clearance and fit on Tongue and Groove Joints

[Nabil Abdo]

Hi all,
I wanted to pick the brains of the resident experts, a very basic question often glossed over in the books.

I am using a T&G joint to put a solid maple edging on the edges of some plywood shelves.

The edging is 3/4" stock with a 1/4" tongue milled into it, the mating groove in the plywood edge is 1/4".

On paper this sounded perfect, my question is more of a general concept:

How tight a fit should this T&G be , I am guessing a mallet should not be needed to fit the edging into the plywood?

Should there be clearance allowed for glue? ie. should the groove be a little deeper than 1/4"?

I am finding that when joints are milled too exactly, then it is too difficult to fit them properly. Am I missing something?


thanks in advance,
Nabil

[Lee Schierer]

You aren't missing anything. You can't put a 1/4" plug in a 1/4" hole, one or the other has to be undersize or oversized. Glue will cause the wood to swell so a glued joint should probably be slightly looser than an unglued joint would be. When I do T&G I like to have a snug slip fit, where the pieces will slide together without a lot of force and come apart. Pounding a tongue into a groove is a good way to split wood. Straightness of the boards will also dictate the tightness of the fit.

I don't think you will find a hard and fast answer for this, it is more of a feel or personal preference thing. I've never seen a reference that says you want the tongue to be exactly .005" smaller in all dimensions than the groove. Likewise if you are gluing you need some space for the squeezed out glue that ends up in the joint to go, so the depth would need to be greater than the height of the tongue.

[Frank Drew]

I like to have a snug slip fit...

Same here.

[Peter Quinn]

Glue film wants to be .003"-.005". The difference between the tongue and the groove needs to be .006"-.010" (glue film times 2) to allow for glue. Perfect glue film is rare, so it is advisable to mill the groove a bit deeper than the tongue to allow for squeeze out. it is also advisable not to glom on the glue like Tom Sawyer painting a fence! On machine milled joints measured with calipers where shimming is possible I like to have .008" difference between parts, .006" is a bit tight, .010" is a bit loose.

At this point I don't check everything with calipers, I just set up, run a few tests till i like the fit, and go.


Can you forward this discussion to the makers of dowels? I actually called a dowel company whose name was on the package i bought once to ask "How am i supposed to get a .375" dowel in a .375" hole in the first place, and where does the .008" of glue go?" They were dumb founded. Crickets on the other end of the phone followed by "Duh, uh, duhm, Ohh, well....."

Can't use a fractional oversized bit in a jig, can't buy undersized dowels in quantities less than 5000 pieces. Their suggesting was to sand each dowel a bit! So i ask "Well why the #$%^ don't you mill them .010 under then? Do you like sanding dowels by hand?" Crickets....crickets....