Plunge Roughing

What is Plunge Roughing?

Plunge roughing is a milling technique capable of removing large amounts of metal very rapidly.  The process is essentially a type of drilling operation, in that the plunging tool used feeds directly into the workpiece in the Z axis only, creating a hole size equivalent to its own diameter.  Moreover, like a drill, the plunging tool — regardless of extensions — can maintain maximum feeds and speeds in cuts of any length.  For rough machining of molds, plunge roughing offers a highly productive alternative to conventional Z-planar facemilling.  The basic process is simple. In successive passes, the plunging tool is fed directly down into the mold in its Z axis only, retracted like a drill, indexed to new X-Y coordinates, and fed down again. The process continues in a pattern of overlapping holes until the entire mold core or cavity is roughed out.

Perfecting the Technique

In 1997, SofTech introduced the first implementation of plunge rough cutter path generation in a commercial CAM system. The center-cut variant uses a counterboring end mill to plunge directly into the work.

Pattern of Plunges for Center-Cut Plunge Roughing

To broaden the application of plunge roughing, a second variation — edge style — was added to Prospector in 1998.  Edge style differs from center-cutting in that a smaller step-over is used between each plunge so that readily available off-the-shelf tooling can be used instead of the purpose-built counterboring end mill.  This form of plunge roughing is better suited for lower horsepower machines and does not require high-pressure coolant.  Advanced algorithms in Prospector determine the correct path to machine either cavity or core conditions such that the tool never bottoms out.

Pattern of Plunges for Edge-Cut Plunge Roughing

A predrilled entry hole is required for cavity areas because the plunge tool can not center cut.  Prospector automatically creates these holes at the deepest part of the job. 

The entry holes are created using a milling cutter and given a conical shape.  It was found through experience that the conical shape of the entry hole facilitates chip evacuation when the plunge roughing operation begins.

Prospector supports a specialized Z-Planar cleanup cutter path to remove scallops and smooth out the irregular pattern of stock left over from plunge roughing.  Regardless of the plunge rough technique used, this final Z-Planar program is required to complete the roughing process.

Benefits of Plunge Roughing

Major productivity gains can be achieved using the plunge roughing process programmed with Prospector software.  Results indicate that plunge roughing metal-removal rates are minimally more than 50% higher than those obtained by conventional Z-planar roughing with button-type facemills.

And this advantage increases dramatically in deeper cuts. While the plunging tool, regardless of extensions, can maintain maximum feeds and speeds in cuts of any length, extended-length facemills require reduced depths of cut to avoid tool deflection and chatter.