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CNC Machining: Flutes and Surface Finish

I am going to attempt something new here so please bare with me! With the launch of my new website I want to take this blogging thing a bit more seriously and post some (hopefully) interesting articles. Furthermore, I am going to commit more of my time to improving the quality and frequency of the videos I upload to YouTube. I enjoy working on this website along with my YouTube channel so it is about time that I start putting in more effort to step up the standard of my content to better engage, thought-provoke and inspire you, the reader.

Anyway, on with my first article:


The topic of selecting tools is a very complex subject within the world of CNC as selecting the correct cutter for the right job is paramount to the ‘success’ of the job in terms of finish and life of the tool.

I recently read an article outlining some of the basics of end mills on where they go through the different types of ‘bits’ their properties and what they are good for. I’m not really going to go through that in too much detail and I therefore recommend reading that article.

In general it is important to select a cutter based on the material you are machining depending on its hardness. Metals require end mills that are typically solid carbide or High Speed Steel (sometimes with a coating) with twisted flutes designed to cut efficiently, clear chips and leave a clean cut. Router bits (for hand operated routers and some CNC router work) are typically made of steel with carbide tips on the cutters aligned parallel to the z axis and are known as straight flute cutters (or some variation of this term), these are used for wood and softer materials but endmills can be used to great result along with dedicated spiral routing bits especially on CNC routers.

But anyway the article I linked above talks through this in greater detail. What I do want to talk about is something I get asked about a lot which is what effect different amount of flutes have and why surface finish doesn’t necessarily improve with more flutes? (this second question is mainly associated with people experimenting machining aluminium on their CNC routers)

All rotational cutters used in CNC applications work best when they take a specific sized chip out of the material they are cutting. This is aptly known as the tools ‘Chip Load’ and it varies from tool to tool depending on diameter, number of flutes, up-cut or down-cut etc… This is mainly important when working with metals as it is a less forgiving group of materials but it is also important to some extent with wood. I use wood a lot on my CNC router and I tend to use as few flutes as I can. Because CNC routers tend to have much higher spindle rpms than milling machines, using endmills with many flutes would require much higher feed rates to take the same sized chip out of the material. This is not always possible as the rigidity of common CNC routers is much lower than mills sot here is a possibility of large deflection under the strain and they may not even be able to achieve such feed rates with any degree of accuracy.

Therefore the trick is to substitute higher feedrate with fewer flutes which reduces the effective feed rate of the cutter and will get you closer to the chip load of the cutter (fewer flute cutters tend to have higher chip loads). In softer materials like wood and aluminium, fewer flutes will also help with chip clearing (removing the chips from the cutting area) increasing surface finish as you are not re-cutting chips. This will also help increase the life of the tool as you are not wearing it out re-cutting chips. Especially when it comes to using small cutters in wood, it is a bad idea to use more than 3 flutes as the flutes simply aren’t big enough to cope with the wood chips. They become clogged quickly and will rub and not cut efficiently. In fact they are likely to break.

This is especially true with very small end mills a the flutes are very small and can become clogged very easily but you can get away with say 3 flutes at 6mm diameter because the flutes are much bigger and so can handle the chip clearing well. A can of compressed air to blow chips out of the cut would also make a huge difference when machining aluminium and harder (than wood) materials on a desktop sized CNC router!

Surface finish is another thing that can baffle people getting started with aluminium on their CNCs. When we increase speed relative to chipload, we improve surface finish (at least until it starts rubbing).  So using more flutes increases the relative spindle speed of the cut. For example, using a two flute cutter at 24,000rm is cutting the material the same amount of times per second a s a 4 flute at 12,000.  So, switching from a two flute to a four flute is like doubling your spindle speed. This works great on low spindle speed milling machines but with routers that typically have a minimum usable spindle speed of 12,000 rpm you are going to get rubbing of the tool because the chip is so thin when you are using a multi flute cutter. The cutter is only taking a tiny slice of the material and so can actually skate off the surface as it can not bite into the material and take an effective chip. This causes deflection and can lead to the cutter taking sudden larger cuts if it skates on the previous flute. This is obviously not good for the cutter, the machine of the finish. Therefore using fewer flutes on a router where the spindle rpm is very high can produce excellent surface finish and maintain a larger but manageable chipload.

This is all I really have to say on this, I may post an updated version in the future! If you have any comments please do leave write them below and I have said anything blindingly wrong please do not hesitate to point it out!

One Comment

  1. jahid jahid

    Great article! At a past manager we had numerous Single Lip Cutter Processors and made numerous ball and shot nose cutters for Mold work. The inflexibility supplied by the minor breadth regularly out played out any purchased cutter. As to warmth, the single woodwind with additional rake and freedom however without a slim woodwind likewise dispersed warmth well. The completions were magnificent!

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