From the classroom to manufacturing enterprises, CNC machining and other digital fabrication technologies are helping to augment education lesson plans as well as product development workflows. There’s a valuable use for digital fabrication in almost every industry.

But what are the basics of CNC machining? What do you have to understand about the machine to accurately translate computer design data into the physical world?

We’ve shared some basics about 3D printing. Now, here’s what you need to know about CNC machining.

Basics of CNC Machines and Tooling Geometry

A computer numerically controlled (CNC) milling machine is used to transform blocks of raw stock into finished parts by cutting away material.

There are generally three axes to a milling machine:

the anatomy of a cnc machine
Basics of a milling machine.

For the purposes of this discussion, we will consider the following directions when referring to the orientation of operations and tools:

  • X axis: Left to Right (Sideways as viewed from the operator’s position)

  • Y axis: Forwards and Backwards (As viewed from the operator’s position)

  • Z axis: Up and Down (Vertical as viewed from the operator’s position)

These three axes of motion let the spindle, which spins a cutter at high speed, carve away material and leave behind nearly any shape desired.

The part being milled is held in a vise, which is in turn attached to the table of the CNC.

Types of CNC Machine Cutters

But not all CNC machines are built the same way. There are different cutters or endmills. Here are some common types:

Flat Endmills: These are the most commonly available type of cutter. They’re efficient and cheap. Flat Endmills are ideally suited for machining of flat bottom cuts and vertical walls.

Bullnose Endmills: The Bullnose profile cutters are similar to Flat Endmill in that they share a common rectangular side profile but with an additional radius at the bottom corner of the flutes. They are great for quickly removing material while leaving a small bottom radius in pockets.

types of milling tools
Side profiles of common milling tools.

Ball Endmill: The Ball Endmill has a full radius at bottom of flutes. These cutters can be utilized for complex surfacing as the contact point between the tool and material surface is constantly changing with variations in the slope of the surface.

ball endmill
Example of Ball End Mill contact point (red) vs centerline of tool (green)

Drills: Drills are used exclusively for making vertical holes in parts. There is no side-to-side or horizontal motion of the tool when using drills. The finished size of the hole will be dictated by the diameter of the tool itself. Besides the diameter of the tool itself, another variable involved with drills is the angle of the drill point. Typical angles are 118° and 135°, generally speaking, the 118° is used for softer materials such as wood or aluminum.

Subsequent operations can be used to alter the size of the hole (reaming, etc.) but those tools are not discussed in this blog post.

Additional Considerations Regarding Endmills: Besides the side profile, it’s also important to understand how the number of cutting edges, or flutes, are formed into the tool as viewed from the cutting end.

More flutes on a cutter restrict the amount of available space for the cut material (chips) to be removed. However, tools with a higher flute count perform better in harder materials.

The cutters shown below are center-cutting. This means that the sharp edge of the flute across the bottom of the mill extends all the way to the centerline of the tool.

the types of cutters

Keep these differences in mind when designing in CAD. Onshape, a cloud-native CAD platform, offers Custom Features that can help make your digital fabrication processes easier. Here are two features that can help you when designing for CNC manufacturing.

Sensor: Measures and compares geometric values. Make sure your tools will fit into corners.

Lighten: Removes material quickly from parts and can account for tool radius.

Why Tool Length Matters

But what about tool length?

Tool diameter, profile, and the number of flutes are important things to consider. Another critical piece of information to consider is the tool length. Especially the exposed length of the tool that is exposed when the tool is clamped in the tool holder or spindle. This is commonly referred to as the tool stick out.

Tool holder

Tool holder and stick out

Tools that have a high ratio of stick-out length to diameter are described as having a high aspect ratio. As the side of a tool comes into engagement with the material while cutting the forces involved work on the tool to create tool deflection. The higher the aspect ratio of the tool, the more deflection will be created. 

Operations that create high side loading and therefore increased potential for tool deflection are:

  • Deep pockets

  • Large step down (changes in Z height) during operations

  • High feed rates (the speed with which the X or Y location of the tool changes while cutting)

One final topic worth discussing regarding tool geometry is the length of cut (LOC) that a tool can achieve. Some end mills have flutes that terminate at the same diameter as the portion of the tool which engages in the holder. Others have a reduced or necked-down profile where the effective cutting diameter is less than the diameter of the holder:


A collection of endmills.

flute length

Flute length and tool diameter.

There are more advanced machines with additional features and axes but understanding the simple three-axis milling machine, different types of endmills, and tool length, is an important first step to mastering manufacturing.

Using Onshape for CNC Milling

We have a number of resources to get you started with CAD, whether you’re an educator implementing a product-based lesson plan or a company looking for ways to improve production workflows.

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(Dave Lapthorne is an Onshape Education Technical Services Engineer.)