Upcut or downcut spiral bit for dados?

[Louis Brandt]

Hello,

Ok, based on what I’ve read from most of you, you’ve convinced me that for cutting a clean dado, a solid carbide spiral bit is better than a straight bit. But now, I’m asking which is best for a clean, flat dado, an upcut or a downcut spiral bit?

My objective is to build a bookcase. I’ll be using solid 1x lumber, either oak or poplar. I’ll only be cutting two dados. They will be 3/8 inch wide dados near the top and bottom of the sides. I’ll have rabbets in the top and bottom pieces that will slip into those dados to join the top and bottom of the case to the sides. The shelves themselves will be supported with pins.

If I understand what some of you have said, an upcut spiral bit will clean out the chips better, but I think that you’ve also said that a downcut spiral will produce a flatter dado.

Since I’m only dealing with a narrow dado (3/8 inch), wouldn’t the downcut be the better choice, since with a narrow dado, there shouldn’t be much of a chip problem?

If it sounds like I’ve already made up my mind, please forgive me. I don’t mean it to sound that way.

Please give me your opinions.

Thanks,
Louis

[Joe Scharle]

What the downcut action does for you, is to 'shear' cut the surface downward, leaving a smooth edge but a dado full of dust. Small price to pay. Whereas, the upcut action tends to lift the surface fibers and may give you a little or a lot of tearout.

[Todd Bin]

The objective is a clean dado. Use a downcut bit. No tearout on the edges and a flatt bottom.

Also for most bookshelf applications the dado is as wide as it is deep. That means chip clearout is not as big of an issue.

[Peter Quinn]

For dados I'd vote neither. A two or three flute carbide tipped plunge bit is more than sufficient. Typical dado is maybe 1/4" or 3/8" deep for case work? No reason to blow your hard earned cash where it isn't needed. I have milled thousands of feet of dados with an over arm pin router, but never a spiral carbide. The bits we use are down shear, but not solid carbide. I know we have some three flute down shear carbide tipped bits, but I can't seem to remember who makes those. Whiteside maybe?

I like spirals for mortises, or holes that are going deeper than they are wide. They are also great for flush trim applications in tough grain. For wide shallow cuts over long distances they are actually harder to control IMO.

[Louis Brandt]

Peter,
You said, "For wide shallow cuts over long distances they are actually harder to control IMO.", but I'm talking about only a 3/8 inch wide dado (not very wide), and the width of my sides (length of dado) will be only 7 1/4 inches (not very long).
Louis

[Peter Quinn]

Peter,
You said, "For wide shallow cuts over long distances they are actually harder to control IMO.", but I'm talking about only a 3/8 inch wide dado (not very wide), and the width of my sides (length of dado) will be only 7 1/4 inches (not very long).
Louis

Sorry, it seems i didn't read your post carefully the first time. As I understand it with spiral carbide down shear produces a cleaner surface but not as good a bottom. Up shear produces a better bottom but more ragged top edges but better chip extraction. So down shear for dadoes, up shear for mortises.

They also make bits with both up and down shear for the best of both worlds. For two 3/8" dados that length I would still opt for a good two flute carbide tipped. I think Infinity has them for $8? I'd make the 3/8" tongue centered on the top and bottom members, and make it a stopped dado, so everything is blind, the case looks clean, and it doesn't make a hill of beans if you get tear out any way as it will be hidden. Otherwise I find Onsrud makes the best solid carbide spirals I have used. Might be a nice bit to have for mortising with a plunge router so its never a waste of money in any event assuming you get a length that works for mortising.

[John Lucas]

While I do use spiral bits alot, I think I would use a 3/8" wide mortising bit. I cuts clean sides AND a clean bottom.

[Louis Brandt]

John,
Help a newbie to understand what you meant. When you say "mortising bit", do you mean just a two or three fluted straight bit?
Louis

[Bruce Wrenn]

I prefer reverse helix, screw on cutters from Her-Saf. These were originally designed for CNC machines. I have cut THOUSANDS of feet of 3/4" X 1/8" deep dados, while building cubbies for local YMCA's. Still using my first cutter.