IS YOUR BUILDING BOARD FLAT?
By Roy Bourke
MAAC 204L
Now that we are getting into the season for building, it’s time to clean up the workshop and make sure everything is ready for the winter’s projects. And one of the main “tools” for any model aircraft builder, regardless of what type of models he builds, is a flat building board. The question is “Is your building board flat?”
Most builders need not be convinced that a flat board is crucial to the proper alignment of the finished model. So much so that some of the top builders, particularly those concerned with building large pattern and aerobatic scale models, go to great lengths to obtain an absolutely flat reference surface for the alignment of their models. They buy old (or sometimes new) pool tables, used metal or granite surface tables from machine shops, glass doors, etc. etc. However, the building board most commonly found in an experienced builder’s workshop is a wooden door set up on legs or forming the top of a bench.
But is it flat? How can you tell? You can check its “straightness” with a straightedge, but the surface could be twisted and still check out straight in all directions. You can also “eyeball” it for any sign of twist or warp with an amazing degree of accuracy. But it would be good to have a mechanical method, either as a double check on the eyeball method, or as the primary method of detecting any twist, particularly on building boards for large aircraft. In the May-June issue of “SAM Speaks” (the bimonthly publication of the Society of Antique Modelers), Charlie Reich describes such a method involving taught threads.
You start by cutting 9 pieces of hard 3/16” balsa into 2” squares. Sandwich a strong thread between two of the blocks and clamp it to one corner of your building board. Then go diagonally to the opposite corner and clamp another two-block assembly, stretching the thread taught. On a laterally opposite bench corner, clamp a two-block assembly with two threads sandwiched between the blocks. Stretch the two threads across the board diagonally and where they cross the previous single thread, pass one over and one under the taught thread. Proceed with the double thread to its diametrically opposite corner and clamp another two-block assembly with the threads pulled taught. You now have an “X” pattern of threads running across your board.
If the board is twisted, it will be obvious from the way the double threads cross the single thread. Any twist will result in one of the double threads being pushed away from its mate by the single thread. Shim the appropriate corners of the board to bring the double threads into direct contact with the single thread.
You can check for any warps in the board by passing a single block (the ninth block) over the surface of the board and under the taught threads. Ideally the block will pass under all threads easily with little or no gap. If the gap widens, or the block will not pass under without lifting the thread, a low or high spot on the board can be detected, and the board needs to be shimmed or repaired as appropriate.
Finally, Charlie Reich recommends a material called “Celotex/Homosote” as a good surface for a building board. It is the same material that 2’ x 4’ ceiling tiles are made from, except that it comes in 4’ x 8’ sheets. However it is hard to find, so he suggests taking a ceiling tile to a supplier and asking if they have it in large sheets. If anyone finds some, please let us know where you got it!
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