# Shank Length vs. Cutting Length (or carbide height) in Straight Bits



## xzJoel (May 11, 2012)

Hi,

I am considering buying some router bits, but I have reached a mental stumbling block. Initially I had thought that the useful depth of cut/maximum usable length of bit was related to the cutting length/carbide height plus a bit of the shank. (I'm new to this and was confused.) After reading some of the posts here, I am now under the impression that the only portion of the bit that has to be in the collet is approximately 5/8 inch for a 1/2" shaft bit. That is to say if I have the following bit

Carbide length 1/2 inch.
Total length 2 1/4 inch.
Maximum distance to cut = 2 1/4 - 5/8 = 1 5/8 assuming that you could press the collet against your work piece.

Given that you should cut in small shaves (around 1/4" at a time or less), what does the additional carbide length do for you?

Take Freud's bits - 
Part # Diamater : Carbide Length : Shaft : Overall Length
12-116 : 1/2 " :::: 1 " :::: 1/2 " :::: 2-5/8 "

12-130 : 1/2" :::: 2-1/2 " :::: 1/2 " :::: 4-3/8 "

The 12-116, by my math, has a useful cutting length of 2" and the 12-130 has a useful length of 3 - 3/4". If my stock is 2" or less in the relevant direction (or 4" or less if I can route from both sides), why would I ever use the 12-130?

Put differently, if I am not planning on using stocks (or combination of stocks) greater than 2", why would I buy a bit with a shaft length greater than 2 5/8"? I don't want to invest in several bits of the same carbide shape if I don't have to, and I'd rather not buy the shortest lengths possible of I am making an obvious mistake in judgment.


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## Mike (Nov 22, 2004)

Joel, removing 1/4" of material at a time is not just plunge or dado cutting depth, it could be on an edge. IE.. you are using a 3/4" diameter bit in a 1" diameter guide bushing with a template you would want to make a single pass cutting the entire width of the board edge. You could safely remove 1/4" of material from the board edge. Many projects are built with 3/4" thick material so a 1" cutting length is fine. You want the carbide length to be longer than the material for a clean cut. So it seems like you should just buy the bit with the longest carbide cutting length you can find for all projects, right? The opposite is true. Carbide is brittle and the longer the cutting length the more flex in the bit. This is why you will often see long cutting edge bits where part of the carbide has snapped off. You are best off to use the shortest cutting length bit that will get the job done. The longer the steel the more flex in the bit. What we end up with is trade off's to be able to make the cuts we need. Needless to say there is a lot more flex in a 1/4" shank bit than there is in a 1/2" shank bit, more vibration too. This is why there are so many different sizes of bits and none are the best choice for all jobs.


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## paduke (Mar 28, 2010)

Mike
Very clear explanation ty


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## Drew Eckhardt (Aug 2, 2008)

xzJoel said:


> After reading some of the posts here, I am now under the impression that the only portion of the bit that has to be in the collet is approximately 5/8 inch for a 1/2" shaft bit. That is to say if I have the following bit
> 
> Maximum distance to cut = 2 1/4 - 5/8 = 1 5/8 assuming that you could press the collet against your work piece.


With a template you loose 1/4" to the template, 1/4" to the router sub-base and another 1/4" to the lock nut. A Porter Cable collet has about 1/4" to the collet face which could fit inside that lock nut and another 1" you'd have for full engagement with the bit which seems prudent since the collet grips tightest where its cones are being compressed by the nut at the top and female cone in the router shaft.

That's 1 3/4" leaving 1 1/8" of reach.



> Given that you should cut in small shaves (around 1/4" at a time or less), what does the additional carbide length do for you?


On a deeper cut taken 1/4" at a time if the round shaft spinning at 24,000 RPM touches the wood it'll probably burn it while carbide will cut cleanly. 

Fixtures aren't perfectly square, hand-held routers aren't perfectly stable, and there's appreciable flex in some plunge bases so that's not unlikely.



> 12-130 : 1/2" :::: 2-1/2 " :::: 1/2 " :::: 4-3/8 "
> 
> The 12-116, by my math, has a useful cutting length of 2" and the 12-130 has a useful length of 3 - 3/4". If my stock is 2" or less in the relevant direction (or 4" or less if I can route from both sides), why would I ever use the 12-130?


Perhaps to cut mortises in a table leg.


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## bobj3 (Jan 17, 2006)

Hi Drew

Say what ? ,,,,,you lost me on that one..


==


Drew Eckhardt said:


> With a template you loose 1/4" to the template, 1/4" to the router sub-base and another 1/4" to the lock nut. A Porter Cable collet has about 1/4" to the collet face which could fit inside that lock nut and another 1" you'd have for full engagement with the bit which seems prudent since the collect grips tightest where its cones are being compressed by the nut at the top and female cone in the router shaft.
> 
> That's 1 3/4" leaving 1 1/8" of reach.
> 
> ...


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