Combination Drills (center dills)

[Marty Feldman]

It is useful to distinguish between "combination drills (center drills)" and center drills, at least for us fussy guys.

A center drill, as I would personally prefer to use the term, is used for locating the axial starter point for subsequent drilling operations. It's the use which most of us have in mind when a hole position is to be established. The so-called combination drill -- the one with the tiny pointy drilling tip on the end of the conical centering portion -- although commonly seen in use as a center drill, was designed to establish the socket of a tailstock center, for holding work between centers. If you subscribe to that usage, it is not the right tool for the job when starting in on drilling a through or blind hole.

-Marty-

[John Hasler]

> A center drill, as I would personally prefer to use the term, is used for locating the axial starter point for subsequent drilling operations.

I had always understood that that is what spotting drills are for.

> The so-called combination drill -- the one with the tiny pointy drilling tip on the end of the conical centering portion
> -- although commonly seen in use as a center drill, was designed to establish the socket of a tailstock center, for holding work between centers.


And that, to me, is a center drill: a drill for making sockets for centers (assuming that the conical section is 60 degrees). I use center drills for spotting anyway, though: I don't have any spotting drills.

[GlennW]

Known as "Spotting Drills" and "Center Drills" to me.

I use "Spotting Drills" if it is to be followed by a twist drill, and a "Center Drill" if it will be followed by a live or dead center.

Center Drill on top, Spotting Drill below it.

Spotting Drill.jpeg (24.18 KiB) Viewed 2772 times

[Harold_V]

The discussion of center drills and spotting drills is an old one, likely never to be fully resolved.
I started in the machining trade back in '57. Never in my many years in the shop did I ever work where spotting drills were employed. Mind you, I didn't work in CNC shops, where they gained popularity. Center drills were used for spotting, and did what was expected of them without issue.

Does it really matter how one creates a proper location? A spotting drill doesn't do anything that a center drill won't do--although it likely is somewhat more robust unless large center drills are in use. And, yes, I've heard the argument about the difference angles. So? They still work.

Conical center drills are not restricted for use as centers, no more than the 60° form is restricted for use as a center. The conical shape is preferred when there's minor misalignment of centers, or when the ends have not been faced. Some center drills provide a counterbore, which eliminates the need to face the end of a shaft. In the end, which approach a person takes is highly personal. There is no right or wrong in this instance.

Harold

[SteveM]

One thing that I found out the hard way, is that if your tailstock isn't really aligned with the headstock, you can snap the tip right off a small center drill, and you can take the tip right off an expensive carbide center drill.

That's why you hear Harold, among others, talk about holding center drills lightly so that they find their center.

I have had good luck as well with leaving the tailstock unclamped and pushing it into the work to get the drill to find center, then locking it and using the handwheel to advance.

If you are using a drill press or a mill, there is no danger in snapping them off, as the drill, by definition, is concentric with the spindle (unless you have a problem with your chuck).

If I am using a center drill to spot a hole, I tend to use the larger ones, as the tip is relatively strong, but the drill is short and stout like a screw length drill.

Steve

[earlgo]

One thing that I found out the hard way, is that if your tailstock isn't really aligned with the headstock, you can snap the tip right off a small center drill, and you can take the tip right off an expensive carbide center drill.

For this reason, I have a very small tool bit that I use in the Atlas, whose tailstock is slightly misaligned, to pre-dimple the center of stock, so that the center drill/combination drill/spotting drill will start in the correct location.
Works every time.
--earlgo

[SteveM]

For this reason, I have a very small tool bit that I use in the Atlas, whose tailstock is slightly misaligned, to pre-dimple the center of stock, so that the center drill/combination drill/spotting drill will start in the correct location.

You are referring to a tool bit held in the toolpost and you are using the carriage to do that?

I have heard suggestions that the best way to ensure that a center is in the center of a part is to start it with a drill but then finish it with a tool held in the toolpost and fed with the compound set at the proper angle.

Steve