Subject: centre drills: bell vs straight

From: [email-masked] (Harold Kroeker)
Date: Tue, 03 Dec 1996 16:30:23 GMT

There seem to be three types of centre drills available and I am not
sure which type to use for which application.  The types that I have
encountered are:

Straight
- 60 degree included angle

Bell
- 60 degree included angle which bells out to 120 degrees.
- produces centre holes with beveled edges to prevent damage to the
centering hole.
- claimed to reduce the bearing surface of the centre hole

Ring seat
- similar to bell type but the 60 degree included angle is followed by
an 82 degree chamfer angle.
- from the McMaster-Carr catalog: "two rings on body provide a strong
bearing point for heavy duty work"

In "The Amateur's Lathe"  it states that one should drill a centre
hole large enough to support the work and no larger.  Is there a rule
of thumb to use or do I need to make a few centre holes too small to
learn this?

Harold

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From: [email-masked] (JD)
Date: Tue, 03 Dec 96 17:31:04 GMT

>of thumb to use or do I need to make a few centre holes too small to
The 82 degree countersinks are used for flat head screws and
rivets.