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Surface Engineered Log Saws for Enhanced Tissue Conversion Productivity & Quality

Dr. Rajiv Ahuja, Dave Graham, Terry Isaacs

International Knife and Saw, Florence, South Carolina, USA &

UCT Forestry LLC, Stuart, FL, USA

INTRODUCTION

An efficient converting production line relies on precise, consistent, high-quality cutting of tissue

logs at speeds in the range of 100-300 cuts per minute (CPM) for bathroom tissue and 20-70

CPM for toweling and JRT applications. Despite significant innovation and automation of

converting machinery including the log saw machining center, the saw blade used for cutting

tissue logs has not seen much innovation over the past several decades. Blade-related cutting

problems are all too familiar to converters worldwide. Whether it is bias in the cut...crushed

cores...or ragged edges, blade-related issues can adversely impact productivity, quality and

manufacturing costs. A majority of these problems can be eliminated by blades that can cut

faster, longer, and cooler.

As a result of recent advances in surface engineering, a new generation of log saw blades is

assisting tissue convertors in meeting the challenges of demanding applications such as jumbo

roll converting. This technical paper details some industrial performance data from mills that

have used these engineered log saws and discusses the unique characteristics of a surface

technology that is of special significance to the tissue converting industry.

BLADE RELATED QUALITY & PRODUCTIVITY ISSUES

Some of the more common log saw blade cutting issues include the following:

Product Quality Productivity

Crushed cores

Constrained cut speeds due to blade

distortion and increasing axial run-out.

Ragged edges Inadequate saw blade life, leading to

frequent changeover related shutdowns.

Bias Cut

These problems originate from one or a combination of the following:

1. Unsatisfactory static run-out of the blade.

2. Overheating of the blade during operation.

3. Dulling of the blade cutting edge.

4. Unsatisfactory alignment of the CBN grinding stones.

5. Inadequate lubricant adjustment leading to gumming of grinding stones.

The factors listed above originate from inadequacies in one or a combination of the following:

1. Machine alignment and adjustment.

2. Blade geometry and metallurgy.

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3. Log saw machining center operational parameters and tissue paper characteristics (log

density, fiber and moisture content etc.)

To extract the best performance from your saw machining center it is essential to pay attention

to all of the three areas listed above. Moreover, safety considerations require that the saw

machining center be maintained to high standards – a blade that requires frequent changeovers,

creates excessive dust and debris during cutting and creates conditions that may lead to spark

generation and subsequent ignition, can be a cause for potential safety hazards.

LOG SAW BLADE PERFORMANCE REQUIREMENTS

Minimum possible static run-out

This is largely a function of the steel metallurgy and heat-treatment of the blade; the

manufacturing and tensioning technique and additional measures taken to ensure that during

transit and storage blades do not stress relive and loose their tension. Higher axial run-out

increases the bias cut which can be a source of quality rejects, especially for larger diameter rolls.

Edge Retention

To ensure a clean cut every time it is essential that the blade has a super sharp cutting edge – this

is the reason that the saw machining center is equipped with a set of grinding stones– after every

few cuts the cutting edge bevel is ground to a razor sharp finish. Edge retention is a complex

function of the bevel geometry; steel metallurgy, microstructure and hardness; residual stress

during manufacture; grinding of the blade during operation etc. Most manufacturers use 2-3

different grades of steel – Chromium-Vanadium alloyed tools steels or high Carbon, high

Chromium D2 steels – depending on the performance demands of the blade. However, all other

factors influencing edge retention vary from manufacturer to manufacturer – so it is essential for

end-users of these blades to understand these differences.

Dynamic / Operating Run-Out

During operations, the saw blade experiences significant forces – (1) centrifugal force from the

rotation of the blade and the reciprocating / swinging action of the arm and (2) friction forces

encountered when the blade is in the cut – the harder/tighter/denser the roll, the higher these

forces. It should also be noted that these friction forces are a function of the cross section area

of the tissue log being cut – a 15 cm (6 in.) roll will have 2.5 times higher friction forces than a 10

cm (4 in.) roll. Both these forces distort the blade and increase blade run-out, resulting in

increased bias cut and reduced cutting accuracy.

The friction forces encountered during the time when the blade is “in the cut” generate heat,

leading to a steady increase in the temperature of the blade. Even a 20-30 degree temperature

gradient from bevel to bore in an 800 mm diameter, 5 mm thick blade is enough to cause

significant increases in blade wobble and hence cutting deviation. If the blade’s axial run-out

crosses a critical threshold, the cutting process has to be halted to allow the blade to cool down.

This can significantly impact productivity.

