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Discussion Starter · #1 ·
I was wondering about how various angles on a router bit would effect a router bit. Basically it has a cutting edge that has to peel away wood in a fashion similar to how a hand plane works. In using a hand plane, you might skew it as you plane along the length to achieve a smooth result, but by comparison, you can't do that with a router bit. Using the same comparison, you can hand plane end grain of a board by skewing a hand plane to get less tearout finish. You can't change the way a router bit paddles it's way across end grain. I was wondering about the shear angles designed into a router bit, and would like to know more about the engineering in their design.

I know that hand molding planes evolved from holding the plane square to the wood edge … into holding the plane at a specific angle along the edge as it's used to reduce scraping, and continue slicing as it creates a profile. I wondered how this relates to router bits.
 

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There are router router bits with skew angles. I'm not sure which one started that. It may not have been possible until CNC manufacturing took over the process.

The principle is fairly simple. Instead of having the cutting edge attack the wood all at one time, as in when standard planer or jointer blades make contact, the cutter only makes contact with a small portion of the wood at any moment. This both requires less force on the part of the cutter and leaves wood next to the cut intact which reduces the risk of having a large chunk break away due to a weakness in the grain. There may also be some gain by coming at the grain from a slight angle, just as hand planing is easier going cross ways than longitudinally.

I see guys occasionally trying to sell router bit sharpeners that just grind the edges as the bit spins in the router. This leaves you with no relief angle. No relief angle means you are scraping wood off instead of cutting it off. The relief angle is fairly important and it's around 25 to 30 degrees as a rule.
 

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Theo
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I make end grain cuts very slowly, sometimes very, very, slowly. Seldom, if ever, have issues doing it this way. An option is to sand the end grain.
 

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Discussion Starter · #5 ·
There are router router bits with skew angles. I'm not sure which one started that. It may not have been possible until CNC manufacturing took over the process.

The principle is fairly simple. Instead of having the cutting edge attack the wood all at one time, as in when standard planer or jointer blades make contact, the cutter only makes contact with a small portion of the wood at any moment. This both requires less force on the part of the cutter and leaves wood next to the cut intact which reduces the risk of having a large chunk break away due to a weakness in the grain. There may also be some gain by coming at the grain from a slight angle, just as hand planing is easier going cross ways than longitudinally.

I see guys occasionally trying to sell router bit sharpeners that just grind the edges as the bit spins in the router. This leaves you with no relief angle. No relief angle means you are scraping wood off instead of cutting it off. The relief angle is fairly important and it's around 25 to 30 degrees as a rule.
I agree with what you are saying. I was thinking more in the line of taking a large spiral type bit with a profile in it. By doing that might create the shear angle that would work better for end grain possibly.... Or using carbide attached at a severe angle?
 

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Discussion Starter · #6 ·
There are many examples of cutting devices that have been adapted over the years to the Archimedes principals. For instance if you put a twist in a flat paddle drill bit...it would work closer to an auger bit or a twist drill bit. I was just thinking about the design principles involved. Wood that is supported on a table saw with a zero insert leaves little tear out, so it seems to me that using a severe shear angle on a router bit allows the yet to be cut wood for added support as the bit shears at an angle.
 

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I would agree Gary. There may be limits to what is practical when doing that to brazed on carbide.
 
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