knives on Kuhn cutter

I have a Kuhn 700 disc cutter that is getting pretty old but it is still cutting very well.  I have one concern, however: the knives. When I install new knives, I torque the nuts to 88 foot lb as specified in the manual.  I noticed that even when I use new bolts for the knives (after evert five knife replacements), some of the knives do not rotate with respect to the bolt.  If I were to use a smaller torque, then all knives can rotate.

 

Once I use the cutter for a couple of hours, I noticed all the knives are now stuck and cannot rotate with respect to the bolt.

 

Is it not supposed that these knives rotate?

 

I certainly do not wait until a significant fraction of the knife (25% or so) is missing.  Is there a better rule for when to change the knives?  

 

Thank you,

Paul

 

 

 

knives on Kuhn cutter