What is the working principle of a rewinding knife grinding machine?

A Rewinding Knife Grinding Machine operates on the principle of controlled abrasive material removal: a rotating grinding wheel is brought into precise, repeatable contact with the cutting edge of a circular slitter knife, removing micro-layers of worn or damaged steel to restore a geometrically accurate, sharp cutting bevel. The entire process is governed by three interdependent subsystems -- the grinding wheel drive, the knife holding and rotation mechanism, and the feed control system -- working in coordinated sequence to produce a consistent, repeatable edge profile.

In practical terms, the machine clamps the circular knife on a precision spindle, rotates it at a controlled speed, and traverses the grinding wheel across the knife face at a programmed feed rate and depth of cut. The result is a restored bevel angle accurate to within +/-0.5 degrees and a surface roughness typically in the range of Ra 0.2 to Ra 0.8 micrometers, depending on the finishing pass specification.

The Grinding Wheel: The Primary Cutting Element

The grinding wheel is the functional heart of the machine. It is a bonded abrasive tool -- meaning abrasive grains (the cutting agents) are held together by a vitrified, resinoid, or metal bond matrix. As the wheel rotates at high peripheral speed, each exposed abrasive grain acts as a single-point cutting tool, shearing off a tiny chip of knife steel with each pass. This is identical in principle to conventional machining, but at a microscopic scale involving millions of cutting points simultaneously.

Wheel Speed and Material Removal Rate

Grinding wheel peripheral speed is typically maintained between 25 and 35 m/s for conventional aluminum oxide wheels, and up to 45 m/s for CBN (Cubic Boron Nitride) superabrasive wheels used on hardened tool steel or carbide knives. Higher peripheral speed increases the number of cutting contacts per second, improving surface finish while reducing the chip load per grain -- which extends wheel life.

Material removal rate (MRR) is expressed as cubic millimeters of steel removed per second. In knife grinding, depth of cut per pass is kept deliberately shallow -- typically 0.005 to 0.02 mm per pass -- to prevent thermal damage to the knife edge. Excessive heat during grinding can reduce the hardness of the steel within 0.1 to 0.3 mm of the cutting edge, a phenomenon known as thermal softening or burn, which causes rapid re-dulling in service.

Abrasive Types and Their Application

• Aluminum Oxide (Al2O3): Standard abrasive for high-speed steel (HSS) and medium-hardness tool steel knives used in paper and nonwoven converting. Cost-effective and widely available.
• Silicon Carbide (SiC): Used for harder, more brittle materials. Less common in knife grinding but applicable to certain ceramic-coated blades.
• CBN (Cubic Boron Nitride): Superabrasive suitable for knives with hardness above 60 HRC. Offers significantly longer wheel life -- typically 50 to 100 times longer than aluminum oxide -- and superior thermal stability (source: Norton Abrasives Grinding Handbook, 2019).
• Diamond: Used for tungsten carbide knife grinding. Diamond wheels are mandatory for carbide blades as conventional abrasives cannot cut carbide efficiently.

Knife Holding and Rotation: Ensuring Concentricity

For the grinding process to produce a usable result, the circular knife must be held and rotated with high precision. Runout (eccentricity) of the knife during grinding directly translates into diameter variation on the finished blade. In gang-slitter applications where multiple knives must match in diameter to within 0.01 mm, any spindle runout is unacceptable.

The knife is mounted on a precision-ground spindle using either a collet chuck, a magnetic face plate, or a hydraulic expansion arbor, depending on knife bore diameter and the machine design. Spindle runout on quality rewinding knife grinding machines is maintained at less than 0.003 mm (3 micrometers) TIR (Total Indicator Reading), a specification verified during machine acceptance testing.

