Are center drills self-centering (in a lathe)?

Thanks yall for the education. It appears that my understanding of the process lacked quite a few of the finer details.

I have occasionally "step drilled" with the center drills (especially before using the large one). My center drill set is not labeled, so I don't have a working knowledge of exactly which one is a #5 vs a #2. I have never broken a center drill which means either that I have been lucky or I don't force the tool. I've noticed the jump when the center drill initially contacts the stock but it settles down quickly and the holes "look" to be on center (I'm not sure how I would go about measuring any eccentricity). I think that given the flexibility built into my Atlas 6x18, if the center drill were off center I'd see the tailstock chuck doing some kind of vibrating number.

If center drills are considered flexible, then rigid must exist only in theory.

Thanks

Bruce

Thanks, Bob.

Harold

With a test indicator. Mount it on a magnetic stand and rest the tip in the hole. Rotate the stock to measure the eccentricity. Sounds from the rest of your post that you'll see less than a couple tenths at most, though.

Jim

Actually a lathe tailstock is so stiff when clamped and massive when unclamped, and a regular center drill so short, hard, and stiff, that the drill usually _bores_ the center hole.

It's _never_ exactly on center. Measure the width of the two curled drilling chips. You'll find they're usually unequal. When they do test equal, rotating the center drill 90 degrees proves that in the other direction, it's off center. That is, after drilling, then rotating the drill 90 degrees and drilling again, you'll find it's off center one way or the other 99.9% of the time.

There is a plane through the axis of the work that the center drill is aligned on, and rotating it so the, um, are they lips?, yeah, the edges of the lips are aligned on that plane will allow both edges to share the load equally, producing the best hole without adjusting or scraping the tailstock.

I certainly don't know whether you get more bang for the buck aligning the lips or adjusting or scraping the tailstock.

Chuck up a bit of stock and put an end mill in the tailstock chuck. It'll bore a hole very nicely centered in the stock. Try it with more accurate stock in a collet. Amazing.

This means your tailstock is too far away from you, just beyond center. When it's nearer than center, it wants to go down, but the design keeps it from moving noticably.

Or that.

Um, no. Drilling is boring with a short drill down to about one diameter depth. The rotation of the work around its own center forces the drill to that center when drilling longer holes.

What happens sometimes is a very long drill and a poorly started hole conspire to produce embarassment. The short, hard, rigid (stiff) center drill gets the hole well starter by boring, and boring produces well center, very round holes.

Yours,

Doug Goncz (at aol dot com) Replikon Research, Seven Corners, VA

snipped-for-privacy@aol.com.bat.exe ( Doug Goncz )

Except that the center drill is not a boring tool, in the sense of a single bit that addresses one side only. It is a combination drill, at least in origin, intended to drill and countersink a pre-punched (bench made) starter indent, so that a 60 deg TS center point can rest on air at the base of the countersunk hole.

One of the problems in this thread is that it's based on work held in the headstock chuck, and go from there. But the older way of working between centers was to work off of stock that was mounted literally between centers (via a lathe dog). The headstock center would be soft, and dressed to 60 deg before the next work was mounted. You would hand punch the center point to be addressed on the TS end at the bench, then use that punch indent on which to drill the 60 deg indent for the TS dead center. "Scrape TS vs align drill lips" is not an issue, in the older setup. [not commenting on remainder of post[ FM