The role of the knifeholder in productive shear slitting

By Reinhold Schable, Applications Technology Manager, Tidland Corp -- Converting Magazine, 4/1/2001

Gone are the days when a shear-slitting knifeholder was a primitive, cast-iron contraption that required a pocketful of wrenches every time a pattern change was made. It had to have the durability of an anvil because repositioning involved a lot of pounding with a hammer. Lost time, lost product and accidents were a fact of life. This, of course, is no longer acceptable.

Current knifeholders must be thought of as precision instruments, rather than blacksmith tools. As such, we expect more from them, and need to be more discerning about their usability, efficiency, design features and safety. This article will address some of these issues.

Rigidity, compliance, and vibration

The knifeholder must be compliant, that is: it must be able to absorb axial runout and lateral vibration without destructive "chatter." The measure of a knifeholder's susceptibility to vibration is not merely its weight and rigidity, but rather its overall harmonic response to the inevitable vibrations present in any machine system.

A massively heavy unit, without adequate compliance may subject the blade to catastrophic lateral impact against the lower slitter, especially if a critical harmonic develops in response to machine vibration or lateral run-out (wobble) of the lower slitter. However, the same condition can occur to a lightweight, flexible knifeholder, so it's important to consider not just knifeholder weight and rigidity but the total dynamics of the unit-including the support system.

The most critical area in this regard is the method whereby the blade/hub assembly is attached to the knifeholder-support body. The use of spring plates favors the creation of resonances similar to a "tuning fork"-with significant blade to blade damage. On the other hand, an absolutely inflexible connection cannot disconnect the ever-present, lateral-machine vibration or absorb lateral anvil blade run-out.

Cant angle control

A basic law of shear slitting requires that the materials enter a "closed nip." This means that the extreme edges of both upper and lower blade must be in contact where the material enters the nip. Beyond this basic requirement, the amount of cant angle is less critical from the standpoint of slit quality but more critical from the standpoint of blade life. This statement can be verified by the presence of a myriad twin-arbor slitters throughout the world which slit critical products-despite the fact that twin-arbor slitters have no cant angle whatsoever.

Cant-angle control is a two-edged sword. The nip must be closed, as noted above, but excessive angle is very destructive to blade life. In practical reality, if all other slitting parameters are being met, 1 deg of cant angle should be considered maximum.

Especially important is the necessity that the blade be held truly vertical (90 deg) to the plane of the web, otherwise, it is impossible to guarantee a closed nip with such low cant angles. Knifeholder designs which incorporate springs in the blade-retaining hubs, or as a part of a flexible hub-support mechanism itself, allow the blade to tilt over the rim of the lower slitter and are troublesome for this reason.

If a knifeholder is to be used on machines where either edge of the lower slitter can be utilized, the ability to reverse the cant angle is a useful feature (Photo 1). This should be possible without the need to disassemble the unit merely to reverse the angle, and the precision, when reversed, should be unaffected. The use of precision-machined components to fix the angle is not an option; any design that requires operator skill to interpret the cant angle, either by "eyeball" or by stick-on labels, is to be avoided.

Set-up and repositionability

Knifeholder design requirements vary according to frequency of pattern changes, machine accessibility, etc. If pattern changes do not interfere with in-line production, a simple, manual knifeholder, which may require a tool to secure the unit may be adequate. In most instances, a simple rack-and-pinion system is fully acceptable. Then, there are applications in which time is of the essence, no interruption of the in-line process can be tolerated, and fully automated systems are required.

Since operator skill is a constant variable, the knifeholder must be intuitively simple. Extensive instruction manuals should not be necessary for day-to-day operation. Setting up a knifeholder during a pattern change should require no more than three separate operations ("place," "set-up" and "engage"), preferably no more than two ("place/set-up" and "engage")-otherwise, operator confusion could result.

Engaging or disengaging a slitter, once it has been set up, should not involve more than one operation; ("on" or "off"). The need to "tap it once or twice" with a hammer is sure evidence that the knifeholder design needs improvement.

