Meaden Printing

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Factors that Influence Wheel / Blade / Knife Life
  
1.  Material being cut ( paper, plastic, metal, etc. )
2.  Speed of material being cut.
3.  Material and hardness of the cutting wheel.
4.  Condition and hardness of the anvil role.
5.  Pressure on the cutting edge.
6.  Shape and condition of the cutting edge.
7.  Number and type of teeth in a perforation wheel.
8.  Regrinding

Materials 

The material normally being cut are usually either paper or plastic.  Papers vary considerably in their quality and abrasive nature.  Environmental laws that require paper manufactures to reduce discharges in rivers have resulted in the manufacture of more abrasive papers.  The increased use of recycled papers has also changed the nature of many of the papers currently in use.  The strength of the fibers holding the paper together and the fillers or chemicals used to whiten the paper change the strength of the paper.  Plastics and non wovens have also come into use.  Materials like Tyvek which in many ways looks like paper is considerably stronger and presents different problems in cutting than paper. In the label industry there are a number of treated or composition materials in use.

Speed

Higher running speeds require better equipment and better cutting tools to avoid problems.  In general, the higher the speed the better the whole processing system must be - more precise and rigid in all areas.  Speed causes vibration of all sorts.  Hence, it becomes more important that the tools and holders be more precise.  In particular, for cutting wheels the concentricity and roundness of the cutting edge is critical.  The cutting edge must be concentric to the bore of the wheel and the outside edge must be round.  Otherwise, the wheel will start to jump and bounce.  One spot on the wheel can become dull or flattened.  This may show up as an intermittent cut or perforation that varies.  Squareness of the cutting edge to the bore become more important at higher speeds because this can cause wobble, scuffing and vibration.  All of these conditions effect the life of the edge.

Hardness and Material of the cutting wheel

The material used to manufacture the cutting wheel will have a great effect on the life of the wheel. General duty requires a material that can be hardened to around 60 Rc and for economic reasons must be as low cost as practical. Typical materials used are O-1 and 52100 steels. Other materials with chromium and vanadium added are available to make the edge last longer. Typical premium materials used are D2 , M2 and CPM-10V tool steels. However, while the use of high alloy tool steels will make the edge last longer they are much harder to manufacture and cost considerably more. The cost of the wheel must be balanced against the additional value of production.

Condition and hardness of the anvil roll

Because all crush type cutting wheels run against a rotating anvil the hardness of the anvil is very important. Normally the anvil roll should be at least 4 points Rc harder than cutting wheel so that the cutting wheel will wear out first. Good anvil rolls are expensive and difficult to replace. It is generally more economical to allow the wheels to wear out faster than the anvil roll. If a wheel runs in the same place on the anvil roll for a long period of time a groove will be worn in the anvil. This groove will cause the knife ( even a good knife ) to cut poorly. Under these conditions the knife should be moved to an un grooved portion of the anvil if possible.

Pressure on the cutting edge

How sharp is sharp ? Unlike a kitchen knife where the edge should be very fine, a circular knife cuts with pressure. The wheel pinches the material between the edge and the anvil roll until enough pressure exists to separate the material. Since the edge of the wheel is rolling on the anvil there can be considerable stress on the edge of the wheel. Depending on the material of the knife, the edge can only withstand a certain pressure level. When that level is exceeded the edge starts to break down. Applying more pressure to the knife will cause it to wear out prematurely. Only enough pressure to achieve the cutting action should be used.

Shape and construction of the cutting edge

A razor sharp edge will cut material with very little force on the knife. However, a razor sharp edge is very sensitive to changes in force. Because the edge of the knife has an extremely small area in contact with the anvil a very small force on the knife can produce extremely high stress at the cutting edge. In fact it is very easy to exceed the breaking strength of the knife material if a razor sharp edge is used. Consequently most high quality crush type knives have a controlled radius on the cutting edge. The size of the radius can vary. Smaller radii cut with less force but generally don’t last as long and are much more sensitive to variations in force on the knife . (You can’t just crank it down.) Larger radii require more force on the knife but don’t cut as clean and generally last longer and are easier to adjust. The size of the radius must be selected according to the type of material being cut in order to achieve the best performance.

Number and type of perforation teeth

Generally perforations are used to allow the user of the paper to either fold the material or tear it off. The selection of the number and width of the teeth in the wheel depends on the strength of the material and the amount of force desired to tear the perfed section off the form.  Small or thin teeth are damage more easily than larger or shorter teeth.  A larger than average tie width will cause more wear than thinner tie widths.

Regrinding

Regrinding incorrectly can cause considerable problems that sometimes are hard to detect. There are several major considerations that must be followed when regrinding a slitter or perforator wheel.
They are:

1. The grinding arbor and bore of wheel must fit as tightly as possible to prevent concentricity problems.

2. Slitter wheel must run true to the axis of rotation of the arbor of the grinder so that there is no
    wobble.

3. It is generally good practice to grind the arbor while it is in the grinder so that concentricity and
   squareness errors are reduced.

4. A premium quality open wheel which is balanced on its own grinding arbor should be used

5. Light grinding feed and good coolant should be used to prevent over heating of the cutting edge.

 
 
 

 

 

 

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Meaden Precision Printing

March 2014

We have been supplying the printing, converting, and packaging industry with products since the late 1950's.  Our products include, but are not limited to- standard punches & dies, special shaped punches & dies, crimp-locks, and perforated, slitter & score wheels / blades / knives.  We manufacture 95% of the printing items in our plant and we are also a distributor for SchoberTechnologies & Sisprint.