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Router Bit Geometry

Posted on 30 July, 2020 at 17:45

How Do I Select the BEST Router Bit for a Job? – Part 3


Let’s recap what we’ve talked about so far:

  • CNC Machines generally use Solid Carbide bits
  • Hand routing generally requires High Speed Steel
  • Carbide tipped bits are a good way to save money, especially in larger diameters
  • Heat destroys bits
  • The more cutting edges you have the faster you can go
  • You need a chip large enough to remove the heat, but small enough to leave a clean edge

If you’re worried that a long list is only going to get longer, the good news is that above won’t change too much from one job to the next. If you have a CNC machine pretty much all of your jobs will be with Solid Carbide bits. And while some will cut a large variety of materials, all with different chip loads. Once you figure out what works in aluminum you can pretty much do the same for all other aluminum jobs. From here out we’re looking more at job specific parameters.


Router Bit Geometry


There are a lot of nuances to router bit geometry. A bit that cuts aluminum will largely look just like one for plastic. But, those specific angles and ridges make all the difference. Luckily, the engineers who design and test these things need to worry about those aspects of geometry. Suffice to say, use aluminum bits for aluminum and wood routes for wood. Any bit technically can be a general-purpose bit. But some tools are designed to perform better in one, or many materials. These are some of the aspects that you’ll just have to explore on your own and see what you like.


However, there is a very important piece of geometry that is a consideration when selecting the right router bit. Up-spiral, down-spiral, straight-edge, compression, mortise, burr, etc… How are you supposed to know what type of bit to use? Let’s begin:




I am always going to recommend to begin your router bit search with an up-spiral router bit. The simple reason is that it pulls the chips up and out of the cut. Which, removes the heat and leaves a cleaner edge with longer tool life. But, that same motion that pulls the chips up and out also pulls on the material. So, if you have poor material mounting, or a laminated material, this type of router bit may pull the material up, or even pull the material apart. Up-spiral bits also have a tendency to blow out and fray the top edge of some natural woods. For these reasons we have a vast array of router bit geometries to choose from.




While a down-spiral does solve for many of the complications that an up-spiral presents, it brings it’s own baggage. With down-spirals the chips are no longer being pulled up and out of the cut. In fact, they can push the chips back down into the cut, resulting in smaller and smaller chips. There are some ways to mitigate these perils, like reducing the depth of cut to ½ the bit’s diameter. But, these headaches often outweigh the benefits. I only recommend going down this route as a last resort if nothing else is working. Using a down-spiral requires a significant amount of babysitting to make sure that the chips are clearing the cut.




If a tradition spiral bit isn’t an option I’ll first turn to a straight edge bit. While these don’t pull the chips up and out, at least they don’t push anything back down into the cut. It solves the majority of the up-spiral issues without bringing in the down-spiral deficiencies. And, as you keep your journal diligently, you may find that for certain materials a straight edge provides a superior finish compared to spiral bits. Low-Helix bits are a blend of traditional spiral and straight edge, these perform exceptionally well in hard plastics like acrylic.




New materials are being invented all the time, and many of them are some form of laminate. These can be tricky little monsters. If spirals and straights just aren’t cutting it for you, a compression is the next step. Though, these can be quite pricey. These tools are beautiful catalogue centerfolds as the complex geometry mixes an up-spiral bit with a down-spiral. A traditional compression bit will have the cutting edge split 50/50 between up-spiral at the tip and down-spiral at the shank. A mortise compression bit adjusts that ratio to have a shorter up-spiral. This is because many materials won’t allow for an initial depth of cut to submerge the down-spiral portion into the material, and if all you’re using is the up-spiral edge you may as well use a traditional up-spiral.




While most router bits finesse their way through the material like a savage ninja, burr bits almost grind their way through the material. These router bits look like a miniature weapon a bad guy would use in Game of Thrones. They are covered in many very sharp little burrs. These have primarily been developed for use in aeronautics materials. Some of the honeycombs and composites are exceptionally difficult to get clean edges. Feed rates and chips loads don’t completely go out the window. But, if you find yourself having trouble routing these materials, Hartlauer Bits has a variety of resources we can connect you with.




What is the difference between an end-mill and a router bit? Well, in smaller diameters, not much. But, once you start getting into ¼” and above the geometry starts to make a difference. As I noted above, router bits are designed to slice like a ninja, removing chips with finesse. An end-mill will have a different helix geometry, how tight the spiral is, in addition to flute geometry. It isn’t so much finessing as it is muscling the material away. Like I said, in small diameters the two will perform in much the same way. But, in larger diameters they will act differently. I would generally reserve a large end-mill for milling large blocks of material. If there is a significant amount of lateral movement you’ll be better served by a router bit


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