The importance of quality tooling on your slitting line can not be over emphasized. It is important that the machine be equipped with the best possible tooling, for this is the heart of the slitting operation.

Slitter tooling consists of:

• Knives
• The arbor spacers (to determine the various widths of the cut)
• Strippers rings
• separator discs for the overarm separator
• and separator disc spacers for the overarm
• shims, if you do not have a shimless operation
• Slitter Knives

The outside and inside diameter of the knives are pre-engineered by the machine builder. The diameter is generally 50 to 60 times the thickness of the material to be slit. It is advantageous to have the diameter as large as possible to facilitate material entering the knives. However, once the material is being slit, as small a knife as possible should be used to minimize the separating forces that will be required.

The thickness of the knife is very important factor, because during the slitting operation there are natural horizontal forces exerted. If the knife is not firm enough, deflection could result, which will cause an undesirable burr or chipped knives.

Ideally the knife thickness should be four times the thickness of the material being slit to provide ample strength particularly when cutting high strength material. The relation of the OD of the knife to the OD of the spacer can effect deflection.

Using the "four times the thickness of material formula" doesn't necessarily require I" knives to slit 1/4" material; in most cases, two 1/2" knives backed-up will prevent the deflection. Two 1/2" knives will cost approximately 30 percent more than a 1" knife. However, using two 1/2" knives, four cutting edges will be obtained, which has to be considered in the cost of operation.

With the following information, the knife manufacturer can make proper recommendations for your slitting jobs:

• The thickness of the material to be slit or trimmed
• The type of material to be slit; mild steel, stainless steel, nonferrous, or high tensile steels. It is also important that the tensile strength of the material be known. Shear strength should not be confused with tensile strength, which is generally half as much as shear strength. (However, both are expressed in terms of pounds per square inch.)
• The condition of the material - whether the material is hot rolled with scale, pickled, or cold rolled
• The make and model number of the slitter on which the knives are to be used.
• The physical dimensions such as outside diameter, inside diameter, thickness, keyway width, and key depth. (Figure 4)
  

The knife must be made with enough strength to withstand the most difficult material to be slit, and tough enough to prevent chipping or spalling during operation. Maximum wear resistance is important to maintain a keen cutting edge necessary for a good slitting job.

The Grades of tool steel available for slitter knives is expanding every day.

High carbon-high chrome steel (and its variations) have been the work horse of the industry for years, but the development of new steels and new processes/manufacturing methods, have allowed increased specialization and the ability to taylor the optimum knife steel for more specific cutting applications. But there still remains a trade off between wear resistance and toughness, and there is no one single best knife steel for all cutting applications.

High speed steels and the high alloy steels are basically used on light gauge slitting jobs generally 1/16 or less in mild steels, or its equivalent in tensile strength. There are many variations of high speed and the higher alloys including relatively new powdered metallurgy steels, which offer an improved combination of toughness and wear resistance at a higher price.

Generally, knives manufactured from a high carbon-high chrome steel have been used for years because of the steel s ability to maintain a keen cutting edge. However, with heavier gauges, an entirely different analysis and hardness is involved. A knife designed to cut 1/4 and 3/8 in. material will not have the proper wear properties when slitting thinner gauges, and this also must be taken into consideration. There are many areas in which the user will have two sets of knives; one for light gauge and one for heavy slitting gauge.

In discussing wear, both hardness and chemical analysis of the knife are factors, and it can be a mistake to discuss one without the other. At a given hardness for instance, a high carbon, high chrome knife will out wear a knife made of a shock resisting tool steel simply because of its chemistry.

The selection of spacers is also very important. Spacers should be of a good quality to resist nicking or movement during the life of the spacer. Spacers are manufactured to the same exact tolerances as knives, and if properly handled and maintained, rarely require replacement. Most spacers are produced from an excellent alloy of tool steel and should be hardened to about 58-60 Rockwell C to prevent nicking in handling. Should these be nicked or bumped on the bearing surface, considerable difficulty can be encountered in set-up or production.

There have been instances where a slit strip had a mark along the edge evenly throughout the length. It was later determined that a spacer had a bump on the bearing surface which was caused by a nick. Every time the knife made a revolution, the clearance changed at the cutting point, thereby causing the marked edge.

Spacers are generally made in one piece to an OD and an ID. However, in some instances they can be manufactured as split or inter-locking spacers. This type spacer is used to facilitate set-up changes without completely opening the machine. It is done by merely loosening the lock nuts, backing them off, and when the tooling is separated, the spacers can be separated and pulled off the arbor.

These rings are available in a range of material and color, and can also be made of a polyurethane plastic material. The most common grade is the Buna N rubber because of its excellent oil resistant qualities. This is readily available in black, white, blue, red, yellow, or green at no additional charge and supplied to a 65 plus or minus 5 durometer reading, which has been proven the best for most slitting operations. However, they are available in ranges from 50 to 85 durometer. Remember that the harder the stripper ring, the more possibilities there will be to mark the material being slit. This could be a problem in the light gauge or non-ferrous materials.

Polyurethane is recommended for precision slitting operations and on non-ferrous materials, because of its oil resistant and non-marking qualities. This grade is available in a wide color range, and it is considerably higher priced than the rubber material. It should be noted that stripper rings can be permanently bonded to steel spacers.

