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Mortise and loose tenon - dimensions

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Forum topic by abehil posted 05-05-2015 10:56 PM 1183 views 0 times favorited 9 replies Add to Favorites Watch
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abehil

104 posts in 800 days


05-05-2015 10:56 PM

I’m interested in learning loose tenon joining. I’m totally confused over the exact dimensions for the tenons.

I’m going to want to make my own loose tenons because buying them is just too costly. I’m looking to save some money over pocket screws which work well but a veritable box of screws goes into every project!

If I were to make a 1/4” mortise, is the tenon also exactly 1/4” or is it slightly thinner?

I see discussions about length, width and direction of grain. But for a given mortise, will the tenon be the exact same size?


9 replies so far

View Gene Howe's profile

Gene Howe

8240 posts in 2890 days


#1 posted 05-05-2015 11:10 PM

I run a length of 2 1/2” wide stock through the planer, taking it down to 1/4” thick. When I’ve got the 1/4”, I then run it through again without moving the cutter head. This makes for a nice snug fit. Then rout the edges to get a rounded edge to fit the mortise. Cut them to length. Store the excess for next time.

-- Gene 'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton

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Texcaster

1138 posts in 1135 days


#2 posted 05-05-2015 11:35 PM

A couple of quirks about slot mortising

1.. The mortise in the end grain will be a slightly different size to the edge grain mortise.

I make the tenons to be a tight, HAND PUSHED fit to the smaller mortise. Hand plane one or both sides, while still a long length for a perfect fit.

2.. Make all the tenons from the same species. At the same thickness setting, different species will be different thicknesses

3.. I only round over for a through tenon. I just chamfer for blind tenons. The important thing is for the tenon to contact the top and bottom of the mortise to avoid alignment problems during glue up.

Slot mortising …http://lumberjocks.com/Texcaster/blog/43424

-- Mama calls me Texcaster but my real name is Mr. Earl.

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abehil

104 posts in 800 days


#3 posted 05-06-2015 01:40 AM



A couple of quirks about slot mortising

1.. The mortise in the end grain will be a slightly different size to the edge grain mortise.

I make the tenons to be a tight, HAND PUSHED fit to the smaller mortise. Hand plane one or both sides, while still a long length for a perfect fit.

2.. Make all the tenons from the same species. At the same thickness setting, different species will be different thicknesses

3.. I only round over for a through tenon. I just chamfer for blind tenons. The important thing is for the tenon to contact the top and bottom of the mortise to avoid alignment problems during glue up.

Slot mortising …http://lumberjocks.com/Texcaster/blog/43424

- Texcaster

I’m sure all this is completely obvious to most but I have to ask…

1. Why is the mortise in the end grain different size to the edge grain mortise? How much different? Is it because of how the wood responds to the mortise being cut or is this a rule to follow?

2. Why are different species different thicknesses?

3. Is the top and bottom the edges or the ends?

View Holbs's profile

Holbs

1372 posts in 1491 days


#4 posted 05-06-2015 02:34 AM

there is a sweet youtube video about the thickness of a mortise going into a tenon. Using 0.005” increments to show you how too loose, perfectly snug, too tight a simple 0.005” measurement makes. I had it bookmarked…but can’t find it.

-- Yes, my profile picture is of a Carpenter Bee! The name is derived from the Ancient Greek "wood-cutter"

View Texcaster's profile

Texcaster

1138 posts in 1135 days


#5 posted 05-06-2015 07:54 AM

A couple of quirks about slot mortising

1.. The mortise in the end grain will be a slightly different size to the edge grain mortise.

I make the tenons to be a tight, HAND PUSHED fit to the smaller mortise. Hand plane one or both sides, while still a long length for a perfect fit.

2.. Make all the tenons from the same species. At the same thickness setting, different species will be different thicknesses

3.. I only round over for a through tenon. I just chamfer for blind tenons. The important thing is for the tenon to contact the top and bottom of the mortise to avoid alignment problems during glue up.

