The main features used to describe an end mill are illustrated below.
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On top of these geometric features, end mills are characterised by their type, number of flutes, material (and coating). Thus, in a classical catalogue, the end mill above would be described as:
- Type: flat end
- Number of flutes: 4
- Material: solid carbide
- Shank diameter: 6mm
- Cutter diameter: 6mm
- Overall length: 50mm
- Flute length: 22mm
- Helix angle: 45°
Each of these features has a direct influence on how the end mill will behave during a machining operation and on the cutting possibilities in terms of performance and shapes.
▶ What Size of End Mill Should I Use?
Imagine you want to mill a very deep pocket inside a high part. You will need a long tool to be able to machine the bottom without colliding your spindle mandrel with the stock of the material. Hence, the length of your end mill will be determined by how deep it must penetrate into the material. The associated concept with this observation is called “stickout”. It is defined by the distance from the end of the tool holder to the tip of the end mill.
Also, keep in mind that the cutting depth should never exceed the flute length of your end mill. If cutting deeper than the length of the flutes, chips won’t clear properly, heat will build up and you risk to damage your tool.
At this point, it might seem smart to buy end mills that are as long as possible to have the flexibility to use them in more situations, right? It’s actually not the case, because the more stickout, the less rigid a tool is. If it sticks out too far and you work it too demanding, the cutting forces will make it bend, which is what is called “tool deflection.”
Tool deflection can actually be very problematic since it induces:
In short, shorter end mills are more rigid and less expensive. So save the extra-long ones for operations where they are really needed
▶ Shank & Cutter Diameter
The diameter of your tool will have a direct incidence on the profiles you can cut. Let’s say you want to make a box and assemble it using 90° interlocking joints.
Since your tool is a cylinder with a certain radius, it won’t be possible to do it as such. Indeed, the tool will leave a circular profile in every internal corner, with a radius equal to half its diameter. The larger your end mill diameter, the bigger the radius of this circular profile. In order to overcome this problem, CNC operators use what is called “dogbones”. Dog bone corners describe the shape of a corner that is extended outside the cut area to create a perfect 90º corner.
On the other hand, increasing the diameter of your tool offer two main advantages.
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First, it makes your tool more rigid, which will allow you to make deeper cuts while minimizing tool deflection. A lot more rigid actually, since increasing diameter by 2x will increase its rigidity by a factor of 16x.
Secondly, it improves your MMR (Material Removal Rate) since the end mill can remove more material per unit of time when it is moving inside the material, allowing you to optimise certain operations and do the same job faster.
▶ How Many Flutes Should I Choose?
Flutes are the deep spiral grooves that allow chip formation and evacuation. They are the part of the end mill anatomy that create those sharp cutting edges (sometimes referred to as “teeth”).
The number of flutes on your end mill is a crucial parameter that depends mostly on the material you want to cut and on the capabilities of your machine. Indeed, the number of flutes on your end mill will impact:
- the feed rate of your machine,
- the surface finish of your piece,
- and the ability of the tool to clear chips.
Feed rate is indeed directly linked to the number of flutes of your end mill: if you add flutes, you will have to increase the feed rate, or decrease the rotational speed of your spindle, to keep a constant chip load. So, depending on the speed capabilities of your CNC machine and of your spindle, you might have to choose an end mill with less/more flutes.
If you are not familiar with those concepts, we recommend you to have a look at our article on feeds and speeds.
Secondly, having more flutes on a tool creates smoother cuts, but it also leaves less space for chips to evacuate. This can be somehow overlooked if you are cutting soft materials, but not at all if you are cutting aluminium for instance. The reason is that aluminium produces very large chips compared to other materials. So when the end mill is cutting down in a hole or a slot, its flutes provide a crucial path for chips to escape.
That explains why it is recommended to use 1 or 2 flutes end mills with aluminium since they have more chip clearance than with a 4-flutes one, where the chips will jam progressively, overlapping the cutting edges of your tool and making it break eventually.
To sum up, fewer flutes are best at chip clearing, while more flutes create a smoother surface finish.
▶ Should my Helix Angle be High or Low?
General-purpose end mills typically have helix angles around 30°. Increasing the helix angle will reduce the cutting forces and the amount of heat and vibration generated during the milling process. Hence, end mills with a higher helix angle tend to produce a better surface finish on the workpiece.
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