Rotation Speed of the Knife

The knife itself rotates slowly during grinding -- typically at 5 to 30 RPM -- allowing the grinding wheel to work progressively around the full circumference. This slow rotation ensures that the wheel-to-knife contact arc is consistently maintained, producing a uniform bevel with no flat spots or high points around the knife perimeter. Some machines index the knife in fixed angular steps rather than continuous rotation, particularly when grinding knives with radial features or damage localized to one sector.

The Feed System: Controlling Depth and Traverse

The feed system controls two independent axes of motion that together define the grinding result:

• Infeed (depth of cut axis): Moves the grinding wheel toward the knife face in increments as small as 0.001 mm per step. This axis determines how much material is removed per grinding cycle and controls the final knife diameter.
• Traverse (cross-slide axis): Moves the grinding wheel across the width of the knife bevel face. The traverse speed -- typically 50 to 300 mm/min -- combined with infeed depth determines the surface finish and heat generation. Slower traverse at shallow infeed produces finer finish; faster traverse at deeper infeed removes material more quickly but with coarser surface texture.

On CNC-equipped machines such as the MCD Series Rewinding Knife Grinding Machine, both axes are servo-driven and controlled by a programmable logic controller (PLC) or dedicated CNC unit. The operator inputs the target bevel angle, total stock removal, number of roughing and finishing passes, and traverse speed; the machine executes the cycle automatically and repeats it identically for every knife in the batch.

Bevel Angle Formation: Geometry of the Grinding Process

The bevel angle -- the included angle of the knife cutting edge -- is determined by the angular relationship between the grinding wheel face and the knife face at the point of contact. This relationship is set by tilting either the grinding head or the knife spindle to the desired angle before the grinding cycle begins.

Common bevel angles for different substrates are shown in the table below. These are industry-established starting points; actual angles are fine-tuned based on knife steel grade and specific slitting conditions.

The grinding wheel profile -- whether flat-faced, angled, or radiused -- also contributes to the final edge geometry. A flat wheel face produces a flat bevel; a radiused wheel introduces a slight hollow grind, which reduces the included angle at the cutting edge tip while maintaining backbone strength behind it. Hollow grinds are preferred for film and foil applications where extreme sharpness is required.

The Coolant System: Preventing Thermal Damage

Grinding generates heat at the wheel-to-workpiece interface through friction and plastic deformation of the chip. Without active cooling, the knife edge temperature can rise to 300 to 800 degrees Celsius within seconds -- well above the tempering temperature of most tool steels (typically 150 to 250 degrees C for hardness-critical applications). Exceeding the tempering temperature reduces hardness and creates tensile residual stresses that promote microchipping in service.

The coolant system on a rewinding knife grinding machine serves four functions:

1. Heat removal: Flood coolant directed at the grinding zone absorbs heat from the interface and carries it away from the knife.
2. Chip flushing: Coolant flow removes metal swarf and abrasive debris from the grinding zone, preventing re-cutting of chips which degrades surface finish.
3. Wheel cleaning: Continuous coolant flow inhibits loading (clogging) of the wheel face with metal particles, maintaining cutting efficiency.
4. Corrosion prevention: Water-based coolants include rust inhibitors to protect both the ground knife surface and the machine structure.

Coolant concentration is typically maintained at 3 to 8% water-soluble oil or synthetic coolant, balanced to provide lubricity without promoting bacterial growth in the sump (source: IMTS Metalworking Fluid Management Guidelines, 2021). Sump maintenance -- including concentration checks, pH monitoring (target pH 8.5 to 9.5), and regular fluid replacement -- is a standard part of machine upkeep.

Wheel Dressing: Restoring the Grinding Wheel

As the grinding wheel works, abrasive grains wear and become dull, and the wheel face loads with metal particles. This progressively reduces cutting efficiency and degrades surface finish. Dressing is the process of resharpening and re-truing the grinding wheel using a diamond dressing tool -- either a single-point diamond, a diamond roll, or a rotary diamond dresser mounted on the machine.

