I am trying to tap a gold ball with a regualr tap and die set to
make this kind of thing you put on the tong, (I don't know the name
in english) but the tap is too long
It sounds like you're using either a taper or plug tap. What you
need is a bottom tap.
Taps for machine screws come in 3 styles, taper, plug & bottom. The
taper tape has a longer starting point than the others. The point on
the tap is about 2 or 3 times the diameter of the tap, with no or
very shallow threads. Depending on the material being tapped & the
size of the hole, the taper tap is usually used 1st.
The plug tap has a shorter point than the taper tap & the threads go
closer to the point. It's usually used after the taper tap, again
depending on hole size & material being tapped.
The bottom tap has threads going just about to the end of the tap.
It's used last & is meant to tap almost to the bottom of a blind
hole (one that doesn't go all the way through). Even then some
bottom taps have a point on them that prevents them from going all
the way to the bottom of the hole by about 3 or 4 threads.
One thing you can do to get very close to the bottom is to grind any
of the excess point off. Grind the tap off until your're just short
of the 1st thread on the tap. Be sure you keep the tap cool
(frequent dips in water) while grinding or you'll soften the tap &
it won't cut effectively.