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Profit Pointer for Design Analysis of Tube Roll Tooling

 

Profit Pointer for Design Analysis of Tube Roll Tooling

By Charles Gehrisch, Robert A. Sladky and Greg A. Watt, Roll-Kraft Ohio U.S.A.
 
Profit Pointers, Design and Analysis of Tube Roll Tooling, is a continuation of ROLL-KRAFT's popular series of technical service bulletins provided to assist our customers with their production of high quality tubing products. The Profit Pointer series is another example of ROLL-KRAFT's commitment to excellence. We believe that an educated customer is a better customer. Our engineering staff reports that our customers have found Profit Pointers helpful in understanding the tube and pipe manufacturing process. The result is better communication regarding all aspects of this business and easier resolution of problems when they are encountered. ROLL-KRAFT is recognized worldwide as the industry leader in tube and pipe mill tooling technology. Our capabilities extend far beyond manufacturing. Our engineering staff, equipped with the latest computer technology, can design tooling to match your exact specifications and analyze existing tooling to ensure that it is correct for your application. ROLL-KRAFT's innovative REWORK PLUS program was designed specifically for this purpose-to analyze your design and tooling- free of charge! After completing the analysis, we'll recommend design changes and/or tooling rework that will improve the performance of your manufacturing operation.

Among the recommendations that might result from the REWORK PLUS analysis are the following:

1. Modify the existing tooling;

2. Add new passes to the mill to accommodate tooling design changes;

3. Rework existing tooling due to excessive wear; or Re-design the tooling.

Incorporating these recommendations will help you achieve your manufacturing objective - fabricating superior tubing and pipe products that are mark-free, with excellent weld and dimensional integrity.

I. Strip Formation Requirements in Each Pass

In order to properly form any tubing or pipe product, the entire roll forming process must be analyzed from beginning (flat strip) to end (finished tube). The amount of work, or forming, done from pass to pass should be even and progressive, provided the stands are evenly FIGURE A illustrates proper strip
formation for a low carbon steel using a standard edgeform design. Notice the equal spacing between each pass. Conversely, FIGURE B shows improper strip formation for the same tube size, material and mill configuration. The irregular spacing between passes indicates overwork in some passes and underwork in others. Many problems can arise from using this improper design. They include marking of the tube, roll over, buckling, bad welds and incorrect sizing.
II. Size Range of the Breakdown Rolls
Multiple tube sizes can be run on the same mill using only one set of breakdown rolls. Many companies do it and it can be accomplished with the four (4) most popular breakdown designs used in the industry today. To review, these designs are (1) conventional, (2) standard edgeform, (3) versatile edgeform and (4) "W", or reverse bend. They are pictured in FIGURES C, D, E and F, respectively. While all four designs are used to make multiple sizes, the versatile edgeform design is preferred for this purpose for several reasons. First of all, and most importantly, this tooling design does not require any roll changes. If you are using a versatile edgeform design, you can make multiple sizes if the material gauge of each size is the same. Secondly, this design will form the most strip into the edgeform radius in the breakdown section. Also, a much rounder tube is produced using this design. Thirdly, presentation of the strip edges into the fin passes is better than with the other breakdown designs. This results in consistent weld quality from size to size. Finally, the versatile edgeform design reduces springback. A design analysis will determine if multiple sizes can be made from the same set of breakdown rolls. To assist in this analysis, the following three questions must be answered:
1. Are the breakdown rolls designed to run multiple sizes?

A limited range of sizes can be made from one set of rolls. It is important that the tube being formed is within these parameters.

2. Does the mill have a separate motor drive for the breakdown section?

A single motor drive for the breakdown and fin section will present problems when the tooling is worn and must be reworked. For example, when the breakdown rolls are reworked (Note: They wear faster than fin pass rolls due to more use), the fin section, regardless of condition, must also be reworked to maintain correct progression, line speed and metal line. With separate motor drives for each section, the breakdown rolls can be reworked and the speed of the fin section adjusted for the reduction in root diameter of the breakdown section. Remember, a three motor drive system can only be applied to mills having universal drive stands that allow the bottom driven shaft to be shimmed to maintain pass line.
 

