The problem of getting sufficient heat to a joint in a heavy object
using a little torch is exacerbated by virtue of the lower potential
temperature using propane or natural gas and oxygen. Acetylene is
far hotter than other gases.
You can mitigate the above problem by using a larger tip and you may
even have to go to a melting tip ( rosebud ) Any way you look at it,
all systems have their limits and you must learn what those limits
are through experience.
Ron MIlls @ Mills Gem Co., Los Osos, Ca.