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

[Chris Skoglund]

I'm building a router table. My base is 3/4 birch ply. It has a slight bow in it and I want it to be perfectly flat before I laminate 2 layers of hardboard to the top and finish it with a Formica top. How to do this? (I see lots of homemade router table plans yet I don't see anything that talks about getting the base perfectly flat. They all assume a flat surface to begin with but in my experience I've never found any plywood or MDF to be PERFECTLY flat).

I'm thinking of getting some 1/4" angle iron from a machinist and running it around the bottom perimeter. It will be thick enough that it shouldn't flex and should therefore pull the plywood flat to it. I'm thinking this should suck it down flat and keep it flat in the years to come.

Or, the other option I'm playing with is to run thin cables off the ends (from the bottom) and attach them by turnbuckles to the leg rails. I should be able to supply enough tension to flatten out the bow I'm thinking. I would do three cables on each end, middle and one on each corner, and attach a separate turnbuckle to each position to add control over the process.

Any thoughts and/or suggestions? I want this sucker to be flat!

Many thanks in advance.

[Jim Matthews]

Hardwood or steel battens beneath the top, with screws to draw it down tight.

Plywood was made to be flat, once.
I'm not sure that this is critical.

It's the sort of pursuit that can drive you mad, with little actual utility.

Unless you're making very long grooves in very thin material, what's really important
is the "landing" around the router base.

I believe their are cast iron inserts (http://www.amazon.com/Bench-Dog-Tools-40-102-Extension/dp/B001TDLSUS)for those seeking precision.

[Todd Burch]

Oh my. Get a new piece of plywood man!

[Charles Wiggins]

Oh my. Get a new piece of plywood man!

Seriously. My experience with plywood is, once a potato chip, always a potato chip. You can probably pull the bow out with bracing, but more than likely, over time, that bow is going to come back to haunt you. Start with quality, flat ply and reinforce it to PREVENT bowing.

[Pat Barry]

Something to keep in mind also is that whatever you add to one side of the plywood you should add to the other side to keep the structure in balance, otherwise you will just create a bowed tabletop.

[Prashun Patel]

You have to mechanically constrain plywood to be flat. IMHO, 3/4" is a little thin for a top. Laminate another piece of 3/4" ply underneath (preferably with an opposite bow) and there's a good chance you'll end up with flat enough.

[Phil Thien]

Something to keep in mind also is that whatever you add to one side of the plywood you should add to the other side to keep the structure in balance, otherwise you will just create a bowed tabletop.

^^^ This.

My router table is a single piece of BB plywood laminated with HPL (High Pressure Laminate) both top and bottom.

If the bow is minor, you might not have to do anything other than make sure the bow is UP when you're done. The weight of the router and any lift will pull the bow down a bit. But a small bow UP isn't really a problem. A small bow DOWN is a problem.

My router table is approx. 20" square (to fit as an extension table for my BT3000 saw). I'd say the bow I had was somewhere between 1/32" and 1/16" over the width of the table. After adding the lift and the motor and the down-draft box, I'd say I'm flat to about 1/64" or better.

[Mike Wilkins]

Since my advice is free, I can offer it. Junk the potato chip shaped plywood and use MDF. 2 layers of this stuff, with a layer of laminate in each side will make for a rock-solid top which will unlikely to sag, even with a monster router hanging below. If you want to further stabilize the top where the insert is routed out, you can add some angle iron in the top of the base cabinet on either side of the insert. You could even make it fancy and pretty by attaching some solid wood edge banding around the edges. Nice, stable, flat, heavy and pretty. Sounds like a nice combination.

[glenn bradley]

Seems like we're open for opinions so here's mine. Plywood is ill suited for a router table top as "flat" is a relative term (which rarely relates to plywood :D). I also agree that 3/4" is too thin for a workhorse router table top. My cabinet is BB ply but, I have ripped "ribs" on edge that provide the uniform surface that the top mounts to. I would stack/glue a couple panels of 3/4" MDF for your blank, then laminate both sides, cut to final dimension, cutout your opening and add trim if you want to improve appearances.

There are good and bad things about an MDF top. MDF is much flatter than plywood. It is malleable insofar as you can pull it down onto a known flat frame and be pretty sure that it will be flat. This same flexibility means that if you do not properly support even a 1-1/2" thick MDF top, it will sag. DAMHIKT :o. My top has seen endless hours of use and remained true for years. A little more effort at the build will pay dividends for years to come.

[Gordon Eyre]

I have two sheets of melamine and it has held up well for 10+ years.

[Conrad Fiore]

Three letters, MDF

[Chris Skoglund]

Thanks for the responses everyone. Seems there as many opinions as options. I went with a frame of angle iron screwed to the underside. Had it made for $20 by a machinist friend. Going to laminate two layers of 1/4" hardboard to the top side and then finish it with a laminate of Formica.

[David Helm]

I use HDO plywood. 1 1/16 inch thick, eleven plies. It is made for concrete forms and is very flat and won't deflect. Not cheap, but one must pay for quality

[Charles Wiggins]

Where do you typically find it?

[Tony Shaftel]

I have the Lee Valley router table. It's made of steel plate and is specifically engineered NOT to be flat--it has a slight bow to it so that the area where the work meets the bit is a little higher. I had to get used to that idea before I ordered it, but decided it solves the flatness problem. What you never want is for the table to be slightly lower around the bit than elsewhere. Slightly higher not only means that isn't true, but it also means the workpiece itself can be a little bowed and still be against the table when it goes past the bit.

[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.

271877

[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....

[Michael Mayo]

I went through this same thing a year or so ago. I got plans from shopnotes.com and made the router table using a really nice piece of white Corian I got from a client. It was beautiful and was going to be a sweet top. But I used 3/4" plywood as a base for the Corian and used West Systems epoxy to laminate the ply to the Corian. Well when it cured I cut the router plate holeand cold not for the life of me get the plate for my lift to sit flush. The plywood bowed something fierce and there was no way to fix it. I ended up using two 3/4" thick pieces of MDF and used a formica laminate over that. The lift plate took about 2 min. to get flush and it has stayed that way for 2 years now. Bums me out that I couldn't use the Corian but live and learn. I did use my router and completely removed all traces of the plywood from the Corian and I may try again to use that but would use MDF instead of the plywood the second time around. So I would not recommend using the plywood for the base of your router table. You will save yourself a ton of pain and time if you just use MDF from the start. It is very flat compare to plywood and is much more stable.

[Steve Juhasz]

Three letters, MDF

Four letters, IRON.

Bench dog and General both have iron tables with cutouts for inserts and rails for accessories. By far better than MDF and all that other soft junk. If you want it to last, make it like your table saw.