When selling slitting equipment and when working as a slitting consultant, I traveled the world visiting web slitting customers and attended Web Converting shows. I often was asked how Shear Slitting knife life could be improved. The reasons for the questions always related to reducing slitting defective rolls and lessening production “downtime,” along with the amount of knife resharpening and too-early replacement-knife costs.

The reality is that multiple factors determine how long shear knives will continue slitting acceptable finished rolls. Two dynamic factors are the amount of friction force required to keep the Top and Bottom Knives in side-to-side contact and the amount of Bottom Knife rotating speed needed to keep the Top Knife rotating faster than web speed. These factors, at excessive levels, quickly can lessen Top Knife sharpness. Another factor is the material from which the knives are made.

Applied pneumatic shear-knifeholder air pressure creates the Top and Bottom Knife force needed to ensure the knives stay in contact. Production department air-supply systems commonly provide maximum pressures around 90 PSI (6.2 bar). That amount of air pressure would create an extreme amount of knife contact force friction, way above what is needed to slit the material. So, holder air PSI must be regulated at a lower level for a longer knife life.

Knowing and regulating the minimum amount of air pressure required to maintain knife contact friction for slitting the web material is important. But, because of potential web thickness and toughness variations, adding a slight amount of (T.B.D.) air pressure insurance might be considered.

Also, knife-life contact friction levels will vary from holder-to-holder based upon varying amounts of Top to Bottom Knife engagement stroke travel distances against a coiled mechanical disengagement spring. Lab test results of side-load force variances from different travel distances of a paper mill duty holder can be seen in Figure 1.

Then, there’s the knife rotation speed. Both the Top and Bottom Knives must rotate faster than web speed in almost all slitting applications. The friction-driven Top Knife rotates slower than the Bottom Knife. The amount of overspeed required varies with the web’s material’s characteristics: thickness, toughness or having an abrasive coating. And because of the many dynamic variables, a common set-up factor is to program the bottom knife overspeed at 10% faster than web speed.

Note: Top Knife diameters lessen from the re-sharpening process. If driven at the previous Bottom Knife speed and contact force after remounting, it will rotate faster than before, with shorter knife life. Some companies have multiple full sets of Top Knives and have full sets resharpened to the same diameter. When reinstalled, this helps them have a close common knife life.

Another major knife-life factor is the material from which the knives are made. The more common Top and Bottom Knife metals are 52100, D2, M2, CPM-10V and Tungsten Carbide (see Figure 2).

In-house empirical studies or slitting lab trials could provide how much holder air pressure and bottom knife overspeed would be best for your slitting knife life.   

Dave Rumson
860-256-5658; drumson@maine.rr.com
ARC Member, ARC TV Presenter,
R2R Presenter, Converting School Educator