up/down spiral router bits

[Zach England]

When looking at spiral router bits, what are the advantages of up and down cut bits? It seems apparent to me that an up bit will evacuate waste more efficiently. Is this true? Are there other reasons to use one over another?

[Thomas S Stockton]

Zach
An up cut bit does get chips out of the hole better and is what you want to use in cutting mortises. The down cut is used more for shallow inlay work as it leaves a cleaner surface although a sharp up cut bit does a fine job. So I mostly have up cut bits because they are more versatile. There are also what are called compression bits which have up cut on the end of the bit and down cut on the other part, these are usually used on CNC routers for cutting out plywood parts were you want two clean faces.
I prefer solid carbide because it last longer but high speed steel does the job also.

[Frank Drew]

I like upcut for the reasons Thomas notes; you might still find, though, that an air gun to blow out chips can come in handy.

[Bill Huber]

Now an up cut bit on the router table is a down cut bit and a down cut bit in the router table is an up cut bit.

Man that always screws me up......

[glenn bradley]

+1 on what others have said; I use upcuts for mortises as they do a better job of clearing the chips. Loss of a clean edge (if any) is generally covered by the tenon shoulders. Downcut generally leaves a cleaner edge, on veneered sheet goods in particular.

[george wilson]

Are you referring to router bits that have BOTH up and down cut,meeting in the middle of the blade? They are for going around the edges of wood,so the wood doesn't get a burr on either top or bottom sides.

Do not cut too rapidly with small dia. downcut bits. They are great for inlay,but the bottom end of the bit of necessity doesn't cut too efficiently,and the bit can snap off.

[Vince Shriver]

Now an up cut bit on the router table is a down cut bit and a down cut bit in the router table is an up cut bit.

Man that always screws me up......

Well, techincally, yeah, the bits point to the floor one way and to the ceiling the other way; but whether cutting a mortise on the router table or hand held, you would want to use the same bit for either orientation. It gets down to whether the bit pulls the chips in or pushes the chips away. A down spiral bit will not pull the chips out of the slot no matter whether the bit is on top or on the bottom of the workpiece - but an up cut bit wil. (At least that's how it seems to me.)

[Matthew Hills]

The down-cut bit is also helpful if you're running across the grain of a plywood's veneer.

Matt

[Cory Hoehn]

A down-cut bit is great when you're cutting MCP or another veneered surface because it forces the veneer down onto the piece, rather that forcing it up and away. You'll get less tear out and chip out that way.

[Prashun Patel]

Downcut - minimizes tearout
Upcut - minimizes chip buildup

Quality spiral bit can compensate for the shear direction, which is why most people opt for a high quality upcut.

[Cliff Rohrabacher]

seems to me I saw one that was both up and down cut just for CNC apps.

[Joe Scharle]

Compression bits cut down at the tip and up the rest of the shank. I use one for deep mortises. Not too good for edge cutting i.e. tenons.

[Anthony Whitesell]

Just remember you reverse what you do by hand when you use it stationary.

Circular saw=face down
TS=face up

Router=right to left
Router table(shaper)=left to right

hand held RH spiral=up cut
router table=down cut

P.S. I probably have some of them backwards as I'm not in front of the tools at the moment.

[Steve knight]

I don't know on a handheld router but I have cut over 2" deep with a 1/2" downcut bit. I mostly use downcuts on my cnc machine. they seem to cut hardwoods better and don't usually cause the blowout on crossgrain cuts. 1/8" really pack the shavings in ( I use that to my advantage) but 1/4" and up tend to not cause too much clogging og shavings. but this may be because I don't ahve any kind of plate blocking the shavings.