drilling concentric holes

[oregonsteam]

when drilling two concentric holes, is it more accurate to drill the larger one first, then extend the depth with the smaller hole, or drill the smaller hole first, then drill out the larger diameter? Any opinions are helpful. Thanks!

 

[vlmarshall]

Drill the smaller first! Always a good idea when drilling "large" holes, anyway. The center of a drill does a bit of brute-force work, just mashing the material out to where the cutting edge can get a bite on it. Starting with a smaller drill makes that mashed area a lot smaller to start with, and non-existant for the following drill.

 

[rake60]

A center drill or spot drill is essential as a starting point.
It's mark gives the drill bit an accurate starting target.
From there the material being drilled becomes the master of the process.
If the material is of a constant molecular structure and tip of the drill bit is
perfectly ground to an absolutely perfectly symmetrical geometry the hole
will drill perfectly straight. Will all of those things come together in our lifetime...
Probably NOT!

Metal is a material in a frozen state.

Let's simplify it.
Take a look at any ice cube from the kitchen freezer.
If it's molecular structure were perfect you should be able read the print
of a newspaper through it. You can't do that. There are foggy spots in it
that contain more air than the clear areas. Those foggy spots are softer
than the clear areas because of a lesser density. Those areas yeild more
quickly to disturbances.

Metals have those exact same properties.
You can't see them but they are there.
It is what causes drill bits to lead off and boring processes to result
in wandering measurements.

Truth is there is no right answer here.
The material is in charge of the result.

Rick