CNC Cutting Approaches

CNC Cutting approaches can bring to mind a few different options when considering how to raw material but, typically, refer to the feeds and speeds of CNC programming to achieve the precision cut.  The approach can also consider the type of machine you use, the choice of tool, and the cutting pattern programmed through the CAD/CAM system. Inevitably, these still involve the rotation direction, chip formation, chip load, and milling orientation of the cutting tool as it passes over the raw material to remove materials.

Tools

Cutting tools are an important consideration in cutting approaches. The quality of any project begins with the quality of the cutting tool and the security of its mount in the tool holder. Typically, tools share these similar characteristics. A tool works best if it is 30% to 50% harder than the material it is cutting. It should be resistant to wear and chemical reactions. High thermal conductivity and low friction materials are also common in cutting tool fabrication. Tools typically rotate in a clockwise motion and must include chip disposition in their design due to the subtractive process of CNC machining.

Tools vary in their approach from operating like a drill bit on an end mill to fly cutters or face mills that cut along the exterior plain of the raw material with a high circular speed. Most other cutting tools are a version of one of these cutting approaches.

Speed

As CNC machining hardware and software continue to improve, the speed rate and number of passes required to chip away material grow inversely. Cutting tools running at high speeds work more effectively with deeper cuts. Deeper cuts on each pass result in fewer passes. There are, of course, calculations to gauge these parameters based on both raw material and cutting tool material composition.

Advances in software help facilitate the use of higher speeds with lower vibration. This makes cutting approaches simpler when tools can remain closer to the raw material with no wasted “air cuts”. High-speed cutting can be intimidating but the results are cleaner lines, higher quality finishes, and, of course, faster machining.

Feed

Feed rate references the speed at which the cutting tool approaches the raw material with a simple x-y axis tool movement. For most CNC machine operations, multi-axis tools are used and the raw material is held by rails and chucks while the cutting tools approach the raw material.

The feed also references the rotational speed of the cutting tool. Conventional Milling or Down Milling approaches the material with shallow cuts to begin the machining process, working towards deeper cuts at the completion of the CNC program command. Climb Milling or Up Milling cuts from deeper to shallower cuts and works more effectively on CNC machines as it removes heat from the operation with the larger chip extraction carrying the heat away with the chaff and chatter is under more control than manual machining.

Conclusion

A formula for calculating feed rate can typically be used with a new machine or cutting tool as a good starting point.

ChipLoad x CutterDiameter x NumberOfFlutes x SpindleSpeed = FeedRate

Machinists with experience can sense where improvements can be made. After a few passes, you can touch the bit to determine the thermal buildup. Look at the raw edge of the cut to look for rough edges or uneven cutting patterns. Chatter that causes finish problems can be adjusted to compensate. Machinists and designers also have a fairly good idea of tool selection, angle of cut, and the toolpath that will best suit the raw material and finish required. With fewer machinists entering the trade, CAD/CAM software has built-in programming to handle many of these variances to refine the process as well as train new operators. The latest addition of artificial intelligence now feeds that information back into the processor for adjustments to be made during the machining process. The complexity of tools and speeds create some intricate programming but have made cutting approaches all the easier.