Is Your Roll Formed Production Scrap Rate Over 5%?
Common Causes & Tips to Reduce Scrap
By: Chuck Summerhill
Ideally, a roll forming system should produce parts at a scrap rate of 0-5%. If your scrap rate is higher than 5%, efforts should be taken to investigate the causes and to take corrective actions. The main causes for scrap are:
Other causes for increased levels of scrap can be attributed to the complexity of the profile, pre-punching or notched features and cut off related problems. These issues are considered less common and require specific details not covered here (check our technical resources section of our website for additional information). This article will cover the main causes for scrap and preventative and corrective actions to minimize it.
Causes for excessive scrap (can be a combination of these):
Is the main cause of your scrap due to the incoming steel?
The saying “junk in, junk out,” is true in many cases. Excessive camber, crown, oil-canning, cross-break, coil set, burred or damaged edges in the coiled material greatly effects the consistency of the roll form production line and specifically to the roll formed cross section. Material defects as mentioned are caused for various reasons in the post processing of the steel, but your production line personnel can make basic measuring and visual inspections of the incoming coils and develop standards that work for the specific roll set and product. In other words, coiled material will always have one or more of these abnormalities and a good tooling design combined with specific setup procedures for each defect scenario will result in acceptable parts and scrap rates while avoiding the most extreme defects by rejecting the worst material before it is ever used.
One other variable that must be mentioned is the incoming steel that is procured from multiple sources. Inconsistencies in mechanical and chemical properties of the steel purchased from multiple sources are often unseen using common inspection techniques, but will become apparent when the parts are being produced. Large variations in these properties create scrap due to cross-sectional or linear (straightness) problems and downtime caused by trial and error adjustments required to “chase” these material inconsistencies.
Tips to improve consistency of incoming steel:
- Develop an inspection process for the incoming coil stock to measure camber, edge wave, crown, thickness at both edges, etc.
- Document tooling settings from each successful run and record the amount of scrap produced to achieve the proper setup. Compare the scrap produced to inspection results of the incoming steel to determine which defect is causing the most percentage of your overall scrap. Communicate your incoming steel problems with your suppliers and develop quality limits.
- Revise components on the machine or tooling set to accommodate the material defects.
- Request material certifications for both mechanical and chemical properties, again comparing to the amount of scrap produced to achieve acceptable parts. If large amounts of scrap are seen when using material with a particular mechanical or chemical variant, reject this incoming material before running it in the future.
Is your tooling set causing you the most amount of scrap?
While the quality of the incoming steel is very important, tooling design, the setup and condition of the roll set contributes greatly to the amount of scrap produced. Too often, scrap produced from a typical tool set is due from the lack of using documented setup charts and settings or from secondary adjustments made by operators on-the-fly after the initial setup does not work. If faced with high amounts of scrap due to the tooling, a process of elimination is needed to determine the corrections needed to minimize it.
Tips to determining and reducing the amount of scrap due to the roll tooling:
- If twist is the main cause of scrap, consider the straightening device at the end of the machine, and if it is adjusted to an extreme position to counteract the defect, consider a tooling change or the addition of other tooling to minimize the defect before the product reaches the straightener fixture.
- Excessive scrap due to angularity problems during a tooling setup may be caused from the lack of specific passes designed to address each bend in a segregated manner. In other words, many tooling sets do not have enough roll passes to properly compensate for material variations that affect the amount of spring back. Additional roll passes or side rolls can help control the angularity of given bends, reducing scrap.
- If scrap is caused from parts that shift from one side to the other in the roll set, causing dimensional or tolerance problems, consider how to contain the strip within the tooling. The use of roll traps, entry tables, side rolls and guide stands are a few ways to control material shift.
- Scrap due to aesthetic problems like scuffing are common. Since the roll tooling can only rotate at the exact speed of the material at one point on the profile (such as located on the web of a channel section), the flange rollers that form the vertical features are larger in diameter than at the drive point on the cross section and therefore have a greater surface speed than the material itself, resulting in friction and scuffing on these surfaces of the product. To minimize this, idled flanged could be incorporated or specific top spindles can be idled. Coolants and lubricants must be specified and applied properly. In some cases, the forming pressures caused by the flange rolls result in scuffing problems. Either reduce the amount of forming angle from one pass to the next or add side rolls between passes to relieve some of this pressure and minimize scuffing.
- Tooling wear contributes to increased levels of scrap. Occurring gradually over time, as the tooling wears, the start up scrap rate will also increase. Conduct an inspection by first looking for wear to the inside bends of the rolls. These areas are the main wearing points, and can be checked by using radius gages. Wear on the top portions of the flange rolls normally appears as grooves. Either replace these worn segments or consider re-grinding the roll contours to remove the worn areas.
Do you see a variation in scrap rates from one shift to another?
We hear this issue all the time and although it seems like a simple problem to fix, it continues to plague many companies. It comes down to three main causes; lack of documented setup procedures, differences between shift operator techniques and the operators’ experience or skill level.
Tips to reducing scrap by incorporating setup and training procedures:
- Use roll tooling setup charts for each tooling set to record the correct locations of each spacer, roll and shim as well as gage hub settings for each station. Be sure to add special notes or comments that each operator follows.
- Eliminate the classic problem of one operator making different adjustments from another operator by communicating which adjustments are the best solutions and updating the setup charts to reflect them. Stay disciplined in this approach to minimize the scrap rate variation between shifts.
- Train mill operators using various methods such as outside training seminars, tried and true techniques developed by previous operator experiences and developing a formal operator training program within your organization. Some companies rate their operators using quantifiable measures and tests that rank them from beginner to expert.
Do mill related problems cause excessive scrap?
Another culprit to unsatisfactory scrap rates is the alignment and maintenance of the rolling mill. Common problems such as spindle shoulder misalignment, bent spindles and spindle deflection cause numerous product defects that are difficult to trace and result in inconsistent setups from one run to the next. A tooling set is machined to specific tolerances and if the machine is not properly aligned, the rolls cannot operate as designed. Bent spindles and spindle deflection are also common problems and are both related to excessive force being applied to the material to achieve a proper bend.
Tips to maintaining your roll forming mill to reduce scrap:
- Conduct routine inspection of the mill, focusing on shoulder alignment from top to bottom spindles of each stand as well as stand to stand alignment. Consult with our technical staff or view related articles on our website for specific information.
- Check for worn and bent spindles and replace or repair as necessary
- Check bearings and pre-load settings and lubricate regularly
- If spindle deflection, caused by excessive pressure on the material, is resulting in increased scrap, consider additional passes to alleviate the forces required to form the metal in one or more passes, or consider the use of another mill that has a larger bearing configuration.
Find more roll forming articles here.