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C. Tate
11-02-2001, 10:08 AM
Here is a question that came up in the shop this morning and we cannot not come up with an answer.

There are standard center drills and bell type center drills. Why the difference? Is there an advantage of using one over the other? We do a lot of cylindrical grinding and I can only think that the bell type may chamfer the hole at a more obtuse angle helping to prevent any burr from causing interferance.

Any comments?

halfnut
11-02-2001, 10:32 AM
My old machine shop teacher always said they were to protect the corners from handling dings. He called them protected centers.

I don't quite understand the bell and obtuse. What I am refering to is a center drill that also counter bores a shoulder. I've seen them but never have used one myself.

Thrud
11-02-2001, 11:53 PM
C. Tate,

Your presumption about the chamfer is correct - I use these most of the time.

The center drills I prefer are the radius drills. These produce a curved surface to allow more accurate tracking of the centers while taper turning. You could also make yourself a set of centers with carbide or chrome steel balls brazed on the end - a "ball center". These give consistant results when turning or grinding cones (turned tapers).

Dave