Upspiral bits for a router.

[Julio Navarro]

What are they for and what do they do?

[tod evans]

shear.....either up or down cut or combo, good for veneered sheetgoods and squirrly solids...02 tod

[Allen Bookout]

Also good for cutting mortises.

[Tom Jones III]

While we are on this topic, when do you use a spiral bit and when do you use a 2 flute straight bit? I know that a spiral bit is good for plunge cuts and deep cuts but other than that I don't know when to choose one over the other.

For example, last night I was cutting stopped dados for splines. The bit was 1/4" and the dado was 3/8" deep in oak. Which bit would have been better?

[Julio Navarro]

I am interested in the mortise part of it.

[Jerry Strojny]

I broke three 1/4" straight bits when doing some stop dado splines in oak earlier this year. Once I bought the spiral, no problems.
Now, I know I am of no help to your question. I guess I'm just saying what worked for me. I never had that problem with straight bits untill that day. Still haven't figured out why either.

[Sam Chambers]

Julio, I use an "upcut" spiral bit when routing mortises, and it does a great job. The "upcut" means that it will pull the chips toward the router (and out of the mortise), rather than pushing them down into the bottom of the mortise. All of my mortises have been 3/8", so I only have one spiral bit. Got it on sale from Grizzly.

A word of caution...Those spiral bits are sharp (DAMHIKT), so be careful handling them.

[Bob Childress]

While we are on this topic, when do you use a spiral bit and when do you use a 2 flute straight bit? I know that a spiral bit is good for plunge cuts and deep cuts but other than that I don't know when to choose one over the other.

On the other hand, for making plunge cuts in ply, such as holes for adjustable shelves, the Down spiral pushes the wood down and minimizes tear out.:)

[Julio Navarro]

So are spiral router bits only for plung cuts?

[Doyle Alley]

So are spiral router bits only for plung cuts?
No, they are also useful for edge routing. I have an 8' Jointability (poor man's jointer) and it calls for a 3/4" diameter shear bit.

[Brian Gumpper]

Here is my 2 cents.

You want to use a spiral bit where the bit is cutting down on the face of the material you are routing. So, if you had a veneered piece with veneer face up and you were routing handheld from the top, you would want a downcut bit. Same holds true for the flush trim versions.

They can be used whereever a regular straight bit can but I don't think your feed rate will be as high. They are the best for plunging and mortises almost always, if not always, use an upcut bit to get the material out of the mortise.

As far as the 1/4" carbide tipped bits, they are very prone to breaking. The two main causes I find are feed rate and excessive vibration from the workpiece. Spiral bits do have a higher shear strength but are brittle so don't drop them or they will shatter.

[glenn bradley]

They also make spiral flush trim bits for the same reasons; shear is cleaner that chop, down-cut from the veneered surface minimizes tearout. Up-cuts work well for 'jointing' on the RT.

[Hank Knight]

So are spiral router bits only for plung cuts?

Julio, I use a spiral bit for just about all straight cuts. I can't think of an application where a straight bit would be preferable to a spiral bit. A good carbide spiral bit seems to stay sharp longer and cut smoother than straight bits. I use an upcut bit most of the time, especially for deep cuts like mortises. But if you are cutting dado's or rabbits and want the top edge of the cut to be crisp, use a downcut bit. As mentioned above, a downcut bit pushes the chips to the bottom of the cut and has a clean, shearing action at the top edge of the cut, so you get a nice clean edge. Upcut bits pull the chips up out of the cut so you are likely to get a little fuzz at the top edge of the cut.

[Jerry Olexa]

Great for mortises becuz they pull the shavings out of the mortise rather than accumulate in the hole.

[pat warner]

They have their place but there are surprises.
See the FAQ (http://patwarner.com/faq.html) link, no 12.

[Ian Barley]

I am not a fan of spirals. I find them much more expensive to buy and to sharpen (my service charges 6 times a smuch to sharpen a spiral as a straight). Every time they are sharpened they lose diameter. Anything smaller tha 1/2" seems to snap easily. Whenever I have to make the choice I pick a good quality straight over a spiral. The only time I choose spiral first is routing plastic.

[tod evans]

i agree with ian, my spirals get used very infrequently, 95% of the time i use straight bits or straight with shear(the carbide is set at an angle). it`s not cost effective to sharpen the spirals...02 tod