By Baicheng Wen, Ph.D. Roll-Kraft, Inc. Ohio, U.S.A.
One of the goals of every manufacturing company is minimizing production costs in order to maximize profits. In the tube and pipe industry, this can be accomplished by selecting the correct style of tooling before production begins. Greater profits will result from increased mill efficiency, reduced downtime, and extended tooling life. Three basic styles of tooling are used to make tube and pipe—(1) solid, (2) split, and (3) carbide inserted. The total cost (initial, rework, and lifetime), as well as the useful production life, for these three styles varies greatly from one to another. This article will examine these cost differences, including the cost effectiveness of mill setup, to help tube and pipe manufacturers select the most profitable tooling for their particular operation.
2.0 Tube and Pipe Tool Designs
2.1 Solid Rolls
Solid rolls are the most common tooling used to make tube and pipe. This one-piece design has the lowest initial cost of any of the three styles of tooling due to its simplicity in construction. Solid rolls are preferred for several reasons. In addition to the initial cost factor, solid rolls with small root diameters are selected to produce high driving forces to maximize mill efficiency. Secondly, physical limitations of the mill, such as vertical center distance, roll space and bottom line, might require that smaller, solid rolls be used. Finally, solid rolls are more economical for limited production runs. The main drawback to using solid rolls is the cost of rework (this includes those additional costs associated with mill re-adjustment) and downtime that results from the rework. For example reworking a set of solid rolls reduces the root diameter. If these rolls are in the breakdown section, the forming line of the mill is lowered, requiring mill adjustment to raise the bottom driveshaft before production can resume. Shims will be required on mills with limited or non-vertical adjustment capabilities, or the side roll stands must be adjusted. These activities increase
downtime and operating costs. Additionally, reworked solid rolls turn slower, reducing forming speed if the drive speed on the mill is non-adjustable. The net result is a decrease in production. Also, reworking the solid rolls in one section of the mill might require reworking the entire set, regardless of condition or need, in order to maintain the correct forming line and/or mill speed.
2.2 Split Rolls
A split roll is a two-piece component bolted together to form a solid roll. A roll of this design is physically larger (wider flanges require more roll space on the mill) than a conventional solid roll used to produce the same size tube or pipe. FIGURE 1 illustrates a typical split roll.
When a split roll is reworked, the roll width is reduced to maintain the original root diameter and contour. As a result, mill setup is unaffected, mill speed and production rates do not change, and downtime is significantly reduced when split rolls are used. The only alteration required to the mill is the addition of spacers to each reworked split roll to compensate for the reduction in roll width. Due to the fact that reworked split rolls do not affect production parameters, only those rolls which are worn require rework. The biggest drawback to using split rolls is cost, especially in lower volume work. Both initial and rework cost for split rolls exceeds that of solid rolls; however, split rolls can be reworked many more times than a conventional design. Combine those savings with significantly reduced downtime for mill alterations and adjustments and the lifetime cost of a set of split rolls is less than that for a set of solid rolls.
2.3 Carbide Inserted Rolls
A carbide roll is a steel split roll with a carbide insert in the work area (contour) of the roll (see FIGURE 2). Due to the cost of this insert, this design has the highest initial cost of the three roll designs discussed. The advantage to this design, however, is found in the lifetime cost of the rolls, especially on large production runs. Carbide inserted rolls last 15 to 20 times longer than rolls made from conventional D-2 tool steel (depending on the material being processed). While steel rolls are being reworked again and again, carbide rolls keep working with the original contour. Another benefit of this extended tool life is tighter production tolerances for a longer period of time compared to standard tooling. Also, the coefficient of friction of the carbide insert is less than steel. Both of
these characteristics result in improved quality of the final product, both dimensionally and cosmetically.
