"Proper Knife Selection, Maintenance, and Safety are Critical to Quality Finishing"
[Column #68, 8/00]
Proper Knife Selection, Maintenance, And Safety Are Critical To Quality Finishing
One of the most important aspects of quality finishing is quality paper cutting. Even the most beautifully printed piece on the finest paper can look shoddy if it is not trimmed properly. I shudder to think of some of the experiences we’ve had in my plant as a result of poor knife choice, maintenance, and safety. No doubt, choosing the appropriate knife for a given cutter and knowing how to maintain it properly are critical to the quality of the cutting it produces and the life span of the machinery. This article will cover basic knife selection and maintenance and safety tips.
The first step in quality cutting comes with selecting the knife itself. All cutting knives used in the finishing process have some type of steel inlay. Three major types of knives are used in the paper cutting process: standard steel, high-speed steel, and carbide.
Traditional standard steel knives, which were once the norm in the industry, are by far the cheapest on the market but they also have the shortest life. Steel knives need more frequent sharpening and simply do not hold up as well as high-speed steel and carbide knives. In contrast, high-speed steel knives have gained popularity because they provide a better quality cut and last much longer between sharpenings. The initial cost for high-speed steel knives may seem significantly higher than standard steel knives (about double), but because the high-speed steel lasts nearly three to five times longer between sharpenings, they are worth the investment in the long run. Both the reduction in down time due to sharpenings and the lowered total sharpening costs can add up to big savings in the end.
The highest quality knives are those made of carbide. They provide a superior cut and last much longer than high-speed steel knives. The downside is that they are much more expensive to purchase than high-speed steel knives and require special grinding equipment to sharpen. Carbide knives also chip easily and, because of their cost, are often limited to cutting only certain types of material.
Knife Bevel, Length, and Angles
Standard grinding bevels are usually 23 to 26 degrees but this can vary based on the material to be cut. If you are uncertain which bevel is best suited for your job, consult a knife supplier. They can provide you with detailed information concerning both blade and bevel choice.
It is just as important to properly maintain your knives as it is to select the appropriate ones for your jobs. Most importantly, keep them reasonably sharp at all times! Operating machinery with dull knives slows the machine down, requires extra power (because of the extra force needed to cut the material), and, worst of all, produces a second-rate product. Other problems downline, such as sealed edges that cause continuous misfeeds in a folder, can occur during inferior cutting jobs.
A cutting machine operator should be well-trained in knife maintenance and safety. He or she should be able to recognize the signs of a dull knife by the quality and appearance of the cut, the sound of the cutter, and the length of time since the last sharpening.
There is no exact time schedule for knife sharpening. On average, most knife suppliers agree they should be changed every forty hours of cutting time. This can vary, however, depending on the material being cut. Heavier coated stocks, for example, can dull a blade fairly quickly, and grain direction can also shorten knife life.
As with most equipment, properly maintaining your knives can save money in the long run. Allowing a knife blade to become dull requires more grinding to restore a keen edge. A properly maintained knife should require only about 1/64" metal loss to thoroughly sharpen its blade. A dull knife (which deteriorates very quickly) requires significantly more ground off to return it to its original condition—sometimes as much as 1/8". With an average of only one and a half to two inches of play, it doesn’t take long to grind the entire knife away.
It is just as important to recognize when a blade needs sharpening as to know how to change it properly. Safe knife changing is critical to the training process and should always be monitored carefully. In addition to the safety guidelines suggested by the guillotine manufacturer, have an internal system in place with rigid steps to remove and replace the blade. On average, a 45" cutter weighs about 40 pounds. An operator can easily lose a finger in a split second lapse in judgment. Modern cutters have safety mechanisms in place to help protect the operator; however, they will not completely prevent accidents and are simply meant to minimize the risk of working with a knife. The following are a few tips for safe knife changing:
Prepare for the knife change before removing it from the machine by placing an empty knife board on the cutter table. This way, the knife can quickly be secured to the board once it is removed and helps minimize risk of injury.
Simply put, do not underestimate the importance of knife selection, maintenance, and safety. All are critical to a successful finishing job and a safe working environment.
Insist that operators always use the proper knife handles and knife changing devices, and be sure these accessories are properly installed. Knife handles and changing devices should also be checked for any maintenance problems on a regular basis.
Be sure the sharp edge of the knife is never unnecessarily exposed to operators or passersby. Even the slightest accidental bump or contact can yield serious injury.
Instill respect in your operators for these devices by knowing and following the manufacturer’s procedures and your internal policies for working with and changing knives.