Router table - how flat is flat enough?

[Greg Woloshyn]

I'm working on a router table for my table saw extension. I've laminated 2 pieces of 3/4" MDF for the table and will be building a plywood supporting frame underneath for extra strength. I'm going to be using an aluminum woodpeckers router plate. Being a machinist, I'm concerned about the overall flatness of my tabletop. It looks to have a slight sag in the middle of about .010. Is this too much? What can I get away with to still make accurate cuts and not have problems in the future?

[pat warner]

10 mils, for me is unacceptable for joinery.
If your doing edge to edge or end to edge joinery, 5-10 mils of misallinement is a mess.
Moreover; it's an unpredictability. Sometimes things will flush out sometimes they won't.
Since you know you have a 10 mil error you won't have to look far to diagnose poor joinery.
If for every ordinary stick (section and length) you pass x this cutter, it never rocks, then you might be home free; but I would not count on it.
Forget this (http://patwarner.com/images/dovetailed-lap.jpg) on a bumpy table.

[Sid Matheny]

If you are saying .010" I would say it would be close enought for most all the type of work I do on my RT.

Sid

[Dwayne Cromarty]

I have a Jessem phenolic router lift and with the router clamped in place it is slightly dished in the middle. This does cause issues with accuracy, for sure. If you are not flat, make sure the router is high, not low. At least that way your stock is tight to the table at the router bit.

[Larry Frank]

The other issues is how level the router plate is with your router table. I think that it will be difficult to get them within 0.010". My router table has the router plate just slightly higher than the table.

[Jeff Duncan]

You'll never get it perfect without driving yourself crazy.....and then even if you do.....it will move!!!

I agree with getting it close enough but leaving a crown vs a dip. A crown allows wood to go through the cutter dead flat at the point of contact. With a bow you have to exert extra force on the stock to push it down into the bow, and any variation in pressure will cause imperfections in the cut. A shim or 2 under the top when you attach it to the cabinet should get you where you need to be.

good luck,
JeffD

[Peter Quinn]

You can probably work out most of that .010" dip when you attach the sub structure. Some creative shimming with metal, plastic or even paper shim stock may work. MDF has a little give to it, even at 1.5" thick. If you can measure it. no reason not to try and eliminate it. That said it is wood you will be working, not parts for the door on the space shuttle, so you can sand, scrape or plane out minor variations quite easily. Ever wonder why wide belt and drum sanders are so popular amount wood workers? As others have said as long as the router plate is not lower than the infeed and out feed edges, the issues are IME minor. Now if you can keep all your stock flat within .010" end to end as you work, let me know how to do that! Luckily wood bends a bit under pressure.

[Peter Aeschliman]

I agree that .010 is too much. I also agree that you can take care of that sag when you attach your sub-structure. I personally would use some scrap hardwood for the sub structure rather than plywood. If you use quartersawn hardwood for the sub-structure and orient the grain such that it's 90 degrees to the table, the expansion and contraction shouldn't affect the table's flatness.

Simply screwing and gluing your substructure to the underside of the table may be all it takes to flatten out the table.

[Greg Woloshyn]

I was thinking of using a few pieces of steel angle iron (that is straight) to screw underneath my tabletop directly to the MDF. I also have some boat epoxy I could use to seal up the MDF bottom from moisture and help it from sinking over time.

[Alan Schaffter]

If you have a typical 11+" by 9+" aluminum or phenolic mounting plate, it would be nice but the table doesn't need to be flat. It can be crowned slightly as long as the plate is at the top of the crown and the edges of the plate and table are flush so the stock can't catch at the infeed and outfeed edges.

[Myk Rian]

When I use MDF to make a table top, I glue the convex side together. )( That cancels out any warps, and produces a flat table.
My RT is still flat after 6 years.

[Mike Goetzke]

I built a router table in the wing of my Uni using the saw board that came with it. It wasn't very flat at all. I attached the top to a cabinet:

http://img.photobucket.com/albums/v212/mbg/Unisaw%20Cabinet/IMG_0675_6_1.jpg




I reinforced the bottom of the table top with several boards and pocket screws:

http://img.photobucket.com/albums/v212/mbg/Unisaw%20Cabinet/IMG_0677_1_1.jpg



The table was still not completely flat so I added some shim stock below it before I fastened it to the cabinet:

http://img.photobucket.com/albums/v212/mbg/Unisaw%20Cabinet/IMG_0678_9_1.jpg


Was still flat even 5 years later.

Mike