Solving problems on the tube mill

THE FABRICATOR® FEBRUARY 2001

August 16, 2001

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Tube mill operators face a variety of challenges every day in their efforts to produce high-quality tubing in a cost-effective and productive way.

Tube mill operators face a variety of challenges every day in their effort to produce high-quality tubing in a cost-effective and productive way.

This article examines some of the typical problems producers encounter, some common causes of these problems, and some ideas for how to solve these problems.

Lost Mill Time During Operation and Changeovers

Often, excessive downtime during normal operation or tooling/job changeover can be attributed to one or more of the following causes:

1. No written procedures for setup. Every mill should have written procedures for all operators to follow. The machine, tooling, and steel are fixed factors in the mill setup equation; the only variable is the human factor. This is why it is so important to have written procedures in place to control the process. Written procedures also provide a tool for troubleshooting when problems arise.

2. No setup chart. Tweaking the mill during setup loses valuable setup time. Operators must work the tooling the way it was designed. This means setting up to the parameters of a setup chart.

3. Lack of formal training. Formal training helps operators perform the procedures for tube mill operation and maintenance and ensures that all operators are on the same track.

4. Disregard of parameters from previous setup. If the tube mill has been set up according to the written procedures and setup chart, the operator can write down the numbers from the digital readout on the single-point adjustment (SPA) unit, allowing the next operator to set up where the first left off. Setting up to the numbers can save as much as 75 percent of total setup time, as long as all the other tips discussed in this article are followed.

5. Mill in poor condition. A poorly maintained mill costs valuable time and scrap during setup and operation. The mill must be dependable so that the operator is not chasing mechanical problems during normal operation and setup. A good maintenance program, as well as rebuilds or upgrades when necessary, is essential.

6. Mill in misalignment. Tube mill misalignment, poor mill condition, and inaccurate setup account for 95 percent of all problems in tube production. Most mills should be aligned at least once a year.

7. Tooling in poor condition. Operators must know how much life is left in the tooling before the next scheduled rework. Running the tooling until it cannot produce tubing anymore not only wastes valuable mill time, but produces scrap and affects delivery schedules. All tube production companies should have a tooling maintenance program in place.

Any of these causes of lost time on the mill can have varying degrees of value, depending on the severity of the conditions. The bottom line is, the more of these items that are in control, the less downtime on the mill.

Splitting in the Weld Zone

Weld zone splitting can be a result of some or all of the following:

  1. Overly narrow strip with insufficient material to forge
  2. Poor alignment or setup
  3. Insufficiently worked fin passes, so the edge is not prepared for welding
  4. Poor slit edge
  5. Off-center strip approach (strip rolled over) to the weld box, preventing forging between the weld rolls
  6. Nonparallel edges entering the welding machine
  7. Inappropriate weld power for mill speed
  8. Poor-quality steel with improper chemistry

Irregular Size in the Sizing Section

When irregular size occurs in the sizing section, the problem may not necessarily be in the sizing section itself. The operator also must check the setup in the breakdown, fin, and welding section of the mill to ensure proper presentation to the sizing section. If the forming section sends improperly formed tube to the sizing section, irregular tube size can result.

The operator also should check for bent shafts, oversized bores on the tooling, or undersized outside diameters (ODs) on the driven shafts. The integrity of the side roll boxes also should be checked.

In addition to these checks, the operator should consider the following questions:

  1. Is the weld size in accordance with the setup chart?
  2. Is the weld size round?
  3. Are the strip edges parallel, with no step going into the weld rolls?
  4. Is the weld scarf smooth?
  5. Are rework shims installed under the bottom driven shafts to maintain the metal line?
  6. Are the correct spacers installed on the driven shafts and to the correct length?
  7. Are the bearings and bearing blocks tight?
  8. Are the side rolls parallel?
  9. Is the tube being cooled properly?
  10. Are all the drives coordinated and adjusted to match the rework of the tooling?
  11. Has the chemistry or hardness of the material changed?

Weld Chatter

Weld chatter is the inability to achieve a clean cut of the outside weld bead after welding. The scarf knife chatters and produces a ribbed or rough cut on the OD of the tube. This is unacceptable in most of the end products produced by the tube and pipe industry.

Several techniques can be used to prevent weld chatter.

The scarf knife should have a slightly larger radius than the tube OD. This will provide a concentric, clean cut.

An ironing pass should be used after the scarf stand. As the name implies, this stand irons out any hot imperfections the scarf knife may leave behind. It also adds a tremendous amount of stability to the scarfing operation.

On mills that employ induction welding, moving the induction coil upstream a bit and away from the weld rolls helps temper the edges of the strip by preheating them before welding. This results in a more malleable material that is softer and easier for the scarf knife to cut.

The heel of the scarf knife or insert should be ground to an angle of 18 degrees from the horizontal, and the tool should be set at an angle of 15 degrees from the vertical. This provides the proper clearance so the knife does not drag on the tube or pipe. A straight up-and-down approach to the tube or pipe invites chatter.

