I did a search but did not find anything like this so here it is.
Ever need to drill a hole that is in some inaccessible location for the drill length you have? This is how an old tool and die maker taught me to do it.
Start with a suitable piece of round stock. (This one has been turned down for a 1/4" drill.)

Chuck it up in the lathe and, using the drill you intend to drill the needed hole with, drill the RS about 1" deep.

Now file or grind a slot approximately 3/4" from the end and half the diameter of the RS exposing the end of the hole. Poor picture BTW.

Now file a flat on the end of the bit so that it laps over the end of the hole.

After drilling the desired hole the drill bit can be removed and returned to the index and the extension can be saved for another project. I have several extensions that I made about 25 years ago. They have been handy many times over the years.
Joe