3. Is special material being used?

It is recommended that multiple sizes not be made in the same breakdown section when using special materials such as stainless steel or titanium. However, if multiple sizes are going to be run, versatile edgeform is the only breakdown design that should be utilized.

III. Mill Configuration

Mill configuration is very important to the engineer when designing tooling. He must know the number of stands and the type of welder used on the mill. The material that will be formed must be compatible to this configuration. If not, certain products cannot be fabricated to acceptable standards. As an example, refer to FIGURE G, which illustrates the flower for a 2.000" O.D. tube made from low carbon steel. The mill configuration for this tube requires two (2) breakdown passes and three (3) fin passes with side rolls between each stand. FIGURE H is the flower for the same tube. Note, however, the change in the mill configuration. Five (5) breakdown passes and four (4) fin passes are required. Why such a difference to make the same tube? Perhaps, the cosmetic requirements for acceptable vary between the two designs. While the tube represented in FIGURE G may be fairly round with a strong weld, marking may be present or the concentricity may not be acceptable to the end customer. Changes to the mill configuration can achieve specific results to produce a more acceptable product and many times it is necessary to add stands and passes. ROLL-KRAFT's engineers can make this determination after analyzing the existing configuration. It is important to remember, though, as a rule, that for a given tube size, the cosmetic quality of that tube will increase as more passes are added.

 

IV. Material Gauge
 
Compatibility of the gauge of the material being run with the tooling in the breakdown and fin sections is essential for proper strip formation. Furthermore, it is critical that the strip form correctly in the early stages of the breakdown section to ensure quality tube production. This is accomplished by using top breakdown rolls that match the gauge of the material. When running a thicker than specified gauge through the breakdown section, the strip cannot conform to the contour of the top roll (see FIGURE I). Obviously, the clearance between the top and bottom rolls is too tight for the gauge being used. This causes several problems. First of all, the edges of strip are pinched resulting in thinning of the strip, affecting its drive ratio through the mill. Secondly, the tube will be heavily marked, which is highly undesirable. Finally, no edge formation will occur, defeating the whole purpose of the breakdown rolls, and causing flat spots on the strip as it enters the next pass. When running a thinner than specified gauge through the breakdown section, the clearance between the top and bottom rolls is excessive (see FIGURE J). This also prevents the strip from conforming to the contour of the bottom roll, resulting in poor strip formation. In addition, slippage of the strip is likely to occur which will affect the drive of the tube through the mill. As is the case with running a thicker gauge material, virtually no edge formation occurs. As a result, the edges of the strip are not stabilized causing it to buckle. Flat spots are also likely. Top breakdown rolls are very important in handling gauge requirements. Most breakdown sets, regardless of design, will have several top rolls to cover the range of material gauges that can be run on a particular mill. In some cases, additional top rolls are required for the second and third breakdown passes. If a gauge which is thicker than the original design is to be run, the top rolls can be cleared out to handle this thicker gauge requirement. If a gauge which is thinner than the original design, or one which is difficult to form, is to be run, additional top rolls can be manufactured to match the bottom rolls. Finally, using the correct top rolls for forming high strength materials is especially important due to the increased difficulty in forming and welding.

