It's generally acknowledged that existing machines exist, and it's challenging to improve or rectify slitting problems that are inherent in the design of an existing machine. There are times, however, when alterations can be made, and giving consideration to some of these suggestions based on actual field experience can be helpful. If you are thinking of buying a new machine, it might be wise to compare the proposed machine's features to the suggestions discussed here.

High-quality shear slitting demands full support of the web at the slitting nip, a service provided by the lower slitter ring. The diameter of the lower slitter relative to the diameter of the upper blade is a significant factor in good, trouble-free shear slitting.

Considerably larger lower rings are the preferred arrangement. A quick look around the industry shows considerable variety in this regard. For instance, the primary paper industry may have a ratio of 1.5:1, with ratios as much as 2:1 being common. The converting and packaging industry, on the other hand, may even reverse these ratios, the lower slitter may have a ratio of 0.75:1, or even as small as 0.5:1. How do these ratios affect slitting?

In tangent configured machines, any variation in blade overlap immediately effects the position of the slitting nip and the degree of web support. Larger lower rings tolerate overlap variations better than smaller rings. In addition, the amount of lateral setback of the upper blade can be reduced with larger rings, maintaining a well-supported nip as well as a significantly reduced amount of blade penetration below the web line. Disproportionately small lower slitters are extremely sensitive to minor deviations from stringent operational limitations.

Wrap-configured machines are not as sensitive to these same problems. Since the web is usually in contact with the anvil assembly well before it reaches the slitting nip, web support is not quickly lost. If the machine is presently wrap-configured with relatively small lower rings, and converting the machine to tangent slitting is being considered, the diameter of the lower rings should be increased for the reasons discussed above.

Isolating the "spreading zone" from the "slitting zone" will prevent slitting disturbances caused by bowed rolls or similar spreading devices, which if improperly placed, could lift the web off the slitting "table." In tangent-configured machines, the slitting table is the region between the webs' infeed support and outfeed support, across the slitters. This is the slitting "zone" for tangent slitters, therefore the web must not be disturbed as it passes through this region. If the influence of a bowed roll or spreader bar is able to trespass into the slitting zone, slit quality will be compromised. This also applies to designs where the spreading effect "fans" the slit webs outward from the slitters, instead of diverging after passing the outfeed support roller.

On wrap-configured machines: don't be too quick to lift the web off the lower slitter assembly and deliver it to the spreading zone. Let the wrap system do its job—maintaining control of multiple narrow slit webs. Keep the webs in contact with the lower slitters until you're confident that the entire slit web is stabilized, and then allow the spreader system to lift it off the slitters.

The slitter section should be solidly incorporated into the basic machine frame, and not be a patched-on addition which could sway and vibrate like the drummer in a marching band. This is especially important on higher-speed equipment, where vibration and movement accelerates as the distance from the core mass of the machine is increased.

While it may be possible to create a machine using "Erector Set" components for light-duty applications, there is no substitute for heavy, welded frames, solidly mounted on a heavy foundation isolated from the various rumbling and bumping inherent in an industrial environment.

Naturally, the slitter support beam itself must be very robust and resistant to even the slightest sag and twist. Filling the beam with sand or concrete to suppress vibration is a low-cost and effective tactic. Do it.

Twin-mandrel slitter systems which have marginal shaft diameters will vibrate as they accelerate through their harmonic frequencies, subjecting the blades to unnecessary wear even if they do not "jump." Because the upper slitters are mounted on hubs, the typical mandrel diameter may be relatively small, making them especially susceptible to speed induced vibration. Wrap-configured lower slitter shafts must be large enough to resist deflection caused by web tension. A center support under the shaft can help control deflection and harmonic vibration.

Slitters which are incorporated into the lay-on roller mechanism are especially unstable and subject to vibration. This design philosophy suggests that minimum interleaving will occur if the web is wound immediately after being slit. The shortest distance is, therefore, to wrap slit and use the lower slitter assembly as the lay-on roll. Thus, the entire slitter section rides directly against (and is subject to the bounce dynamics of) the rewinding roll.

Is your entire slitting section "operator friendly"? The operator should be able to make set-ups and blade changes in a safe, efficient manner. The height of the slitters, for example, should be about belt to mid-chest height for ideal visibility, especially for operators who wear bifocals. (Having to work overhead while wearing bifocals is an intolerable arrangement).

How is the lighting? Good lighting means no shadows, especially in the crucial nip area between the blades, since this is where the operator judges overlap and set-up. The slitter section should be the best illuminated place on the machine. Fluorescent lamps across the entire width of the machine, close to the slitters, makes working at the slitters efficient and pleasing. A 60-watt bulb in a clamp-on lamp or a flashlight held between the teeth is not considered "good lighting".

