CNC Milling

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Compared to 3D printing and laser cutting, CNC milling is a more complex and challenging manufacturing process. Unlike 3D printing, which is an additive process, CNC milling is a subtractive process. This means that instead of using the material to fill the space your model takes, you have to start with a larger stock of material that is at least the same size as your model on all axes. As a result, CNC milling can be more expensive since all the material you cut away is turned into small chips and cannot be easily reused. However, CNC milling allows you to work with a variety of materials such as hardwood, MDF, waxes, plastics, and metal (not in FabLab), and provides greater accuracy and tolerances than 3D printers.

FabLab provides access to two CNC machines:

Which machine to choose

Shapeoko Modela MDX-50
Material Woods and plastics Only wax
Axes of freedom XYZ XYZ+A
Working envelopes Suited for plate materials,

it has only 10 cm of Z travel.

It has smaller XY travels than Shapeoko,

but a much larger Z travel distance.

Mills

Mill types

A CNC router uses mills, which are specialized tools designed to remove material from a workpiece.

These tools look like regular drill bits but are optimized to handle side loads and facilitate better material evacuation.

While there are many variations of mills, we will focus on just a few.

Flat end mill

Flat end mill
Ball end mill
V-bit / Engraving mill

This mill is highly precise and excels at producing completely flat surfaces. However, it may encounter difficulties when working on sloped surfaces due to the flat end of the tool, which can cause the slope to appear as a series of stairs. As a result, this mill is generally not the preferred choice for artistic projects that involve organic shapes, but it is ideal for creating functional parts.

Ball end mill

In some ways, the ball end mill operates in the opposite manner of a flat end mill. While a flat end mill is effective at producing flat, horizontal surfaces, a ball end mill can struggle to achieve such surfaces, as its curved end creates a series of grooves that often result in cusps between them. However, a ball end mill is ideal for machining organic shapes and features, and surfaces at an angle will typically be much smoother.

V-bit / Engraving mill

V-bits get their name from their shape, and while they aren't well-suited for removing large volumes of material, their pointed tips make them ideal for engraving text or images onto a material.

Tool library

For more information on mills Fablab has in stock, visit the Tool library

Mill parameters

Several important parameters are shared across different types of mills, including their size, the number of flutes they have, and the materials they are made from.

Size

The cutting abilities of a mill depend on its diameter. A thicker mill can handle more sideload and remove more material at once, but it cannot machine features smaller than its diameter. In contrast, thin mills are delicate but allow for more detail and smoother finishes. As a result, large mills are ideal for fast material removal, while small mills are slower but deliver better surface finishes. This is where the concept of roughing and finishing passes comes into play. Using a large-diameter mill for roughing quickly removes a lot of material, leaving the finishing thin mill with less material to work with, and thus reducing the overall operation time.

The length of the mill, or stickout, between the holder and the cutting end affects how deep you can mill into your workpiece before it collides with the tool holder. However, too much stickout (more than 5-6 tool diameters) can cause the mill to bend or deflect, and in some cases even break.

Flutes

The flutes on a milling bit are the spiral grooves that run up the length of the bit. They are designed to remove chips and debris from the material being milled, allowing the bit to move smoothly and efficiently through the material. The number of flutes on a milling bit can vary, with more flutes generally providing better chip evacuation and less vibration, but also requiring more force to mill. The shape and angle of the flute can also affect the cutting ability of the bit, with different flute designs suited to different materials and cutting operations. -

Material

The type of material used for a mill can have a significant impact on the speeds and feeds that can be achieved. High Speed Steel (HSS) and Carbide are the two most common materials used, with Carbide being the preferred choice in Fablab. Carbide bits offer greater resistance to wear and temperature, which makes them more difficult to dull. However, they also have lower tensile strength than HSS tools, which means they can be more brittle in certain situations.