# Downcut spiral bit in a router table



## cagenuts (May 8, 2010)

Yeah it's wrong, well at least that's what I've always thought. However Shopnotes #113 advises such an event on page 43.

Granted he's only trimming veneer but surely the bit will try and push the stock up off the table?


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## kp91 (Sep 10, 2004)

Hilton,

It can be used in a router table, especially in situations where the wood tends to fuzz or tear out easy. Remember that it's harder for a downcut to eject chips than an upcut. Use featherboards or push blocks to keep a downward force on the stock to ensure an even depth, but never directly over the path of the bit


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## Quillman (Aug 16, 2010)

Some notes on spiral ground router bits:
The spiral cutter (solid carbide) has its place and since it does slice continuously (not an unbroken cut like 2 straight flutes) it can produce an impeccable finish; it may still tearout, however. They do bore well too and plunge better than any flute design. They are harder than brazed-on carbide tools, last a lot longer, cost more, and are difficult to sharpen. They have associated vectors (force with direction) that make them unsafe except in production situations where the work is well fixtured and feed rates constant.

The down spiral will produce a top clean corner (in a mortice e.g.) and fire the chip into your socks on outside cuts. Its force vector wants to pick up the router and has scared this furniture/maker silly, but only once.

The up spiral will fray x-grain , perform well down grain, and pull its chip from the excavation. The force vector in the up spiral wants to pick up the stock but at the same time pulls the cutter from the collet, the most likely of the 2 cutters to change depth of cut. Expect analogous and perhaps unexpected performance in the router table. Keep east/west cutting depths to a minimum for safety.

Few diameters are available in solid carbide. All tools (for 1/2 shank routers) have been ground into their shanks and none have cutting diameters >.500". Skinny and long cutters whip and deflect whether solid carbide or steel. Expect some chatter as the cutting depths increase. 2+hp routers flywheel the cutter better than small routers and will help smooth out the ride and reduce chatter. Larger straight (>5/8 CD) brazed on carbide cutters are preferred by this furniture maker, especially for safety and outside cuts. There is a lot more to the story, but in my view, their risk and expense is not worth their performance benefits, except in a production environment.


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## bobj3 (Jan 17, 2006)

HI

The best bit to use is one that can do both jobs in both router types.

Compression Up/Down Spiral Router Bit
MLCS solid carbide router bits

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cagenuts said:


> Yeah it's wrong, well at least that's what I've always thought. However Shopnotes #113 advises such an event on page 43.
> 
> Granted he's only trimming veneer but surely the bit will try and push the stock up off the table?


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## RJM (Apr 11, 2010)

Quillman said:


> Some notes on spiral ground router bits:
> .... They have associated vectors (force with direction) that make them unsafe except in production situations where the work is well fixtured and feed rates constant.
> 
> but in my view, their risk and expense is not worth their performance benefits, except in a production environment.


 
I completely disagree. The forces created by the spiral are very small. I notice no difference in the forces induced by upcut and downcut spiral bits when using a handheld router. 

When used in a table, an upcut spiral tends to to pull the workpiece toward the table. 

Some (better) brazed bits also have a shear angle which will also induce a small upward force on a handheld router.

Solid carbide bits are much more expensive, often 2 to 3 times more than a similar size brazed bit, but as an absolute value, this is only $10 to $25 more and is often worth the the added expense when considering the cost of the wood being cut and the desired outcome for the project. Also, when amortized over several projects, the cost is nil.


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## cagenuts (May 8, 2010)

Ah so I can use my downcut spiral bit with bearing in the table, I just need to be careful.


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