How to Mark an Arc for Wood Cutting

Written by mark morris
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How to Mark an Arc for Wood Cutting
A roll of string and three nails can help create the perfect arc. (string image by Edsweb from

Dimensional lumber is square with lots of straight lines and 90-degree angles. Envisioning a curved line in all those straight edges can be tricky but breaking up harsh angles with the occasional rounded edge pays off big in woodworking designs. Marking that curved line, or arc, may be simpler than you think. At least two methods to do this require no special equipment and can be done by even a beginning carpenter.

Skill level:
Moderately Easy

Things you need

  • Round objects for templates
  • Nails or screws
  • Mason's string
  • Tape measure
  • Carpenter's square
  • Pencil

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    Using a Template

  1. 1

    Choose an object or existing piece that has the curve and dimensions you need. Objects like paint cans and planters often have good curves that can be imitated. You'll need something light enough to move easily so that you can bring it to your material without too much trouble. This will be your template.

  2. 2

    Set the material to be marked on a sturdy, flat surface. If your template object is heavy at all, clamp your material to the work surface with C clamps before continuing.

  3. 3

    Set the object on top of the material and position it so that the curve of the object places the arc where you need it. Trace the arc of the template onto your material by dragging a pencil along its edge.

    Outlining Arcs Using String and Nails

  1. 1

    Make larger arcs with a piece of mason's twine and a few nails or screws. Drive a nail or screw at each end of the proposed arc. Make sure they are positioned where you want your curve to begin and end.

  2. 2

    Draw a straight line between these two points and mark the centre of this line.

  3. 3

    Use a carpenter's square and measure up or down the distance you want your curve to bow and add a third screw or nail at that point. For example, for a curve four feet long with a six-inch bow up, set screws 48 inches apart, draw a line between them and mark the centre. Mark a point six inches above this centre and place a third screw or nail.

  4. 4

    Tie the mason's string tightly to one of your end nails. Stretch it snugly around your centre point and to the opposite end point and tie it off tightly. This forms a peak, similar to a gable end.

  5. 5

    Run a pencil along the side of the string away from the bow. If the bow goes up, run the pencil along the bottom and visa versa, pushing the string out so that the pencil creates an arc the width and height of your three points.

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