During dressing, the diamond tool traverses across the wheel face at a controlled feed rate, fracturing and removing the outermost layer of the wheel to expose fresh, sharp abrasive grain. Dressing also corrects any out-of-round condition that develops as the wheel wears unevenly. On CNC machines, dressing is programmed as part of the automatic cycle and executed after a set number of knife passes or when a force or power threshold is exceeded -- ensuring the wheel is always in optimal condition without operator intervention.

Wheel wear compensation is a related function: as the wheel diameter decreases through dressing and normal wear, the CNC control automatically offsets the infeed position to maintain the correct depth of cut. Without this compensation, a shrinking wheel diameter would produce progressively undersize knife bevels. On machines like the MCD Series Rewinding Knife Grinding Machine, this compensation is handled automatically, eliminating the need for manual diameter offset corrections between cycles.

The Complete Grinding Cycle: Step by Step

Understanding each phase of the grinding cycle helps operators optimize machine settings for their specific knife type and condition:

1. Knife mounting and datum setting: The knife is mounted on the spindle and the machine probes the knife face to establish the starting position. This datum ensures that the programmed total stock removal is applied from the actual current knife surface, not from a theoretical position.
2. Roughing passes: The grinding wheel removes the bulk of the worn or damaged material at a higher infeed depth (typically 0.01 to 0.02 mm per pass) and faster traverse. Multiple passes may be executed in this phase depending on the extent of edge damage.
3. Semi-finishing passes: Infeed is reduced to 0.005 to 0.01 mm per pass and traverse speed is reduced. These passes correct the bevel geometry established in roughing and bring the surface roughness into an acceptable range for the finishing phase.
4. Finishing pass: The final pass uses the minimum infeed (often 0.001 to 0.003 mm or a spark-out pass at zero infeed) and the slowest traverse to produce the final surface finish. Spark-out passes -- where the wheel traverses without additional infeed -- allow residual grinding forces to relax and produce a finer finish than the roughing or semi-finishing phases.
5. Diameter measurement and verification: After grinding, the knife diameter is measured on the machine using a contact probe or offline with a micrometer. The result is compared to the target diameter and tolerance band. If within tolerance, the knife is released; if outside, additional corrective passes are executed.

CNC Control: Automating Precision and Repeatability

Manual grinding machines require a skilled operator to set depth of cut, traverse speed, and angle for each knife -- introducing variability between operators and between shifts. CNC-controlled rewinding knife grinding machines replace these manual inputs with stored programs, ensuring that every knife ground to a given program receives an identical edge geometry regardless of who operates the machine.

A modern CNC grinding controller stores multiple knife programs (typically 50 to 200 programs on mid-range systems), each containing:

• Bevel angle setting
• Number of roughing, semi-finishing, and finishing passes
• Infeed depth per pass for each phase
• Traverse speed for each phase
• Knife rotation speed
• Dressing frequency and dresser feed parameters
• Target knife diameter and tolerance

This programmability is particularly valuable in multi-substrate converting facilities where the same machine must grind knives for paper, film, and foil lines. Switching between knife types requires only a program recall, not a mechanical reconfiguration -- reducing setup time from 15 to 30 minutes (manual) to under 2 minutes (CNC program recall).

How the Working Principle Translates to Real-World Performance

The working principle described above -- controlled abrasive removal, precise knife rotation, programmed feed axes, active cooling, and automatic wheel compensation -- combines to produce measurable outcomes in converting operations:

Data in the table above is based on industry benchmark comparisons published by the AIMCAL (Association of International Metallizers, Coaters and Laminators) Technical Committee, 2022. Actual results vary by knife steel grade, substrate, and machine condition.

The extended regrind cycle life achievable with a properly operated CNC machine results directly from the controlled thermal environment (preventing edge softening) and consistent material removal (preventing over-grinding that accelerates diameter loss). Over a knife population of 200 blades, the difference between 6 and 14 regrind cycles represents 8 additional service lives per knife -- directly reducing annual blade procurement cost.