how "flat"building board
 

I am in the process of making a build board out of mdf. I have it built and it has a sag of about .009 in the center. The table is 6ft x2ft and will be topped with steel. I have not ever built a plane before and want to get this right but not chase my tail.

Thanks krank

9 thousands. You'll never know it. Rule of thumb. If it looks right it will fly right

If you can't get by with shimming it underneath with a piece of Monokote it will just have to do.

I'm curious how you measured an error that small in a table so big. Expensive straightedges?

I used a straight edge and then used sheets of paper to touch the straight edge. Then measured the stack of paper it took to fill the gap. .009

thanks krank

0.009" ---- You'll never see it from 100' away.

here is a picture of the structure of the table. it is all made from 3/4 in plywood. 16 in on center supports.

http://i1328.photobucket.com/albums/...psyo9yxqf5.png

Holy Crap! look at all those outlets! Planning on lots of kitchen appliances? ;) With those tolerances you will be fine, plenty of good airplanes built on the dining room table.

lol, I hate having something to plug in and no outlet to plug it into. So when I built the room I put an outlet on every stud except the north wall which is where my tool box and the laundry sink will be. Each wall has its own breaker. The outlets recessed in to the wall in the white area are for the tv/monitor and stereo and they have there own circuit as well. the room is only 8x14 so not huge but built just for me.

I was always a bit anal abut getting stuff as true as possible. At this stage could you not shim the center support of the board. I would if it were mine.

For future reference the most stable board I ever had was a slab of marble rescued from a junkyard. It one formed the top of an old fashioned washstand.

The bottom sheet is screwed to the supports and the top sheet is screwed up through the bottom sheet into the bottom of the top sheet. So I think what I may do is unscrew the top sheet and use some sheets of paper in between the 2 layers to shim it up.

I have used solid core doors and topped them with Homosote they stay straight and true for ever. These are my two building tables one has a 36 inch Xerox machine inside the other has a 2 X 2 steel frame.

Dennis

Definitely shim. Recheck, recheck, recheck. Then go build a straight wing. You're wise to get it right.

You might consider adding an adjustment screw. I built my board with adjustment screws. I check for flat before each build. Two years now, no adjustment yet.

Thanks for all of the info. I think I am going to replace The mdf on the building side with 3/4 plywood and shim it flat.

I love the idea of adjustment screws, then you can crank in or out incredibly small amounts.

9 thousands? Everything has a tolerance, 9 thousands would be well within

All wood and dimensional lumber will move with temp and humidity. Engineered lumber such as plywood has a flatness tolerance that is pretty generous. So .009 is nothing I would worry about. Shim it out today check it when the season changes it may surprise you how much it can move. Especially if two different materials are fixed together with glue. I screw the homosote to the solid core door and the screws allow the homosote to move eliminating warping.

There is more than a good chance simply shrinking the covering will result in more than .009 change.

Dennis