In today’s world of complex steel fabrications, we are all governed by allowable tolerances. Tolerances are important as they ensure a clean/neat fit up and or proper clearances. This confirms that at time of assembly everything fits as how it was designed; thus alleviating headaches or issues at critical points in the project where any mistakes could lead to bottlenecks and negatively impact the manufacturing or fabrication process.
But many times, and especially in the bender roller industry, we are presented with contradictory tolerancing. This is especially true on frame rail members made for the construction and or agricultural industry. These parts usually consist of a single kick bend (primarily less than 45deg), with two long tangents coming from both sides of the bend; with the critical dimension to hold, being the offset from straight at the far end of the member. (This issue can be observed not only in these specific parts but any bent member where the far end tangent position is critical). This offset from straight is usually toleranced at +/- ¼” deviation. At the same time, the degree of bend is toleranced at +/- 1deg deviation. It should be noted that in most applications +/- 1deg allowable deviation on the bend made is industry standard.
The issue between these two tolerances increases as the length of the tangents increase and we move further away from the bend. In looking closer at the +/- 1deg tolerance on the degree of bend, this allowable deviation is problematic in nature as it is derived from the slope and is linear in its function. This deviation can be expected to exhibit itself incrementally, and as tangent lengths increase the deviation equally increases. This places the measurement of the offset at the far end of the tube out of tolerance while maintaining a +/- 1deg .
This issue can be observed in almost all parts with tangent lengths longer than 1ft or 12”. In order to hold the +/- ¼” tolerance placed on the deviation in the offset from the straight dimension, the tolerance placed on the degree of bend must be fractional at best, which in most cases is extremely difficult to hold and or maintain.