A homemade straightedge, version 2

6 07 2010

The previous straightedge I made was from narrow pieces of pine. Pine is relative stable, as woods go, but it still does move a little  and change shape with the weather. The straightedge consists of two pieces of wood; the reason there are two pieces is that they are used to check each other for straightness — no external reference object is needed, as long as the two pieces of wood are made into exactly the same shape. (You can read more about how it all works at the previous post.)

A little bit of warping is OK for some purposes. The straightedge is about 16 inches long, and when I put the two pieces together, it’s possible to fit three sheets of standard 20 lb. office paper between them. Assuming that they changed shape in exactly the same way (this may be an unwarranted assumption), each one warped about the thickness of one and a half sheets of paper, which is equivalent to .006 inches, or .15 millimeters. This is OK for some things, but it isn’t good enough for checking the flatness of a sole of a plane, for example.

I decided to make another straightedge, but this time around, I used 24 inch lengths of  1/2 inch thick Baltic birch plywood. This stuff is much higher in quality than regular construction-grade plywood: the wood is harder, there are almost no voids, and the plies are thinner, which means there are more plies for a given thickness. In this application, the most important property of plywood is that it’s stable; it won’t change shape with changing humidity.

The straightedge and the tools to make it. The plane was better for removing material quickly and getting it to shape, and the sandpaper on glass was better for fine tuning (and not having to sharpen).

I made it in much the same way as the previous one. First I sawed out two pieces of plywood, then I marked a corner of each piece so that I would know which edges to line up, and in which direction. I set the two rough cut pieces side by side and ran a hand plane over them until they were same shape. Once they were the same shape, I could start checking them against each other by “unfolding” them and holding them up to the light to look for high spots that need to be planed down. This process took a little while, partly because planing plywood edges is very hard on blades and I needed to stop and sharpen the blade a couple of times. Eventually — after it was pretty close to the right shape and after I got tired of sharpening — I switched away from the plane, to a piece of sandpaper glued to glass. Regular old glass from the hardware store is very flat because it’s made by floating molten glass on a layer of molten tin, and gravity does the work of flattening out both sides. As much as I hate to admit that sandpaper ever works better than a plane, that was the case here, at least for the fine adjustments, and I didn’t have to sharpen it.

I used this method on both sides of each piece of wood. At the end, this gave me many more surfaces that I could check against each other for flatness, and fortunately, it all worked out — there’s not a peep of light visible between any of the combinations of edges. If the plywood is as stable as everyone says it is, then there will be no need to true up these straightedges ever again.

If you want to make a straightedge yourself, you don’t need a plane, as I had previously thought — you just need sandpaper stuck to a flat surface. And this will probably work just fine for other materials, like plastic and metal.

The straighted and tools again. The plywood is butted up against a planing stop, which keeps it from sliding around.

One corner on each piece was marked to make sure that the two pieces were always lined up the same way when shaping and checking for gaps.

I added an alignment pin so that they can be stored together. It also serves as a permanent reference for which edges match up (for checking straightness), and in which direction.

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2 responses

23 09 2011
Dave W.

Hey there, nicely thought out process for making a straight edge.

Thanks.

2 05 2012
Ecuadave

Actually, comparing two pieces will not guarantee a straight edge because they may have complimentary curves.Three pieces should be used to guarantee an absolutely straight edge.

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