Sheet length accuracy is of primary importance when converting rolls to sheets. In order to insure a level of accuracy, some converters are forced to sheet over-sized lengths and then guillotine trim to a finished size. Besides requiring an additional operation, this method can reduce the roll to sheet yield by at least five percent.

All rotary cutters have certain inherent design features which contribute to sheet length variation. This article will identify the causes of sheet length variation, and offer solutions for improving sheeter productivity and reducing waste.

Web Tensioning and Conditioning
Proper web tensioning and conditioning plays a major role in sheet length accuracy.  Maintaining proper brake pressure at each unwind station, as well as smooth web flow into the cutter is essential for precision sheeting.

Excessive brake pressure at the unwind creates web slippage, in the draw roll section, and as a result, the sheet length becomes shorter as the roll diameter decreases.  A properly trained Operator will set the brake pressure strong enough to insure the web feeds into the cutter without causing wrinkles.  In low speed operations (300 fpm (90 mpm) or less), the Operator can manually maintain proper tension by occasionally relieving brake pressure at the unwind.  At higher speeds, automatic tension control is necessary to compensate for the rapidly reducing roll diameters.

Rolls that have been damaged during handling also affect sheet length accuracy. For example, egg-shaped rolls produce a “taut-slack” web condition that produces a succession of short and then long sheets.  Many Operators compensate for egg-shaped rolls by running the sheeter slower to minimize the “load-unload” condition. A better alternative is to add a dancer roll between each unwind stand and the cutter.  The dancer roll will absorb the web fluctuations and provide a smooth flow into the cutter.

Sheet length control problems also occur because of excessive decurling.  In an effort to delivery flat sheets to the pile, an Operator may increase brake pressure at the unwind to improve decurling, and as a result, short sheets occur. Consider using larger roll core diameters to help minimize curl at the end of a roll. Or, invest in a more efficient decurl system.

If the sheet length varies randomly, check the roll itself. Make sure that the roll’s core is properly seated on the chucks or airshaft, and also be sure that the roll is not turning on its core.

Mechanical Integrity of the Cutter
Old stationary bed knife/rotary cutters can be outfitted with mechanical systems to maintain sheet length. Random sheet length variation is usually associated with worn components in the mechanical gear train. A quick mechanical test to determine the amount of backlash in the gear train is:

• Shut off the power to the cutter and follow safety lock out procedures.
• Rock the gear (or timing pulley) mounted on the draw drum.
• As the gear turns, watch the gear (or timing pulley) mounted on the knife revolver.
• The amount of movement on the draw drum before the knife revolver turns is the slack in the gear train. The equivalent of one tooth movement can be as much as 3/8″ variation.

To locate the source of variation within the gear train, the following components need inspection:

• Replace loose change gear (or timing pulley) keys
• Tighten set screws on loose coupling hubs
• Eliminate backlash between gears by properly positioning intermediate gears.
• Examine gears for missing teeth or replace timing belts for stripped cores. Replace them accordingly.
• Check for bent or misaligned shafts, and correct as required. Note:  Failed bearings are associated with bent or misaligned shafts.
• Inspect the mechanical gearbox that actually varies the sheet length per the manufacturer’s recommendations.

Proper Set-Up & Operation
Even with the proper tension control and a tight mechanical cutter drive, sheet length accuracy can still be affected by improper set-up and operating procedures:

• If sheets begin to shorten, check the squeeze roll pressure.  Generally, nip pressure on board should be set at 45 psi, and on multiple webs of paper, no more than 60 psi.  If the nip is too light, slippage occurs, and if is too heavy, stair cut when sheeting multiple webs increases.
• Another cause of short sheets, particularly on coated materials, is slippage at the draw roll.  This slippage is due to a glazed build up on the rubber covering on the draw roll.  Remove the glaze, by either washing the squeeze roll or rubbing the surface with fine grit sandpaper.
• Variation in cutoff during speed changes on older stationary bed knife designs is attributed to the mechanical gear box in the drive train.  As a rule, sheets tend to run longer during speed increases, and shorter during slowdowns.  Even cutters with new PIV transmissions experience variation up to ¼” during rapid speed changes.  To minimize this occurrence, the Operator should slowly increase line speed from stop to running speed over a 60 to 90 second interval.
• Poor web flow through the cutter also influences sheet length.  On long cutoffs, the knife revolver turns slowly relative to the web speed.  When sheeting lightweight material, a phenomenon called “blocking” occurs.  As the fly knife begins to cut, the web buckles, and backs up between the point of cut and the squeeze roll. “Blocking” results in longer cutoffs and slower speeds.  

If the cutter is not equipped with air-washed doctor boards, a wooden bar ¾” thick and 2” wide can help. Lay the bar across the width of the cutter roughly halfway between the draw roll and the bed knife. Block up the bar about ¼” above the before knife doctor board.  By threading the web under the bar, the web buckles will be flattened out.

Improperly mounted doctor boards can also contribute to oversized lengths and out of square sheets.  This condition usually occurs after a jam-up at the knife or a knife change.  Check underneath the doctor board for debris that may be altering the web flow.  Make sure that the location of the before the knife doctor board relative to the bed knife is in accordance with the manufacturer’s specifications.

By understanding the causes of sheet length variation, and taking steps to locate its source, you may find that there are ways to improve accuracy without a major capital expenditure.  If the suggestions do not bring your sheeting operation up to the desired level of efficiency, then it is time to investigate either a retrofit program, or the purchase of new equipment.



Skip Hoffman is Customer Service Manager at Maxson Automatic, an innovator in rotary cutting and sheet handling equipment. The company is based in Westerly, RI, USA.