Centre Drills

[jamesthomsan]

Hi Everyone, Please share some advise.

Which Centre Drills is good for spotting the location for the drilling operation?

[Latticino]

I'm a little confused by your question.  For the tolerances typically needed for blacksmithing I just use a square and pencil to mark the location and a center punch I forged myself to spot the steel.  If you are looking for information on precision machining you would probably get better information from an alternate forum, but a typical center drill consists of a combination pilot drill and countersink.  There are certainly machinists on this forum who might be able to recommend a source, if that is what you are looking for.

They typically look like this:

[SLAG]

In a pinch,   riveting kits are a good, cheap source for the drill-countersink combination drills that Mr. Latticino has alluded to, and illustrated.

SLAG.

[Kozzy]

Spotting drill bit without the extra step of the centering bit is fine too--the goal being to have a super short shank so the bit is rigid and doesn't immediately try and wander (bend) as it makes it start.  It's amazing how much a full length drill bit can actually bend under the pressures involved. 

They are only useful with a rigid set-up--both the work and bit fixed in place on their respective axis--end drilling in a lathe being the most common example.  Very little benefit without that rigidity, though if I'm hand drilling a pilot hole in fairly thin material I sometimes use a spotting bit in a center-punched divot just because I like the stiffness.

On deep hole drilling, especially with aircraft length bits, the exit is NEVER perfectly in line with the entry.  With care, one can sometimes drill a bit undersize and use a reamer--most reamers are a bit stiffer than spiral flute bits and can slightly straighten a hole..bot it's not a miracle-worker (requires rigid set-up).  Parabolic flute bits are also great to use on deep holes as they clear chips better than your standard drill bit.

If you are trying to get super accurate hole locations, there is a machinist trick to that but I don't want to ramble on unless that is the actual goal.