Tolerances
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Tolerances on part dimensions are largely dependent on the tolerances
of the material being formed, end flare and springback, notwithstanding.
A dimensional tolerance of ± 1/64” is commonly applied to
cross-section dimensions and a tolerance of ± 1° to 2° to
angles. Given a specific set of conditions, closer tolerances can
be held, e.g., ± .005”. But, whenever such tolerances
are specified, it should be recognized they usually represent additional
tryout time, more tooling expense and possibly even the need for premium
priced material that has special thickness and mechanical property controls.
The manufacturer is often asked to propose equipment to make a given shape
from more than one gauge of metal and in the interest of minimum tool cost,
to do so in one (1) set of rolls. Remembering that rolls must be
fitted for the maximum metal thickness and also that the only adjustment
available to the operator is vertical to bring the rolls closer together
or farther apart. Figure #3 shoes the minimum and maximum conditions
that exist in a typical case. Note the change in the angle of the
vertical leg, the change in overall height and the change in arc length
at the bends.
Figure 3: Minimum / Maximum Relationship - Multiple Gauge Forming
Strip width variations also affect section tolerance.
In the case of the hat section pictured here, width variation would be
reflected as a variation in the length of the legs. The part designer
should be governed accordingly.
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The straightness of a formed section varies due to any one of several factors,
strip thickness or hardness changes in a given coil, or for that matter, from
coil to coil, roll pressure adjustments, lubricant or temperature changes and
camber. The operator must recognize the affect of these conditions and be
able to compensate for them. A straightening attachment on the exit end
of every machine provides this facility.
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Straightness consists actually of three (3) considerations - camber, or
deviation from a straight line in a vertical plane; sweep, deviation from a
straight line in a horizontal plane; and twist, the order of magnitude, for
camber and sweep being about 1/8” or ¼” in about 10’
and twist about 5° to 15° in 10’.
