A few words of encouragement. Don't beat yourself up
In a rush to do something productive during my toddler's nap, I inserted the crank puller into the drive side crank arm. When I met resistance, I just gave it all the muscle I could. I snapped the crank puller off in the threads of the crank arm because I was too stupid to realize I didn't remove the crank bolt first!
Within the last six months I have myself neglected
to remove the washer that accompanies some of these
bolts, much to the detriment of puller, washer, and
self esteem. Things happen. Even to those who are
supposed to know better.
Use a Dremel with a small cutting wheel to cut the broken ring out of the crank. Then remove bolt. If you can still thread a new crank puller in, use that, and if not, simply use a gear puller to pop the crankarm off.
If you're careful with the Dremel, you can re-use the arm, though I'd install a self-extracting bolt set.
This is about where I would start, because I
am notoriously cheap and would be about
reusing the arm as a priority. Sometimes the
guys that do this for money like Burton have a
tendency to unconsciously factor time spent
into the equation (rightly so, shop time being
a rational consideration in any repair.)
So they have a tendency to adopt the shock
and awe approach out of the gate.
Still, you may end up eventually at destructive
removal. If that becomes the case:
DEWALT DW400 4-1/2-Inch Small Angle Grinder
They used to sell these at Home Depot here
for 40 bucks and I see they still list them on
Amazon at that price. They work so well for
so many cutting applications that I would not be
without one.
This is the tool that singlehandedly made the U-lock
obsolete. Use eye and hearing protection, make
sure someone else is responsible for your toddler,
and as always, YMMV.
Empathetically,
Mike Larmer
p.s. My impression is that a decent quality crank
puller will strip the threads in an alloy crank before
it leaves a part of itself in your way. You might
want to go a little higher end with the replacement.