The bottom knife is the reference, not the follower

The top knife gets the attention because it is the one you can see cutting. But the bottom knife is the reference the whole set is built from. Its outside diameter and face run-out decide where each slit lands, and the top knife simply follows it. When slit width drifts across a web, or one lane is consistently out while its neighbours are fine, the bottom knife is usually the place to look first.

What we make

• Bottom slitter knives — flat counter discs
• Grooved bottom rings for top knives that run in a groove
• Spacer rings and shim sets to set lane positions
• Matched top-and-bottom sets, ground together

What the geometry has to get right

Outside diameter, bore, thickness and face parallelism are the four numbers that matter. The face has to be flat and square to the bore, because any tilt shows up as a step in the slit. On a grooved bottom ring the groove width and depth have to match the top knife’s section, and the groove wall is what actually does the shearing on that side.

Bore fit matters more than people allow for. A bottom knife that is a loose fit on the shaft will not repeat its position when it is remounted, so lane widths move between setups and nobody can tell why.

Where a bottom slitter knife fails

• A visible step or groove worn into the face. The top knife has been running in one track. Once the step is deep enough, a new top knife cannot recover the cut.
• Slit width drifting across the web. Face run-out, or the bottom knife not seating square on the shaft.
• Burr on the underside of the web. Bottom edge rounded, or clearance opened up.
• One lane consistently out of tolerance. That station’s spacer stack or bottom ring, not the top knife.
• Vibration at speed. A set that is no longer balanced, often after mixing new and worn rings.

Regrinding shrinks the diameter — plan for it

Bottom knives are reground on the face, which reduces thickness, or on the outside diameter, which reduces the size the top knife registers against. Either way the geometry changes, and the set has to be re-shimmed afterwards. There is a limit: once the diameter drops far enough the top knife cannot reach its working overlap, and the ring has to be replaced. We will tell you the usable range when we quote, based on the diameters you give us.

How we match an existing bottom knife

Send the worn ring and tell us the slit width it was set for. We measure outside diameter, bore, thickness, face flatness and groove dimensions where present, then confirm how much material is left for regrinding. If you are replacing a full set, send the whole stack — mixing new rings into a worn stack is the most common reason a rebuilt set will not hold tolerance.

Bottom slitter knife questions

Do I have to buy the top knife at the same time?

It is the safer route. The clearance is set between the two, and a new bottom knife against a worn top knife usually needs more shimming to behave.

My bottom knife is reground — will the top knife still work?

It depends on how much diameter was removed. Below a certain point the overlap cannot be reached and the top knife has to be changed too. Send both and we will tell you which is the case.

Can you make grooved bottom rings to match my top knife?

Yes. Send the top knife section and the groove dimensions you need, or send the worn ring and we will measure the groove.