July 26, 2001
Tube mill changeovers involved more than just tooling. Fast changeovers require control over many variables (entry equipment, side pass stands, and so on). Above all, two factors are critical in fast, consistent setups: regular maintenance of the mill and tooling, and reliance on written procedures.
A changeover on a tube mill does not mean just changing the tooling. It means changing from running good tube of one size to running good tube of another size.
Many machine configurations can speed up a changeover, such as rafted sections, quick-change fasteners, etc. However, fast changeover is usually just a matter of having the mill and tooling in good dependable condition and organizing the rest of the efforts of the changeover process.
How long should a changeover take? This question has no specific answer because the variables are too numerous and widespread. Not all mills are of the same configuration. Some are more operator-friendly than others. Some have more passes, which require more tooling than others. Is the mill in good, dependable condition? Some claim to change a mill over in an hour, but then spend hours, if not days, chasing all the problems the mill or tooling might have before good tubing is produced.
Other factors that affect the speed of a changeover are:
The mill and tooling are fixed items that, if maintained properly, will perform the same way each time they are used. The biggest variable is the human factor—how each worker maintains, sets up, and operates the mill. One worker has a way to set up the mill, and another has a different way. Day shift has one way, and night shift has yet another.
The whole process can be predictable each time, which is why written procedures must be a part of any operation. These procedures not only allow for predictable and consistent setups but can be used for troubleshooting and for training of new personnel. This article outlines some important tube mill variables that must be identified and maintained to effectively speed up mill changeover, regardless of the equipment used.
The condition of the mill and its alignment must be reliable during a changeover. If the mill is in poor condition and out of alignment, changeover times will take longer.
The entry table has a very important job to do. It must center and guide the material to the first breakdown pass. Material introduced off-center from the start will cause problems all the way through the mill.
The following checklist can help prevent problems with the entry equipment:
On driven passes, operators should ensure the following:
On side pass stands, the shaft ODs must be within tolerance, and the shafts must be tight within the blocks to which they are mounted. The blocks also must be tight within the gibs that retain them.
In-and-out and side-to-side adjustments should be tight, with no end play. Tie bars, where used, must be in good repair, with all fastener parts intact. Units that are adjustable in height should be set to the proper metal line.
If brass washers are used on the top and bottom of the side rolls, they can be converted to thrust-type bearings. This eliminates the wear variables of the brass washers.
On "M"-style mills using solid tooling, the metal line drops, thus requiring the side roll passes to be lowered to match the new metal line. A good tooling maintenance and setup program requires that personnel know exactly how much to drop or raise the metal line of different roll sets at the time of setup. This precision not only saves valuable setup time but affects the quality of the end product as well.
The weld box must be in good mechanical repair, and it must be dependable. A variety of weld box configurations and features is available, so common sense must be used in evaluating the integrity of the components.
For those units in which the weld rolls are held in by clevises, extra clevises should be kept on hand with weld rolls already installed for fast setup during a changeover and for quick replacement during a run.
The metal line is affected by roll changes with certain weld box designs. The ODs of the weld rolls that are being installed, as well as how they will affect the metal line of the weld box, should be known.
The weld rolls should be set up with a plug gauge of the same diameter as the welded tube size to match the radius of the weld rolls. This is especially effective in three- and four-roll weld boxes.
As with the weld box, for those units with clevis-mounted rolls, extra clevises should be on hand to speed up changeovers. Once the rolls are set up with a plug gauge, they should be checked for even clearance between all rolls. The use of a plug gauge is especially effective for square and rectangular setups.
Tooling should be organized and ready to install on the mill. Jib cranes with tooling fixtures can save time in handling tooling and help reduce operator safety risk during the removal and installation of roll sets.
Knowing the rim clearance figures from the last successful run can also save time, because the operator can start where he left off. This can help ensure that prime product is run with minimum scrap.
Proper tooling maintenance is essential in achieving faster changeovers. A program should be in place to ensure maintenance is being performed and that tooling is not just taken off the mill and put in a box or on the shelf until the next installation. This maintenance program should include more than just the tooling itself. Records should be kept of tooling prints, setup charts, rework, in-house maintenance, rim clearances, and costs.
With proper tooling maintenance, the operator should be able to answer the following questions:
No matter what type of equipment is used in a tube mill, fast changeovers depend on the steps discussed in this article. Even a company with all the latest quick-change equipment needs to perform outlined procedures in maintenance, setup, and operation to make changeover successful.
Achieving faster changeovers requires consistency and discipline. Changeover programs are often set up but then not monitored. The standards that are set in any operation require the participation of everyone involved: management, maintenance, operators, supervisors, and general laborers. All need to share in the responsibility of making the process of faster, more efficient changeovers work.
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