Is there a place to put things down for a moment while changing blades, or must the operator run in and out of the machine to perform blade changes or other simple tasks? Juggling sharp objects while working on machinery is not an ideal arrangement.

Are the necessary controls nearby? For example, must the operator run over to the control panel to turn on the air every time just to verify the blade overlap? Naturally, some fail-safe interconnects will be needed between the control panel and the machine controls.

How accessible is the slitting section? A machine which forces the operator to his knees (or worse yet, to crawl under a machine) to access the slitters is almost as much trouble as a machine that requires a ladder. Having to lean over a beam or roller, or reach around something to access slitters at arms length, makes set-up very difficult. Slitters will not get the attention they need under such circumstances. They will be troublesome. Guaranteed.

Are the slitters visible from the control panel during a run? Can you see the actual blades, or only the guards? If you need to monitor slitting performance, you need to see the blades and the nip itself.

In addition to the bowed roll or spreader bar, some other less obvious troublemakers adversely affecting slitting include:

Turning bars. On a printing press, for example, a turning bar has been observed steering the slit webs against each other, creating clouds of dust. Make certain any downstream web steering devices do not create conflict between slit lanes.

Misaligned rollers. Infeed and outfeed rollers are responsible for controlling web stability over a tangent slitting "table." Baggy edges from a slightly misaligned infeed roller, for example, will make trouble at the trim slitters. Misaligned outfeed rollers can scatter the slit webs like traces on a dogsled team at lunchtime.

Trim-chute misalignment is another outside influence that will make trouble. The trim strip must not be skewed away from the cut point, or slit quality is compromised. A common problem at the trim slitters is to set the trim chute to one side, with the result that the center of the air velocity now pulls the trim strip away diagonally, creating a "tearing" strain at the cut point, and degrading slit quality.

Air turbulence (from fans, blowers, jets. etc.) can be troublesome to thin webs under low tension on tangent configured machines. A thin, unstable web is especially difficult to slit under such conditions, and a veritable tornado at the trim slitters is certain trouble for webs of any type.

In many instances, the existing web path through the machine is fixed, and not much can (or should) be done about it. However if an alteration is possible, here are some options to consider:

Is the web path tangent/horizontal? If so, keep the "draw" across the slitting table as short as possible so as to minimize web flutter, and make certain that spreader rolls do not lift the web off the table. Gravity is an ally in that the web rests positively on the lower slitter rings, even if tensions are low. Tangent/horizontal paths require more machine space, and do not always provide easy access for set-up, blade changing or making minor adjustments during a run. On the other hand, wrap/horizontal configurations are much more compact, and are usually more accessible, even during a run.

Is the web path inclined or vertical? Does the web run upward or downward, and how high is it relative to the operator? If the slitters are below eye level and the web runs upward, set-ups and monitoring the slitting nip are easy, but are usually more troublesome at the trim slits unless the trim-removal systems are overhead. Slitters that are above eye level, with the web running downward, make inspecting the slit edge a perfect arrangement, but can complicate slitter set-up.

Slitters are designed to cut things. With this basic fact in mind, some safety measures are self-evident:

• control lock-outs to prevent accidental starting during maintenance, and
• brakes on high-speed slitter motors to prevent free wheeling for long periods after being turned off.

Blade changing is also an issue; on individual knifeholders which have removable blade cartridges, the guard serves to protect the hands during handling. But on twin-mandrel systems, an entire mandrel full of exposed, sharp blades confronts the operator. It's definitely more hazardous to handle a heavy mandrel filled with unguarded blades.

Consider the speed at which the slitters engage: If they are individual knifeholders, do they slam down abruptly, startling the operator as he makes a set-up? Is the system web break tolerant? On the other hand, are the slitters in the way? Are they obstacles to work around?

Machine designs of almost infinite variety populate our industry, and while there is no perfect machine, some configurations are better suited for specific applications and materials. Often, subtle modifications can significantly enhance performance. A few basic principles should be kept in mind, namely:

• The slitting zone must be isolated from adjacent zones, or any other function which can disturb the slitting function.
• It should be accessible and operator-friendly.
• The slitting section should be rigidly designed for stability and immunity to vibration.
• Safety is the first priority.
• Wherever possible, use a lower slitter of maximum diameter, relative to the upper blade.

If these simple principles are satisfied, the slitting of most flexible web materials will be much more reliable.