Using rolling mills correctly is surprisingly complicated. All the
different rules of thumb were created as an attempt to simplify.
Sometimes they work and sometimes they do not, because there is no
real substitute for knowledge.
There are two type of damage that can be produced, - damage to mills,
and damage to ingot. Let's take damage to mills first:
The maximum amount of bite should be stated in documentation that
come with the mill. If it not there make an effort to find out. If
maximum is exceeded, the gears will be damaged. Some say it can be
estimated depending on roll diameter, but in our corner cutting
world, good rolls can have crappy gears, just to save a cent or two.
Maximum amount of bite also depends on condition of rolls surface.
Maintaining pristine polish is very important. Metal that touches
surface of rolls moves at different speeds as compared to rolls.
There will be a point, where speed of rolls and metal will be the
same. Metal prior to this point moves slower, and metal past the
point moves faster. There is great deal of friction between metal
that moves slower and the rolls. Rolls have to slide past this metal
and if surface looses it's polish, the forces acting on gears can
exceed original design specifications, with subsequent damage not far
Let's deal with ingot damage. On first pass through rolls, bite must
be as large as possible. 30% reduction in thickness in one pass is
recommended. This is necessary to engage the core of ingot.
Otherwise, rolling will only affect outside layers, leaving core
untouched. Unless you have a motorized mill capable of such bite,
this is an impossible task. Therefore, ingot must be forged prior to
milling. After forging small bites are not ideal, but with hand mill
there is no other choice.
Theoretically, milling should continue until 70% of original
thickness is reduced. I said theoretically, because in practice it
would require ingot to be perfect. I am quite happy to anneal after
50% reduction, If 50% percent cannot be achieved, the ingot should be
It is advisable not to anneal at theoretical annealing temperature
if maximum deformation of 70% has not been reached. There is definite
relationship between amount of deformation and annealing temperature.
However, to practice this theory requires great deal of experience
and the damage may not even be noticeable, unless whatever you doing
requires a lot of annealing and deformation.
To summarize: Large bites are better than small and try to achieve
at least 50% of deformation. If you can do more even better, but do
not exceed %70. Do not anneal unless absolutely necessary and if
deformation is less than 70%, use slightly less temperature.
Deviations from recommendations are not deadly and only show up in
projects with many annealing/deformation cycles.