The Modern Art of Milling

Tips and techniques that can turn a standard milling job into a work of art.

Recent developments in milling have the potential to save manufacturers substantial time and money by increasing throughput. From selecting the right milling cutter for the job to using a roll-in approach to facemilling and employing milling tools for holemaking when appropriate, manufacturers can improve milling productivity without investing a dime in new equipment.

Courtesy of All images: Sandvik Coromant Selecting the Right Tool

When choosing which milling cutter to apply, a host of factors must be considered, from the geometry and size of the part being produced to the workpiece material being machined.

When walking through a manufacturing facility, it is common to see a 90° mill for machining square shoulders being used for facemilling. In some instances, this is justified. If milling irregular shapes or castings with surfaces that cause the DOC to vary, this tool may be the best choice. In other cases, a standard 45° facemill can provide a significant benefit.

Because of chip thinning, where chip thickness is less than a cutter’s feed rate and occurs axially when a milling tool’s entry angle is less than 90°, entry angle has a substantial impact on the appropriate feed per tooth for a milling cutter. A facemill with a 45° entry angle causes chips to become thinner as they travel across the workpiece. The chip thickness is lower than the feed per tooth as the entry angle decreases. This, in turn, allows the feed rate to be increased by a factor of up to 1.4. Applying a mill with a 90° entry angle in this situation, however, reduces productivity up to 40 percent, due to not obtaining the axial chip thinning effect provided by a 45° entry angle tool.

End users often overlook cutter size—another important aspect of milling tool selection. Many shops facemilling large parts, such as engine blocks or airplane frame components, use relatively small-diameter tools. This leaves significant productivity savings on the table. Ideally, 70 percent of the cutter should be engaged. For example, a 2 ” facemill will only have a 1.4 ” engagement and offer low productivity when cutting multiple sides of a large part. A bigger tool will save significant time.

Facemill Optimization

Another way to improve milling operations is through facemill optimization. When programming a facemilling operation, end users must first consider how the tool enters the cut. Typically, this is done by simply coming straight into the workpiece (Figure 1). Such a method is usually accompanied by a banging sound that results from the tool producing a thick chip when the insert exits the cut. The insert pounds against the material, introducing vibration and producing tensile stresses that reduce tool life.

Figure 1. When facemilling, entering a cut straight on introduces vibration and produces tensile stresses that reduce tool life.

A better approach is a roll-in technique, where the mill rolls into the cut without reducing feed rates and cutting speeds (Figure 2). This involves rolling the cutter in a clockwise motion, ensuring that climb cutting takes place. This produces chips that start thick and end thin, minimizing vibration, reducing strain on the tool and increasing the amount of heat entering the chips. Tool life can double or triple by changing the way the tool enters each cut. The programmed radius of the toolpath to implement this technique is to use half the diameter of the tool and add the distance of the offset from the cutter to the workpiece.

Figure 2. By rolling into a cut, a thick-to-thin chip is produced, reducing the strain on the tool and transferring a greater amount of heat into the chips.

While the roll-in technique refers to a tool entering a workpiece, the same general principle applies to staying in the cut whenever possible. Typically, a large facemilling application is programmed so the tool cuts along the full length of the part, steps over and completes the next cut in the opposite direction. It is usually better to integrate a spiral morph, or box method, rolling around the part’s corners to maintain constant radial engagement, eliminating the step-over. (Figure 3).