How can I know how straight a jointed edge is?

Once I've jointed an edge, I'd like to know that there are no hills or valleys in the edge over the length of the board. For example, it's hard for me to know if there's a concavity a 16th deep if it's spread out over half of an 8' board. How can I tell how straight an 8' edge is without an 8' straightedge?
-Tom
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Buy a long straight edge, but then you are subject to the slave labor in china that makes the straight edge. Or do as the rest of us do, sight down your jointed edge to check. Also place against the other piece to be joined and you'll see any problems.
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Back to the reality that two edges to be joined need only be complementary. Then there's the reality that 1/16 inch is clamp closeable over three feet.

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Joint 2 8ft pieces, flip one over and butt up the jointed edges together, If they are NOT flat, it will show up like a sore thumb
John
wrote:

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"nireedmot" wrote in message

Often, you can just sight down it with a practiced eye, Or, lacking that, there is generally a surface in your shop that is "flat" (a relative term), i.e, your table saw top, a bench top made "flat" (a relative term) for the purpose, a wing of a jointer, a factory cut plywood edge, etc, that can be used for comparison.
Flat enough is relative, and generally perfect enough for woodworking.
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Stretch a piece of colored monofilament fishing line?
If you cannot see it how would it matter?
If you are edge glueing 2 pieces together then hold the 2 together looking for gaps.

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wrote:

Sighting down the board is probably the easiest method, although not quantitative. Winding sticks are also useful. You could also use a laser. A board can be out-of-whack in other ways too: cup, bow, crook, and twist. A well-tuned long-bed jointer should give you a straight edge.
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wrote:

I use my bench... if that isn't flat enough, I use the cement garage floor.. If you need a higher level of accuracy, build an 8' outfeed table for your jointer... *g*
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Hold the board up to an identically jointed board; there should be little or no daylight visible between the two.
David
mac davis wrote:

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One thing that works, trace the edge on whatever paper you have. Rotate the board axially and compare edges. Any error is now 2 times as great.
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: Once I've jointed an edge, I'd like to know that there are no hills or : valleys in the edge over the length of the board. For example, it's : hard for me to know if there's a concavity a 16th deep if it's spread : out over half of an 8' board. How can I tell how straight an 8' edge is : without an 8' straightedge?
A very sensitive spirit level will do the trick.
Jeff G
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On Sun, 12 Dec 2004 07:55:33 -0000, "POP_Server=pop.clara.net"

That is the most accurate. This is how they make those huge machines flat. It takes longer than other methods, but works. Just use a level that is calibrated and write down the numbers as you move it along. Once you have a good straight board, keep it upright in a dry area to use as your standard. Check it once in awhile. BTW, you can also plot a large panel this way.
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