 

V. Strip Formation by Side Rolls

One of the old myths in the roll forming industry states that side rolls only guide the tube; they do not perform any work. Closer to the truth is that with today's designs, side rolls are an important and necessary part of strip formation. Many times their contribution is neglected by designers, causing major strip formation problems. A review of the function of side rolls in each section of the mill is in order. In the breakdown section, side rolls contain the strip through the breakdown passes and keep it off the bottom breakdown rolls to help prevent marking, assist in its formation and allow a better presentation into the fin section. Round design fin pass side rolls hold the round shape of the tube and prevent peaking and marking. In the sizing section of the mill, the side rolls oval the tube and help the driven sizing rolls make the final tube round and also prevent marking. FIGURE K illustrates the use of the wrong side roll. The roll shown is a single radius design that is being used with edgeform design breakdown rolls. Notice that the roll does not contain the radius formed in the previous driven pass. This allows the material to spring back causing marking of the tube, strip roll over, buckling and flat spots on the strip in the next driven pass. This situation is remedied by using the correct side roll as illustrated in FIGURE L. The side roll contains the radius of the strip. It also helps to form and bring up the strip, keeping it off the sides of the next driven pass. The problems mentioned previously are prevented.

VI. Type of Material

Tooling design is directly affected by the material being formed. A design that works very well for low carbon steel may cause major problems if it is used to form stainless steel. Remember, one design does not fit all! Many companies try to use existing tooling to form high strength materials and they usually encounter problems. An analysis of the tooling could have prevented these problems before they occurred. Physical characteristics, especially the yield, or springback, vary from material to material. They must be taken into consideration when designing tooling. FIGURES M and N illustrate the flower design for two 2.000" O.D. tubes with a .120" maximum wall thickness. The difference between the two tubes is the material from which they are formed. One is low carbon steel, the other is stainless. Obviously, the same tooling cannot be used to form both tubes, unless the tooling is designed to make stainless tube. The springback of the material must be addressed in designing the correct tooling for each tube. Depending on the condition of existing tooling and the mill configuration, an alternative to new tooling exists. A REWORK PLUS analysis may determine that the existing tooling can be reworked to accommodate different materials.

VII. Breakdown and Fin Pass Design

Many factors influence the design of tube and forming rolls. In the breakdown section, the material being run is a major design consideration. When high strength materials are being formed with the standard edgeform design (see FIGURE D), the edgeform radius should be smaller than that used for low carbon steels. This will reduce springback of the material. The fin section of the mill prepares the edges of the strip for welding. The design of this section is influenced by the type of material being formed and the welding process used. FIGURE O illustrates the four (4) major fin designs used in the industry today. A brief description of each design is in order.

1. Single radius or round design.

This design is used primarily for low yield materials such as low carbon steel. The springback of the material does not cause major forming problems. The edges of the material remain parallel and the tube round as the strip enters the welder.

2. Compound flat oval design.

This design, in which the bottom roll may have a double radius, is used when forming materials having a significant amount of springback. This includes high strength materials and those with high or low thickness to diameter (T/D) ratios. The disadvantage to this design is the shape of the tube as it enters the welder Ñ it is not round. However, the advantage of this design is consistent, quality welds. The fin passes keep the edges of the strip parallel as they enter the welder. A slightly different version of the flat oval design uses a top roll with a smaller radius than the bottom roll. This will form a rounder tube compared to the compound design while keeping the strip edges parallel for welding.

3. High oval, or peak design.

This design is used for materials with very high T/D ratios (greater than 15 percent) or mill configurations that do not have fin pass side rolls. High T/D ratios require a significant amount of work between the last breakdown pass and the first fin pass. This design allows for gradual strip formation through the first fin passes and the use of a round or flat oval design in the last fin pass.

4. Finless (no fin blade) design.

This design is used primarily in the last fin pass for TIG or MIG welding when the strip edges must be butted together as the tube enters the welder. It can be either a round or oval design. If oval, the pass can be adjusted down to keep the edges parallel without marking the tube, as can happen with a round design. Before implementing a finless design, it is important to analyze the mill configuration, including the number of fin passes and the horizontal distance from the last fin to the welder. If the mill has side roll stands between the last fin pass and the welder, a finless last fin pass may not be needed.

