Roll Forming U-Section Unequal Legs

Disclaimer: Since you said you are new to roll-forming, I'm going to approach my response as such. Forgive me if I cover topics you are already aware of.

There are a couple potential causes for this. It could be one or a combination of the following:

First check the strip for consistent gauge across the strip-width. This mostly becomes an issue with edge-mults that are slit off the master coil. Steel is like dough with the edges becoming thinner as it is rolled out. We call it the crown. So, check your master coil for how much crown it has. The amount can be changed by specifying tighter tolerances during the coil ordering process. The reason I mention the material first is it should always be the first thing you verify when you have problems. I don't want to say "garbage in garbage out" but rather there are differences in gauge control depending on the product. HR Black for structural has more crown than P&O ran through a tempering pass. They both have their place. The key here is to make sure your strip is rolled to the tolerance needed by your mill and tooling design to make the product you need. For non-welded open profiles you need a consistent gauge across the width.

How does gauge affect the rolling process? It changes the drive point of your die pair. Roll tooling is designed with progressive profiles that ensure a constant drive point. If a roll pair profile is designed to have a drive point at the center of your roll space but you're running an edge-mult, your gauge will be thicker on one edge making the outside of your roll space the drive point. Usually this is at a different radius making the surface feet per revolution faster, or slower, at that point. Your mill starts to fight itself with each pass trying to drive the strip at a different speed. The strip reacts by pivoting off center. If each pass is independently driven you can look at your motor loadings, it help paint the picture of what is happening in your mill.

Second check your roll-pair relationships; there are three different things you're checking here... flange gaps, roll alignment to its mating roll, and overall mill alignment.

For flange gaps this ensures your top roll is parallel to your bottom roll for your driven passes and that your inboard is parallel to your outboard for your idler passes. If roll pairs are not parallel, you're forcing an unintended change in the drive point of that pass; we already talked about what that does.

For roll alignment to it's mating roll, place a straight edge across the two roll faces. If one roll is off-set from its mate, even if they are still parallel, you are forcing a change in the drive point. Depending on the profile you are making, a 0.003 offset between mating rolls will give you grief.

Overall mill alignment ensures your strip isn't going up and down hill or skewing left to right as it travels from one pass to another. While diagnosing this problem, if you see the strip starting to skew off center in a particular pass, remember, it is not necessarily that pass creating the problem and is often two passes prior.

Lets say you see the strip skewing in pass 5... pass 3 could be the one with the issue and using pass 4 as a fulcrum.

So, you've checked your strip, it's good... you've checked your mill alignment, it's good... but you're still having the problem. Start looking at your tooling and it's set-up.

Tooling should be procured from a tooling design company that has engineering horsepower and experience in designing tooling for your application. And the tooling you are using should be designed specifically for your mill, for your product profile, for your steel gauge and grade.

A set of rolls designed to make a profile using Low Carbon HR Black will not make the same profile using HSLA or Full-hard cold rolled. These materials work harden different, they elongate different, they have different tinsel and yield values, all of which required different forming progression through the mill.

Your tooling provider can confirm the tooling design you are using is suitable for what you are trying to make and they will provide a Set-Up chart that will show desired gap values for each pass and what the desired growth and reduction is per pass. Since you are making an open profile, you'll be looking at growth per pass, not so much reductions.

Steel elongates as it is worked and your tooling designer will account for this in the tooling design. Also, a you create radii, the wall will want to thin, the tooling design should account for this as well; so long as the tooling is designed for the grade and gauge you're using it for.

All the processes involved in forming steel are based in science and physics. The strip in your mill is doing exactly what you're telling it to do, the art behind the science is finding out what to tell it. It boils down to quality of the strip, mill alignment, and tooling design.

Good luck.