how to flatten a piece of plywood for a router table?

[Erik Christensen]

If I want a really flat plywood surface, especially over long spans I have had good luck with this process - take 2 pieces of 3/4" BB ply, cut a matching set of 9/32 deep grooves in the inside faces of long dimension of both pieces, get some 1/8"x1/2" flat bar stock at the borg, cut it to match the length of your grooves less an inch or so leaving you room to trim up the finished top, put some gorilla glue in the grooves & insert bar stock, yellow glue on the faces and clamp flat or put in a vacuum bag. I made the top of my wife's office desk this way - it spans 40"+ and she often climbs up on it to reach stuff stashed on top of the upper cabinets - there is no noticeable deflection when subjected to a 135# load mid-span.

[Leo Graywacz]

If you want flat the do particle board or MDF. Plywood is no longer stable as the have tried to make it affordable.

[johnny means]

Why two layers of hardboard? If you think you may ever want to add miter slots or t-tracks, you may find your top is a bit thin. Also, a single piece of 3/4 for the top layer makes it easy to create adjustable router plates from 1/2" material if you ever go the multi router route.

Your angle iron frame will keep the sides of your table straight, but odds are with one side laminated with three layers of different material and the bottom exposed to the environment, it will eventually sag.

[johnny means]

Why two layers of hardboard? If you think you may ever want to add miter slots or t-tracks, you may find your top is a bit thin. Also, a single piece of 3/4 for the top layer makes it easy to create adjustable router plates from 1/2" material if you ever go the multi router route.

Your angle iron frame will keep the sides of your table straight, but odds are with one side laminated with three layers of different material and the bottom exposed to the environment, it will eventually sag.

[David Helm]

Where do you typically find it?

Any well stocked builder's supply yard.

[Chris Skoglund]

I'm following a plan from Shopnotes. They recommend the hardboard to give you a smooth inside edge for the miter gauge to travel against. However, after reading these posts (thanks everyone) I'm thinking that maybe I'll glue another piece of plywood to the top instead of using the hardboard. That way I'll have 2 layers of 3/4" BB plywood instead of just one. (Although there are some recommendations for MDF here I've read enough contradictory posts on the MDF vs plywood that I'm just going to stick with the plywood since I already have a big sheet of it and don't feel like lugging back a piece of MDF home).

If I use 2 layers of 3/4 plywood plus the steel battens that should give me a long lasting table, right?

[Todd Burch]

Yes, longer than you'll want.

[Chris Skoglund]

It's already been in my life longer than I planned

[Chris Skoglund]

What's the best glue and screw option for laminating the two pieces of BB plywood together? Wood glue or contact cement? I was going to rough the surfaces up and use wood glue.

[Chris Fournier]

The structure you fix your table to is key to flatness. Your laminating scheme will likely make matters worse regarding flatness.

[Chris Skoglund]

So if I'm going to fix two 3/4 layers of BB plywood together and then mount that to a steel frame should I just screw the plywood together, and not glue it?

[Vince Shriver]

Erik, I was going to suggest the same thing. That "fix" comes from Wally Kunkel (Mr. Sawdust) for making a RAS table flat. Another idea (although more involved) would be to make a torsion box (building in a frame for the router plate), and finishing it off with laminated MDF.

[Todd Burch]

Chris, if you have a really good known flat surface, like a tablesaw, you can simply glue and clamp your two pieces of ply to the top of the saw. No screws required. Use wax paper underneath to protect your saw. Leave it in the clamps for at least 24 hours. If you cut your pieces a bit oversize, and stagger them 1/8" or so diagonally, that will make cutting them to the exact size desired later really easy. Don't make the mistake of cutting each piece just the size you need, and then have to get an exact alignment during the glue up. If you have a nail gun, you can shoot a nail or two into the pieces to hold them steady as you draw the clamps down. If you apply yellow glue (that is all that is needed) to both pieces, you can let it set up just a tad, to allow it to go from slippery to sticky, before you put the two pieces together.

There are a lot of different tricks you can use like these to make your job easier. You may find, if you follow this procedure, you don't need the metal frame to keep or hold it flat.

Todd

[Keith Hankins]

No offense, but I think your way over doing it (I know hard for me to say with the window, but the wife did that). Build a good box and cantilevered with the top. I took 2 pieces of 3/4" mdf wrapped in oak and topped with a piece of formica. Been using it for almost 10 years no issues. It has a woodpecker PRL with a PC7518 (heavy) and no sag and plenty stable.

router table.jpg

[Chris Skoglund]

Actually, what I'm looking to make is exactly what you have, using BB plywood instead of mdf and minus the nice window. Did you glue the two pieces of mdf together? I'm trying to figure out if I should glue and screw the plywood together or just glue or just screw. And if I glue, should I use wood glue or contact cement? New to this....