[sploo]

Hi all,

I'm working on a plan to create spherical objects by stacking circles cut from MDF, then trimming it smooth on lathe like jig (using a router).

The circles would be cut from 3/4" MDF, in three passes on a CNC machine. As a result, the inner diameter will remain the same, but I can cut three different diameters on the outside, to make it a little smoother before trimming.

The image "slices.gif" shows what would be a cross section of an approximately 19" diameter sphere, created from stacks of MDF circles. Each green rectangle is 1/4" thick, with each group of three representing a single 3/4" sheet. The object would be spun about the axis shown by the white horizontal line.

I've worked out a way to hold a router, so I can move it along the center line of the object, with the bit always normal to the sphere surface (i.e. 90 degrees). The image "slices-solid.gif" shows the rectangles filled in, with the various positions on the router bit in red.

What I'd like to know is, what sort of RPM speeds should I use to spin the object?

If I think in terms of the CNC machine, 120ipm with a 1/4" depth of cut is pretty normal. I doubt I'd be trying to trim that kind of depth off in a single pass, but 120ipm would equate to 2rpm at the largest diameter point of the sphere*.

This seems kinda slow to me (and it would take ages to trim the whole object).

* 19" diameter x PI = approx 60" => 120ipm is 2rpm