I’m currently working on a Child’s Fan-Back Arm Chair. The original was built in about 1760 in New England. However, I am converting this original design to a Rocker.  I chose a knife-blade style for the runners where slits in the bottom of the legs capture the runner. A pin is also driven through this joint.

While assembling this model, I became concerned about a proper position of the runner component on the chair legs. After all, I would like the Rocker to sit in a proper position at rest.

Here is a picture of the SketchUp model (albeit without sculpturing/shaping the seat, the arms, or carving the volutes in the crest rail.

The runner position is important to the resting configuration of the chair and I would like  a slight tilt backward in the seat when the rocker is at rest. I can guess at this ideal arrangement of the runner in the legs, but it would be nice to have some back-up data.

Knowing the Center-of-Gravity of the overall assembly would be helpful data. Interestingly, I found a plug-in to SketchUp that calculates the C of G location, and displays on the model. I found this in the SketchUcation website. Here it is loaded Plug-in in my Extensions Tab.

First you select the Assembly of your model, then select the C of G extension, and then click on  “Find C of G” in the pop-out menu. Then you wait – depending on computer speed, etc. it can take a while for the calculation. The result will look like this.

Note the East/West and North/South Lines that indicate the C of G. In SketchUp, I’ve set up a side view with Camera in Parallel Projection. This shows that the C of G centers on the lowest point of the Runner. Therefore, I believe this will be a good set up for the Runners.

Nevertheless, I will be doing some tests in the shop with temporary connections of the runners.

Tim

@KillenWOOD

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