In accurate large scale work, long straight edges are often essential.  But how do we get to an edge that is really straight over a long distance?

We can use chalk or string lines or lasers, but here is a low tech method that does the job very well and quite fast.

This came up in a project involving constructing a 12 segment yurt floor in plywood, and I wanted a straight edge 8 feet long to draw along, and also as a fence for a circular saw.  I happened to have some 6mm ply in 300mm widths, so I used a piece of this.

A good width for a straight edge is very useful.  It makes it much stiffer, and also offers clamping opportunities well out of the way of the business edge.

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Laying out the stock on a surface

To check whether an edge is straight or not is really quite simple.  Lay the piece on a surface and draw down the edge.  I used the back of one of the plywood floor sheets. I use weights to hold down the ply, otherwise the pencil can creep underneath and give a false edge.

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Draw tightly down the edge.  Use weights to avoid any gaps

Then flip it over and match it up with the drawn line.  If the edge is straight, it will perfectly match the line.  If not, the actual edge and the drawn line will be mirror image curves.  My edge was not straight, but convex.

Now I can use the drawn line to help mark a proper straight line on the stock.

I aligned the stock with the drawn line, closing the gap as far as possible.  In this case the edge was slightly convex, so the stock touched the line in the middle, leaving gaps at the ends, which I equalised.

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Transferring the line

Then I set up a gauge to the widest gap.  Moving along the drawn line I transferred the drawn line at regular intervals onto the stock.  You don’t need anything fancy, a piece of wood with a 6mm notch cut out of it would do.  The main thing is to have something set up to the fixed distance. This is much better than repeated measurement.

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Near the centre at the widest bulge

This left me with a line of marks on the stock that was a mirror image of the actual edge.

Then at each transferred point I marked, by eye, half way between the point and the edge.  This new line of points had to be a straight line.

IMG_0157Then it was simply a matter of joining the dots with a batten,

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The straight line is half way between the edge and the transferred points

and planing down to the line.

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When I retested it, the edge was spot on straight.

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The drawn line fits the flipped straight edge exactly

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A clearer view of the flipped line, shifted sideways

A simple but effective way of creating an accurate straight edge!

Points to remember are:

  • make sure the edge of the stock is firmly down on the drawing surface
  • use a sharp pencil for a clean line
  • use some kind of gauge to transfer the line.  A piece of wood with a cut out the thickness of the plywood would do.  The great thing is to have a fixed reference.
  • a long soled plane will be quicker and more accurate.  Clamp the stock firmly.  I just clamped it to the stack of plywood, because it was more convenient than taking it to a bench.
  • Label the straight edge!  It is easy to forget which edge you have straightened…
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