 

When using modern breakdown designs, especially the versatile edgeform (see FIGURE E), the round, or conventional, fin pass design can be used more often than in the past. This is due to the fact that the edges of the strip are stabilized in the breakdown section. As a result, a simpler fin design an be employed. This is a very desirable situation in tube forming. Using a round fin design translates into less work required by the welding and sizing rolls to make the tube round. Finally, fin blade width and angle design should not be overlooked. Both parameters are important to correct strip formation. Incorrect fin blade widths and angles can cause marking of the tube and poor quality welds.

VIII. Fin Pass/Welder Compatibility

Formation of the outside periphery (girth) of the strip is very important when designing fin passes for different weld and material applications. The girth must be reduced evenly and correctly before presentation of the tube to the welder. Also, proper formation of the girth will assure that the edges of the strip are free of any burrs or roughness that would inhibit a good weld. Each welding process is unique in its requirements for strip presentation. Low frequency welding (ERW) does not require a large vee at the apex of the tube. A copper wheel pushes the material down, providing the electrical contact necessary for forging, or welding, the tube. High frequency welding requires a larger vee at the apex than low frequency welding, especially for tube sizes less than 1.5000". The larger vee prevents pre-arcing back to the last fin pass or any other rolls before the welder. Pre-arcing must be avoided to prevent poor welds, pickup on the last fin pass, loss of power at the welder and damage to the edges of the strip. Several factors affect the formation of the vee angle. Among these are the fin design, fin width and the horizontal distance between the last fin pass and weld rolls. This distance should be kept to a minimum, with the last fin pass and the first weld roll being as close as possible to each other. This will ensure correct vee angle formation and prevent roll over of the strip in this area. TIG and MIG welding require a butt seam presentation to the welder. To accomplish this, the last fin pass is often a finless design. As stated previously, the entire mill configuration must be analyzed before implementing a finless pass.

IX. Cosmetic Requirements

As mentioned in the introduction of this Profit Pointers issue, industry standards for quality have become more demanding over the years. Customers insist on and expect high quality tube and pipe products. The definition of quality frequently includes mark free tubing. The primary cause of marking, or pickup, is incorrect alignment and set up of the tube mill. After checking the mill for alignment and set up, an analysis of the tooling can be made to determine if this is causing the cosmetic problems. Corrective action can then be taken. Several design changes can be utilized to prevent marking. Floating flanges on the outside of the bottom breakdown rolls have proven to be very successful in this regard. Marking results from the extreme difference in surface speed between the throat, or root, of the roll and the higher contour of the outer flanges. The higher surface speed of the flange tends to pull the edges of the strip ahead of the center of the strip. By using this design, the strip actually floats on the outer flange preventing scuffing and marking (see FIGURE P). Marking can also occur in the fin and sizing sections from incorrect girth, fin blade width and side roll design. In attempting to eliminate this marking, some manufacturers put angles on the outer contour of the driven and side rolls near the rim. But this is not a solution to the basic problem of marking and pickup on the tube because the material is pushed into this angle resulting in cosmetic flaws.
ROLL-KRAFT solves this problem by using one of two approaches: (1) provide special clearances in the driven and side rolls, which has proven to be quite successful; (2) the use of floating flanges in the fin and sizing sections, which is simiar to the technique used in the breakdown section.

Both bearing and bearingless floating flanges are utilized. A typical bearing floating flange is shown in FIGURE Q. As its name implies, the flange floats on bearings for support and to reduce the friction between it and the root of the roll. With the bearingless design, the flanges turn on plain D-2 steel collars (see FIGURE R). Bearingless flanges work very well in preventing marking on tubes made from high strength and specialty materials. An alternative approach to eliminating marking and pickup is to change the roll material. The vast majority of forming rolls are manufactured from D- 2 tool steel; however, two options exist. One is the use of ampco bronze. Excellent cosmetic quality results when this material is used, especially when difficult applications are encountered. The biggest drawback is faster wear rates compared to D-2. The other option is carbide. The extreme hardness of this material prevents galling and marking and provides superior wear profit characteristics compared to conventional tool steels; however, carbide rolls cost more than those made from D-2.
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