Video: Machining API Groove with “Spirograph” Tool Path

A system for turning oil-industry face grooves on a machining center uses an adjustable toolholder along with an NC code generator for creating the unusual path.


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Face grooves in circular flanges are often turned, but turning is a challenging way to produce the special grooves in oil-industry valve bodies. The American Petroleum Institute (API) requires this groove to fit tightly with its sealing ring by means of a ±15-minute tolerance on its 23-degree walls and finishes of 32 and 63 microinches RMS. Those tolerances can be hard to hold when plunging the turning tool into the material rapidly wears the cutting edge. In addition, the valve bodies themselves are simply awkward to turn.

Cutting tool maker Sandvik Coromant has developed an alternate system for API seal ring groove machining that does not use turning at all, or at least not turning on a lathe. The company’s “SpiroGrooving” system instead uses an adjustable toolholder to make face grooves on a machining center. Two turning and boring inserts are simultaneously rotated and helically interpolated to generate the groove.

The toolholder in this system positions the two V-style inserts at a distance appropriate to the groove diameter. The other important component of the system is a software code generator in which the user inputs the groove diameter and the desired cutting pitch and chip thickness (both dependent on workpiece material) to obtain the NC code particular to this groove.

The tool path is not a simple helix. Instead, as the spindle spins the custom toolholder in time with the helical orbit, each of the two inserts alternates between cutting the inner and outer wall of the groove, and the diameter of the helical path tapers to create the 23-degree walls. Sandvik calls the resultant path an “intelligent spirograph.” Watch slow-motion footage in the video above.