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How Many Flutes (Cutting Edges) Do I Need?

Posted on 30 July, 2020 at 17:40

How Do I Select the BEST Router Bit for a Job? – Part 2

Hopefully part 1 was a helpful start for selecting the best router bit. Our approach for selecting the right router bits is to use the information we do have to narrow the field. That way when we get to options where we don’t have the information our field of options is smaller. Then picking based on price or availability won’t box us into a bad place.


How Many Flutes (Cutting Edges) Do I Need?


Imagine for a moment that you are alone in the woods chopping a huge tree into firewood. You’ve got one big axe; how long do you think it’ll take you? I’m no Paul Bunyan, so I’m going to estimate 5 days… Now, let’s take that situation and imagine you and a buddy both have 2 medium size axes. With two of you the work should get done faster… but the smaller axe means that each axe will be a little less effective. So, instead of it taking 5 days… It’s safe to say with two medium sized axes, we’re looking at something closer to 3 days. We could keep increasing this example, but essentially, the more cutting edges you have the faster you can cut (higher feed rate).


There are a few caveats to keep in mind though. Notice that in my example, when we increased the number of axes… they got smaller. That’s because as you add more cutting edges to a router bit the smaller the flute becomes. The flute is the area behind the cutting edge where the “chip” or debris is developed. This is really important to be aware of because the chip is what removes the heat from the cut. And, as we discussed last post, heat breaks down the cutting edge.


So, how do we balance the amount of heat coming out of the cut with the size of the chip and the feed rate? Well, luckily there are formulas and ratios that provide a predetermined range for optimal edge quality and bit longevity. We’ll explore that in future posts, but some immediate resources can be found in our bit list, routing guides, and of course from calling us directly (800-644-2487). For this post we’ll focus more on selecting the right bit that can lead to optimal performance.


While making sure that you’re properly accounting for the amount of heat is the most important consideration. An often over looked consideration when determining the number of cutting edges is what your machine can handle. Machines will have a speed that can be maintained in straight lines, and a speed that can be maintained while cutting corners. Your machine may be able to run at 600 inches per minute (ipm) in straight lines, but if it doesn’t change direction well, you’ll find yourself dwelling in the cut in every corner. That makes complex shapes into much riskier jobs. But, this risk can be mitigated by choosing the appropriate number of cutting edges.


This is the straightforward part. If your machine has a speed limitation and you need to remove as much heat from the cutting edge as possible, then single edge bits are going to be your flavor. But, if you’ve been endowed with one of the super CNC’s, you can afford a few more cutting edges. Or, say you’re routing lots of small intricate shapes. In that case, you’ll generally want fewer cutting edges. This is why so many table surfaces have a bunch of cutting edges, it allows for less down time while you’re resurfacing the table.


Another consideration is edge quality. Often, you’ll find that additional edges can leave you with a cleaner edge. There are a variety of reasons for this, but it makes sense. Just like higher grit sandpaper leaves you with a smoother surface, the addition edges, taking smaller bites, will do the same. Just imagine not having to flame polish that acrylic. It sounds like blaspheme, but those are just some of the promises of choosing the best router bit for the job.


The number of cutting edges your router bit has is one of the most important considerations you’ll have to make when choosing the right router bit. But, what I want you to take away from this post is that you want to work within your machine’s limitations when making this decision. And, the big question is, “will my machine be able to move fast enough to remove enough heat from the cut?” I can promise you now, if you’re making dust instead of chips, you need to try again.




I can’t end this post without at least some specific information about chiploads. The chipload is the size of the debris you are removing from the cut. This is what removes the heat from the cut, increasing the tool’s life and quality of the material’s edge. The magic formula is:


Feed rate=Chipload × RPM × # of flutes


In today’s post, we discussed the # of flutes, and we’ll explore the other variables in future posts. But, you can start seeing the relationship between the # of flutes and the feed rate. I apologize for the math lesson. But, holding chipload and RPM constant, increasing the # of flutes increases the feed rate. But you can also see that while holding the # of flutes and RPM constant the chipload will drop if the feed rate drops. This is why it is important to know your machine’s limitations when choosing the right bit for the job!


This is another good time for me to make a plug for the router journal. Once you find a feed rate that gives you a good finish take note of your settings. That way, when you, or your cousin Larry, need to cut that material again you won’t have to recreate the wheel. This may sound trivial, but it makes a big difference when your training the new guy.


Top tip – Looping the corner


Dwelling in the cut is never a good idea. The small chips you already have keep getting smaller because the tool isn’t moving laterally and it just keeps cutting what’s already been cut. This builds up a considerable amount how heat. A few seconds of this can be enough to break even the most durable bits. So, instead of pausing for the machine to completely change directions, have the tool continue to move in a small loop, so that the tool is continuing to move while the router changes direction. Just make sure that you loop into the refuse, not the finished product! ;)


Top tip – Bit running hot?


Chipload and formulas are nice… but is anyone really going to pull out a micrometer and measure enough chips to get an average size? What if the edge looks fine? Or, the bit seems to last long enough? What even is long enough? A quick way to know if you aren’t pulling enough heat out of the cut is the temperature of the bit after you’re done cutting. If the tool is so hot that touching it would burn you, then you are not using a high enough feed rate. Make bigger chips and get rid of that heat!


Thanks for reading this post. In Part 3, we’ll continue with router bit fundamentals while we take a look at the geometry of the tool.


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