CNCzone Network:

1. ## Milling a Sphere?

Basically I want to make a bowl without using CAM software. I'm still fairly new to CNC but believe I have the basics down. I know how to do radius' in the z axis with g18/g19 but can't find a way to curve (or wrap) the radius around another radius (or full circle) in the xy plane. Am I going about this the wrong way? I'm using Bridgeport DX-32 controls, any help is appreciated.

2. 070612-2117 EST USA

NotDylan:

Suppose the the Z axis is the axis of the sphere. Note you can do a swept arc, parabola, or portion of an ellipse around the Z axis with the following:

Use a ball endmill with a radius as large as is reasonable, but less than the smallest arc radius in the surface being milled.

Mill multiple circular paths in the X-Y plane with a changing radius, Z depth, and feed rate.

Create a subroutine that will be called with for each new Z depth. In this subroutine calculate the feed rate. I suggest a finishing operation separate from the roughing one.

In this case I would probably make the center of the sphere Z = 0. Normally I use the top of the material as Z = 0. But here I would initialize N = 0, or whatever to put you near the material top. N is my layer count and you increment N going down. Obviously you quit at the bottom.

In HAAS G12/G13 are circular pocket milling operations. With the proper parameters this starts in the middle and works outward, and would be used for roughing. Change the parameters and it only does the outer edge of the pocket.

You need to write the equations for the cross-sectional shape of the spherical pocket. These equations are used on each call to the subroutine to determine the new radius based on the cut depth for the Nth layer.

For a circle in the X-Z plane the equations are as follows using a fixed angle increment:
z = Radius of sphere * SIN [ N * Angle increment ]
radius to put in G12/G13 = Radius of sphere * COS [ N * Angle increment ]

Can you work from this general outline?

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