On Sat, 13 Sep 2003 15:16:39 -0400, Mike Graham
wrote:

In article , Will Self wrote:

What is causing this, and what can I do about it?

Drill the cast iron with a backing-pad of mild steel. That will keep the
bit from breaking. What's happening is that the drill is self-feeding like
a corkscrew into the work. When you break through you lose the resistance
of the chisel-edge going through the work, you advance the drill too fast
and the edges catch. If you're drilling on the mill then a good way to
avoid this problem is to use the knee to feed the last bit. Drill most of
the way through then lock the quill and raise the knee to drill the rest of
the way. This also works in other ugly situations like when you're drilling
into a cross-hole.
You may find, Mike, that quicker than locking the quill and using the
knee to feed is to use the quill lock to put enough drag on the quill
so that the drill can't self feed the last bit. I do this all the
time.
ERS