Endmills are the things that look like drill bits. The difference is that a drill bit cuts vertically, and an endmill cuts horizontally. As you drag a spinning endmill perpendicularly, it carves out material along its circumference.
We use “endmills” and “bits” interchangably, but it’s important to note that you can’t mill with a drill bit, and vice versa.
We generally use two sizes of bits: 4mm and 6mm (0.157” and 0.236”). Wider bits are more rigid and can take on more load, so we like to use 6mm in 0.25” or 0.1875” aluminum, where 4mm might break. 4mm bits are smaller, and we can only mill our #10 hole pattern with 4mm (0.157” < 0.196”).
Since 6mm bits are bigger, there’s more cutting area, so it’ll get through areas faster. If you have big pockets or contours, 6mm can save you a lot of time.
Switching out bits is an easy way to save time and chance for a mistake, so often we’ll try not to switch bits if we can avoid it.
If you need to use a 6mm in anything thinner or softer, it’s probably a problem with your feeds and speeds for the 4mm.
| 4mm | 6mm |
|---|---|
| smaller, works for hole pattern | thicker, more rigid and higher load |
| breaks more easily | rarely ever breaks (still gets dull, so needs replacing) |
| used in 0.09” - 0.125” aluminum, any plastics, etc. | used in 0.1875” - 0.25” aluminum, big pockets and contours |
A flute is the sharp groove that spirals around the bit. There are bits that have multiple flutes for more aggressive jobs, but for what we’re doing, single flute bits are plenty enough.
Cutting length is how far up the flutes go. Use the shortest cutting length you can, because longer cutting lengths are more prone to vibration, deflection (bending), and breaking.
Tightening Bits and Collets | How to Use Your CNC Correctly
This is a banger video on how to put a bit into a collet. Tip #3 isn’t relevant to us, and also we’re guilty of overusing our collets, but the visuals are great here.
The only part the video doesn’t show is removing a bit from a collet.