more rigid rail. How are you set up for mortising the end of a rail? There a lot of jig designs out there to facilitate this.

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Oldelm

75 posts in 923 days


#5 posted 10-11-2012 02:57 AM

This may be a single face tenon as the instructions are saying a 1/4” off each “face” of the board. A single face 1/2” tenon on each end of the rail. Pretty typical on mission style work. Hope this helps.
Jim

-- Jim, Missouri

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TopamaxSurvivor

15076 posts in 2424 days


#6 posted 10-11-2012 04:07 AM

Oldelm Are you saying when it says ”a 1/4” off each “face” of the board” you only take 1/4” off one side? What is the back side called? I thought they were both faces.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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MNgary

236 posts in 1165 days


#7 posted 10-11-2012 04:32 AM

I would be more concerned about the size of the shoulders than thickness of the tenon. On a properly fitted tenon and mortise glued into one solid mass, the shoulders will be subjected to most of the stress. And your tenon will still provide adequate glueing surface to make a solid union and hold the shoulders tight against . . . Yes, the tenon will add something to resisting failure, but the major strength of a mortise and tenon joint is having shoulders outside the joint and, secondarily, the tenon inside the mortise (the physics of compressing end grain directed forces versus longitudinal grain compression).

There is also the shear strength of the tenons to consider, though. In my opinion, the 1/4 inch tenon is still thick enough to provide the shear strength needed if the 1/4 inch tenon is a couple inches high and you are using a wood with not unreasonable shear strength.

For your rail, 47phord, what’s the top and bottom shoulder dimensions?

-- I dream of the world where a duck can cross the road and no one asks why.

View Fred Hargis's profile

Fred Hargis

2037 posts in 1241 days


#8 posted 10-11-2012 11:53 AM

When I use loose tenons, I cut the mating mortises at the same time with the same jig to ensure alignment of the mortises. Doing a second mortise for a loose tenon at this point may make for some fairly precise positioning. Use the existing tenon, gluing additional wood on to it will be fine, if you choose to do that.

-- Our village hasn't lost it's idiot, he was elected to congress.

View Oldelm's profile

Oldelm

75 posts in 923 days


#9 posted 10-11-2012 01:53 PM

Yes I am saying 1/4” off of the face side of the rail on each end. The loose tenon IMHO seems problematic to me because the strength is dependent on the thickness of the remaining wood left for the shoulder. Again IMHO a 1/4” tenon with all of its grain still holding would be stronger. I guess it is all terminology, I have been taught that the “face is the show side of an element and all of the measurements and gauging is done from that side. If you look into lots of styles of furniture from tables to joint stools to chairs you will find tenons with a single “cheek ” on the face side.

-- Jim, Missouri

View 47phord's profile

47phord

175 posts in 985 days


#10 posted 10-11-2012 10:49 PM

The rail is 3/4” thick x 7 1/4” tall. I’m using red oak. The dimensions for the tenon are 1 3/8” off the top, 1/4” from each side and 1/4” from the bottom. What I am most concerned with is whether the tenon would snap if any twisting force were applied to the sofa (like when it is being moved, for example) as a 1/4” thick tenon isn’t very thick.

View teejk's profile

teejk

1215 posts in 1432 days


#11 posted 10-11-2012 11:23 PM

was it a home depot plan? my wife found one of their plans for a mission style table once. It was the first project that I ever did from a plan (and other than a huge birdhouse, was the last).

mission style would normally have thick legs (say 2×2 or more). if so, max the tenons (you only need 1/8” shoulders or less to cover any flaws on the mortise). and modern glues are amazing if you need to start over (realize that once assembled, the glue-ons really have no place to go in M&T joinery).

just my opinion as usual.

View TopamaxSurvivor's profile

TopamaxSurvivor

15076 posts in 2424 days


#12 posted 10-11-2012 11:53 PM

Thanks Oldelm

I’m just learning by causal observation. I have never really be taught anything about wood working except for a bit of basic carpentry like rafter cutting, ect.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Oldelm's profile

Oldelm

75 posts in 923 days


#13 posted 10-12-2012 02:46 AM

47phord, I hop you got enough info to proceed with your project. Sometimes purchased plans are not quite as clear to us or just have errors. I believe it was horizontal ice that has a thread going about this exact issue. If I were building your project I would at least have a 3/8” wide tenon at the seat rail joint just for shear strength. Gluing pieces back on will probably work. I’m just guessing that there is a slat rail on the back of that 3/4” rail that will help back up that 3/4” piece so it does’t deflect enough to stress the tenons. Let us see your work when completed. I like mission style pieces

-- Jim, Missouri

View Oldelm's profile

Oldelm

75 posts in 923 days


#14 posted 10-12-2012 02:50 AM

I apologize for the spell check thing. I need to just turn it off. It does more harm then good. It even changes after you proof it sometimes.

-- Jim, Missouri

View MNgary's profile

MNgary

236 posts in 1165 days


#15 posted 10-12-2012 06:45 AM

47phord, the twisting force is going to be dispersed across a 5 and 5/8 inch tall tenon solidly glued into the mortise. Further, there are going to be other resistances to the torque from racking (back rail, box formed with the arms, front to rear stretchers, etc,).

I appreciate your concern and not wanting to put the time, cost, and effort into a project that will fail. But just step back from the problem and, in your mind, do you know anyone strong enough to tear a quarter inch thick piece of oak by twisting it? Even if someone attaches a 3’ lever to the quarter inch board (maximum depth of a sofa) I doubt they could.

In my personal opinion, after looking at the shoulder dimensions you’ve provided, I think you should do this joint as described in the plan The shoulders are sized to handle the various stress’ on a front or rear sofa rail and the thickness of the tenon is right at one-third the board’s thickness which is the dimension multiple field tests demonstrate as appropriate.

Worry less and enjoy more your project.

-- I dream of the world where a duck can cross the road and no one asks why.

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