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FLEXO Magazine : November 2008
TECHNOLOGIES & TECHNIQUES tion. The point of interest plays a large part in measuring TIR in some cases. If you are measuring a conventional wide-web print cylinder with a potentially bent journal, it is likely that the further out the dial indicator is placed along the journal, the greater the TIR will be. The cylinder or roller is then rotated and the total dial indicator reading is observed while the component is rotated. The indicator reading should not exceed the tolerance given by the specification of either the manufacturer of the component or within the industries recommendation. CONCENTRICITY & TIR TIR is sometimes confused with concentricity. Concentricity is two shapes sharing the same center point, or the degree to which a given dimension resembles a perfectly round circle or cylinder. Concentricity is more complex than TIR, as it controls opposed median points to a cylinder axis. It can be verified with two or more dial indicators. The two diametrically opposed dial indica- tors are placed on either side of the cylinder or roller and posi- tioned and rotated about the area of interest. Several readings are obtained at each selected cross section along the entire length of the cylinder. This measuring technique is much more commonly done during the manufacturing and QC process of the cylinder or roller, versus in the field where a TIR measurement is more commonly used. The question one needs to ask when deciding between TIR and Concentricity is: What do you want to mea- sure and control? If the objective is to control surface devia- tions from a datum axis, then TIR is your area of interest. If op- posed points (sum and difference) relative to a datum axis then Concentricity is the focal point. BALANCING ACT One additional consideration is, when the part rotates at high speed, balance can become an issue. Initially, balance has some- what less of an effect on TIR, but can have a significant effect on concentricity. When looking at unbalance in a print cylinder, a practical definition may be: The uneven distribution of mass about the print cylinders rotating centerline. There are many causes for an unbalanced print cylinder. If proper care is taken in the manufactur- ing process to ensure machining is concentric, then it stands to reason that the two axis 44 F LEXO will coincide and the print cylinder will be in a state of balance. As wear occurs or cylinder lockdown remains tightened as the cylinder is hoisted from the press, TIR and balance is affected. Other factors include corrosion, distortion from handling and storage, and ink build up. If balance is not maintained, the life of the print cylinder and the press diminishes. Large unbalances require large weight corrections, whether adding or taking away weight. If left unat- tended, the cylinder may deflect when running at production speed, having a detrimental effect on TIR, concentricity, integrity of the cylinder, and ultimately the quality and consistency of the printed product. It’s easy to see how a well-maintained print cylinder or roller can affect your company’s operating cost by minimizing extra cylinders, consuming energy, and adding capacity. Increasing uptime and availability for production is a prime benefit. The added bonus is an increase in print quality and repeatability. It is also very apparent that cylinders and rollers with little or no TIR are a critical part of the print station. Well-maintained print cylinders have many benefits to the printer. Here are just a few examples: ??Dangers associated with press failure are minimized, while presses running smoothly have more “uptime” available. This increases productivity, and extends the life of the cylinder. ?Minimizing vibration produces better and more consistent ? print quality, and print repeats. ? Airborne noise is often attributed to mechanical vibration. ? Exposure to high levels of vibration can affect operator efficiency. ? The press absorbs the forces produced by TIR and unbal- ? ance; a well maintained cylinder or roller therefore mini- mizes structural stress. ? ABOUT THE AUTHOR: Tim Reece is a member of All Printing Resource’s Technical Solutions Team, where he has been employed for the past 10 years. He has 26 years of experience in flexographic print- ing, and has been certified by the NCSS (National Council of Skill Standards) in Graphic Communication at an Expert level. NOVEMB E R 20 0 8 www. f l e x o g r a p h y. o r g