Table / board flatness

[Paul Kobe]

Hi, I've seen a few comments on using glass, granite and other materials for a flat table for building.  What's an adequate table flatness?  The table I've been using measures 0.003 to 0.006" across 48", that is one or two sheets of looseleaf paper thickness.  The table is a heavy sheet steel work bench with 3/8" thick  sheetrock on top.  It bows in the center. 
Thank you
Paul

[Brett Buck]

Hi, I've seen a few comments on using glass, granite and other materials for a flat table for building.  What's an adequate table flatness?  The table I've been using measures 0.003 to 0.006" across 48", that is one or two sheets of looseleaf paper thickness.

   Good enough, as long as it stays that way under load.

    Brett

[FLOYD CARTER]

So. Go to a lot of work and expense getting your table top perfectly flat. Then, after covering and painting your perfect wing, it warps!
They all warp somewhat.  All your hard work making your table flat has been wasted.

[Brett Buck]

So. Go to a lot of work and expense getting your table top perfectly flat. Then, after covering and painting your perfect wing, it warps!
They all warp somewhat.  All your hard work making your table flat has been wasted.

     Oy gevalt.

    Brett

[Dan McEntee]

So. Go to a lot of work and expense getting your table top perfectly flat. Then, after covering and painting your perfect wing, it warps!
They all warp somewhat.  All your hard work making your table flat has been wasted.

     So, just so I understand this, you are supposed to use a warped building board, build a warped wing, then paint it flat??

      If it warps when you paint it , it wasn't right in the first place. A wing of proper design, build correctly should not warp at all through the finishing process. I have found that the biggest cause of most warps are people forcing things to fit when assembling things and building in stresses into the structures, along with bad fitting glue joints.

    Type at you later,
   Dan McEntee

[Brett Buck]

     So, just so I understand this, you are supposed to use a warped building board, build a warped wing, then paint it flat??

       Today's philosophy lesson, for all those kids out there - "Trying is the first step towards failing - so never try.  NEVER!"

      Brett

[Mike Greb]

.003 to .006 over 48" will not really affect anything .  I would love to have a board that flat.

[Paul Kobe]

Thanks all.

[Brett Buck]

Thanks all.

   The unfortunate negativity aside, there is a kernel of a point. Having a good reference plane to work from is a necessary, but not sufficient, condition for building straight airplanes. Dan notes some of them, forcing parts into place, glue shrinkage, covering shrinkage, etc. It's really an art.

   For what it is worth, a sheeted foam wing is probably the most stable and reliable way to make a wing that is straight and stays that way. A rigid table that will stay within .010-.015 under the 200 lb loads required to weight it down is a great place to start with that, and pretty much mandatory for success.

    Every kind of built-up construction is much more prone to shifting it's shape as the various components shrink or warp. some more than others.

     Brett

[Ken Culbertson]

Some of us don't have the luxury of a great big flat surface to build on so we use what we can.  One of them is the good old rod jig.  You can build on almost anything if you can keep the rods straight.  I have built wings on hotel closet doors laid out on the 2nd bed while on TDY in the service.  I have used dressers, floors and dorm desks all, well most, of which flew without the need of trim tabs.  I envy those of you who have "shops" equipped to build perfect airplanes but not having one is not an excuse not to try.

The trick in "free form" building is to NEVER BUILD IN STRESS and use CA wherever possible.

Ken

[Curare]

   The unfortunate negativity aside, there is a kernel of a point. Having a good reference plane to work from is a necessary, but not sufficient, condition for building straight airplanes. Dan notes some of them, forcing parts into place, glue shrinkage, covering shrinkage, etc. It's really an art.

   For what it is worth, a sheeted foam wing is probably the most stable and reliable way to make a wing that is straight and stays that way. A rigid table that will stay within .010-.015 under the 200 lb loads required to weight it down is a great place to start with that, and pretty much mandatory for success.

    Every kind of built-up construction is much more prone to shifting it's shape as the various components shrink or warp. some more than others.

     Brett

Ahh, the key element! A flat REFERENCE! How are you gonna know what's warped without a decent datum?

[Randy Powell]

i have a flat bench. Then I use adjustable jigs and a laser level. Works for me.

[FLOYD CARTER]

One of the best-flying models I have built was framed up on top of a dresser in a motel room in Sunnyvale, CA. (1962)
We lived in a motel for several weeks while our newly-purchased home was getting a restoration.

I wonder what the motel people thought of all that balsa dust and shavings. I tried to be neat and clean up every night, but that wasn't easy.

The point is; I have no idea how flat the dresser was, but apparently within the limits of the "flatbuildingboard" folks.