Pencil Milling Part 1

“Pencil Milling” is the name given to a machining strategy where a small-diameter ball end mill is used to perform finish milling in small nooks and crannies of a complex mold or die.  The word “pencil” is appropriate because the tool size is close to the diameter and length of a pencil.  Pencil Milling is a finishing version of rest machining.  In NX CAM and CAM Express, we call our pencil milling option “Flow Cut”.

To understand how Flow Cut works, you should first learn about bi-tangencies.  A bi-tangency exists when a round object (like a cutting tool) contacts the workpiece in more than one place (the two areas marked in red).Flow Cut will discover those bi-tangencies and then create a machining strategy for the area between those bi-tangenies (area in blue). Flow Cut has an additonal option to specify a reference tool (last cutting tool diameter to touch this area).

The simplest machining strategy once the bi-tangencies are discovered is to simply run a ball-end cutting tool (of the correct radius size) through that area.  We call that the “single pass” method.  However, this strategy might break the tool because it is the size of a pencil.  Remember our lesson on leverage?  So we have an additional strategy for multiple passes.

Multiple Passes is self-explanatory.  Instead of a single pass down the middle of the bi-tangency, several passes are generated, each at an offset from the middle.  The part I really like about multiple passes is the option to machine steep geometry differently from non-steep.  Yes, just like in my last post, I can use z-Level method for steep, and a different strategy for non-steep.  Best of all, I can do it all within one operation.

This video will help to show the different cutting strategies.

My next post will discuss some of the Flow Cut options in more detail.

Jim has a Master’s degree in Business Administration from the University of Central Missouri. He has over 30 years of experience in the manufacturing industry, and over 25 years of experience in the CAD/CAM field. He serves as Field Support for Siemens PLM CAM products. Prior to his current role, he held stints as CNC Programmer, CAD/CAM Department Manager, CAD/CAM instructor, and Project Coordinator for Siemens PLM CAM software development.  He also made the Kessel Run in slightly over 18 parsecs.