Roll Forming Tolerance: Navigating the Gray Area of Precision

In the world of roll forming, controlling tolerance is the balance roll formers constantly navigate; it's a subtle art and a complex science – similar to pitching in baseball. 

The degree to which we can maintain tight tolerances depends largely on moving targets. For instance, a tool’s full contact on all surfaces of the metal being formed allows tighter control as compared to box channels where the tools aren’t in full contact with the metal. There, tolerance can vary significantly.

The roll forming tolerance game is played on a spectrum. With that understanding, let's delve deeper into the nuances of roll forming tolerance, exploring:

• Factors that influence it
• Strategies for controlling it
• Real-world implications of variations in tolerance.

Finding the Strike Zone in Roll Forming Tolerance

Just as a baseball pitcher targets the strike zone to achieve the ideal pitch, so, too, must roll formers maneuver the gray area of roll forming tolerances to hit accurate targets. And, as each batter has a specific strike zone determined by their height, each roll forming process has its specific tolerance range. 

This is more than just a game of inches; it's a delicate dance of precision and adaptability, requiring adjustments.

Like a pitcher honing in on that strike zone to deliver a pitch that is neither too high nor too low, a roll forming engineer has to maintain a careful balance to ensure the process stays within the established tolerances. 

An expert pitcher or a seasoned roll forming engineer can play with the edges, flirting with the limits of the tolerance range or strike zone, but always with a keen understanding of the risks involved. 

Though there's a world of difference between a ball and a strike, a slight variation in roll forming can mean the difference between a useful part or costly scrap.

The Cost of Precision | Pay to Play

When you're willing to pay more, you can aim for a tighter tolerance. It's a direct reflection of the principle, "You get what you pay for." 

So, what exactly are you paying for? Let’s look at: 

• Tooling
• Bends
• Time
• Material
  
  

Tooling

The number of passes – say, 10 passes vs. 25 – has a direct impact on tolerance. The more passes you configure for your part, the more roll form tooling that’s required. 

Having more forming stations relaxes the material, allowing more focus on each individual bend rather than performing multiple bends at once.

Tighter tolerance = higher cost due to more tooling requirements. 

However, there comes a point where no amount of additional passes is going to increase the part’s tolerance.

Bends

The number of bends in a part plays a crucial role in roll forming tolerance as well. 

A part with five or six bends may not hold the overall width as well as you’d like. 

Dimensioning a part can be a challenge. It's generally advisable to hold to the outside dimension rather than attempting to go to the inside. 

The internal variations are larger due to contact with the part. It's always a good practice to send your drawing and negotiate if all tolerance requirements can be met. 

Time

Time is money. 

Costs mount in terms of staff time spent on checking the part for range. 

It’s also more time spent setting up the roll forming machine’s stands to accommodate additional tooling if you’ve increased the number of passes.

Material

The higher the yield strength of your material, the more difficult it will be to form. 

Roll forming creates stress to counteract a material’s original shape, so choosing the right material for your specific needs is key to your part’s tolerance range. Some metals are easily formed, while others take more coaxing.

Factoring in the Quirks

This isn’t “Field of Dreams.” The roll forming process isn’t magic.

Despite how accurate your CAD drawing is (“If you build it…”), roll forming has built-in variability. 

Engineers often desire a perfect part. But when you're limited by the process and your budget, you either need to spend more or cede some tolerance. Baseball-playing ghosts aren’t coming out of cornfields, and roll formed parts aren’t always 100% perfect.

Just like real-life baseball has its quirks (isn’t it fun that every MLB outfield is different?), so, too, there are phenomena unique to roll forming: 

• End flare
• Springback
• Bow, camber, twist & webbing

Understanding and managing these key factors are crucial in the roll forming process.

End Flare

End flare, or distortion at the cut points, occurs as a material is progressively shaped. Residual stresses introduced during the process are most apparent at the ends of your components, sometimes leading to significant distortion.

Methods to minimize end flare include roll design procedures and stretch forming, which involves overforming, underforming, and finally finishing the section. However, there’s a point where adding more passes doesn’t reduce your end flare. 

Sometimes this leads to tough conversations that end about so: 

‘This is the shape, and these are the tolerances up to 3” from each end.’ 

Another approach is annealing, a process of heat treatment followed by cooling, which improves ductility, reduces brittleness, and importantly, relieves stresses absorbed during roll forming. 

Springback

Springback is the distortion of a part when forming pressure is removed. This happens because the metal attempts to return to its original shape as the inner bend is compressed and the outer bend is stretched. 

The degree of springback varies depending on the metal in use. 

The main predictors of springback include the yield point (the stress at which material deformation begins) and the elastic modulus (resistance to deformation) of the metal. 

Handling springback is more about preparation than prevention. There's no perfect solution to reduce springback in bending, but overforming can usually compensate for the inevitable. 

Remember, the higher the yield strength, the more challenging it will be to form because more stress is required to counteract the material’s original shape. 

Bow, Camber, Twist & Webbing

Bow refers to a lateral curvature or deviation from a straight line in the horizontal plane. It can occur as a result of uneven stress across the width of the part during forming, causing the part to curve like a bow. This deviation from the ideal straight form can impact the precise alignment and fit of the roll-formed component in its end application. 

Camber, on the other hand, is a curvature or deviation from a straight line in the vertical plane. This can happen due to uneven bending at different points along the length of the part. Like bow, camber can also impact the component's usability in its final application due to deviations from the desired shape.

Twist is the rotation of a part's cross-sectional shape around its longitudinal axis, often caused by uneven cooling or improper alignment of forming rolls. This can also significantly affect part tolerance, as a twisted part may not fit or function properly in its intended application.

Non-symmetrical parts are more likely to bow, camber, and twist due to uneven stress. 

All three of these issues can be minimized with careful control of the roll forming process, including using the correct tooling, ensuring proper alignment of the forming rolls, and maintaining consistent material properties throughout the process.

Webbing refers to the integrity of your material as holes and stretching can affect your part’s tolerance, too. If they’re spaced every couple of inches and consume a major part of your web, your forming will be affected – lots of waviness may occur.

Roll Forming Tolerance Ks

Roll forming is the most cost-effective way to achieve continuous production of complex shapes with multiple bends. However, it does have its quirks. Understanding and managing them are crucial to delivering components that meet the agreed tolerances – your strike zone.

The key to maintaining the process within the agreed tolerances lies in control. Increase the number of passes and frequency of checks for better control. Pay attention to tolerances related to dimension profile, width of end flare, and how symmetrical your part is. The higher the yield strength of your material, the more challenging it will be to form.

Options and variables are endless. From holes and stretching impacting tolerance, to the effect of long flat surfaces that tend to wave. To navigate this complex world of roll forming tolerances, open conversations and understanding the material you’re working with are vital.

Great Teams Need Great Communication

Remember, we're here to help you understand the factors that affect tolerance. Want to know if roll forming is right for your part? Send us a drawing and we can start a conversation: