Ensuring flat door panels assembled on a sagging MFT

[jarbroen]

I'm getting ready to assemble a kitchen's worth of cabinet doors and drawer fronts and realized(not sure why I didn't before) that both of my MFT's are not flat.  There's about 3-4mm of sag.
I noticed this when I put my stacks of freshly milled stock on top of the MFT's before I cut to length and route the profiles.
There was a gap under the stacks and I thought at first it was a jointing fail on my part, but a check with multiple straight edges shows it's the tops.
I know they are MDF panels and not supported.  And both tables are at least a couple years old.  Probably not a defect and just something to deal with.
The question is how do I deal with that to ensure I'm not getting wonky doors?  I'm doing inset so they need to be dead flat.
I already scrapped a set of doors that I now realize were tweaked because they sat on the not flat MFT's for a few days - they are all warped in the same direction.

What are some tricks the cabinet makers use to work with the MFT as an assembly table for something as finicky as cabinet doors?

*edit - I use Bessey bar clamps when assembling the doors - the saggy top shouldn't be a problem at that point.  What I'm concerned about is after gluing and I need to lay the doors flat for sanding/painting/storage.

 

[AlexThePalex]

*edit - I use Bessey bar clamps when assembling the doors - the saggy top shouldn't be a problem at that point.  What I'm concerned about is after gluing and I need to lay the doors flat for sanding/painting/storage.

Then temporarily put a flat board on top on your MFT, or work on another table.

 

[Birdhunter]

I bought the cheapest hollow core door at Home Depot. It has a plain front. It’s totally flat. I paid less than $20 USD. I use it when I need a truly flat reference surface. It’s not as flat as a surface plate, but it’s good enough for woodworking.

I flip the tops on my 2 MFTs when the sag gets too bad. They are still not really flat.

 

[Bohdan]

Put a suitable piece of timber on a car jack under the centre of the MFT and adjust until it is level.

 

[jarbroen]

I really appreciate all of the suggestions!
It’s funny how improving one part of the process (now that I have a jointer) reveals flaws in other parts.

[member=15289]Birdhunter[/member] - I think your idea might be the winner.  Fits my needs of fast and inexpensive.  I have so much work to do that building my own assembly table isn’t in the cards.  And, there’s a reclaimed building supply place on my errand route today that should have a cheap door.  I picked a door up there for a workbench in my last house and that sucker was near dead flat and stayed that way for years.

And I won’t feel bad about getting paint on a cheap door rather than an expensive piece of ply I can use for a project.

The timber table idea is good too, but I don’t have anything milled.  I have a bunch of rough 8/4, but haven’t determined lengths for my projects.

I also flipped one of the MfT tops, but now it bows...lol

 

[Michael Kellough]

The consistently warped doors could be the result of a twisted MFT instead of the sagging top.

If the MFT frame was truly flat, and the top was sagging, and the doors were set so that one corner was over the sagging middle, then they could/would set up twisted, same as if the frame was twisted.

As suggested, easy solution is to put something flat across the MFT, door or mdf panel, but make sure/shim the auxiliary flat thing to make sure it stays truly flat.

 

[jarbroen]

Good point on the possibility of twisted - especially since I have the two MFT's joined side to side with the MFT joiner bars.
It's challenging to avoid having a high spot in the middle of the two tables, the sides tend to droop.  That could be tweaking the tables.

I loathe MDF, but that's another good idea for a cheap and easy assembly surface with shims to keep it flat.  I have the STM that I could throw that on.  Just need to figure out how to keep it flat on there - across more than two of the wood bars there are dips or high spots.

 

[ChuckM]

I recall seeing Birdhunter's suggestion before of using a hollow core door as a platform for assembly/glue-up. A local cabinet shop owner told me to use a 3/4" ply sheet. I have not had issues with my workbench in terms of flatness, but its surface is not very wide.

Seeing Burdhunter's post today (again) drove to look for some doors. I found some free doors on Kijiji (Canada's Craiglist), but came across a building center there, listing new raw hardboard hollow doors (2’8” x 7’10” x 1” 3/8” ) for $5 plus 5% tax. So I drove down and grabbed two (I don't have a pick-up truck, or I'd get four). I have Krenov-style sawhorses to use with them.

Thanks, Birdhunter.

 

[Michael Kellough]

Good point on the possibility of twisted - especially since I have the two MFT's joined side to side with the MFT joiner bars.
It's challenging to avoid having a high spot in the middle of the two tables, the sides tend to droop.  That could be tweaking the tables.

I loathe MDF, but that's another good idea for a cheap and easy assembly surface with shims to keep it flat.  I have the STM that I could throw that on.  Just need to figure out how to keep it flat on there - across more than two of the wood bars there are dips or high spots.

Since you have two joined tables you probably don’t move them often. It would be worth the trouble to shim them level all around. Tape the shims to the floor and table legs so they can’t easily be moved out of place without your knowledge. If the tables are on a soft surface put a piece of ply under each leg, at least 9 square inches, the bigger the better if the surface is soft. Since this is a tedious process and you won’t want to do it again, go ahead and bevel those pads at 45 degrees. It makes it easier to tape the pad to the floor and makes it harder for the toe of your shoe to push the pad out of place.

With a dead level flat workstation you can use it as a reference for measuring in three dimensions. If the mdf portion is not flat it can be replaced, or bridged as in this thread.

MDF is one of the flattest materials you can buy and is a valuable resource in a shop. While you can use it like natural wood it’s not so pleasant to work and for some uses should be treated like acrylic or aluminum. It’s great for jigs and can do the work of thick acrylic or aluminum in many cases as long as you keep it away from moisture or high humidity.

 

[jarbroen]

Good score on the doors!  I think hardboard would be a good surface - oil it and glue just pops right off.

Great suggestion on leveling the joined tables.  I've made do with using the leveler thing on the MFT legs, but those don't keep their setting(and don't level every corner).  I'm in the garage on concrete so regular shims should work.  I'll definitely remember the tape suggestion.  I've shimmed before, but kept dislodging them so I gave up.  And I used to re-arrange my shop more often.  Now I've got the space pretty well figured out and positioned.
Or, just thought of this, I could use your pad idea and use double sided tape to hold those in place.  Make them just big enough that I could bore holes for the legs to sit in.  I could even mark the spots in case I need to detach a table for an out of shop project.
Man, now I'm getting crazy... haha

If I was able to get the MFT's co-planer I can try new MFT tops or bridge it with a sheet of MDF like mentioned.  A sheet of something would be a lot less expensive than two new tops.
I usually have sheets of 3/4" BB which is great, but not always perfectly flat.  I think my aversion of MDF was established years ago before I had Festool dust extractors - probably not a big deal now with proper dust control.
There's also MDO - just not sure if that would be as reliably flat as MDF.

 

[Birdhunter]

Glad the door suggestion worked. When I do a glue up on the door, I cover the surface with waxed paper. I usually can reuse the waxed paper several times.