So reducing friction forces during the cut can have a significant impact not only on the operating

temperature of the blade but can also lower the horse power requirement for rotating a blade at

300-400 rpm– especially blades diameters of 800mm+.

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Dissipation of the heat generated by friction forces can also be an effective way to keep the blade

temperature low. This can be achieved by either a complex blade cooling mechanism installed in

the log saw machining center or better still, by using a blade with inherent thermal management

capabilities.

Lubrication issues

Some sort of lubrication is commonly used on the body of the blade to reduce friction forces.

However, this lubricant can frequently gum up the grinding wheels reducing their efficacy and

eventually leading to dulling of the cutting edge. Also, lubricant contamination can transfer to

the tissue rolls, increasing the % of ‘broke’ rolls. Saw blades with a ‘lubricious’, low coefficient-

of-friction surface can significantly reduce lubrication consumption and all of the associated

drawbacks of using lubricants.

SURFACE ENGINEERING OF LOG SAW BLADES

It is clear from the above discussion that blade performance and cutting productivity is greatly

influenced by its (a) bulk properties (such as metallurgy and microstructure); (b) design and

manufacturing processes and tolerances; and (2) the surface properties of the blade. The first

two factors have been addressed by tool manufacturers over the years, but surface engineering

aspects have been largely ignored.

For traditional industrial tooling it has been well recognized over the past 30 years now that

surface coatings can enhance tooling performance significantly – notably, tool life by protecting

the cutting edge. In the metal cutting industry the prevalence of Physical Vapor Deposition

(PVD) and Chemical Vapor Deposition (CVD) coatings is widespread and 70-80% of all new

tooling is coated. These surface engineering advances have not been carried through to Wood,

Paper & Tissue cutting. There is very limited use of some coatings, most of them now traditional

and with very marginal performance improvement – this includes Teflon, Chrome and Nickel

plating, PVD Titanium and Titanium-Aluminum nitride. Coatings such as Teflon are used

primarily to improve surface lubricity and reduce friction, whereas Cr and Ni to improve the

wear resistance of the cutting edge and the body of the tool. The latter two coatings have higher

hardness than the steel used to manufacture the tool, but offer no significant friction reduction or

thermal management.

The recently emerging EXO technology offers a coating that possess a combination of hardness,

lubricity and thermal management that make it ideally suited for creating a premium log saw

blade – the EXO-THERM blade.

EXO TECHNOLOGY

EXO technology is a proprietary metal finishing process developed by UCT Coatings Inc, based in

Stuart, Florida. This coating was initially developed for applications where the surfaces were

required to have high lubricity and adequate hardness – example guns that can operate without

grease. Further testing revealed that the coating had good thermal dissipation and anti-sticking

capabilities – this opened up several new opportunities – especially in wood working and tissue

cutting applications.

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EXO Technology applies patented inter-metallic nickel-boron based family of coatings to metallic

surfaces – one popular version of this coating has been marketed under the trade name of

UltraCem™. The properties of these coatings makes them suitable for applications requiring high

wear resistance, friction reduction, uniformity of dimensions on complex geometry, strong

adhesion to substrates, and inherent solid lubrication. Corrosion inhibition is an added feature.

Some of the relevant properties of the coating have been shown in Figure 1.

EXO Technology is not a line-of-sight technology, compared to many other coating deposition

technologies. All surfaces are coated with a uniform thickness including inside and outside

corners and blind holes, ensuring uniformity of properties on all surfaces. The rate of plating

deposition is about 20 microns/hr and well-controlled, providing a high production throughput

rate. The entire coating process can be scaled and automated as needed to accommodate most

sized part. Pre and Post-plating processes may include a variety of surface finishing techniques,

including polishing, stress-relieving, heat-treatment etc.

The structure of the coating, both in cross-section and top view has been shown in Figure 2. The

structure of the coating is columnar, with individual columns ranging in diameter from 15 to 30

microns. Each of the columns is further composed of micro-nodules with a diameter range of 0.5

to 1 micron. From the top-view it can be seen that the coating has a large actual surface area

compared to the apparent and projected surface area. This topology makes the coated surface an

ideal heat radiator and dissipater. Also, the nodular structure offers limited contact area of any

external surface sliding past this coated surface result in lower friction forces.

Figure 3 illustrates some horse-power and cutting force data derived from cutting tests using

EXO coated surfaces. This data, while not directly from a tissue cutting application, clearly

indicates that the coated surface compared to an uncoated one results in 30-40% lower

horsepower for cutting. The exact reduction in horse-power while cutting tissue logs still needs

to be evaluated, but it is expected that similar reductions are possible.

