The Fundamentals of Metal Straightening

10 Apr.,2023

 

Every company that uses metal coil in their products knows the drill–you uncoil the metal into a straightener, feed it into your press and, voilà, metal parts are ready to go. Have you ever really thought about the metal straightening part of it, though?

As in, is it even necessary? And if so, how in the world does it actually work? Why is that machine so big?

Fear not, dear metal processor, we’re here to answer all of those questions for you.

Let’s start with the basics of metal straightening.

Why Is Metal Straightening Necessary?

From the time the original metal is made to the time it’s shipped to your facility, it goes through a number of processes that change it. First, it’s run through a series of rollers that make it thinner and thinner. It’s tightly wound into a coil. It’s uncoiled and slit into smaller strips, and then rewound. And every step provides more opportunities for imperfections to be introduced into the coil.

So, when it comes to your facility, you can’t just run it straight from the coil into your press. Doing so could jam your machine or result in bad parts that won’t work for your product. Either way, you avoid those issues, and possible downtime and loss of revenue, by properly running the coil through a quality straightener, which can remove a number of conditions from the coil.

6 Coil Conditions

There are six main conditions in coil that comes from the mill.

  • Camber (shown above) – When the coil slitter wanders and causes one edge of the strip to be longer than the other, resulting in an S-shaped strip
    • Fixable? Yes, with a corrective leveler.

  • Center buckle (shown above) – When the center of the strip is longer than the edges, resulting in waves down the center
    • Fixable? Yes, with a corrective leveler.
  • Coil Set – The lengthwise curvature in a strip of metal due to winding tension
    • Fixable? Yes, with a straightener or corrective leveler.
  • Crossbow – A bow in the underside of the strip
    • Fixable? Yes, with a straightener or corrective leveler.

  • Edge wave (shown above) – When the edges of the strip are longer than the center, resulting in waves down the edges
    • Fixable? Yes, with a corrective leveler.

  • Tapered edges/center crown (shown above) – When the strip edges are thinner than the rest (tapered) & the center is thicker than the rest (crown)
    • Fixable? No.

Introduction to Straighteners & Levelers

Now that you are aware of the various conditions in a coil, let’s look into the machines that can fix them. As noted above, they fall into two categories–straighteners and corrective levelers.

Both use a series of rollers, with the top row of rollers set at different heights (more on that later), which bend and stretch the metal strip to re-flatten it. The rollers are usually made of extremely hard (and extremely heavy) 52100 heat-treated steel. They are available with a number of different coatings, such as matte chrome (the most popular), Teflon, titanium nitride and tungsten carbide. Depending on the material coating, they can also be diamond cut with a pattern to fit your needs even further.

Flattening the metal sounds simple, but it is really complicated–the metal must be stressed 20–30% beyond the yield point (the maximum stress point before a metal begins to change shape permanently). This is when the strip exits the elastic range and enters the plastic stage. If you do not stress the metal enough, the coil conditions can return, resulting in the aforementioned downtime and bad parts.

You must also note that different metals, e.g., steel vs. aluminum, have different yield points. So, there is not a one-size-fits-all solution for metal straightening. Plus, the thickness of the material results in different roller diameter needs–smaller-diameter rollers are better for thinner materials and larger-diameter rollers are better for thicker materials.

Different Types of Machines

Straighteners

Straighteners come in two main flavors–pull-through (unpowered) and powered.

Pull-through straighteners (like the example shown above) rely on a feeder to pull the metal through the rollers. While these machines typically cost less and save space, inertia (resistance to any change in velocity, whether from rest to motion or vice versa), can result in less accuracy, less effectiveness and even marked materials if the rolls slip during starts and stops. While the feeder is usually separate from the straightener, there are all-in-one straightener-feeder machines. The same issues apply to these combo models, though.

Powered straighteners (shown above), while bigger and more expensive, do not have the same issues and therefore produce better parts. Instead of relying on the feeder to pull the metal sheet, a powered straightener has rollers that are driven to move the metal through. On some, the bottom roll is driven, while the top row is free. Others use a combination of both.

Speaking of the rollers, on a standard straightener, the rollers tend to be larger, because there’s only one row of them, to prevent deflection, i.e., rollers bending. Standard powered straighteners are suitable for removing coil set from most metals and thicknesses.

There are also straighteners specifically designed for wires (example above), with either two planes for round wire or a single plane for flat wire.

Corrective Levelers

Rather than large rollers, corrective levelers, (shown above) employ a large number of smaller rolls that are backed with more rolls. This creates a very rigid, tightly spaced roller structure that can gradually work out tighter strip bends, as well as provide deeper stress relief for greater flatness and reduced metal-weakening stresses.

These are the only kind of straighteners that can fix camber, center buckle and edge wave, because they can change the shape of the metal from edge to edge and be adjusted to stretch just the sides or just the middle.

Calibration

As previously mentioned, you need to go 20–30% beyond the metal’s yield point to ensure the metal will stay flat even after it goes through the press.

Unfortunately, there’s no one-size-fits-all solution for metal straightening. That’s because there are a wide variety of factors that need to be considered, including type of metal, material thickness, coil set amount and roller diameter and spacing. The required settings can even change for the same coil, due to increased coil set closer to the center, where the metal is more tightly wound.

That being said, a good place to start is by lowering the entry roll to the thickness of the material BELOW 0, and raising the exit to the thickness of the material. So, for example, if your material is .125” thick, you’d put the entry roller at -.125 and the exit roller at .125.

Straighteners and corrective levelers come with either computer or manual adjustments–both with readouts to make it a very simple task to adjust the height of the rollers to meet your needs.

When adjusting the roller heights, regardless the type of straightener, aluminum is much harder to straighten than steel due to it being more elastic. You need to come down harder on aluminum coils to get them straight.

Picking the Right Straightener

So, now that you know the basics, the question is–which machine fits your needs best, and what size do you need?

Before you call up a manufacturer, do some research into your needs and have the following information ready:

  • Type, thickness & width of material(s)
  • Voltage
  • Speed needed, i.e., how much material you’ll be running in a certain amount of time

This information will help the manufacturer design, develop and build the machine that meets your needs.

 

Press Room Equipment specializes in producing custom-built, high-quality powered or feed-driven straighteners (we do not offer corrective levelers) in a variety of sizes to fit your needs. If you’re interested in learning more about how we can meet your metal processing needs and to get a quote on your new straightener, call our sales specialists at 417.864.3636 or request a quote online.

If you have any questions on straightener feeder. We will give the professional answers to your questions.