Choosing the right milling bit for the job at hand can be an extremely important factor in whether a milling project comes out amazingly, alright, or not at all. Combine that with having more choices of bits than materials, and choosing the right bit can be quite a difficult task. However, by answering a simple set of questions, you can greatly simplify your choice of bit. Here are the things you need to ask:
- What material am I cutting?
- What is my machine capable of?
- What shape am I cutting?
Let’s quickly go through each of these and what they mean.
What material am I cutting?
Your material is one of the most important single factors in choosing a bit. Materials have a lot of properties that matter both for your design and cutting (hardness, density, size) as well as just for cutting (how does it chip? melting point, thermal conductivity). For any combination of these properties, one thing remains constant: your bit must be sharp. A dull bit will always cut poorly, and can actually be dangerous to use due to increased likelihood of breakage. Most of the bits sold by Inventables are made of solid carbide, which is an excellent choice for small machines because of its stiffness, durability, and versatility.
Your material likely breaks down into one of three categories: plastics, woods, and metals.
Plastics are fairly soft and extremely versatile materials. They cut easily and tend to take a very smooth finish with no further processing. Plastics also form very nice chips off of the bit when they are being cut, which makes choosing your cut settings very predictable (but we’ll get to that later).
Wood is beautiful and strong, and although it cuts smoothly and quickly, it doesn't tend to be as smooth as most plastics. This is because wood has grain and fibers, which tend to tear and bend when you cut them, rather than cleanly shear. Wood also forms small chips and dust when cut rather than continuous curls like plastics tend to.
Metals (particularly softer metals like aluminum and brass) are the strongest, hardest, and most difficult to cut of the three groups. You can do amazing things with metals, but only if your machine can handle it. Speaking of your machine, that’s the next thing to think about.
What is my machine capable of?
Cutting any material puts a strain on your machine. For smaller machines like the Shapeoko, this often shows as the spindle twisting side-to-side, or the whole x-axis twisting front-to-back. Whenever your machine moves like that, you lose accuracy and unnecessarily wear your machine down. Cut settings and bit choice can minimize the force needed to cut a material, resulting in less strain on the machine. That being said, a stiffer machine will allow use of larger bits and higher speeds. The other thing to consider is how fast and accurate the spindle of the machine is. In general, for small machines, the higher the speed the spindle can reach, the better. It is also important to have the spindle be accurate, with very little wobble. When a bit is in the spindle, if it is not straight or wobbles (the spindle has runout), the bit will not cut evenly or accurately. Smaller bits can even break if the spindle is inaccurate enough.
What shape am I cutting?
Most jobs fall into two categories of geometry: 2.5D and 3D. 2.5D jobs have two-dimensional shapes that are cut to different depths. 3D jobs have complex, 3D surfaces. 3D machining requires a different bit shape than 2.5D machining. The other important thing with both 2.5D and 3D is what the size of the smallest detail is. In general, you want to choose the largest bit that can both cut your part and be safely used in your machine. A larger bit is stronger, and allows you to remove more material faster. However, larger bits can’t cut smaller corners or details, so you must consider the detail you need when choosing bit size.
So far we’ve been pretty general, so let’s dive into some specifics about the actual cutting bits.
This is a generic, 4-flute, square end mill. It’s one of the most common bits you can get. Let’s go over what that means. There are channels between each cutting edge called flutes. They act to carry away the chips from the cut. In this case, there are four of them, meaning there are also four cutting edges. The flutes spiral up towards the shank of the bit, so we call it an upcut bit. Finally, the edges of the bit all the way at the bottom are square, making it a square end bit. They could also be chamfered (bull end) or rounded (ball end). There are a few measurements on the bit itself that are important. There’s bit diameter (how big around the part that actually cuts is), shank diameter (how big the part that goes into the spindle is), bit length (how deep the bit can cut), and overall length (how long the whole bit is). Additionally, there’s the number of flutes (usually 1, 2, or 4) and the angle of the flutes (although we really only care if it’s up, down, or straight).
Now let’s focus on the cutting action. When the machine is running, the bit is spinning and being pushed into the material. Whenever one of the cutting edges comes into contact with the material, it cuts it away into a chip. The chip curls into the a flute of the bit, and is flung out from the cutting area to contribute to the mess around your machine.This is where the angle of the flutes comes into play. If you use a standard upcut bit, then as the bit cuts, it also pulls slightly up on the material. This is great for pulling chips out of deep, narrow cuts. However, on woods or laminates, that slight upward pull can cause a some chipping of the grain around the top edge of the cut as the grain is pulled upward instead of shaving cleanly off. A straight flute bit pulls material neither up nor down, and so behaves well on wood. Straight flute bits are especially great for plywood, as they reduce chipout on both the top and bottom surfaces. Downcut bits push material slightly downward, which is good for cutting thin laminates as it leaves the top surface very clean. However, chips can build up in the cut, affecting deeper cuts. This is somewhat true of all bits. If chips are not cleared from the cut, then when the bit comes back to the same location, it will be re-cutting those chips as well as the existing material. This shortens the life of the bit, makes cuts less accurate, and can reduce the quality of finish that your bit will leave on the material. Bit shape is one way to clear chips from a cut. Brushes, vacuums, and small blasts of air are also effective.
So with that being said, let’s look at some bits and when to use them.
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| 25295-02 |
Two-Flute Square-End Mill
This is the workhorse bit. It’ll cut just about anything but the despair around a broken heart. |
| 25295-01 |
Two-Flute Straight Cut End Mill
It’s awesome for cutting plywood because the flutes don't pull upward as they cut, which reduces chipping in the top layer.
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| 25294-02 |
Two-Flute Ball End Mill
The end is round, which allows for fine detail to be cut on 3D surfaces. It will not, however, cut flat horizontal surfaces.
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| 25295-03 |
Single-Flute Upcut End Mill
This is another upcut bit, but it only has one flute and the tip of the cutting edge resembles a hook. It cuts plastics and other really soft materials beautifully, and the hook tip helps cut through thin materials better.
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| 30423-01 |
Fishtail End Mill
This is a slight variation on the standard square end mill, in that the tips of the cutting edges extend down past the center of the bit. It’s good for punching through thin material and getting fine detail.
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| 26007-01 |
Engraving Bit
The tip is really tiny, only 0.01” in diameter! It’s tapered to make it stronger, which also has the effect of making the cutting diameter increase the deeper you cut. These bits are only for engraving very fine detail, but they’ll do it in any material.
With that selection of bits, you can mill just about anything that will fit on your CNC router. So make a choice, and go get your mill on!