I find myself repeating the same thing over and over again when servicing a paper cutter; “You’re not replacing your dull knife blade often enough!” Many owners and operators of guillotine cutters do not truly understand how the type and condition of the knife blade affects the quality of the cut.
It’s amazing how many times I service a cutter that isn’t cutting so well and all I have to do is install a sharp knife. Presto, the machine cuts great. Most operators however, will cut until they have problems and only then decide it is time to change the knife.
Well that’s probably way past the point the knife needed changing. Power cutters today are extremely strong and will cut though a lift of paper with a very dull knife. What operators don’t realize is how much wear and tear this puts on the cutter.
It’s hard on the hydraulic cylinders and clutch systems. Operators will run the maximum clamping pressure in an attempt to compensate for excessive draw on lift of paper. It’s also hard on the clamping linkage.
With repeated use of dull knives, you cause a lot of premature and expensive damage to the machine. And of course the cut quality also suffers with a dull knife. This could all be avoided by simply keeping a sharp knife in the cutter.
Here’s my rule of thumb, using 50# Offset as the standard:
- Standard Steel Knife Blade: will cut about 8 hours or about 1,200 cycles
- High Speed Steel Inlaid: will cut about 16 hours or about 3,000 cycles
- Carbide Knife Blades: will cut about 10-20 times longer than High Speed Steel
One of my favorite tales of woe is this: A client is cutting about 2-3 hours a day, everything from plain paper to chip-board. They say, “I don’t understand why we’re having problems…we just put a sharp knife in the cutter.” When I ask, “How long ago?” they respond, “Last month”. Now you do the math…if they’re using a standard steel knife, it probably should have been changed at least 6-8 times!
Some other factors that can affect your results:
- The expertise of your sharpening service
- Types of paper or other substrates you might be cutting
- Knife angle
For example; cutting chip-board will dull the knife rapidly to the point it will need replacing as soon as you are done with that chip-board job. Another example; your sharpening service doesn’t do a good job and you assume you’re installing a good sharp knife. I have run into this many times.
The grinding angle on the knife also greatly affects the performance of the cut. Generally, printers use a 24 degree angle. But a 24 degree angle will create draw when cutting spongy or NCR stocks with low clamp pressure. Many cutter manufactures include some guide lines on knife angle in the manual. These guidelines vary by manufacturer. Generally speaking you can look at it this way:
Knife Grinding Angle
- Soft Spongy Paper: Smaller angle, 19 – 20 degrees w/low clamp pressure
- Regular Paper: Med angle, 23 – 24 degrees w/med clamp pressure
- Hard Paper: High angle or double bevel up to 30 degrees w/higher clamp pressures
Always refer to the OEM manual or discuss this with your sharpening service and the manufacturer. There are far too many different configurations and materials to list here to give a one-size-fits-all answer.
Just to summarize, it sounds like three simple practices will keep your guillotine cutter humming along:
- Keep a sharp knife in the machine at all times
- Use the right knife steel to suit your operation
- Use the right knife angle for the job you’re cutting