Hi Orchid,
I am trying to refine my prong setting techinque andhave realized
that I don’t really know what is the proper size bur to use for
notching prongs. In the past I always used a small (like 1.5 - 2mm)
90 degree hart bur but it took a lot of moving it back and forth to
notch out a decent seat for the stone and I wasn’t always satisfied
with the results. Now I’m wondering if I should be using a bur that
is closer to the diamater of the stone so that it makes a cut that
is closer to the circumferenceof the stone for a better fit.
Also I’m thinking of buying a prong cutting guide to control depth
and height. Any recommendations?
Thank you all 
As a general rule cut the seat about 40% to 50% into the prong for
harder tougher stones. diamonds and the corundum's. And, up to 60%
for lesser hard or tough stones such as emeralds.
I thought the rule regarding notching prongs is to never cut into
the prong more than one third (ie no more than 33%) of its diameter.
I presume this is for strength and to avoid prong failure.
Helen
UK
I always use a burr that is the same diameter of the stone or a tiny
bit less. I prefer to cut my seats only 40% to allow for filing and
polishing later. A seat cut should never ever be more than 50% when
finished.
If you have a fragile stone the rule still holds. Though I will
sometimes use a higher carat metal on a really scary stone for ease
of setting.
Have fun and make lots of jewelry.
Jo Haemer
timothywgreen.com