T i m wrote:
Hi folks,

I'm trying to make a special drill for drilling out the stump of a 9"
sq wooden gate post but want to leave the surrounding concrete intact.
snip
(Someone kindly suggested 'sharpening' a masonry bit but looking at
it's design I'm not sure it would work very well through 4.5m (9 holes
x .5m deep) of 'good' wood?)


Why not?
Looking at one, if you take a fair bit off the top, to leave a
90 degree cutting edge, with some relief, then it will cut fine,
though perhaps not as fast as a proper bit.
Cutting away a large portion of the bit support that the carbide is
soldered into to enhance chip removal would be a good idea.
Alternatively, a simple auger worked very well for this sort of thing.
If I could have welded it on a long shaft, it'd have been ideal.
(it was around 1")