Set-up cannot be taken lightly, as a fair amount of precision is needed. Most shear-slitting applications require the operator to place the unit within about 1.50-2.0 mm (about 1/16-in.) of the edge of the lower slitter edge. A significant amount of time and productivity is lost as the operator tests and re-tests the blade set-up. False starts because of a missed set-up is particularly wasteful.

Incorporating preset stops, or position locators to facilitate correct placement of the knifeholder, has been an important addition to knifeholder design in recent years. Look for design features that are robust and do not require an extra operation, such sliding a tab in and out, or locating a reference edge before securing the unit to the guide bar.

Side-load force regulation

When springs are used to regulate sideloading, the pre-load placement of the blade has a significant impact on the actual sideload force. This means that the operator must place the slitter even more precisely and consistently at a predetermined distance away from the edge of the lower slitter edge during each setup. This may call for the use of shims, spacers or jigs to preset the unit, but because their use is dependent on operator competency, such knifeholders should be avoided wherever possible. Also, spring uniformity between spring-regulated knifeholders may be inconsistent, introducing yet another variable into the slitting equation (Figure 1).

When pneumatics are used to control sideloading, initial blade placement is less critical. Unfortunately, most pneumatically actuated systems have the air cylinder axial to the blade centerline. This means that they are subject to "stiction" and "crabbing", which is the tendency of a piston to resist movement at low pressures, and/or bind in the relatively short cylinders typically used in knifeholders (Figure 2).

The use of double-acting cylinders in an attempt to control these problems ignores the fact that it is the pressure differential between the loaded side of the piston and the unloaded side that determines if a system is subject to stiction and crabbing. A simple test of the ability of a knifeholder to respond to side-load force is to apply the desired force (about 2-3 lbs.) against the rim of the blade, instead of against the hub center. The blade should side-load smoothly, with no crabbing or binding.

Diaphragm-actuated slitters are not as vulnerable to binding if the diaphragm is off-axis, and they are more responsive to air-pressure regulation. They are also more compliant to lateral run-out and vibration. Contaminated air supply is also less of a concern with diaphragm-actuated knifeholders.

Finetuning the side-load force must not be dependent upon the need for the operator to adjust the position of the knifeholder body relative to the lower slitter edge. This is equivalent to using a hammer to adjust the side-load-something that is still practiced, sad to say.

Currently, the pneumatic approach is the preferred method of regulating side-load because it's easily controlled and reliable. Increasing or decreasing the side-load force is as simple as changing the air pressure at the control console, in contrast to adjusting the placement of the unit itself (i.e.: "Hand me the hammer, Harry.").

Engage and disengage sequencing

The knifeholder's vertical and lateral strokes must be independent and sequential on both the engage and disengage cycles. The vertical stroke should be fully extended before the lateral stroke begins extension, otherwise a blade "crash" can occur. (Blade "crash" is usually defined by the operator as: "A condition, in which the @!!* & @!! blade has landed on top of the @!!* & @!! anvil ring!!") In the same manner, the lateral stroke must be fully retracted before the vertical stroke begins retraction, otherwise the blade will "drag" over the edge of the lower ring, and chipping may result (Figure 3).

This sequencing action must be followed for all shear-slitting systems, regardless of slitter design and is equally applicable to twin-shafted slitter systems, as well as individually mounted, traction-driven slitter systems.

For individual, traction-driven knifeholders, sequencing must be positively valved, rather than relying on a series of cascading orifices or sequential pressure chambers. These designs may become more imprecise and unreliable as blade diameters are reduced during regrinding, creating a condition which requires that the knifeholder must extend further to maintain correct blade overlap.

A good test of the sequencing characteristics of any knifeholder is to set it for maximum vertical stroke and apply air pressure, noting the path the blade takes as it extends. It should fully extend its maximum vertical stroke first, and only then should it begin its lateral extension. If the lateral movement begins before the blade is fully extended vertically, the blade may come to rest on the perimeter of the counter blade, instead of against its side-the condition previously referred to as a blade "jump" or "crash."