Figures 2 and 3 depict how rubber rings are utilized. The stripper rings should be packed loosely on the arbor. This will provide some gap, allowing the ring to compress without exerting pressures against the knife. The diameter of the stripper rings between the male knives should be equal to or slightly larger than the diameter of the knives. This helps support the strip and on light gauge and softer materials can keep the non cutting edge of the slitter knife from marking the material. These male stripper rings are often harder than their female counter parts. The stripper rings between the female knives should be less in diameter than the knives, and the thicker the material being slit, the greater the difference should be.

In the rubber grade, there is a tendency for the material to shrink after manufacturing, so this must be taken into consideration if these rings are laid aside for an extended period without use. It has also been found that rubber rings will tend to distort and lose their original shape during use. Dual Durometer stripper rings solve many of the problems of distortion by using a harder rubber on the ID to hold the shape, and a softer rubber on the OD to help grip the steel and reduce the chance of marking softer steels.
Precision slitting of very light gauge steel strippers can be used, however, your machine must be specifically designed for this type stripper (Figure 3-A).

Three major clearances are horizontal, vertical positive and vertical negative (see Figure 5). There are various charts and recommendations relating to suggested clearances for various gauges. These clearances are most important to achieve the desired cut. Improper clearances can cause premature dulling and possible chipping of the knives. Of course, the slitter should be set up for a balance cut. (Figure 6).

The following photos depict edge conditions resulting from two set-ups. Figure 7 shows results from a tighter clearance which allows more material to be pushed down, resulting in a fracture that is not clean. Figure 8 shows a fairly good cut, and also indicates that the steel being cut is of a higher tensile. It requires a smaller amount of actual penetration before fracture.

The importance and the care in handling of tooling cannot be over-emphasized. It has been manufactured to precision tolerances, and should be handled accordingly.

Cleanliness is one of the most important factors in making a set-up. If the arbors and all the tooling are not perfectly clean, this will result in a poor set-up, causing poor quality edges and possible damage to the knives. Spacers especially should be wiped again and again to make sure there is no dirt or foreign material of any kind to interfere with the set-up. The use of tooling cabinets offers an advantage in protecting tooling when not in use.

Knives must be reground regularly. Most knives are ground by commercial grinders. It is best to consult with your grinding wheel supplier for the proper wheel, however, we would suggest an aluminum oxide medium grade #54 H hardness 8 structure. Knives improperly ground or used beyond the grinding stage can also result in chipping. During the slitting operation, the force required to separate the metal and the heat generated in the slitting operation will cause a fatigue of the metal at the cutting edge. It is then necessary that all this fatigued material be removed in the grinding operation, otherwise very minute hairline cracks will exist which will ultimately result in a chip. It is generally recommended that approximately .0 10 to 0 12 be removed on a side which would mean a .020 to.024 overall stock removal to get below this fatigue condition. Of course, in slitting very light gauges, the fatigue is not as prevalent as in heavier gauge slitting.

It is also imperative that these knives be ground in perfect sets as far as the OD is concerned. Also, visibly inspect reground knives looking for chatter marks on the periphery of the slitter knife. This condition is generally caused by improper grinding of some sort and can lead to reduced knife life. Should these chatter marks be discovered, consult with your grinder to correct the conditions at once. Chatter marks are generally caused by using too hard a grinding wheel, not dressing the wheel often enough, feeding the grinder too fast, taking too heavy a cut, or possibly a worn spindle bearing on the grinding machine. Also, visibly check for discoloration from overheating in grinding.

It is also important that the knives are secure on the arbor and that there be no movement of the knives during the grinding operation. After the knives are ground, they should be lubricated and properly stored to prevent corrosion and possible nicking. Of course, any burr that may be left from the grinding operation should be immediately removed.

Clearance is one of the most important items and extreme care should be exercised at all times to make sure the proper clearances are preset and maintained during the operation. Dull knives also present a problem. When the knife begins to dull, it loses the keenness of cutting edge, and it will necessitate more power in the machine to make the cut. The horizontal pressures exerted on the knife can increase by as much as 400 percent. When these horizontal pressures increase something must go, and it generally results in a spalled or chipped knife. Observe the chip in the form of a half moon shape (as seen in Figure 10). Look at the bottom of the arc, and if there is the appearance of a square shoulder effect at the bottom of this arc, it generally indicates excessive pressure has been applied to the knife.

This excessive pressure can be from dull knives, improper set ups, knives not running true and parallel to each other and knives not properly ground. It could also be caused by arbor deflection, worn bearings which would cause too much lateral play in the arbors. The parallelism of the arbors should also be checked occasionally. This is readily done by mounting knives at each end of the arbor directly above one another, then bringing the arbor down and checking with a feeler gauge to ascertain that the arbors are parallel.

Will knives tend to lose their hardness after continued use? The highly alloyed steels used in the manufacture of knives is of a thorough hardening nature and the hardness will be maintained regardless of the diameter of the knife. One reason tonnage might drop off is that all the fatigue must be removed from the cutting edge to prevent premature dulling or chipping. Another factor to remember is that as the diameter is reduced by subsequent grindings, there would be a change in the peripheral speed, which means the knife is doing more cutting.

For more information about anything you see on this web site please call:

The Wapakoneta Machine Company
TEL: 419-738-2131
Toll Free: 800-837-2131
Fax: 419-738-5828
E-Mail: sales@wapakonetamachine.com