If weld chatter occurs after these preventive measures have been taken, the cause of the chatter may be one or more of the following:

  1. Insufficient upset, or weld bead, for the scarf knife to cut
  2. Insufficient weld power to forge properly
  3. Scarf stand too far away from the weld box, making the weld bead too cool to be cut effectively and smoothly
  4. Weld is cooled too fast after the weld box or before the scarf stand
  5. No V-style support roll under the scarf knife, allowing the tube to drift and jump
  6. Improper scarf knife radius
  7. Dull insert or cutting knife

Strip Rolling in the Breakdown Section

Sometimes, the breakdown section can be the site of problems with strip rolling from side to side, or rolling high on one side. Some possible causes include:

  1. Loose entry table.
  2. Uncentered strip entering the mill.
  3. Loose driven stands.
  4. Unparallel breakdown stands.
  5. Improper design for extremely light or heavy gauges.
  6. Breakdown rolls worn beyond tolerance.
  7. Loose side roll boxes.
  8. Side rolls not set to the metal line.
  9. Side rolls not set to the setup chart.
  10. Tie bars not installed on the side roll passes, causing the rolls to float instead of to control the strip.
  11. Camber from slitting.

Tube Marking

In Breakdown Section. Tube marking in the breakdown section is often a result of the following:

  1. Incorrect strip width
  2. Uncentered strip entering the mill
  3. Driven rolls improperly adjusted to gauge
  4. Loose driven stands
  5. Unparallel driven rolls
  6. Driven rolls uncentered with side rolls
  7. Driven rolls not set to metal line
  8. Drive revolutions per minute (RPM) not coordinated with fin and sizing RPM
  9. Improper design for extremely light or heavy gauges
  10. Loose side rolls
  11. Side rolls not set to metal line
  12. Side rolls not set to setup chart
  13. Rolls worn beyond tolerance

In Fin Section and Weld Box. Tube marking in the fin section and weld box can be caused by:

  1. Overly wide strip.
  2. Driven roll settings adjusted too tight.
  3. Unparallel driven rolls.
  4. Loose driven stands.
  5. Driven rolls not set at metal line.
  6. Drive RPM not coordinated with the breakdown and sizing RPM.
  7. Fin blade worn beyond tolerance.
  8. Overly wide side roll settings.
  9. Loose side roll boxes.
  10. Side rolls not set to metal line.
  11. Side rolls not centered with driven rolls.
  12. Unparallel side rolls.
  13. Improper tube size between each driven and side roll pass.
  14. Improper forming in design.
  15. Overly tight weld roll settings.
  16. Rolls worn beyond tolerance.

In Sizing. In the sizing section, the following can cause tube marking:

  1. Overly tight driven roll settings
  2. Loose driven stands
  3. Driven rolls uncentered from pass to pass
  4. Unparallel driven rolls
  5. Driven rolls not set to metal line
  6. Drive RPM not coordinated with the breakdown and fin RPM
  7. Bent shafts
  8. Overly wide side roll settings
  9. Loose side roll boxes
  10. Unparallel side rolls
  11. Side rolls uncentered with driven rolls
  12. Side rolls not set to metal line
  13. Improper tube size between each driven and side roll pass
  14. Rolls on the incorrect stand
  15. Improperly cooled tube
  16. Overly tight Turk's head roll setting
  17. Rolls worn beyond tolerance

No Weld, Caused by Improper Forming

The following may cause improper forming, which leads to an inability to achieve a weld:

  1. Overly narrow strip
  2. Uncentered breakdown and fin pass rolls
  3. Incorrect settings in breakdown and fin pass sections
  4. Fin pass not working strip
  5. Fin blade worn beyond tolerance
  6. Weld box uncentered with fin pass section
  7. Weld box not set to metal line
  8. Overly wide weld roll settings
  9. Mill speed too fast for welded capabilities
  10. Weld rolls worn beyond tolerance

Check It Often

In general, several tube mill areas should be checked on a regular basis. This should be done at least monthly, but should be based on the usage. A higher production rate or running heavier metals through the mill requires more frequent checks. Shafts should be checked for OD, looseness, bending, and parallelism. Shoulder alignment should be checked, and the integrity of the entry table, drive stands, side roll boxes, weld box, and Turk's head units should be ensured. Of course, rolls should be checked to ensure they have been installed on the correct stands.

Once a year, the mill should be aligned. A mill alignment usually takes one or two days and is most often done by a professional. Every day, the mill operator should use a setup chart and follow all operating procedures.

The operator also should know the chemistry, Rockwell hardness, width, and thickness of the strip entering the mill and should document these values. Tube size should be measured between each pass.

Most important, for high-quality, consistent results in tube producing, an operation standard should be established for all employees to follow.



Robert Sladky

Vice President - Tube Mill Engineering
Mill-Tech, a Division of Roll-Kraft Inc.
10940 North 142nd
Waverly, NE 68462
Phone: 402-786-2601

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