October 11, 2001
This article discusses the prevention of problems associated with tube mill tooling, touching on maintaining tooling, as well as troubleshooting common problems that may arise during the tube production process.
In today's tube and pipe producing industries, the need to run the tube/pipe mill faster, reduce changeover times, and produce a higher-quality tubular product has put an increased demand on the tube mill tooling.
The tooling's job is to take a continuously coiled flat strip, form it so that the two edges come together, hold the edges in place while they are welded, and then size and straighten the finished tubular product to the required specifications. With some tube mills running at 1,000 feet per minute, the tooling must be able to perform consistently.
This article discusses some troubleshooting practices to help mill operators correct and compensate for some of the most common tooling problems that arise in tube production. However, before production troubleshooting is discussed, consideration should be given to preventing trouble before production begins.
Specifying. When requesting a quotation for tube/pipe mill tooling, the tube producer should specify all required tooling set parameters, including the tube size, wall thickness, material type, and required tolerances. The mill type also should be specified, along with any new features or recent modifications to the mill.
Ordering. The tooling order should be placed with an experienced tube/pipe mill tooling designer/manufacturer. All necessary tube parameters should appear on the purchase order when the order is placed.
Receiving. Upon delivery of the tooling, the tube producer should receive from the manufacturer a complete set of tooling drawings and a tooling setup chart, along with written verification that the tooling was physically inspected and that it conforms to all of the design parameters specified on the drawings. The tube producer also should visually check the tooling when unpacking it from the shipping container to ensure no damage occurred during shipment.
Maintaining. Storing the tooling in an organized and clean fashion will assist the mill operator when it is time to set up on that size again. Tooling should be cleaned as it comes off the mill and inspected for damage or wear; bad bearings should be replaced at this time. Special attention should be given to worn faces, bores, or contours and for pickup on the contour areas; these problems should be taken care of before the tooling is stored.
All tooling racks should be labeled clearly and correctly to help prevent the loss or misplacement of tooling, spacers, or shims.
Keeping Records. The purpose of keeping good records is to achieve an adequate tooling condition for each rolling, maximizing the footage run and quality of the product while minimizing reconditioning costs and providing for timely reordering of worn-out rolls.
A chart of the actual setup gaps between rolls should be kept for every run. This chart should be filled out for each pass on the mill at or near the end of each production run. This will allow the operator to start up the next run where the last one left off and will allow for a comparison between the actual running gaps and the gaps specified on the tooling manufacturer's setup chart. When the two gap measurements begin to differ excessively, tooling reconditioning may be required.
Reconditioning. A tooling set may need to be reconditioned when:
Reconditioning should be done by an experienced tube/pipe mill tooling designer/manufacturer. It usually involves taking a light cut of material off the tooling contours and returning all of the contour dimensions and root progression back to original specifications. The diameters, however, typically are undersized by a certain amount and need to be compensated for accordingly.
If excessive wear appears on the roll faces or bores, they must be welded and/ or plated and then ground back to original sizes. In some cases, some of the tooling may even need to be replaced.
The results of the tooling recondition process should be documented and submitted on a regrind chart that shows print size, before size, and after size. This chart should be kept with the other tooling set records.
Roll tooling often is considered the most critical part of the tube mill operation because it forms the tube shape that controls most of the mechanical factors that influence weld quality. However, the tooling will perform only as well as the mill on which it is installed.
Therefore, the mill itself, its maintenance, and its alignment are the foundation that determines the tooling's ability to control the mechanical factors critical to the welding process.
Some potential mill problems include bent or worn shafts, worn housings, worn bearings, and general mill misalignment. Other problems can be generated by the mechanical drive system, the welding equipment, and/or the coiled strip.
Following are some common problems that could arise during a production run on a tube/pipe mill, along with possible causes and solutions. Only the most common problems and solutions are discussed here.