Slot mortising …http://lumberjocks.com/Texcaster/blog/43424

- Texcaster

I m sure all this is completely obvious to most but I have to ask…

1. Why is the mortise in the end grain different size to the edge grain mortise? How much different? Is it because of how the wood responds to the mortise being cut or is this a rule to follow?

2. Why are different species different thicknesses?

3. Is the top and bottom the edges or the ends?

- abehil

abhil,

1 ..The size difference is just something to be aware of. If it was a big deal no one would use a slot mortise. You would think mortising end grain would be harder to do but it cuts easier than edge grain. Maybe it’s to do with that, I don’t know.

2 .. I used to think I could use all my rubbish up to make tenons but I’ve found not to mix species. Especially hard and soft.

3 .. Top and bottom of the mortise, it’s easy enough to align outside rails but a mid rail must be in an exact spot. If the tenon fits top and bottom, the rail goes in it’s place without any bother.

4 .. If you have to belt the tenon with a hammer it’s too tight. If it will seat with arm pressure it fits.

the drawings and photos are a bit dodgy, I hope they tell the story.

-- Mama calls me Texcaster but my real name is Mr. Earl.

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abehil

104 posts in 800 days


#6 posted 05-07-2015 05:37 AM

Texcaster,

The pictures and explanations do help.

From my novice initial point of view, having not made a single mortise/tenon yet, I thought the mortises to be whatever the router bits size is. But the tenon’s seem more finessed into the right thickness. And also from my novice brain I see that nobody seems to know exactly what that thickness is ahead of time (or they don’t want to just say it) that I had to deduce that it varies based on some some set of random temporal factors (mystical forces) :)

Anyway, I’m now taking from what has been said here as that the tenons are “almost exactly the same thickness, but not quite.” So now I’m saying it with as much accuracy as anyone I believe. I also can see that not having a thickness planer yet might make this a little trickier than I first thought. I do have a jointer and a sander though so maybe I can make some.

View Mark Kornell's profile

Mark Kornell

1061 posts in 1992 days


#7 posted 05-07-2015 06:26 AM

The reason you won’t get exact numbers is that wood – and working with it – isn’t exact in the way metalworking is. A 1/4” router bit should be exactly 0.250”. But when you apply that router bit to wood, the resulting hole is rarely exactly 0.250” for a lot of reasons.

The first is called runout. That is, how close to perfect is the spin of the router. It should be a very small number, but likely isn’t 0.000”

The second is slop in the jig used to hold or locate the router. The third is how the wood fibers compress when cutting force is applied – which depends on wood species and orientation of the grain being cut. The fourth is humidity changes – and resulting wood movement – from day to day.

No mysrical forces involved. All quite predictable based on the exact router, the specific bit, the jig or.fixture used, the wood species, wood.dryness and humidity history. Just a lot of variables involved which are specific to you.

There were.two answers given. A) about 0.005” clearance and B) you should be able to fit the joint with hand pressure and it should hold under gravity. Both describe the same end result – a perfectly fit tenon.

By “sander” do you mean a drum or widebelt?

-- Mark Kornell, Kornell Wood Design

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runswithscissors

2183 posts in 1487 days


#8 posted 05-07-2015 07:10 AM

“Trial and error” sums it all up quite well.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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abehil

104 posts in 800 days


#9 posted 05-08-2015 02:14 AM

Mark Kornell,

It’s a wide belt. Wish I had a drum sander. I’m really just starting out. And your explanation is very helpful. I know it isn’t really mystical, just poking fun. I guess I’ll get a better idea when I cut some. I’m in the process of making a jig that holds the router horizontal and the board is clamped down. The jig slides in four directions but keeps it straight; as long as I can build it right.

I recently built a router table and a nice straight flat fence and a lift with a crank handle. It’s a steep grade on the learning curve when the budget is tiny. I bought an old grizzly table saw and it’s needing some serious tlc. I’m just slogging through on getting workable tools.

Eventually I’m planning on building a drum sander but I think that will be a while.

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