Loose dados

[Naomi]

I’m making the castle bookcase that Marc Spagnolo made in 2016 for woodworkers fighting cancer. It’s 3/4” Baltic birch ply. I didn’t have an appropriate router bit for the dados so I used my tracksaw. I’ve done this for one dado before but it was fine. All my dados turned out a bit tight and I sanded and now they’re too loose! Like flopping over loose. I have a few questions:

1. Recommended router bit for 3/4 dados for ply?

2. Does ply expand and contract at all? There’s a high humidity where I live and it’s been raining and my shop is an agro hoop house

3. Does it matter once it’s all glued and screwed? Ie shall I just go for it and hope the glue swells the wood? 

[Mark J]

Hey Naomi, I can give you some quick responses.  

3/4" and 18mm plywood panels always run a tad thinner than 3/4" or 18mm.  There are such a things as a "plywood bits" which are sized a tad smaller than the nominal dimmension, but be aware that the size of a "tad" changes from one plywood manufacturer to another.  

If the joint requires more than moderate hand pressure to go together when dry fitting it is too tight.  Wood glue will swell the wood and the joint won't go together.  Conversely if the joint has any play or gap it's too loose and typical wood glues will not form a good bond, even though the wood will swell.  If the gap is not excessive you could use epoxy which has good gap filling properties and should make a good bond.  Screws, if you're planning to use them, will certainly increase the strength.

[gee-dub]

Undersized bits for plywood may or may not work as the dimensions of ply vary even when specified.  I use a jig that is set using the material that is to fit in the slot.  A router bit smaller than the end dimension allows a more exact fit.  I use a 3/4" bit as it meets 99% of my needs.  Shopnotes and others have variation on this setup.

I use a jig guided by a template collar as opposed to the side of the router.  This has given me the best result.  I think this is version 3(?).

The material is clamped between the long guides to set the width.  The zero edge of the guide is set precisely on the line of your cut and the jig is clamped to your work.

The following just shows the result of the original zeroing cut for the jig.

This edge becomes your zero edge that you match to your location marks for the dado.

The one from ShopNotes No. 76 can be made form easily acquired materials.  I modified mine to use an edge guide clamp.  In fact a built in clamping mechanism is not required and I used one that I clamped to the work with f-style clamps for quite awhile.

 

[Mark J]

I have seen router carriers for track systems that do something similar.  Cut a 1/2" wide dado, then the carriage slides over just enough so the second pass widdens the dado just enough to fit the particular plywood piece.

[wtnhighlander]

The router guide jig is definitely the way to go. Trying to match a router bit to the exact size of the plywood is an exercise in frustration.  Oh, in regards to your track saw method - the sanding likely rounded the shoulders of the dado, and that's why it is "flopping over loose".  I think making tiny adjustments to the track would be better to widen the dado, or lightly sand the shelf until it fits the dado.  It is common practice to hand plane the edges of a panel to fit into a dado or groove, such as with drawer bottoms.

[krtwood]

Plywood does expand and contract in thickness just as much as solid wood.

[Coop]

27 minutes ago, krtwood said:

Plywood does expand and contract in thickness just as much as solid wood.

Never thought about it. I bet you’re right! 

[krtwood]

I did a test where I glued up 3" of solid maple, cherry, baltic birch, and MDF and measured it with calipers in summer and winter.  The MDF changed the most, followed by the baltic birch.