Troubleshooting a cold tube reducer starts when it is being designed.   When the decision to use PLC controls instead of computer controls (or the opposite choice) is made, a major decision concerning troubleshooting is also set.   The reliability of the mill is determined before it is purchased.    Preventive Maintenance programs can not build reliability into a machine.   Predictive Maintenance only determines when parts will fail.   The percentage of downtime, the length of time for operator tooling changes and the cost of operation are determined in the design phase.   Examples of decisions that influence troubleshooting would be:

AC Variable Speed motors instead of DC motors

Servo controlled feed instead of a Cam mechanism

Cartridge Rollstand instead of a Standard Rollstand

Tapered Mandrel instead of a Cylindrical Mandrel

End Loading machine instead of a Side Loading mill

Three Die Rollstand instead of a Two Die Rollstand

Entry Chucks only instead of Entry and Exit Chucks

    The prices of seamless tube mills today are measured in millions of dollars.   A One Inch Mill can cost between one or two million dollars.   A Four Inch Mill can cost around five million dollars.   After the mill is purchased, the installation costs can exceed one hundred thousand dollars. Standardization of parts, where possible, is necessary to keep inventory costs down.   This includes the rollstand area of the mill.   It is very important to make the right decisions to remain competitive in the world market.

    Most of the mills in use today use a crankshaft to drive the reciprocating rollstand.   There are many styles of cold tube reducers.   Some examples are:

A single rollstand driven by two connecting rods

Two separate rollstands driven by one con rod each

Stationary rollstand with reciprocating mandrel & starting shell

Hand size three die roll assemblies that are removable

Refrigerator size rollstands driven by two light weight con rods

Hydraulic feed & rotation

Cam or servo motor driven feed & rotation

Multiple dies sets in the same rollstand

    Many different designs have been tried and most of them work with varying degrees of success.   The one item that remains constant with all of today’s designs is the rolling principle in the rollstand area.   It doesn’t matter whether the mill has three dies or two dies, the principle used is the same.   As the dies reciprocate and rotate along an axial path, the pitchline of the dies must match the surface of the reduction cone to produce blemish free surfaces.   Whether the mill uses linear drives and computer controlled servo motors or a crankshaft and a rack, the metal forming principle in the rollstand area is the same.

    The goal of a cold tube reducer is to produce metallurgically correct, blemish free surface finishes, within specified tolerances, at the highest possible feed per stroke and at the maximum speed that the mill is capable of.   There is somewhat of an order that must occur to accomplish this goal:

                1.    Define the Reduction Schedule

                2.    Develop the Tooling Designs

                3.    Obtain Blemish Free Surfaces

                4.    Highest Possible Feed & Speed

                5.    Meet Metallurgical Properties

                6.    Return to Step #1 and Re-evaluate Each Step  

    It usually takes several iterations of these steps to develop a process which will produce a quality product that exceeds feed rate expectations.

    The reduction schedule is one area of troubleshooting that is often overlooked. Reduction Schedules are determined based primarily on the die technology available when they are developed.   If your reduction schedule was developed years or even decades ago, it’s time to revisit the issue.  Tooling technology has improved starting with the availability of high quality die grinders in the 60’s.   When the high quality die grinders became available, it became possible to control the tolerances and the repeatability of the die groove designs.   The same technology improvements have occurred with mandrel grinders. Therefore, if your reduction schedule was derived years ago, it may be a primary source for troubleshooting your particular rollstand issues.   The capability of your mill to handle the forces required to deform a particular metal may also be an issue.   The length of the mill stroke and the available working cone length is definitely an area for concern.   Some of the older mills have very short strokes that make it very difficult to perform higher reductions.   Mills with longer strokes and working cone lengths give the tooling designers more latitude with their designs.   If you are using a reduction schedule that was designed for softer, more malleable material to form a high strength material, you will increase your chance of having difficulties in the rollstand area at production time.   No one wants to revisit reduction schedules because it’s a lot of work but if you ignore it, you will be trying to troubleshoot the problem at the mill - which may be too late.