Pinching. Putting a strip that is too thick into the tooling set usually causes pinching or deformation of the strip edges in the breakdown section. When tooling is designed, the tube producer specifies a maximum strip thickness. The tooling then is designed based on this parameter; anything thicker causes the strip edge to be pinched. To run a thicker strip, the top breakdown tooling must be modified.
The pinching condition also can be due to problems on the mill, such as unparallel roll shafts, roll shaft shoulder misalignment, or tooling misalignment.
Rolling. This problem can occur anywhere in the mill; however, it is most common between the welding section and the sizing section. Tooling surfaces that are not aligned properly usually cause rolling. This problem could be in the location of contours on the tooling, or it could be a mill alignment problem.
All driven shafts should be checked for parallelism, and all side passes should be checked for on-centerline adjustment. In most cases, the problem can be narrowed down to one stand. It sometimes can be corrected by adjusting the mill, or it may require some shimming.
Rocking. The problem of strip rocking back and forth usually takes place near the end of the breakdown section and/ or before the fin section. Rocking can be caused by inadequate or nonexistent edge guiding or edge trapping, as well as improperly adjusted tooling and excessively narrow strip.
The easy way to compensate for this problem is to adjust the entry guide, trap tooling, and/or fin tooling and make them tighter to accommodate the existing inadequacies. The long-term solution is to have the tooling designer review the design and/or to modify the incoming strip width.
Buckling. Buckling is an undulating (waviness) of the upper part of the strip near the strip edges. This problem usually is found in the transition between the breakdown section and the beginning of the fin section. Buckling typically is a problem only on light-wall tubing products; it generally appears on products having a 40-1 or larger ratio of diameter to wall thickness.
This condition usually is caused by a speed differential between the strip's upper and lower areas of the strip and a lack of tension on the strip. It can be contained by keeping a constant pressure on the upper part of the strip, which provides additional support to the strip's upper edges. This pressure can be built into the tooling, but usually it needs to be supplemented with an additional mechanical device placed between the breakdown section and the fin section.
Additional tension can be created on the strip in a number of ways, such as increasing the root progression, using a different forming application, and/or incorporating a downhill forming technique. However, care should be taken not to create so much tension that the strip experiences stretching problems.
Breathing. This term describes the in and out motion of the strip as it is being formed. Although it occurs naturally throughout the forming process, it must be kept to a minimum in the welding section by proper root progression in the tooling.
Tooling that is running out excessively is a major contributor to breathing. The runout can be caused by incorrectly made tooling, worn bores on the tooling, worn or bent mill shafts, or general misalignment. Good tube/pipe mill maintenance procedures can help minimize this problem.
Fluctuating Size. Size fluctuations usually result from problems with the sizing section tooling. If the problem is with a new tooling set, each pass, from the weld pass to the last sizing pass, should be checked to determine where the problem is occurring. If the problem cannot be compensated for on the mill, the tooling most likely needs to be corrected. If the problem is with an existing set of tooling, a recontour of the tooling set probably is needed.
Marking. Marking is very common and can occur in just about any pass on the tube mill. It can be caused by anything from incorrectly made rolls to worn-out rolls, from misalignment of the mill to the use of incorrect lubricants. It most commonly occurs at the gap area where the two rolls meet.
A marking problem often can be narrowed down to one pass. When the problem pass is located, the operator should analyze the tooling and mill in that pass for possible causes, verify that the tooling in that pass matches the print, and verify that proper relief angles are machined onto the tooling.
Once the cause of marking is corrected, accumulated pickup should be removed from the tooling. If the problem persists, the tooling manufacturer should be consulted.
Eliminating potential problems before production start-up greatly reduces trouble when the product is run. Fewer problems during production usually result in a better-quality product, more uptime, faster production rates, and less scrap.
Mill operators should get involved with the tooling design process and present the designer with questions, concerns, and ideas. Supplying feedback to the designer helps ensure that problems are not repeated on tooling sets in the future.
Along with establishing this relationship, tube producers must maintain the tube mill and tooling in the best possible condition and keep accurate records of production runs and tooling. The result of these actions should be increased mill production and a better bottom line for the tube/pipe producer.