Tapered drill bits - why do they work?

Kettletrigger · Post by **Kettletrigger** » Tue Jun 30, 2009 6:42 pm

Hi everyone, I've been lurking for awhile and finally joined up. I've been plotting and planning some pipes and am excited to get started! I'm a weekend machinist, which brings me to my question:

Why do tapered drill bits cut? Everything that I've ever read, been taught, and experienced with drilling holes is that only the leading edges of a drill bit cut and that the spiral flutes exist solely for chip extraction. I've sharpened an awful lot of drill bits, and I've never sharpened the flanks–only the leading edges. Could it be that pipe stem materials are so soft that the drill bit just forces its way through and hogs the material out?

Based on my experience working with metal, I would think that a tapered reamer would be the way to go, but I haven't seen any discussion about that method.

Obviously, lots of people are using tapered bits with success and tool suppliers sell them, so I must be missing something. I'm hoping that one of the more experienced machinists here can shed some light on this for me.

Thanks!

Jim
Dana Point, CA

Frank · Post by **Frank** » Tue Jun 30, 2009 7:48 pm

They don't taper all the way to a sharp point. There is a small cutting edge/face approx. 2mm wide, standard drill point angle (118 deg.?).

It's the same as a tapered wood screw drill bit - just has a longer taper and longer shank.

Kettletrigger · Post by **Kettletrigger** » Tue Jun 30, 2009 11:02 pm

Ah, maybe I wasn't clear in my question. I realize that they do have small cutting edges at the tip, but as the drill is forced into the hole the sides of the drill bit are removing material as the taper gets wider. That's the part that I don't understand.

FredS · Post by **FredS** » Tue Jun 30, 2009 11:27 pm

The leading edges of the tapered flutes do indeed cut, not just the tips. They work just like a tapered reamer only the flutes are twisted instead of straight. That means that there is a lot of cutting surface in contact with the workpiece which means more heat (a lot more heat) is generated. It also requires more feed pressure and a slower feedrate (not neccesarily lower RPM though). Briar & ebonite are relatively soft, so it works OK.

Sasquatch · Post by **Sasquatch** » Tue Jun 30, 2009 11:30 pm

They have cutting edges all along the shaft too (or good ones do). Are they incredibly good cutting edges? Nah.

Take a regular drill bit, say 1/4", and drill a hole about 3/4 of an inch deep. Then just angle the drill over about 45 degrees. The bit will cut on the sides, just not super duper well.

Kettletrigger · Post by **Kettletrigger** » Wed Jul 01, 2009 12:31 am

Thanks for responding to my question guys...I thought that the sides of tapered bits might be different in some way from normal drill bits, but it sounds like they're the same (aside from the fact that one is tapered and one isn't).

What I'm taking from this is that they cut because the material is soft enough to allow them to. I don't think the 1/4" tilting drill bit trick would work too well if I tried it in a piece of steel.

Sasquatch · Post by **Sasquatch** » Wed Jul 01, 2009 9:06 pm

Just use lots of WD40 and the steel will cut like butter. Kinda.