How tight for M+T joint?

[Mike W.]

I have some M&T joints that I've made with a couple of homemade jigs over the last couple of days. I am wondering whether I need to tighten up my tenoning jig based on the fit. At first, I had to really push hard with my hands to get the tenon in all the way, to where it seemed like no glue at all would fit... now its just a bit loose. I'm not sure whether -in between- would be perfect because I haven't added any glue yet and I don't know how much expansion there will be.

So I'm wondering if anyone can explain in text how a joint like this is supposed to fit? Is there an online resource that gives a good verbal representation of this kind of thing?

Thanks, Mike

[DJ Delorie]

From what I've heard, the rule is this: You should need to tap it in with your shop mallet, but you should only need light taps. However, I like to be able to put it together without the mallet, but still snug enough to stay put when I let go.

But anyway... if the joint can't hold its own against gravity, it's too loose. If you can't get it apart to put the glue in after test fitting, it's too tight.

[Eric Lund]

David Marks uses a slip fit on his show, so he can slide the tenon in and out of the mortice by hand. Works for him.

Cheers, Eric

[Charlie Self]

DJ Delorie responds:

I do NOT like having to use a mallet to tap fit a joint for a test. A tight push fit with hand pressure, albeit a lot of it, is more sensible to me.

Makes sense.And a tight push fit will hold against gravity unless the piece being held is huge.

Charlie Self If God had wanted me to touch my toes he would have put them higher on my body.

[Andy Dingley]

So work it out. If you're starting to care about the fit of a M&T joint, then it's time to get your head around timber movement and understand how to predict just how much it will move.

Hoadley's "Understanding Wood"

is the classic book here, or the US gov wood products handbook (Lee Valley sell a paper copy, or it's downloadable)

Then you'll need a few cheap $5 air hygrometers for your living space, workshop space and timber storage space. Maybe a pin meter too, but these are expensive and less useful than you might think.

Armed with the knowledge of timber movement, you can start telling whether your joints will get tighter or looser in the future. It doesn't really matter how they fit right now -- it's going to change when they sit in the central-heated lounge instead of the workshop. There are all sorts of theories about how tight a joint should be on assembly, and whether you should be able to assemble them repeatedly by hand, or if you need a one-off with a mallet. Personally I think anything that I can pull apart by hand is OK. Pegged tenons though should need a mallet to drive the pegs in.

-- Do whales have krillfiles ?

[Stephen M]

I suspect that you can get most of the way there w/o and investment in technology.

If you can acclimate your stock well, (a month indoors before a project), you should be able to do this by looking at a calendar. Here in the Northeast, indoor heating will reduce relative humidity. I can expect my stock to expand from it's current state if I mill and assemble it today. If August it particuclarly humid, I can expect it to go the other way.

-Steve

[jev]

In the Rogowski book , complete Illustrated Guide to Joinery, he describes it this way:

"The fit of a ten>I have some M&T joints that I've made with a couple of homemade jigs over

[Andy Dingley]

I half agree with this. IMHO, a cheap (literally $5) _air_ hygrometer and waiting for equilibrium is as good as, if not better than, a pin meter that just tells you one reading on one day.

But you need something, because you have to make comparisons between the workshop space and the living space where the piece will finally end up.

-- Smert' spamionam

[Mo' Sawdust]

Or as some old sage once remarked: If you need a hammer/mallet, tis too tight. If you can tap it together with your hat, tis too loose. Hand fit, and tis just right.

[Swingman]

I agree wholeheartedly ... especially the part where it "may require a hammer to tap it apart". Most of my M & T joints will go together with firm hand pressure when dry fitting, however, even those on the loose side may require a tap or two with the deadblow to pull apart ... when that happens, you know you've got a good fitting joint.

[Wood Butcher]

Probobobaly krill filters instead.

Art

[Eric Lund]

Which makes sense, since the purpose is the peg is to pull the shoulders tight.

Cheers, Eric

[Andy Dingley]

That's another question ! Some of my pegs are 1/4" diameter oak or apple, going through a 1/2" tenon in a table stretcher. Others are 1" oak, through a 3" tenon in a timber-framed building. If I "pull the shoulders tight" equally in each case, I'll pull the tenons off the table.

-- Do whales have krillfiles ?

[Charlie Self]

Andy Dingley notes:

Of course. So don't. Reduce the pull on smaller pieces. Which I am sure you know.

Charlie Self If God had wanted me to touch my toes he would have put them higher on my body.

[George]

"Tight" means mating the surfaces with no gap, what does that have to do with the diameter of the pegs or the thickness of the tenon? The pegged mortise/tenon, so long as it does not rotate, won't wrack, and bears weight downward without the pin.

[Andy Dingley]

Not for a drawbored peg in a tenon - they're bored "over-tight", especially in green wood. But knowing just how much "draw" to use isn't always easy.

-- Do whales have krillfiles ?

[George]

Yes, just that the draw is its own clamp.