    The cross sectional shape is the most significant element in a cold tube reducer die design.   The reduction cone shape is the second item of importance

    The side relief in an HPTR groove is approximately equal to the starting tube diameter and the base groove is approximately equal to the final tube diameter.   Therefore, we can refer to it as a very wide groove.

A Typical , Two Die, Wide Groove Design

    The side relief in a wide groove, two die design, usually is not as great as the side relief in the HPTR design.   A wide groove design is capable of high feed rates and smooth OD/ID surfaces.   The base groove can be narrow to control roundness.   This design can be the best of both worlds.   Usually, the base groove is kept very narrow and the relief area is as wide as possible to provide plenty of space for the deformed metal to flow without scuffing.

A Typical Two Die Narrow Design

    The side relief in a narrow design may consist of a very shallow relief. Usually the narrow designs have a minimum of side relief which is almost unnoticeable.   The narrow groove designs usually have low feed rates but the tube roundness can be controlled easier than the, high feed, wide designs.   Die to die alignment is critical to prevent surface scuffing in narrow designs.

    The fourth die cross section type is the single radius design which is very similar to the narrow design except the single radius can be wide or narrow.   The single radius is capable of good feed rates when wide and low feed rates when narrow.   The single radius is a good general purpose cross section design.

    Reduction cone shape is not an easy subject to explain using only text.   The cone shape is usually an exponential curve which is nearly flat at the die size point and increases in steepness as it approaches the starting size.   Almost any cone shape will make a tube but the goal is usually to improve die and mandrel life.   A satisfactory cone shape is one that produces a good tube, good die life and random location mandrel failures.   Sometimes lubrication failures, from excessive pressure or heat, are caused by pressure points in a poor cone shape design or a cross section shape that is too narrow.

    Surface quality is a function of the indexed location of the dies relative to the axial position of the reduction cone.   Most mills use a rack and pinion to nearly accomplish this task.   The key elements are:

Passline Alignment

Die to Die Alignment

Pinion to Rack Backlash

Die Bearing Radial Clearances

O.D. Lubrication

I.D. Lubrication

Controlling I.D. Freesink between the tube & the mandrel

    Since a pinion has a constant pitch diameter and a reduction cone wants a variable pitch diameter, a balance between the pinion size and the die O.D. must be found.   A mill with a severe mandrel rod snap, during operation, is a sign that the optimal pinion to die relationship is missing.   Severe mandrel rod snap pushes and pulls the reduction cone causing O.D. & I.D. surface blemishes and usually creates tubes that require some rework.   When the almost perfect match between the pinion and die diameter is found, the mandrel rod will be motionless, except for rotation and show no indication that material is being feed into the mill and deformed.   Some people have never witnessed this situation and probably won’t believe it until they see it.   When the mandrel rod stops snapping, either the mandrel came off …..or the machine is finally running the way it should.   It’s worth the effort to synchronize your mill until the mandrel rod stops snapping & jumping around.   The tube surface condition will be the reward. Both the O.D. & the I.D. will be smooth and rework will not be required.

    High feed rates are achieved when the available mill stroke provides the designer with more reduction cone length than is required, the straight section is not too long, the groove cross section is of the wide design and the mill synchronization is nearly perfect at the desired high feed.   At this point it is possible to change the effective pinion diameter by changing the feed rate.   A feed increase is the same as increasing the die diameter.   A feed decrease is the same as decreasing the die diameter.

The key items to focus on when troubleshooting your mill are:

The Reduction Schedule

Tooling Designs

Rollstand Clearances & Alignments

Reduction Cone Quality

High Feed Rates

Maximum Mill Strokes per Minute

    I hope that these tips will help you to troubleshoot your mill. Troubleshooting starts in the design phase not when the tooling is installed in the mill.  Once the tooling is placed in the mill, there are only a few things that the operator or engineer can adjust to make good tubes.        Good Luck!