3.0 Comparing Tooling Cost, Tooling Life and Productivity Among the Three Designs
3.1 Total Tooling Cost
The initial cost of a complete set of tube or pipe rolls varie greatly by design; the carbide rolls being the most expensive. But that is only a part of the cost. Rework cost, the number of reworks performed and tooling life are all factors in determining total tooling cost. The following chart illustrates this point. It is a cost comparison between solid roll and split roll tooling. The initial cost of the split roll tooling is 33 percent more than the solid tooling and the cost of one rework is 77 percent more. However, the solid roll tooling can only be reworked three times (depending on mill configuration) before total replacement, while the split rolls can be reworked as many as 15 times before replacement. As a result, five sets of tooling must be purchased to equal the life of one set of split tooling. The cost of downtime must also be considered when determining total tooling cost. The approximate cost to shut down a mill using split rolls is $2000 (15 reworks cost $30,000). The downtime cost for a solid roll mill is double that of the split roll mill due to the required adjustments to the driving speed and forming line (15 reworks equal $60,000). Combined with the initial and rework costs, split roll tooling achieves savings of $32,000 (18 percent). Tooling costs can be reduced further by using carbide tooling in the driven passes. While this tooling might cost as much as four times that of the solid tooling, it does not require rework over the life of the 15 sets of tooling. FIGURE 3 illustrates the cost comparisons between solid, split, and carbide inserted tooling.
3.2 Tooling Life
Tooling life is determined by the footage of product produced before the tooling must be reworked or replaced. Many factors affect tooling life including tooling material, tooling design, and mechanical properties of the material being formed (such as yield strength and toughness), material gage, surface finish and tube size. Consider cold rolled carbon steel being formed into a median tube size with a median t/d ratio. The tooling is installed on a mid-range mill with a three motor drive. Solid and split roll tooling can produce as much as 1 million feet before requiring rework. On the other hand, carbide inserted rolls might run up to 15 million feet! (see FIGURE 4) This graph also illustrates total footage for the life of the tooling (tooling life). It can be clearly seen that the tooling life of carbide inserted rolls far exceeds that of both solid and split roll tooling. In addition, the tooling life of reworked split roll tooling is greater than that of reworked solid rolls. The mill drive configuration and the amount of material removed impact the tooling life since the root diameter of solid rolls can vary by more than +/- 10 percent. Assume that 0.070” of material is removed for rework. A roll in a specific section of the mill, with a 7 inch root diameter, can be reground ten times (0.070” per rework) before the whole set must be reworked. A typical set of split roll tooling can be reworked 14 times without changing the root diameter. Depending on the minimum flange width of the roll, the root diameter could be reworked an additional 4 to 5 times.
Productivity (feet produced per dollar) is calculated by dividing the production footage by the total tooling cost. This parameter can be used to evaluate each of the three different tooling designs. The results are shown in FIGURE 5. This
graph shows that productivity is increased by at least 30 percent (8 feet/dollar vs. 6 feet/dollar) by using split roll tooling instead of solid roll tooling. However, significant gains in productivity can be achieved if carbide inserted tooling is selected. On average, productivity can be increased by a factor of three (greater than 25 feet/dollar) when utilizing this design of tooling. It should be noted that these productivity increases are more likely to be realized in long, continuous production runs.
4.0 Other Cost Reduction Considerations
A common cost reduction practice among tube and pipe manufacturers is the use of more than one style of tooling on a mill. For instance, using split roll tooling in the fin or sizing section and solid tooling on the rest of the mill. Success in reducing costs can be achieved if the mill operator is experienced in determining the
correct combinations of tooling to use. On the other hand, inexperienced operators can make the wrong decisions that result in both increased downtime and operating costs. Proper training in mill operation, forming processes and forming can help these less experienced operators make better tooling decisions to obtain greater mill efficiencies and lower operating costs.
The comparisons of tooling costs in this article were a general discussion. For a specific forming operation, the manufacturer needs to gather as much information as possible in order to select the correct, low cost tooling. The first step in the tooling selection process is gathering all the mill specifications, including the mill layout. Secondly, define and verify the material that will be formed, the mechanical properties of that material and the anticipated production volume. Finally, refer to the discussion in Section 2.4 to calculate the projected tooling cost for each tooling design in order to select the lowest cost tooling for that particular production run. Tube and pipe manufacturers should also consult with their tooling suppliers to answer specific questions about their particular forming operation. Technical assistance from suppliers on cost comparisons and tooling performance is invaluable in the tooling selection process.