The knives on self-propelled choppers are of higher-quality materials than they used to be and don’t need to be sharpened as often, according to a study by Karl Wild, University of Applied Sciences, Dresden, Germany.

The study’s purpose was to evaluate knife wear and sharpening using a large self-propelled forage harvester and current operator management practices in regards to knife sharpening.

Self-propelled machines have automatic sharpening systems that consist of grinding stones that sweep across rotating cutting knives. A grinding stone removes a small strip from the clearance sides of the cutting knives to remove dull edges and leave sharp ones.

In his research, Wild cited a 1999 study that suggested that knives be sharpened several times a day. “But since then, the quality of the hard facing has improved greatly.”

His 2009 study surveyed German and North American harvester operators. The smaller survey of 11 American and Canadian chopper operators showed some sharpened knives hourly and others, once or twice a day.

“The number of grinding stone passes in a sharpening ranged from 10 to over 60. The minimum number of passes per day was 30 to 60. It is apparent that some North American chopper operators probably sharpen an excessive amount,” Wild reported.

The German research showed no change in fuel consumption after knife grinding and cutterbar adjustment.

“It is believed this is due to the knives still being sharp before grinding because the chopper operators followed a traditional grinding schedule of once or twice daily with 10 to 20 grinding-stone passes every time.

“Even after these preliminary results convinced the chopper operator to grind every second day in corn (with) less than 10 passes, the resulting longer harvesting times . . . did not cause the fuel consumption to increase significantly before grinding. These results indicate that the modern high-quality tungsten-carbide-coated knives used in these tests stay sharp and useful for a long time.”

But because of the self-sharpening feature, sharpening frequency may also depend on the need to maintain cutterhead cylindricality and a consistent and small knife-cutterbar clearance, the study points out.

Not only should the intervals between sharpenings be longer, the number of grinding passes should be reduced, Wild’s findings show.