Bowed MDF

[Rob Holcomb]

Recently I made a post about buying a router table because I didn't have the time to build one. Well I decided that the long term satisfaction of building one outweighed buying and I'm just making the time to build it. So I got some plans and have started to build the base. It's about half done. For the top, I planned on using MDF and bought two 3/4" X2' X 4' MDF Handypanels from Home Depot along with a sheet of laminate, contact cement, J-roller etc. I ordered a Porter Cable 7518 & INCRA Mast-R-Lift and I'm waiting for them to be delivered. Since I want the lift to know exact plate size before making the top, I set the MDF panels on end up against the wall of my shop for the time being. Yesterday, I noticed a significant bow (3/4" in the center) in both MDF Handypanels has developed. Of Course, I will have to return them because a router table top that's bowed is unacceptable. Has anyone else experienced this? Just about every home made router station post I've read says that Laminated MDF was used for the top. I thought MDF was a stable product and was used because it resists bowing. Is there another type of MDF that I should be using or a different material altogether? Thanks for your replies.

[Peter Aeschliman]

I have a love/hate relationship with MDF.

People say it's very stable, and by some measures, it is. It won't expand and contract as much as other materials with changes in humidity. But, if you don't store it flat, it will always bow... period. And if you get it wet at all, it's toast (only use oil based finishes and paints on it). It's also very heavy, it throws fine dust everywhere when you cut it, and the dust is bad to breath in.

Also, if you hang heavy stuff from it (say, a router) it will sag over time unless you design it with proper support.

Now that my rant against MDF is over, I must say that I made my router table out of MDF and it's great. The key is that I screwed and glued quartersawn hardwood supports to the underside of the MDF to keep it flat... and it has stayed flat for about 6 months now. My first version was made up of two face-glued 3/4" pieces of MDF and it eventually sagged.

So proper support structures underneath the table are a must.

[Rob Holcomb]

No one has any other suggestions?

[Ken Higginbotham]

I think the point being that the bow in your pcs were induced. You should be able to 'induce' them flat again. once cut to size and secured to a good base they should be fine. If you get ANY pc of material big enough if can be twisted out of shape...

[Rick Pettit]

This is my problem with using mdf. If you have to do something to support it like adding hardwood strips to the underside or any variation of that then you have to figure more time and material to the job. If you had used a sheet of baltic birch your problem is solved. No added time and material and no wasted money on fuel going back to HD.

[Ken Higginbotham]

I'm not trying to be an advocate for mdf but how ever the mdf was stored or handled that caused it to warp could cause almost any sheet good to warp.

[Thomas Hotchkin]

Rob
You are in luck. Just glue both bowed sides to each other, and clamp to a flat surface, and you will have a very usable surface for your router table. I did this same process with my router table, and it has stayed flat for over 10 years. And if you do not install laminate to both top and bottom, three to four coats of poly will work. Tom

[Don Jarvie]

My suggestion is to use 3/4 birch ply and then put the MDF over that. When I made my extension table that holds my lift I made the frame out of plywood and added some plywood strips a few inches before and after the lift. Then dropped the ply/MDF top in and then laminated the top and sides. With the stips and ply the MDF won't sag.

[Rob Holcomb]

Thanks everyone. Gess I'm heading to Home Depot to make another purchase

[Chris Tsutsui]

I work a lot with MDF and I feel as though it behaves similar to paper. Imagine if you had a very thick piece of copy paper. If you sanded one side, that opens up the fibers / removes the luster on that side. Then that side will absorb more moisture than the other and cause it to warp because it won't have that glossy flat surface.

I've seen some sheets of MDF that look as though they passed through a very coarse grain drum sander while the other side it was glazed over and smooth. That leads me to believe that the sheet may warp if exposed to humidity on both sides.

Just like a sheet of wood that is stained on only one side may warp...

So there are two ways to fix this. Buy a sheet that has both sides that have equal surface texture / grain structure. Or sand both sides yourself, or stain/paint both sides yourself to equalize both sides.

If the MDF has a high moisture content at the time of purchase then you'll have to dry it out equally on both sides just like a piece of timber... Laying flat with spacers, etc...

My router table is made of 1.5" thick lightweight MDF and it has been great. My only concern is Rockler stopped making inserts for the original Jessem Mast R Lift that twists in place so i'm running out of luck.

[Kurt Cady]

MDF is never perfectly flat. Glue two piece together like this )(. Liberal glue. Predrill the holes before adding the glue. Make it long and drill two dowels in before predrilling the holes. This allows you to align the holes after separating the pieces for glue.

And counter sink (small depth) on BOTH faces that get glue. And the screw side. Leaving one side untouched and flat. If you don't countersink, MDF will breakout of the hole a bit as the screw is driven and will have no where to go, causing a lump.