Figure 4 illustrates the corresponding thermal imaging data from uncoated and EXO coated

surfaces. Again it is evident that coated surfaces surface dissipate heat mush more effectively

than uncoated ones – in most cases 2 times the run time was possible before the coated surface

reached the same temperature as an uncoated one.

EXO technology needs to be combined with a blade design that has been optimized to not only

cut tissue material of different grades and logs of different dimensions, softness and density but

also adapted for the deposition of a coating on the blade surface. All coatings have intrinsic

stresses that need to be considered when tensioning the blade (it should be noted that in general,

EXO coated blades in storage, retain their tension over longer time periods than uncoated

blades). Also, blade heat treatment may need to be modified somewhat to extract the full benefit

of the coating.

From the above information it is clear that EXO-THERM blades integrate EXO technology, blade

design, manufacturing processes and blade metallurgy to generate a product that has a unique

set of properties and performance characteristics. These characteristics are derived from

optimizing and integrating the four factors to produce a high-performance, premium log saw

blade.

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THE EXO-THERM SAW BLADE – SUCCESS STORIES

To derive the full range of benefits from the EXO-THERM log saw blade it is also important to

ensure that the log saw machining center is in good operating condition and is maintained well.

Also, the EXO-THERM blade can impact productivity significantly, so all processes up and down-

stream of the log saw machining center need to be re-balanced, to take advantage of the higher

through-put rate from the log saw machining center.

The EXO-THERM blade has been accepted by US tissue convertors over the past 2 years (under

the brand name of UltraCem™ blade), with very good success. This blade is rapidly replacing

Teflon coated blades and a majority of the 800mm+ blades produced by International Knife &

Saw in Florence SC are now of the EXO-THERM grade. 600 mm log saws also derive substantial

benefits from EXO-THERM – longer tool life, lower lubricant consumption, improved cut quality,

higher productivity etc. Some, typical success stories have been indicated in Figure 5. Figure 6

shows the break-down of benefits derived from using EXO-THERM blades – this is based upon

the accumulated experience with several leading tissue-convertors,

It must be emphasized that the performance of this blade does not derive from just the EXO

technology but a combination of this technology with an appropriately designed and

manufactured blade and the fine-tuning of operating parameters within the log saw machining

center – cutting head speed, blade rotation speed etc. These parameters are adjusted for

different grades of tissue paper, to extract the best performance from EXO-THERM blades.

THE EXO-THERM BLADE – AN ENABLING TECHNOLOGY FOR CONVERTING MACHINE OEMs

Over the years the metal cutting machine tool industry has evolved in symbiosis with the

advances in tooling technology – engineered surface coatings on tools, integrated with

innovative tool designs and tool metallurgy revolutionized the performance of metal working

tools. This advanced tooling then became an enabling technology for innovative machine tool

manufacturers – higher speeds and feeds, higher manufacturing tolerances, dry-machining etc

were all made possible because premium tooling gave machine builders the technological edge

to design machines with expanded capabilities. In a similar fashion, premium EXO-THERM log

saws blades where the best of surface engineering had been combined with manufacturing

technology and product design can be an enabling technology for innovative converting machine

OEMs. These EXO-THERM blades can permit operations at higher cutting speeds & productivity,

lower power requirements (which not only reduces energy consumption, but also allows for

smaller, lower HP motors – helping in reducing machine footprint), lower rejects (further

lowering the carbon footprint) and improved cutting tolerances.

SUMMARY

EXO-THERM log saws blades engineered and manufactured specifically to work in conjunction

with a proprietary inter-metallic coating, possess a protective surface that facilitates a major

reduction in friction – which in turn leads to a significant decline in blade surface temperatures

when operating at higher blade speeds. The plating’s unique thermal management property

allows operators to maintain consistent cut quality while enjoying increased blade life, extended

grinding stone life, reduced lubrication requirements, and overall improved blade performance.

Hardness

As Plated: 850-950 Knoop

Ppt. Hardened: 1150-1400 Knoop

Coefficient of Friction

0.0800.20 depending on mating material

and lubrication used

Adhesion

Before Ppt. Hardening: 48 MPa

After Ppt. Hardening: > 69 MPa

Roughness

Original surface + 30-40 rms (micro-

inches)

Coating Thickness Range

10 – 100 microns

Uniformity of Deposit

Uniform thickness, regardless of geometry

• Conformal coverage on complex

geometries

• Uniform coverage in through-holes

and cavities

WEAR RESISTANCEWEAR RESISTANCEWEAR RESISTANCEWEAR RESISTANCEWEAR RESISTANCEWEAR RESISTANCEWEAR RESISTANCEWEAR RESISTANCE HARDNESSHARDNESSHARDNESSHARDNESSHARDNESSHARDNESSHARDNESSHARDNESS

Figure 1. Properties of EXO coating

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Nodular surface

0.5 µm (500nm) nodules

25 µm nodules

COLUMNAR GRAIN STRUCTURE

• LARGE SURFACE AREA TO DISSIPATE HEAT.

