Cutting a Large Radius

I just finished a job building a few control panels for a customer.  In the design phase he wanted a large radius along the front of the panel.  I drew the panel up in AutoCAD and the result was exactly what he wanted.

I go down to the shop and I start working on laying out the panel and then it dawns on me.  How am I going to draw a 57″ radius?  I don’t have a plotter to print out a template and I certainly don’t have a compass that large or something to use as a template.  In that past for a curve like this, I used excel and plotted out the points using the standard circle equation, x² + y² = r² .  But I didn’t feel like following that approach this time.  I remembered the old “string and pencil” trick, which works well as long as you hold the pencil consistently along the curve.

I laid the panel down on the workbench alongside another board that allows a measurement greater than my desired radius.

A common midpoint is found between the main panel and the extended panel.

A screw is placed at this point, the center of the arc.

Now typically, this is where you use string. It’s nice and easy to work with. But in the mess that is my shop, there was no finding string anywhere, so I went with the next best thing, wire. I wrapped the wire around the pencil. The lower the attachment point on the pencil the better. The other end is wound around the screw with the distance between the screw and the pencil tip being your desired radius.

The real trick here is having a solid hold on the pencil when drawing the curve. You can’t tip the pencil at all or this will affect your radius of your curve by +/- an eighth to a quarter-inch. If you really want to get fancy, you could hold a square up against the pencil to keep it vertical. To do a sanity check, you should be able to place the tip of the pencil at your three known points along the curve; the starting point, mid-point and end point. If these all line up with your pencil tip the you are good to go. Draw away!

The completed curve using this method.  If you look close at the right hand side of the curve, you can see two lines.  The pencil tipped and that added about an eighth inch to the radius, extending the curve out past my end point.  A second pass starting at the end fixed that right up.

There’s no easy method to cut the board out at this point. You just need to have a steady hand with the jigsaw. Go slow and use a sharp blade that’s made for a smooth cut on wood and you should end up with something that looks like this.

Not too shabby for a piece of string (wire) and a pencil.