High-Speed Pocket Machining

A common problem in CAD/CAM is to find a suitable tool path for milling a pocket that is defined by a shape in the plane. CNC machines can be programmed to follow a path that consists of straight-line segments and circular arcs; recent NC controllers do also support (specific families of) spline curves. When using high-speed machining (HSM), the spindle rotation speed and the feedrate are higher than for conventional milling in order to minimize the manufacturing time without a decrease in the part quality. The high rotation speeds and feedrates of HSM impose new constraints on the tool path: Due to its kinematic characteristics, the spindle has to slow down and stop its movement in a sharp corner, change its direction and accelerate until the desired maximum speed is reached again. The reduction of the speed has also thermic effects on the tool and on the currently machined location of the workpiece: The temperature at the point of contact rises if the feedrate shrinks.

It is folklore that overheating might cause chipping of the tool and degradation in the part quality. Thus, in order to meet the kinematic constraints of HSM, it is essential that the path is smooth, i.e., at least G1-continuous.

In joint work with Christian Spielberger an alternative to conventional contour-parallel machining was proposed. Let R be an arbitrary point inside of (the machinable area of) the pocket, chosen by the user. We compute a spiral-out tool path that exhibits the following properties:

The tool path starts at R and stops once the boundary of the pocket is reached.
It has no self-intersections.
It is G1-continuous. (Further smoothing yields G2-continuity.)
The cutting width is strictly greater than zero and strictly less than a user-specified threshold along the entire tool path.

Related publications:

M. Held, C. Spielberger (2009):
"A Smooth Spiral Tool Path for High Speed Machining of 2D Pockets".
Computer-Aided Design, 41(7):539--550, July 2009.

M. Held, C. Spielberger (2008):
"A Smooth Spiral-like Tool Path with Controlled Cutting Width for Pocket Machining".
Proc. 5th Int. Symp. on Voronoi Diagrams, p. 88-100, Kiev, Ukraine, Sep 2008.

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