Why can' I make square flat stuff

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Forum topic by mark76wa posted 11-13-2012 07:26 PM 1590 views 1 time favorited 7 replies Add to Favorites Watch
View mark76wa's profile


97 posts in 3594 days

11-13-2012 07:26 PM

Hi all,

I am trying to make an extension table and work surface for my table saw. The plan was to laminate three layers of MDF, add oak sides and a replaceable hard board top. The table was to have two holes for clamping, a down draft box for sanding (shown) and two t-tracks running the length (not cut yet).

I made all the cuts to the first layer of MDF than laminated the second layer on. I used a pattern bit in my router to cut the holes. Than I worked on the hard board top. Lastly I added the third layer of MDF.

When the glue dried I notices the whole thing was tweaked. What will be the front of the table is nice and flat with the sides square to the top. But the back of the table twists up at the corner by 1/8th an inch! I trued to fix it when I added the oak sides but now I just have out-of-square sides.

I have no idea what happened. I did the third layer the same as the second. I was using my table saw top because it’s the only thing that I have that is really flat. I used screws to hold everything together as the glue dried and the glue was even from the same bottle. The only thing that was different was that the third layer of MDF was from a different sheet. I didn’t think that would make a difference because of MDF’s stability.

I cut kirf’s along the bottom thinking that would releave some stress but that didn’t work at all.

Any ideas on how I can save my table top and/or what happened?

7 replies so far

View yrob's profile


340 posts in 3851 days

#1 posted 11-13-2012 07:34 PM

how was your mdf stored ? i can tell you from experience that mdf can still be
out of flat especially if you leave it on edge for a few days. i had this happen
to me with a 1/2” sheet.

-- Yves

View mark76wa's profile


97 posts in 3594 days

#2 posted 11-13-2012 07:51 PM

Everything was stored on edge for quite some time. I share my 1 car garage with all the other junk we have so I have no space to lay things flat.

Is that the problem. The wood was bad from the get go? Can that be fixed with some magical wood working technique?

View RussellAP's profile


3104 posts in 2485 days

#3 posted 11-13-2012 09:51 PM

This might happen if you didn’t drill pilots for those screws. MDF is dense and will spread.

-- A positive attitude will take you much further than positive thinking ever will.

View nwbusa's profile


1021 posts in 2485 days

#4 posted 11-13-2012 09:55 PM

I know this doesn’t help with your current table, but if you end up remaking it at some point you might want to consider building a torsion box design, which would be lighter and very resistant to going out-of-flat. Good luck!

-- John, BC, Canada

View mark76wa's profile


97 posts in 3594 days

#5 posted 11-13-2012 10:02 PM

Russ, I drilled pilot holes all over the place.

NW, I was thinking torsion box but those things are complicated and are kind of deep. I’m not sure I have that top to bottom space. But it looks like that might be what I end up doing. I need to keep it under 4.5 inches.


View a1Jim's profile


117328 posts in 3776 days

#6 posted 11-13-2012 10:04 PM

Are you sure the surface you have it on is flat ? 3 sheets seems like a bit of an over kill. If you had a bow in all of the sheets I could see them having a twist or bow but only to happen on the last sheet seems a little odd. Have you used a straight edge or string line to check the top?

-- wood crafting & woodworking classes

View nwbusa's profile


1021 posts in 2485 days

#7 posted 11-13-2012 10:15 PM

I hear you Mark. Maybe try with 1/2” material on a torsion box if you go that route. Yours would be a bit trickier to build due to the cutouts, but I think it could be done. I am gong to have to go that route on my assembly table (same problem, laminated two layers of 3/4” MDF and 1/4” hardboard on top, and not flat…)

-- John, BC, Canada

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