There are many different factors that go into the objects you can create with a CNC mill, but the biggest contributor is going to be your end mills. At the most basic level, end mills are like drill bits but instead of only drilling vertically, they can also cut horizontally and have several form factors to get the cleanest cut for specific materials. Some are used only for plastics to keep from heating too much, some are designed to keep wood from chipping or tearing out, and some are designed for ultra fine detail work. With all the different specifications you can find with end mills, explaining each part individually would better serve your understanding, so my goal today is for you to have the considerations necessary to lead you down the path to success. Let’s get started!

Shape

From one end mill to the next, the most obvious difference you will find is that end mills come in many shapes and sizes. Some are thin and pointy, and others are wide and rounded. Some of the most common shapes you will find are fishtail (or flat), ball-nosed, and bullnose, and each of these can be a straight cut or a tapered cut.

Size

Size is the biggest determination of what you can do with any given end mill. Large ones excel at grinding through a lot of material at once, but you don’t get a lot of detail out of your parts. With CNC milling, the radius of your end mill is the radius of any internal corner, so you will almost never have a perfectly square corner on the inside of a milled object. Smaller and smaller end mills can be used for each pass to clean up an edge and get the part to the final dimension and shape. However, the smaller your end mill, the more fragile it gets, so if you try to cut through material too fast with a 1/16” end mill and you’re snapping it off into the workpiece. In most cases the most efficient use of a tool is to cut at around half of the diameter of the tool; with a ¼” tool, have a maximum depth of ⅛”.

End mills come in all sizes and determine how fine of a detail you can carve or how long it takes to do it.
End mills come in all sizes and determine how fine of a detail you can carve or how long it takes to do it.

Straight vs. Tapered

There are generally two forms of end mills: straight and tapered. This is a choice based on the geometry of your finished part because a tapered end mill won’t be able to do all the same things as a straight end mill, and a straight end mill may not be the most efficient choice. By using a tapered end mill, the cross sectional area is larger than a straight end mill of the same tip diameter, creating a much stronger end mill that is less likely to bend while milling. For perfectly vertical walls you will need to use a straight end mill as the taper just won’t reach. However for angled walls, using a straight end mill is not the most efficient, ideal choice.

Straight end mills, as seen here, create smooth vertical walls which are great for pockets and profile toolpaths.
Straight end mills, as seen here, create smooth vertical walls which are great for pockets and profile toolpaths.

Fishtail (Flat) Endmills

Fishtail end mills are generally used to cut simple profiles out of a medium, like big letters out of a piece of wood. They cut best using the side of the mill, so most cutting software will slowly ramp the end mill down into the material, rather than a simple plunge. With fishtail end mills, you will have nice square corners at the bottom of any inset section of geometry and a smooth, flat surface anywhere it passes over the top of.

Fishtail or flat end mills are the most commonly used end mill shapes.
Fishtail or flat end mills are the most commonly used end mill shapes.

Ball-Nose End Mills

Ball-nose end mills have a dome-shaped tip. These excel at high-detail contours like relief artwork or mold and die making, but have what is known as “scalloping.” Since the tip of the end mill is round, having a perfectly flat surface is a challenging feat and will take many more passes than a simple fishtail to smooth out.

For high detail work, a ball nose end mill will allow you to capture the most detail at the cost of more time.
For high detail work, a ball nose end mill will allow you to capture the most detail at the cost of more time.

Bull-Nose (Corner Radius) End Mill

Bull-nose end mills are often called corner radius end mills, and are a combination of fishtail and ball-nose. These have a flat bottom, but rounded corners, so you can have a filleted inner corner while also avoiding the problem of scalloping. These are commonly used to mill molds as you don’t need to use nearly as many tool changes to get flat bottom pockets and rounded contours.

Flutes

The tip profile isn’t the only thing that differentiates end mills. The spiral channels on an end mill -  called flutes - determine which materials you can cut. Generally, less flutes equals better chip clearing at the expense of surface finish. More flutes gives you a nicer surface finish, but worse chip clearing. The softer and gummier the material, the quicker you need to remove the chips away from your part. Using a 6 flute end mill on plastic is going to melt the material more than it cuts it, and if you use it on aluminum you run the risk of generating enough heat to friction weld the aluminum to your end mill, completely ruining both pieces. The guideline for soft metals, plastic, and woods is to use one or two flutes; for high-detail milling use three or four flutes, and for carbon fiber, six or more flutes.

Flutes can helix in different directions, depending on the intended material it will be used on:

  • An "up-cut end mill" removes the chips in an upward direction, which creates clean pockets at the cost of a slightly rougher top surface, like tearing out some of the plywood veneer on the top surface, but leaving a clean bottom surface as it mills through it. If a smooth bottom to a pocket is the goal, up-cut is generally the ideal choice.
  • A "down-cut end mill" pushes the ships downward into the material, which will leave a much smoother top surface, but have rougher bottom surfaces. These bits excel at through cuts, since it keeps the surface you see crisp and clean but leaves the opposite face a little rough. If a smooth top profile is the goal, down-cut is generally the ideal choice.
  • A "compression end mill" is a combination of both up and down cut. These tend to be pricier than the rest due to the machining method needed to make them. With an up-cut helix at the base and a down-cut helix at the top, the two helix meet in the middle preventing tear out on either side. If price is no obstacle to the finish surface for your plywood projects, compression end mills are the right choice.

If you're a newcomer to the CNC milling, try starting with a two-flute, up-cut end mill and see how that works for you and your material. Considering that the material options for desktop CNC milling aren't too crazy, you can mill most of the different materials these machines are capable of using a two-flute end mill, but you will need to adjust feedrate and spindle speed.

Different flutes perform better for different materials or details of the finished part. Maker sure to choose one accordingly!
Different flutes perform better for different materials or details of the finished part. Maker sure to choose one accordingly!

Coatings

End mills are made of a few different materials, but “high-speed steel” (HSS) and tungsten carbide are two of the most common. The HSS tools are more forgiving than carbide, as carbide is brittle and can chatter and shatter. HSS is also cheaper than carbide, but it tends to dull faster than carbide.  In order to improve tool performance, manufacturers apply different coatings to extend the life of the tool and keep it sharp longer.

Finding the right end mill for the job is all about finding the balance between the different factors that make up the tool. Don’t forget that standard procedure for CNC milling is swapping out your tools depending on which step of the process you are working on. It’s perfectly normal to have dozens of tools - end mills, drill bits, engraving bits, and others -  that you rotate through to get a progressively closer shape and finish to your final product. I hope that I’ve either gotten you interested in using more end mills or at least given you a better understanding of how once differs from the next. 

Have fun milling!