When I first got mine a few years ago, I didn't check the tightness
of the saw mandrel very well. It worked the first time, then began to
wobble as the screw loosened. This made 'double cuts' on the wire. I
figured it out before I damaged another coil of wire. Now, I check
the tightness (and make sure it is centered) before every use.
Also, I was tighteneing the top plate on the cutting guide way too
tight...and unevenly! I would tighten one end, then the other. Not
only was it too tight, but it was sloped and pinched the coil so
that the blade began binding in the coil (couldn't see to confirm all
that, but that's what it felt like).
Now I put the coil in place, place the entire cutting contraption on
a shelf at eye level, and tighten both screws at the same time. This
keeps even pressure on all parts of the coil, and prevents
over-tightening. It must be firm enough to prevent movement of the
coil, but not any more than that.
One last thing: mine (an older model than yours, this may not apply
to you) came without the little spacer that keeps the 'front' end of
the coil at the proper distance from the post. I made a suitable
spacer from scrap copper. Check yours and make sure the coil is not
too close to the screw-down post.
Since I don't need so many jumprings anymore, I just cut the coils
with my saw. The winding device is wonderful and prevents a lot of
stress on my hands, but I'm fast with a saw, and there is no waste.
I hope this helps!