• LOW SLIDING CONTACT AREA MINIMIZES FRIC-

TION FORCES.

Figure 2. Micro-structural features of EXO coating

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Uncoated Uncoated

Temp after 5 min at 7200 RPM

Coated Coated

Species Coated Uncoated Difference

MDF 3.57 5.18 31.1%

MDF 1.63 2.36 30.9%

Particle-board

3.57 5.18 31.1%

Pine 7.05 11.67 39.6%

Cutting load ~ RMS milli-volts from 3-axes force dynamometer

Thermographs of coated bladesThermographs of coated blades

At 800 lineal ft At 2800 lineal ft At 3200 lineal ft (350°F) Blade failed at this point

At 1200 lineal ft At 1600 lineal ft (346°F) Blade failed at this point

Thermographs of uncoated bladesThermographs of uncoated blades EXO saws dissipate heat more effectively – hence remain cooler over a longer period of time �

�Improved mechanical stability

�Less Resin / Lubricant build-up

�Improved cut quality

�Less cutting deviation

�Lower “dynamic” kerf

DISK HP DRAW % CHANGE AVG. TEMP DIFFERENCE

UNCOATED 0.90 HP 142° F

COATED 0.46 HP - 49% 102° F -40° F

Figure 3. Illustrative horse-power and cutting force data derived from cutting tests

using EXO coated surfaces

Figure 4. Thermal imaging data from uncoated and EXO coated surfaces

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Figure 5. Success stories from converting mills using EXO-THERM log saw blades

Who? One of Top 3 global Tissue Converters

Where? Wisconsin, USA

What? 24" / 610 mm EXO-THERM Log Saw Blades

Details: PCMC and Perini Converting Machines

Small diameter (4.25"/107.95mm)) and mega size rolls of tissue and towel

Blade life ends at 18-19" in diameter

Benefits: Reduced core crushing

Longer Blade Life by about 25% minimum (dependant on product mix)

Who? One of Top 3 global Tissue Converters

Where? Wisconsin, USA

What? 32" / 810 mm EXO-THERM Log Saw Blades

Details: MTC Converting Machines

Jumbo Roll Tissue (JRT) 8.5"/216 mm, 1000'/304.8m single ply white paper.

Roll is very dense and hard.

Blade life is between 10-14 days. 40 cuts per minute, blade RPM 350

Lube spray on blade every 3-4 cuts, blade temperature 100-110 degrees F.

Blade temperature: 140°F uncoated, 100-110°F EXO-THERM

Using IKS copper bonded grinding wheels

Benefits: Reduced core crushing and bias cuts

Reduced blade damage due to tension loss and lower blade temperature

Longer Blade Life by about 25% minimum (dependant on product mix)

Who? Tissue Converter

Where? Wisconsin, USA

What? 32" / 810mm EXO-THERM Log Saw Blades

Details: MTC Converting Machines using IKS lube system

Cutting JRT towel and tissue with diameters of 6-9"(152-229 mm)

Benefits: Reduced core crushing.

Longer Blade Life

Who? One of Top 2 global Tissue Converters

Where? Alabama, USA

What? 24" / 610 mm EXO-THERM Log Saw Blades

Details: PCMC and Perini Machines

Cutting variety of product, with various diameter roll and sheet count. Some extremely soft rolls.

20% of product has cores, 80% coreless. 3.70 cuts per log, 2k logs per day per saw

Benefits: Reduced lubrication and in some machine centers totally eliminated lubrication.

Eliminate core crushing and bias cuts

Blade life increased from 30 days to 45-60 days

Customer "Overall we are very pleased with the UltraCem™ blades - the initial cost is higher,

Testimony: but they save us money on life span and lube cost"

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REDUCED

DOWNTIME

REDUCED

GRINDING WHEEL

CONSUMPTION

IMPROVED

SAFETY

IMPROVED CUT

QUALITY

INCREASED

PRODUCTIVITY

REDUCED LUBE

CONSUMPTION

REDUCED BLADE

CONSUMPTION

Figure 6. Summary of benefits derived by customers using EXO-THERM log saw

blades

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