Reply To: How to know if wooden beams are completely straight?
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Instead of ordinary string, use dental floss for Paul’s method. Floss is remarkably strong and can be pulled piano-string tight with no measurable sag across tens of feet.
Tradesmen who hang suspended ceilings use an orange-colored version to get room sized ceilings as flat as with a laser. ( laser beams are much thicker than the floss). There are about 50 meters of floss in a small dispenser, so you won’t run out any time soon.
Or make your own straightedge of any length you want.
Pick out a board that’s quartersawn or even riffsawn and plane one edge as striaight as you can by sighting down it as a starting point. Then use your prepped edge to draw a line with a fine tipped pencil. Spin the straightedge 180° and compare the edge to the line you drew. That will show twice the error in your board. Tune the edge and repeat and test until line and edge agree. This will get you within a couple thousandths of an inch pretty easily as long as you are even a little fussy.
For accuracy as good as a machinists edge, use three boards to compare them to each other. If they all agree when you test them against each other, all three edges must be straight. . ( with just two edges to compare, you might have complementary curvedi edges)
When you are done you will have three perfectly straight edges, or one straightedge and two boards with perfect edges on them, or at least as straight as you are fussy. It might take a while. This method is how machinists proof their metal straight edges.
Then make your new proofed board look like a tool.
You can dress the board up with a curved back edge that tells you it’s not the straight edge you proofed, and cut fancy ends so the board doesn’t look like a piece of scrap and get cut up for shims or something. A coat of finish will slow wood movement, but you can always repeat the proofing process if you suspect your new tool has gone out of true. Finally, drill a hole in one end so you can hang your new tool vertically on a nail in your shop off the floor. It will likely stay true for years.
I have a 7’ straightedge that’s maybe 30 years old and still true when I test it.
Finally, for a quick and dirty test for straightness, I’ve found that the finished edge on melamine shelving from the box store is remarkably true in most cases. If you have a piece in your closet, use that as a test edge to get you started. If I was in the field I’d test my edge against a customer’s shelf or a granite counter edge to see if I needed to tune my edges. Granite is often cut with CNC water jet cutters to very high tolerances.
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This reply was modified 2 years ago by
Larry Geib.