Blade changing

Removing a large, heavy knifeholder from the machine to change a blade is cumbersome and dangerous. Being forced to replace a blade while the unit is on the machine is also dangerous, especially if the slitters are in an awkward or inaccessible location. The use of fork wrenches, spanners, or hex wrenches to change blades on the machine should be eliminated for safety and efficiency reasons. Even the ubiquitous Allen key can cause problems if the operator cannot directly approach the blade-retaining screws. Allen keys tend to accumulate under machines, like golf balls accumulate in water holes.

The blade-clamping mechanism should be hassle-free and secure. Especially troublesome is the large rotating retainer nut, commonly used to secure the blade. This design requires an anti-rotation pin, which fits into a notch in the blade's bore. Without this pin, the blade can rotate, which, in turn, can dangerously loosen or overly tighten the retaining nut, creating a dangerous condition (Photo 2).

To overcome this problem, some knifeholders fix the blade using multiple screws in conjunction with slots in the blade. This puts extra cost into every blade, places significant strain on the retaining screws, and is not as secure as a physical clamping ring using a similar slotted screw-hole concept.

A related problem is the unintended rotation of the blade hub as the operator loosens or tightens the retaining screws. Designs that require the operator to use a second tool to prevent such rotation should be avoided. The inclusion of an anti-rotation lock for blade changing gives evidence of the manufacturer's concern for safety during the blade-changing sequence.

One of the most important innovations of the last few years is the advent of the tool-free removable blade cartridge, which allows blades to be changed off-machine, in a safer environment. This feature has done much to reduce hazards, machine downtime and improve blade-changing efficiency (Photo 3).

Safety

Knifeholder adjustment controls should be away from the rotating nip. The knifeholder's engage and disengage actions should not be sudden, rapid movements which may startle an operator, or be too fast to get a hand out of harm's way during setup.

Is the knifeholder easy to reposition without placing hands close to the blade? Tools, shims, spacers or jigs should not be needed when making pattern changes. Blade guards should be robust and able to tolerate the occasional snarl that occurs during a web break. In some instances, automatic self-retracting, full-coverage blade guards may be required.

Is the air system integrated with the E-stop control, so the slitters will automatically disengage in an emergency? Does the lower slitter shaft have a brake to arrest free-wheeling in an E-stop?

The blade-retaining system must be fail-safe (avoid spin-on blade retaining nuts, as described earlier). Is there any possibility that the operator could install key-slotted blades so they can spin off if a screw works loose? On extremely high-speed machines, do the blade retaining screws have counter-bored sockets to prevent the screw heads from releasing the blade? A blade flying off at 8,500+ fpm is a serious hazard.

Blade changing should be done off-machine, in a safe environment, especially where access to the slitters is precarious. Unfortunately, many machines require the operator to hang from his toenails in half-light to change slitter blades. Blade-mounting systems that require the use of both hands to manipulate spanners, or wrenches to loosen the blade are especially dangerous under such conditions. The tool-free, removable-blade cartridge is a major improvement to safety on these machines.

Blade handling should be minimized and done in a safe environment. The use of wax coatings to protect the blade edge is important, but the coating should be easily removable after the blade has been mounted on the slitter. This means that blade guards should not prohibit access to the blade once it had been mounted, nor interfere with complete removal of the wax coating.

If the guards do interfere with blade changing, then safety gloves should be worn. Large, heavy blades should be supplied as individually wax dipped rather than in a bundle, since separating the bundle itself can be dangerous-not to mention the possibility of nicking a blade's edge.

After all is said and done, the knifeholder is a pivotal instrument in most industries involved with flexible webs. The tendency to think of slitting as an "afterthought," and treat a knifeholder with the attitude that "any old device will do" is being replaced with a greater appreciation of the importance of this seemingly insignificant part of most web-processing equipment.

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Reinhold Schable, applications technology manager for Tidland Corp., Camas, Wash., a division of Maxcess Intl., has 36 years of converting, primary-paper, film and light metals industry experience. Schable is the author of several technical papers published by TAPPI and other industry magazines. He can be reached at 800/426-1000, fax: 360/834-5865.