Knife Blade Life and Materials

1. SHEAR KNIFE BLADE LIFE - TWO SCHOOLS OF THOUGHT
By Dave Rumson – Slitting Educator/Consultant
Preface: There are numerous factors that can affect shear knife blade life. These include blade
speed, blade side load force, amount of shear angle, bottom knife axial run-out, amount of
overlap setting and blade metals (materials) composition. This article focuses on blade material
composition differences affecting a blade’s sharpness and two different approaches to assuring
acceptable slitting performance.
A few years ago, while representing a major slitting company, I was invited, along with a winder
O.E.M. representative, to a technical meeting at a 24/7 production facility of a large nonwovens
customer. The purpose of the meeting was to develop a standard winder/slitter design to be
employed worldwide at corporate plants. The intent was to produce a standard winder/slitter
design that could be ordered and released quickly to O.E.M. manufacturers, shortening the
normal 18 to 20 week delivery time. In addition, since all the machines would have common
components that could be purchased at quantity discounts, troubleshooting and problem solving
should be quicker and more cost effective.
Many topics were discussed, including, HMI Interface, communications network, web path,
automatic gluing and splicing design, winding shafts, slitting layout, etc.
When discussion centered on the Slitting Section I was asked what blade materials we had
provided with our Slitting Systems currently employed at their plant. I told the customer they
were using D2 steel blades, a moderately priced alloy steel with nearly 12% chromium and
around 1% each of carbon, vanadium and molybdenum to better withstand the kinetic friction
and abrasion developed as the bottom (female) knife blades drive the D2 top (male) knives. The
high percentage of chromium also offers more corrosion resistance.
When asked, “Is there a better material available to achieve a longer knife life?” I responded
that there were two alternative common blade materials, M2 and CPM-10V, which would offer a
longer knife life, but their cost was considerably higher than the D2 blades.
M2 contains 6.5% tungsten, nearly 5% molybdenum, 4% chromium, 2% vanadium and smaller
amounts of carbon, manganese, silicon and sulfur. M2 has excellent wear resistance but less
corrosion protection.
CPM-10V is a blade made from a blend of nearly 10% vanadium, 5% chromium, 2.5% carbon
and 1+% molybdenum, pressed, sintered and hardened as required. The alloy content is much
more uniform as particles of the above noted metals are blended in powder form before
hardening and machining. CPM-10V has been considered historically the “top shelf” knife blade
because it is known for its longer life, wear resistance and toughness capability.
I asked our customer host, “What kind of knife life are you seeing here at the mill with the D2
blades?” He said he didn’t know and immediately left the meeting room to ask the question of
production personnel. When he returned he said, “D2 is fine. The machines have a scheduled
down every six weeks. All the blades are changed as a set whether they need it or not.” The
school-of-thought at this 24/7 nonwovens mill was to work within the planned down time and
pre-empt dull blade situations by changing the blades on a schedule, within the acceptable
slitting time frame of the running blades.

2. A second school-of-thought on shear knife blade life is to maximize the run time on any single
blade. At a well run label plant, I was once told that by switching to M2 steel knives, their blade
changing time was extended by many weeks over D2 steel blades and months over 52100
steel blades. 52100 steel was developed by the Timken Company for better ball bearing life,
combining toughness with wear resistance. Containing only 1.5% chromium, 1% carbon and
less than ½% manganese and silicon, it does corrode/rust easily. It is the most common blade
material and has the lowest pricing.
Maximizing run time of shear knife blades may save production time but the risk is that some
blades will begin cutting poorly; resulting in reduced cut quality on some rolls. Maximum blade
run time requires a clear understanding of your slitting set up and may involve a shut down at
odd times of the day. You also need to be diligent about monitoring your finished roll quality and
dust levels to help determine when to shut down for blade changes.
Either method of determining blade change can work for your organization. If I were running a
slitting department I would opt for a scheduled down-time change.
In both cases, when it is time to change one blade, I suggest changing them all, unless, of
course, there is a total knife failure for some reason that requires changing a blade before your
usual time. In this case, if less than half way through your normal life time I would recommend
changing only the failed blade. If past the ½ life point I would change them all and then get back
to normal schedule. Reason for failure should be noted.
Keep your blades in sets if possible. All knives in a set should be close to the same diameter
after regrinding which can help save set-up time. Another advantage for keeping knives in sets
is that it’s easier to monitor and control usage and replacement. And remember it is very
important to treat the knives as sensitive tools as their sharp edges can be damage easily or
can severely injure personnel if not stored and transported properly. Safety gloves, edge
protection and storage containers should be mandatory.
As with most things in life, there is more than one way to accomplish the same goal. A review
of your total slitting process could save considerable money over time and provide a rapid return
on investment.