Hardware/ End Mill Guide

End Mill Guide

The Basics

If you want to create anything on your machine, you're going to need to learn some end mill basics. These cutters are used by the Mill One's router to carve away at materials in order to make the object you're trying to create.

Creating a project with a nice surface finish on any CNC router usually comes with its difficulties. With the Mill One, we've continued applying our industry experience so that you're chances of success when creating something awesome are as high as possible.

If you think an end mill is like a drill bit, you’d be partially correct. Not only can end mills drill downward like drill bits, but they can also cut side-to-side. This enables the CNC router to cut away at your material in any fashion required to make your final 3D object!

When cutting material with a rotating bit, you can imagine that going too slow will dull or break the bit very quickly. For example, imagine trying to cut through a plank of wood with a saw except instead of moving the saw back and forth you're just trying to push it directly through. Similarly, a bit that's moving too fast can burn or melt the material that you're cutting. Finding a happy medium between these two extremes is a balancing act between three main factors:

  1. How quickly the bit is translating (feed rate/plunge rate)
  2. How fast the bit is spinning (spindle speed)
  3. How much material the bit is removing at any given time (depth of cut & step over)

Each of these factors must be suited to the properties of the material that you're cutting; cutting through foam can happen much faster than cutting aluminum. We generalize these variables under the term 'feeds and speeds', and each end mill has a different set of ideal feeds and speeds.

Since there are a confusing amount of end mill types, end mill sizes, and types of materials available for you to use, we've created a list below of just 3 different end mills that we strongly recommend to get started on your Mill One. We've created feed and speed tables for these end mills for cutting in various materials so that you can copy/paste the values you need for your first projects. Luckily, we also sell two of these end mills on our store!

With one of these end mills in hand, you can start making cool and good-looking things on your Mill One without added complexity! Ensure that any end mill you end up using is installed as deeply as possible into the router, with a bare-minimum of 3/8” of the end mill being inside the router for light cuts.

End Mill Essentials

Advanced Reading (optional)

Learning how to tie end mill size and shape to bit movement, rotation speed, material type, and material removal rate is nearly an art form. There are engineers whose job is knowing how to properly balance all these numbers and apply them based on part geometry, desired finish, and the total job cutting time. We at Sienci Labs are continuing to develop smarter algorithms so that this work can be done more autonomously through software. In the meantime, these more complicated cutting operations will be up to your discretion.

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. The following is more in-depth information on terminology and the theory behind how end mills are applied in a cutting job. A summary and some suggestions can be found at the bottom.

End Mill Lingo

There are end mills for every possible cutting application. Features such as tip shape, shank diameter, cutting diameter, tool length, flute count, helix direction and end mill material can all vary from end mill to end mill. The material that you wish to cut, how fast you wish to cut it, and your desired surface finish will decide what features you want on the end mill you end up using. 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.

There are many types of cutting operations that you can run on you Mill One:

  • Surface engraving
  • 2D profiles
  • "2.5D" objects (profile & pocket combinations)
  • 3D "relief" cuts

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. The smaller the diameter the end mill is, the finer the details it can carve, but the slower it will remove material.

The detail accuracy that you can produce with your machine is primarily determined by the diameter of the end mill, with the maximum depth determined by the length of the end mill. Some of our go-to sizes include:

  • 1/4" diameter end mills to remove a lot of material at once but cannot create detailed reliefs
  • 1/8" end mills for smaller items, offering a balance between speed and the level of detail
  • 2mm end mills and under for more precise 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.


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.

  Ballnose end mill vs. Flat end end mill vs. V-bit end mill
1/4", 1/8", and 1mm flat end end mills
3/8" and 1/8" ballnose end mills
90°, 35°, and 20° v-bit 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.

Flute number, direction, and chipload

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.

2-flute vs. 4-flute flat end end mills

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. You can also clamp a scrap piece of material either above (for upcut bits) or below (for downcut bits) the part that you wish to cut in order to reduce the amount of burring.



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.

Solid carbide vs. HSS end mill

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.



Carbide end mills have the ability to be covered in fancy coatings like TiN (titanium nitride), TiCN (titanium carbonitride), TiAlN (titanium aluminum nitride), and AlTiN (aluminum titanium nitride). All of these coatings can enhance the end mill's abilities by increasing its wear resistance, making it more thermally stable, lowering its coefficient of friction, or increasing its ductility.

For more non-abrasive materials, coatings will only have an affect on the lifetime of the bit. However, for ultra-precision machining or when milling carbon fiber or hard metals, the wear resistance introduced by these coatings are worth their added cost.

To conclude…

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


  • 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)


  • 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


  • 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 admit to, then stick with the cheaper bits


  • If you're on the lookout for a reliable set of bits, make the jump and invest in some coated end mills. Otherwise, if you're on a tight budget or you're still learning how to use your machine then hold onto your money for the time being.

End mill suggestions

If you’ve come looking for some bit recommendations, this is a list that we've put together for the slightly more experienced CNCer, or a beginner that's ready to try something new (in your choice of either HSS or solid carbide):

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

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 bits during the learning process, so consider holding off on buying expensive end mills until you're more comfortable with your machine.

This goes the other way too. If you find yourself looking at a price tag that's half the cost of your machine then maybe you should consider upgrading to a new machine. The Mill One was built to be a simple learning platform, not the be-all and end-all of CNC machines :)