undersize dados for plywood

[softop41]

Hello All,
I hope someone out there in this vastly experienced group and help me understand something about fitting plywood into dados.
I started a project using 3/4" Oak veneer plywood.
Knowing that plywood is always undersize and finding that router bit manufacturers offer a 23/32" bit for plywood, I bought one and went merrily on my way.
Imagine my dismay when I cut a perfect dado in exactly the right place and the plywood wouldn't go into it! I now, 'Measure first...."
Next, I got out the ol' digital calipers and did some checking. The undersized dado is .715" and the 'undersized' plywood is .740"! Even NEW math won't make that work! []

Then I cut a dado with a regular 3/4" bit and it measured .748". Well within woodworking tolerances I guess; my automotive and machining background says no, but I'm trying to 'loosen up'.

If the dado is .748" and the stock is .740", is that glue room or 'slop'? It seems to me that the fit should be a bit closer than that for appearance and stability.
I sure hope someone out there and help clarify this mystery for me.

Thanks,
Jerry

[Pappy]

The problem you are having is common. Thethicknes of the plywood will vary a bt between mfg's. It will also often 'swell a little, especially along the edges.

[bmyers]

Plenty of room for glue thats for sure. You'll probably be ok. What you can do instead of chasing odd plywood sizes to try and make a dado that matches the plywood, make the plywood match the dado. Just rabbet the top side of the plywood and use your 23/32 dado bit for the groove.

I say the top side of the plywood because that will hide the dado a little when it's together.

If you need to fix your original dado, just make a magic shim and glue it in there to take up some space when you assemle it all.

Bill

[LCHIEN]

Another approach is to take that .740 plywood and .715 groove,
knock .025 off the plywood in a slight bevel by running a hsanding block down the bottom side of the shelf. Should make a nice tight wedge fit.

But yeah, measure before you cut next time.

P.S. Also set up your dado, make a test cut in a scrap of wood, (just needs to be a partial cut) and check the fit before cutting all your good lumber.

[Ken Weaver]

I had to learn the same lesson - now I always measure the ply width before cutting any rabbets or dadoes and make them to the width of the stock I'm using.

[LarryG]

This is precisely the reason I think a dado set is a better investment than a set of undersized plywood router bits. With the dado set, you can play with the shims to fine-tune the width to suit the plywood (which, as others have noted, WILL vary). With the router bits, you get what you get.

[DonHo]

You can also solve the problem by building one of these adjustable dado jigs for your router. It's easy to build and makes perfect dados everytime.

http://benchnotes.com/Router%20Dado%...dado_guide.htm

DonHo

[Tom Miller]

If you want to get an idea if your dado is too wide, try a "wet" fit, which is a dry fit with water. Wet the piece going into the dado and see how much it tightens the fit. It's a good thing to get an idea about this prior gluing.

Given the different thicknesses I've seen in ply, and the fact I use more baltic birch than ply anyway, I usually make my dados in two passes (that is, unless I'm using my dado blade ).

Since I have a tendency to make my dados too snug, I usually end up making a dado sanding block to take a few mils off the piece I'm putting into the dado.

To make the block, I cut an extra wide dado in a piece of scrap at the same depth that I'm using in the project. Then I put a piece of adhesive-backed sanding paper on one side wall of the dado and trim the excess, like so:




Regards,
Tom

[sweensdv]

Thats a pretty slick idea Tom, thanks for passing that one along.[8D]

[JimD]

.008 clearance is not a sloppy fit in my book. That is much less than 1/64th of an inch. I do not typically use a router to make dados, however. I also like to use my stacked dado set on the BT3100. There is a lot more carbide teeth doing the work with the dado blade.

[atgcpaul]

I think DonHo's method works great. I made something similar this
morning to cut some dados with my router. Sometimes it's not always
practical to use a dado set. With the router dado guide, you use
the thickness of the wood to determine the width of the dado. Dados
come out perfect width everytime. There's no trial and error like
there is with a dado set. With a plunge router, you can also make
stopped dados which you can't effectively do with table saw.

Paul

[Tom Hintz]

Problems with dado fit caused by irregular wood thickness is nothing new. See the link below for a jig I made that is based on one I saw an old neighbor of mine use. He had built it now almost 50 years ago to deal with this same problem in his cabinet shop. I updated it some but the principle has been around, according to him, far longer than his version.

http://www.newwoodworker.com/autodadojigpln.html