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

From the pic it looks like you are making the walls kind of thin. I suggest making the disk at the equator solid with just a small hole ~0.75" or so thru the center, put the same hole thru the polar disks. Then when putting it together glue a hardwood dowel in those holes, leave it long and use it as the axis for spinning the sphere. Cut it off flush when done and cover with the final polar disks if you want to hide the end grain, you would have to shape them by hand or with some different setup.
That will make the sphere a lot stronger, thin sections of MFD like you have tend to come apart.

As far as turning it, low RPMs is the nature of larger diameters I would start at about 10 RPM and see how it goes, a variable speed motor is real nice for this type of thing.

You may want to consider doing the initial roughing with something like this, rather then the router.
http://www.timbecon.com.au/products/angle-grinder-attachments-349_0.aspx
Could just do it by hand as the sphere spins and check with a template as you get close. Then just do a finish pass with the router.

Rig up some kind of power feed for the router then the slow speed is not as much of a problem.

Andre, thanks for the reply.

I need the sphere to be largely hollow, but it will be extensively braced (I didn't show this in the pic for clarity). The walls would be around 1" thick.

I have a few mains powered motors, and was indeed planning to set up a variable speed machine. I want to work out a safe range of RPM speeds so I can choose suitable gear/pulleys to bring the motor speed down to something sensible (it's variable from 800-3200rpm).

A router power feed is a great idea - but that's for version 2! I'd love to make it a full CNC type setup, then I could control the rotation and router movement in order to create spirals along an object. But that's a job for tomorrow. ;)