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Length and Diameter

How Do I Select the BEST Router Bit for a Job? – Part 4


Hopefully this series on router bit selection has been helpful. Like I have said in previous posts pretty much all router bits can be used as general purpose. But, when we start to understand the nuanced differences between them, it enables us to make better selections for improved edge quality, speed or even bit longevity.


Choosing the length and diameter may seem like the easiest part of choosing a bit. But, there are some guidelines that you’ll want to make sure you follow. Router bits can be very brittle, so we want to make sure that as we route we are supporting them and working with them, rather than forcing them to work against their design.


Diameter and Spindle Speed (RPM)


Smaller diameter bits perform better at higher RPMs. Each router bit has a specific rack angle, the angle of the cutting edge itself, that is designed to cut into the material at a certain speed. If you can imagine an old vinyl record for a moment. The further you get from the center the faster the record is moving. So, if we want to maintain the same cutting-edge speed, we need increase that speed for smaller diameters and decrease it for larger diameter router bits.



1/16” dia 24,000 - 30,000 rpms

1/8” dia 21,000 - 24,000 rpms

3/16” dia 19,000 - 21,000 rpms

1/4” dia 16,000 - 18,000 rpms



This brings us back to that equation for feedrate (feedrate = chipload x RPM x # of flutes). If we increase our RPM for a smaller diameter router bit, we’ll also need to increase the feedrate to maintain the chipload. If you’re unable to maintain a feedrate that supports the required RPM you won’t be making big enough chips and the bit will burn out.


Top Tip


When working with small radii you can rough the cut with a larger diameter bit and then go back and run your finishing pass with the smaller diameter router bit. Manufacturers have specialized hoggers and roughers designed specifically for this task. But, it can be a great problem solving technique as well when you are routing tricky materials and need tight corners.


Router Bit Length


It is very straightforward to say that you want cutting edge length to be longer than your material is thick. And, 99 times out of 100 you won’t have any issues with the cutting-edge length. But, I’d like to cover some of the basics that may just save you a headache or two.


Solid Carbide is a great material for a cutting edge. It’s strong and holds a good edge. But, the same chemistry that makes it strong also makes it vulnerable, you’ll generally get about half a bounce out of a router bit. Router bits are weakest when pushed perpendicularly on their cutting edge. The longer the cutting edge, the more leverage is being applied to the fulcrum. There are two things we can do to balance this out. First, use the shortest cutting-edge length as you can. Second, use the material to support the cutting-edge length and minimize the exposed portion of the cutting-edge length.


If you are blessed with a tool changer neither of the above recommendations will be difficult for you to adhere to. But, for the rest of we now get to play a fun balancing game of length and support. While it seems like the solution to all cutting-edge length problems is to cut everything in one pass, I would only recommend doing that with foam (and that is what I recommend). The general rule for depth of cut is the diameter of the bit itself. Why? Because we want to make sure the chips we’re creating have a place to go, especially if you’re not using an up-spiral router bit.


The solution is to balance all of the recommendations as best you can (and record the results in your trusty little journal). It’s important to always remember that routing isn’t black and white, and that sometimes, even when you’re following every recommendation perfectly, things still may go haywire.


Specialty Router Bits


Another very important thing to remember is that when you’re routing, you’re not alone. It’s very common that some other poor soul has been tasked with the same conundrum you’re in. This is a great time to call Hartlauer Bits. If we can’t help you find a solution to what you’re facing we have relationships with the manufacturers and can work through solutions with them as well.


It’s through these conversations that specialty bits were born. Relevant to our router bit length discussion are extended reach router bits. These bits have a generally short cutting-edge length, then where the flute fades out the shank has a spinback applied. This means that you can bury the entire cutting-edge length into the material and not suffer the consequences of the shank rubbing on the material. This helps you balance a few more of the recommendations a little bit easier.

Top Tip


When you’re looking at the ratio between router bit diameter and cutting-edge length look to keep the length less than 3 times the diameter. Most router bits follow this as the maximum cutting-edge length available. The exception to this rule is high density foam. Hartlauer Bits does carry a variety of extra-long small diameter foam cutters. Be sure to only use these in foam, and make sure to always cut in one pass. You’ll support the router bit better, and will have a better finish.


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