Inspection and evaluation
April 11, 2006
Although most tube and pipe producers don't get too involved in the regrind process, it is crucial—reconditioning roll tooling can extend its useful life by 15 or 20 times. The regrind process reduces the producer's overall out-of-pocket tooling expenses, while helping to ensure the tooling continues to produce a consistent-quality product at the required speeds. A better understanding of the process, especially familiarity with the types of flaws that reconditioning can and cannot resolve, can go a long way toward a better working relationship between a tube and pipe producer and its regrind contractor.
Editor's Note: This is the first article in a two-part series on regrinding tube mill tooling. Part I discusses the first two steps of the five-step process: (1) receiving and inspection and (2) analyzing and creating work instructions. Part IIwill discuss (3) machining, (4) final inspection and documentation, and (5) packaging and shipping.
Most tube and pipe producers don't get heavily involved in the regrind process. In most cases, when their tooling shows signs of wear or can no longer make an acceptable product, they send it to a regrind contractor for reconditioning. Most roll tooling today is produced from AISI D-2 cold-worked tool steel. Because it is through-hardened during the initial manufacturing process, it can be recontoured or reground to restore the contours to their original design with no loss in hardness. The regrind process restores the tooling to original quality while minimally reducing the size of the tooling, which can be compensated for during the mill setup process.
Regrinding can extend the usable life by 15 or 20 times. The regrind process reduces the producer's overall out-of-pocket tooling expenses while ensuring the tooling can continue to produce a consistent-quality product at the required speeds.
Today, because of smaller inventories and shorter production runs in the tube and pipe industries, the amount of time a regrind contractor has to turn around a set of tooling has become significantly shorter. The regrind contractor must have a thorough knowledge of the customer's exact needs and requirements, which could include having preset standards of wear for particular actions to take place, requiring a certain number of spare fin blades with each regrind tooling set, or knowing if bearings or additional hardware may be needed with each tooling set.
A good, sound regrind process consists of five essential steps:
While these regrind procedures can be performed in many ways, they net the same final result.
The first step of the regrind process is the regrind contractor's receipt of the tooling. The contractor then assigns a tracking number to the set; removes all bearings; and cleans, degreases, and disassembles the tooling if necessary. The tooling is organized by pass numbers, skidded, and sent to the inspection department.
The inspection department performs a thorough physical and visual inspection of the incoming tooling and records the results on a regrind chart (see Figure 1). This chart is used throughout the regrind procedure for documenting all facets of the process and eventually is sent to the tube or pipe producer for its records.
A partial regrind chart shows the root diameter and roll widths of several sets of roll tooling. A complete chart includes the contour radius, roll width, face wear evaluation, bore, and notes.
The average regrind chart has six sections: rim diameter/outside diameter, root/throat diameter, contour radius, roll width, bore diameter, and special notes. Each section contains columns for the print size, the before-grind size, and the after-grind size. This allows for accurate information on how much material was removed from each roll during the current regrind and how much every roll is undersized from the original size.
Each roll is measured physically on the rim, root, radius, width, and bore, and those measurements are recorded in the before-grind column on the regrind chart. Each roll also is visually inspected for any obvious wear, cracks, chip-outs, or inclusions. All findings are noted in the special notes area on the regrind chart. Once all measurements have been taken and recorded, the regrind chart is passed on to a regrind engineer.
After receiving the chart, the regrind engineer uses the tooling prints to fill in the values of the print size column. The engineer reviews all of the values from the inspection process and performs a visual inspection of the complete tooling set, finally determining how much material needs to be removed from each roll.
When determining how much material to remove from each roll, the regrind engineer takes into account the pass configuration and tooling parameters of the tube or pipe mill on which the tooling mounts. At this stage the engineer does a tooling design review if the tube or pipe producer has indicated any particular areas of concern or desires improvements in the forming or welding processes.
The regrind engineer must be familiar with each tube or pipe mill, including variables such as number and placement of drive motors, gear ratios, root progressions, drive ratios, and any tooling subset parameters. Most important, the engineer needs to analyze the amount of wear on each roll.
After considering all of these items, the regrind engineer computes a stock removal value that will clean up the roll contours 100 percent while taking the least amount of material off the contour. Accurately figuring this number saves both the regrind contractor and the tube or pipe producer a sizable amount of money over time. Taking off too little material costs the regrind contractor because additional recutting will be needed to achieve 100 percent cleanup of the contour on all rolls. Taking off too much material costs the tube or pipe producer because it decreases the number of regrinds the tooling can have over its life.
The engineer then converts the amount of stock removal to an undersize value and subtracts this from the original print size to determine the value for the after-grind column. The engineer also uses the undersize value to determine the size of bearing block shims or adjustments that may be required when setting up the reground tooling on the mill. This value also is helpful in the future for determining when a tooling set has reached its minimum size.
If a tooling set has a split or constant bottom line design, side grinding of the inside faces is necessary to reduce the overall roll width and maintain the root diameter at original print size. The same considerations are used to determine the amount of stock removal necessary to grind off the faces on these types of rolls. Outside shim spacers or new roll shaft spacers are required with this type of tooling to maintain the overall roll width.
Also determined at this time is the need for welding or grinding of worn outside faces. In some cases, the area where the shaft spacer rides up against the roll face can become worn (known as face wear) because of misalignment in the tube or pipe mill. The misalignment causes an indent to form on the outside face around the bore.
This problem can be repaired with welding and grinding of the worn area or chrome plating and grinding of the outside face back to the original print size width. Another solution is to grind the roll to a revised width and supply new, revised-width shaft spacers to compensate for the revised roll width.
Bore sizes and conditions also must be analyzed to determine the need for plating and grinding the bores. Bore wear usually is caused by worn shafts, so the bore wear usually is not uniform. In fact, it often appears as a tapered roll bore. To repair this, the operator first grinds the roll bore concentric to the contour of the roll, then chrome-plates it to a sufficient thickness to allow for finish grinding. Finally, the operator grinds the plated bore to the print size specifications, concentric to the existing roll contour.
In some cases, it is necessary to weld some chip-outs or inclusions that appear in the contour area to reduce the amount of stock removal necessary for 100 percent cleanup. While chip-outs and inclusions may be repairable, large cracks that appear on a roll contour usually are not.
If a large crack does not clean up during the standard regrind, it usually is not a good idea to try to "chase" it. The best option usually is to replace the roll, if the tube or pipe producer agrees. In most cases, the regrind process cleans up small surface cracks.
Welding of tube or pipe tooling should be avoided if at all possible. Even when done properly, applying weld to a hardened tool steel roll can cause cracking because of the amount of heat that is put into the rolls during this process.
Once all the work details have been determined, the regrind engineer issues the work instructions to the regrind shop along with the regrind chart for the actual regrind machining of the tooling. If new rolls, fin blades, spacers, shims, hardware, bearings, or other items are required, they are specified on the work instructions or regrind chart.