CNC Turning or CNC Milling: The Difference And When To Use

In most cases, the choice between using CNC turning or CNC milling to produce a part is obvious.  There are times, however, when the decision is less cut and dried.

CNC milling holds the raw material still while tools move around it, removing what doesn’t belong in the final part.  CNC milling is the obvious choice for flat, square, and otherwise irregularly shaped parts.  CNC turning, on the other hand, spins the raw material while the tools move very little.  This lends itself to cylindrical parts, of course.

The Symmetry of the Part

Parts which can be produced from symmetrical stock are usually best suited to CNC turning.  Symmetrical stock isn’t just cylindrical stock.  Square and hexagonal stock are sometimes turned.  The connotation of symmetrical stock is simply stock that’s about the same thickness no matter which way you turn it around its long axis.  Two-inch by half-inch bar stock is something that would almost never be used for CNC turning -it’s too asymmetrical.

The Roundness of the Part

While round parts are made with CNC milling, it’s far more common to find CNC mills cutting parts with corners and angles.  All things being equal, if you need a round shaft, you will look to CNC turning to produce it.  But if you need a round shaft protruding out of a large flat plate, you need to consider CNC milling.  Flat or boxy shapes are definitely candidates for CNC milling.

On the other hand, consider something like a crankshaft from an internal combustion engine.  It’s not exactly what you’d call symmetrical, with off-axis journals along its length to push pistons up and down.  Yet probably every crankshaft on earth was finished on a lathe.  CNC turning is the go-to technology to balance the weights and fine tune the bearing surfaces to give the crankshaft years of reliable service.  The key is to notice the roundness.  Pretty much every point on a crankshaft is curved.  That’s how you can tell it’s best suited for CNC turning.

The Surface Features on the Part

CNC milling excels at producing surface details.  Milling sealing surfaces perfectly flat, cutting gasket channels around cooling ports, drilling screw holes, even carving decorative or functional finishes or cutting irregular shapes on a part -these are all the bread and butter of CNC milling.  CNC turning has some limited ability to produce surface features -knurling handle grips, for example.  But for the most part, this is the domain of CNC milling.

Sometimes the Answer is Both

It’s always economical to simplify the design of a part.  Minimize the number of features, or concentrate them on one side of a part so it doesn’t have to be repositioned in the mill.  Plan the design so the CNC lathe can form the part while holding it at one end without having to remove it and turn it around.

That’s not always possible, though.  Often your part will require repositioning, which involves truing it up in the machine all over again.  And sometimes a part may require both CNC turning and CNC milling.  It happens that one is far superior at one function you need, while the other is the only way to include another required feature.  Consider that crankshaft again.  Many crankshafts depend on oil being sent through special passageways, and those holes can’t be drilled on the CNC lathe; the part has to be set up on a CNC mill just for that feature.  Features like that obviously increase your costs quite a bit, so consider them carefully, and consult with your CNC professionals to see if you can find another way to accomplish the same job for less money.

It may seem like a simple question at first, but like most things, the more you dig into the details, it gets more complicated.  That’s why working with qualified engineers who are well versed in CNC milling and CNC turning is critical to getting what you want at a price you can afford.