The Importance of Tolerances in Custom Roll Forming
Ryan Hernden
Learn about the critical factors affecting roll forming tolerances, common industry guidelines, and how to minimize tolerance-related cost.
Learn about the critical factors affecting roll forming tolerances, common industry guidelines, and how to minimize tolerance-related cost.
Process variation and measurement uncertainty are facts of life in manufacturing, and tolerances exist to handle them. Used wisely, they provide an allowable range for a dimension to vary over and help ensure satisfactory assembly.
Tighter tolerances can simplify assembly, but maintaining them requires closer control over the manufacturing process. It can mean manufacturing tools to tighter tolerances and stepping up to more accurate methods of measurement. For this reason, one way to keep manufacturing costs down is by putting the widest possible tolerances on the parts being made.
In this blog, we’ll discuss some of the factors that influence tolerance in roll forming, including material properties, variation, and required deformation.
The tolerances achievable are influenced mostly by material properties, size variations in the material, and the amount of deformation needed to produce the required profile.
Some materials bend more readily, while others need more force and display more springback. The differences are due to two properties: the yield point and the elastic modulus (resistance to deformation) of the alloys. For example, aluminum and copper (both low yields and low elastic modulus alloys) are readily deformed, while high carbon and stainless steel (high yield, high modulus) are significantly more difficult.
Springback is addressed by overbending the metal strip. Gauging the amount of overbend needed requires considerable experience and often some trial-and-error.
The strip used in roll forming operations is usually slit from a wider coil. This coil has a small camber across its width, and its thickness varies slightly. Width can also fluctuate along the length of the strip. As a result, strip varies from batch to batch, and there is also some variation throughout the length of each strip. Collectively, this will create a small amount of inconsistency in how the material bends as it passes through the sets of rolls.
Variation on each bend tends to add up, so the number of bends required, their spacing, and their angles influence final accuracy. This is especially true with the ends of the legs.
Bend radii influence accuracy through the impact of the elastic modulus. Holes and cutouts placed at the pre-press operation can also affect overall accuracy, especially when they’re placed near edges or bends.
Deformation is best controlled by using rolls that constrain the strip completely rather than leaving regions free to move as stresses and loads dictate. However, this can increase the number of roll sets needed, and, depending on the design of the profile, may not be practical.
A closely related aspect to consider is the manufacturing tolerances placed on the forming rolls. Tighter part tolerances require these to be machined to higher precision but will also lead to more frequent refurbishment/replacement as the rolls wear outside their dimensional limits.
For simplicity, we can split our discussion of roll forming tolerances into two parts: tolerances applied to the overall formed part and tolerances applied to the cross-section of the part being formed.
Strip tends to bow (bend left or right), camber (take on an arched form), or twist as it’s formed into a profile, and the ends also tend to flare out.
Achievable tolerances are generally specified per foot of length, although some manufacturers will quote tolerance per 40 inches of length. To address flare, tolerances on the cross-section are often specified at a distance from the ends.
Industry guidelines are:
The profile is dictated mainly by roll geometry and material properties, although variation of the strip material plays a part, too. General industry tolerances for profile features are:
Tooling and setup costs inevitably increase as tighter tolerances are placed on the roll-formed part. While part function dictates what’s allowable, always strive to use the widest possible tolerances.
For specific cost minimization opportunities:
Roll forming can produce high volumes of very consistent profiles, but you have to start with a good design. If you’re considering roll forming, please contact MMC Roll Forming today for a project analysis. Founded over 40 years ago, we have an extensive inventory of no-cost tooling, work with a wide range of materials, and can manufacture to tight tolerances. Our experts can help you understand the best approach for the product you’re trying to manufacture, including selecting tolerances.