Use Plunging to Increase Metal Removal Rates

Plunging—or as it has been recently called in the Midwest, stomping—typically has been an approach saved for long reach roughing applications. When performing roughing applications there are many ways to get the job done—from basic “Z” level roughing using conventional methods (large depth-of-cut, slow feed) to the high speed/feed methods (shallow depth-of-cut, increased feedrates) applied when using modern technologies. For a variety of reasons (programming, power consumption, etc.), plunging is usually turned to only when other methods have proven ineffective/problematic.

Recent cutting tool innovation, as well as new CAM programming techniques, should make plunging a more popular approach, especially on older machine tools that lack the ability to operate at high feedrates.

 

CAM Programming for Plunging

One of the major roadblocks when attempting to plunge mill is programming.

Toolpath generation for plunging applications can be tedious because most CAM packages have lacked user-friendly options with the ability to adapt to various workpiece geometries (2-D, 3-D, etc.), and plunger styles (side cutting, center cutting, etc.).

An important programming technique for side plunging is the ability to perform some type of angular retract at the bottom of each plunge (see Figure 1). This angular retract becomes necessary when the forces acting on the plunger push it away from the part. What can happen next—without the angular retract—is insert failure due to interference with the workpiece as the tool rapids out of the part.

 

Cutter Design Improvements: Handling Forces

In side plunging applications, the forces acting on the cutter will cause the cutter to push off/away from the workpiece. How much the cutter deflects is based on a number of factors: tool engagement, feedrate and the rigidity of machine tool to name a few. The geometry of the cutting tool and insert will also greatly affect these forces and this is an area of recent improvement for a new side plunger (see Figure 2).

The ability to press sophisticated carbide inserts has enabled the development of a plunger that cuts using less force and therefore reducing deflection. In most instances, this new technology eliminates the need for the aforementioned 45-degree retract at the bottom of each plunge and programming becomes much easier by allowing the use of the machine tools canned programs for drilling (G81, G85), especially for simple two-workpiece geometry.

Inevitably, some bending forces will result and the insert will engage small amounts of material during retract motions. With this in mind, side relief was added with lead angle, which gives the tool/insert the ability to handle small amounts of material during retract (see Figure 3).

 

Advantages of Plunging

Machining the workpiece from top to bottom at given intervals (i.e., “Z” level roughing) is the most popular method used for roughing. Recently, machine tools and cutting tools capable of high speed/feed machining have increased the productivity of “Z” level roughing.

This approach can have its limitations when workpiece geometry, machine tool or workholding limits the ability to operate at constant high feedrates (sometimes operating at 200 to 300 inches per minute feedrates). When any of the aforementioned occurs, the advantages of plunging include:

Constant Engagement
When using the Z level machining approach, chip loads (IPT) increase and decrease as variations in radial and axial engagement occur. With plunging, although radial engagement and step-over varies, a constant chip load is maintained.

Optimized Toolpaths
When compared to the Z level approach, plunging is usually a more direct approach requiring less movement (linear inches of machining) to machine a given area. This is especially true when encountering geometries that require multiple XY directional changes (such as a pocket with support pillars).

Optimizing Machine Tool
Plunging removes large amounts of material while moving at relatively low feedrates (more than 50 ipm would be unusual). By implementing plunging, shops with older machine tools can achieve metal removal rates that compare to—and sometimes exceed—newer machine tools running with a high feed approach.

Handling Interrupted Cutting Conditions
In many cases when encountering interrupted cutting conditions, one is forced to reduce the cutting parameters in order to reduce/eliminate the chances of chipping or fracturing inserts. Plunging tends to handle these conditions with more consistent results. This could be attributed to the approach into the material being axial and to the consistent chip load per tooth maintained during plunging.

Dealing with Cutting Forces
The axial forces of plunging typically allow aggressive operating parameters with less concern of moving the workpiece.

 

Summary

Advanced pressing technologies are leading to the development of cutting tools that reduce cutting forces, are stronger in design and make better use of the carbide raw materials used to make them (i.e., saves money). Plunging will gain in popularity as shops learn they can increase efficiency through greater metal removal rates across a wide variety of applications and materials, as well as increase the capacity of older machine tools.