Choosing End Mills

When it comes to CNC routing, the most important aspect of creating your object on the first try with a nice surface finish is taking the time to choose the right end mill, and its feeds and speeds, for the material you’ll be milling. Without the proper planning, you can expect many headaches along the way, including broken bits, broken material, and an uneven or rough surface finish on your project.

Since you’re probably not keen on spending excess additional time trying to give your milled parts a nice finish, we’ve tried to put all the essential information regarding end mills and how to use them into this comprehensive end mill guide. Prepare to learn!

What’s an end mill?

If you think an end mill is like a drill bit, you’d be partially correct. Drill bits are meant to only drill downward and make holes, while end mills can mill both downward and side-to-side. Since end mills are designed to cut in all directions, this enables the CNC router to cut away at your material in any fashion required to make your final 3D object!

There are an endless number of end mills designed for an endless number of applications. Features such as tip shape, shank diameter, cutting diameter, tool length, flute count, helix direction and even the material they’re made from can all vary from end mill to end mill. Your end mills features will mainly depend on the material you’re cutting and your desired surface finish, and since there are a lot of variables we’ll try to stick to the most commonly available end mills that you can use with your Mill One.

Tip shape

End mills tips can take on many shapes, with each tip designed to be used for a specific application. The three most common end mill tip shapes are flat end/fishtail, ballnose, and v-bit/engraving end mills.

Flat end mills are typically used to mill most 2D parts and any 3D parts that do not have complex features, while ballnose end mills excel at cutting 3D curves and contours to a smooth finish. Increasing the overlap between cutting passes for either of these end mills will also boost their ability to produce a smoother surface finish.

Lastly, v-bits have a much more specialized usage in sign-making, PCB milling and other forms of engraving where a 2-dimensional pattern is cut into a material blank. On the Mill One you can vary the depth of cut of the v-bit to vary the width of the engraving. This means that for very fine lines, the Mill One will make very shallow cuts with the v-bit, and for wider lines, make deeper cuts. Since v-bits can come in a variety of cutting angles, ensure that your bit has the right angle for your application. Usually larger (60 degree and 45 degree) v-bit end mills are used for making signs and doing other detailing whereas smaller angles (30 degree and lower) are used for PCB milling to achieve small trace paths.

The difference in cutting shapes between flat end, ballnose, and v-bit end mills
3/8" and 1/8" 2-flute ballnose end mills
1/4", 1/8", and 1mm 2-flute, upcut, flat end end mills
7/16" 90°, 1/8" 35°, and 1/8" 20° v-bit end mills

Size (shank diameter, cutting diameter and tool length)

Depending on if you're looking to carve fine details or just want to hog material out fast, choosing the right end mill size is critical. A basic rule of thumb is that the smaller the diameter the end mill is, the finer the details it can carve, but the slower it will be.

The details that you can reproduce with your Mill One is first determined by the diameter of the end mill, with the depth that you can cut being determined by the length of your end mill. Some of our go-to sizes include 1/4" diameter end mills for jobs that don't require a lot of detail but have a lot of material which needs to be removed, 1/8" end mills for smaller items that offer a balance of speed and level of detail, and 2mm end mills for more precision parts.

One final deciding factor that needs to be accounted for is the hardness of the material. While it may be possible to cut away at aluminum with a 1mm end mill, we would certainly not recommend it because you'd surely end up with many end mills in the garbage. Instead, an end mill should be chosen that you're sure will be able to handle the stress of cutting the material you want to cut.

End mills should also be installed as deeply as possible into the router, only being long enough to make your deepest cut. The bare minimum we’d recommend that 3/8” of the end mill is in the router at all times (although this depth is only enough to make very shallow cuts).

Flute number and direction

End mills have flutes, cutting edges that remove material but also act as channels which move chips and dust away from the cut. Most end mills have two or four flutes, but you can find end mills with any number of flutes. When an end mill has less flutes, more material can be cut away per flute, per rotation. This means that fewer flutes will remove material faster, at the downside of making rougher cuts, meanwhile having more flutes increases the chance of a smoother surface finish.

