The Fundamentals of CNC Milling and Turning


CNC milling and turning are two subtractive machining techniques which are controlled by numerical control (NC) code. When we receive your model, it gets converted into a series of codes that precisely control the cutter in order to form your part from solid bars or rods of metal or plastic. At first glance, the two processes appear to be rather similar, but each is unique. This blog post covers the fundamentals of CNC milling and turning.

CNC Turning Operations

CNC turning example

Lathes are high precision instruments that create round, cylindrical parts. Typically, the workpiece is secured to a chuck, which rotates around an axis while the cutting tool(s) remain stationary. This ensures that you’ll end up with an object which has symmetry about that axis. It’s for that reason that you can’t create off-center axis features such as holes, bores, and grooves on a 3-axis lathe. A secondary milling operation is required to complete those types of geometries. The most common operations performed on a lathe include:

  • Facing: Reducing the length
  • Turning: Reducing the diameter
  • Boring: Enlarging a hole
  • Threading: Creating holes that allow for a screw to securely fasten to the part
  • Drilling: Adding a through hole to a part
  • Knurling: Adding patterned indentations, such as diamond shapes
  • Internal/external grooving: Channeling a part to accept O-rings, or snap rings
  • Parting: Parting off geometries from the bar stock

CNC Milling Operations

CNC milling example

Compared to turning, CNC milling reverses the physical roles that parts and cutters play in manufacturing. In milling, it’s the spherical cutting tool that rotates, while the workpiece remains stationary. Milling machines are extremely versatile and can create an array of shapes. Also, they can produce 3D curvature features using simultaneous moves along the X, Y, and Z axes, using a bull-nosed and/or ball-nosed cutters. While a milling machine has high precision capabilities, it can’t make a perfectly round part as other precision machining equipment can, such as a lathe (<0.0015 in. or 0.0381 mm). Common milling machine operations include:

  • Face milling: Reducing overall thickness
  • End milling: Milling with the end face of an end mill to achieve: profiling, slotting, pocketing, boring, etc.
  • Side milling: Milling with the side face/profile of an end mill
  • Key slot milling: Slot milling with a T-slot style cutter
  • 3D surface contour milling: Simultaneous X, Y, Z axis moves to create 3D features, such as a golf ball shape
  • Pocketing: Removing internal material on the machined work surface
  • Drilling: Creating a through hole to the part via a drill
  • Reaming: Creating a tight tolerance diameter bore (+/-.0005 in. or 0.0127 mm) via a reamer

In the end, the process we use to create your part is dependent on the design itself. Knowing these the fundamentals of CNC milling and turning can help you better understand how to craft your part to take advantage of these capabilities.

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