"David Littlewood" wrote in message
news
In article , Nick Müller
writes
wrote:

Centre drill are often used as spotting drills in the lathe
but there's a pretty wild mismatch between center drill point
angles and the the usual jobber drill that follows. Special
spotting drills are produced for NC use at about three times the
price of similar centre drills.

How do the special features work and are they worth the
premium price? Some seem to be single flute designs and the
included angle is usually less thqn 118 deg jobber drill angle.

I use them a lot on the mill, seldomn on the lathe. I like them!
The keep long, because you won't drill deep holes, and you can't and
should not drill holes. Just the conical part.
Mine are two-fluted.
Buy them!

I have never used a spotting drill - and don't know what their
construction is. However, just an alternative idea for pentagrid: have
you tried stub drills. I almost always use these for starting holes,
especially in small sizes. They are about half the length of a normal
jobbers drill bit, and hence much stiffer, but only cost about the same
or a little more. I do often spot with the very tip of a centre drill,
but I'm not sure it is necessary.

David
--
David Littlewood

What David said, but insist in a split point to insure free cutting without
deflection. The web of a drill has extreme negative rake and doesn't
cut worth a damn. By splitting the point, the negative rake is eliminated,
replaced by a cutting edge that cuts instead of plows. The difference
in starting and drilling is remarkable.

Folks that came up in the shop the old way did *not* routinely use spotting
drills. That's what center drills were for, along with actually drilling
centers. Don't ask me how I know this-----it's hard enough getting old
without someone bringing it to your attention.

Spotting drills are really a tool of the CNC age, where it's important to
have a device that will start a hole where it's desired and not worry about
breaking the tip off, which is common in center drill use, particularly when
you get down to a #1 or smaller. By virtue of the grind design, spotting
drills have virtually no web, so they behave much like a split point drill.
Their short length provides rigidity and insures that the hole you drill
will be where desired, and the grind permits easy evacuation of the chips
that are generated. The tip is far more robust than that of a small center
drill.

Because of my training, and the era in which I spent my time in commercial
shops, I do not use them, nor have I ever.

Harold