The ‘chipload’ on an end mill is important because as material is removed it takes a little bit of heat along with it, and if the tool or the material starts to heat up then you’ll get poor cuts and bad edge finish, you may even burn or melt your material and dull your end mill. This is why end mills with fewer flutes are generally used on materials which can easily melt or weld themselves to the end mill, such as plastics or aluminum.

The spiral direction of the end mills flutes will also have an impact on the surface finish of the material. Since the shaft on CNC routers always spins clockwise, if an end mills flutes wrap in a counter-clockwise direction then the downward spiral will eject the material downward, meanwhile the opposite will eject them upward. The downside to both downcut and upcut bits is that they can cause ‘tearout’ (material splintering or burring) on either the bottom or the top of the material respectively.  The solution to this is compression end mills which spiral in both directions to ensure a smooth finish on both sides, however these bits are much rarer and come with a few of their own problems.

4-flute versus 2-flute flat end end mill

Material

Your end mill must be harder than the material it is cutting. Thankfully, the materials that are used to make end mills are some of the toughest of the tough. The most commonly used materials for end mills is high speed steel (HSS) and solid carbide.

HSS usually combines with a variety of other alloys to have a high wear resistance and durability for cutting both soft and hard materials. Since HSS is softer than solid carbide it’s less likely to crack and instead wear out over time, so it’s limited to slower cutting speeds. HSS end mills are also usually much cheaper than solid carbide end mills and can be additionally improved if they come with a coating.

On the other hand, solid carbide tools are very hard so they’ll stay sharp for longer and won’t wear as much at higher temperatures. Recently, carbide tools have become much cheaper and accessible for use in CNC routing, however it’s important to note that carbide end mills require much higher speeds in order to mill properly. This makes it a great candidate for cutting finishing passes, milling PCBs, and cutting materials that won’t easily burn or melt.

Solid carbide vs. HSS end mill

To conclude…

Here’s what we can apply to the most general of cases:

  • Size
    • Use the largest diameter end mill you can bear if you don’t mind surface finish and want to remove material quickly
    • Long end mills can make deeper cuts but usually deflect more and affect surface finish, try to use the shortest bit you can afford (or mount the bit deeper in the router)
  • Flutes
    • One flute is best for rough cutting materials that can melt or weld
    • The higher the number of flutes, the nicer your surface finish will be at the cost of being able to cut away less material at once
    • Downcut and upcut spiralled flutes both have a downside, although upcut bits are more commonly used since they eject material upwards; pick your poison
  •  Material
    • You should expect more expensive tools to last longer; if it’s your first time using a CNC router or you tend to make more mistakes then you’d prefer to say, then stick with the cheaper bits

 

Other helpful sources

Makezine- The Skinny on Endmills
CNC Cookbook- Total Guide to Choosing the Best CNC Milling Machine Cutters
Guerrilla guide to CNC machining, mold making, and resin casting
Tinker and Futz- A Guide to CNC Bits
CNC Zone- Router Bits and End Mills: Pictures, Descriptions, and Uses
Wikipedia- Milling Cutter
Shapeoko Wiki- End Mills

 

End mill suggestions

If you’ve come looking for a shopping list of bits you can start out with, we’ve put together a starter list of end mills that we’d suggest to a first time CNCer (in your choice of either HSS or solid carbide):

Essentials

  • 1/8”, 2-flute flat end end mills (for woods and plastics)
  • 1/8”, 2-flute ballnose end mills (for contours and detailing)

Experimental bits

  • 1/8”, 1-flute flat end end mill (for foam, plastics, and aluminum)
  • 1/8”, 4-flute flat end end mill (for soft metals)
  • Some smaller flat end end mills around 2mm (for more detailed cutting)
  • Smaller ballnose end mills around 2mm (for more detailed contouring)
  • 1/4", 2-flute flat end end mill (for bulk removing wood)
  • 60° or 90° v-bit (great for cutting hardwood signs)

Specialty bits

  • Large planar bit for surfacing material blanks
  • Small-degree v-bit (for fine engraving, detailing, and PCBs)

If you find any end mills being sold for dirt cheap, steer clear. When it comes to cheap bits you’ll find that the money you save ends up costing you in headaches from broken end mills. If it’s your first time using a CNC router, expect to make mistakes while getting used to the Mill One. You'll probably break/damage expensive bits during the learning process, so consider holding off on buying expensive end mills until you're more comfortable with your machine.