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Managing chipper knife wear to increase chipquality and reduce chipping cost

Raffaele Spinelli a,*, Sotir Glushkov b, Ivailo Markov b

aCNR IVALSA, Via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italyb Forest Research Institute, 132 St. Kl. Ohridski Blvd., BG-1756 Sofia, Bulgaria

a r t i c l e i n f o

Article history:

Received 10 August 2013

Received in revised form

10 January 2014

Accepted 11 January 2014

Available online 2 February 2014

Keywords:

Productivity

Wood

Fuel

Maintenance

Sharpening

* Corresponding author. Tel.: þ39 335 542979E-mail addresses: spinelli@ivalsa.cnr.it (R

0961-9534/$ e see front matter ª 2014 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2014.01.

a b s t r a c t

Wood biomass is turned into industrial fuel through chipping. The efficiency of chipping

depends on many factors, including chipper knife wear. Chipper knife wear was deter-

mined through a long-term follow-up study, conducted at a waste wood recycling yard.

Knife wear determined a sharp drop of productivity (>20%) and a severe decay in product

quality. Dry sharpening with a grinder mitigated this effect, but it could not replace proper

wet sharpening. Increasing the frequency of wet sharpening sessions determined a mod-

erate increase of knife depreciation cost, but it could drastically enhance machine per-

formance and reduce biomass processing cost. Since benefits largely exceed costs,

increasing the frequency of wet sharpening sessions may be an effective measure for

reducing overall chipping cost. If the main goal of a chipper operator is to increase pro-

ductivity and/or decrease fuel consumption, then managing knife wear should be a pri-

mary target.

ª 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Many countries support the increased use of energy biomass,

for its potential contribution to climate change mitigation and

rural development [1]. The European Union has set ambitious

new targets for biomass use, boosting the demand for wood

fuel in all member Countries [2]. However, wood fuel prices are

relatively low, which makes biomass supply a risky business,

where production cost may easily exceed market price [3].

Hence the interest in recovering wood residues, which carry a

lower cost than the primary resource [4]. At the same time,

there is a need for optimizing wood fuel supply chains, in order

to reduce the cost of collection, processing and transportation

[5]. In this endeavour, onemay start fromchipping,which is the

essential element of all modern fuel wood chains, and also a

main source of financial and energy cost [6].

8; fax: þ39 055 5225507.. Spinelli), sotirgluschkovier Ltd. All rights reserved007

Optimization of chipping operations requires improve-

ments on logistics [7], machine selection [8] and operator

training [9]. However, chipper performance is also affected by

other variables, among which knife wear is especially relevant.

As they go through the wood, chipper knives lose their sharp-

ness, which decreases productivity and fuel efficiency [10].

Knife wear has a strong impact on product quality, as well [11].

Wear occurs in the knife’s edge, which is in direct contact

with the processed material. Wear is the result of a complex

combination of mechanical, thermal, electrical and chemical

processes [12]. Their interaction is not fully understood, but

the mainstream opinion is that heat development in the cut

interface may explain most of the wear of a cutting tool. The

high concentration ofmechanical energy in the edge results in

high pressure and significant tensions. These are transformed

into thermal energy, which the processed material cannot

@abv.bg (S. Glushkov), imarkoff@abv.bg (I. Markov)..

b i om a s s a n d b i o e n e r g y 6 2 ( 2 0 1 4 ) 1 1 7e1 2 2118

dissipate easily, being a very poor conductor [13]. Heat, pres-

sure and abrasion determine an increase in the blade front

angle, leading to inefficient shear action [14]. Then, the only

remedy is to restore blade angle through periodical

sharpening.

After sharpening, knife angles must be back to the original

values of about 30� for the front angle and 35� for the rear

angle (Fig. 1). Angles are checkedwith simple protractors, with

a tolerance between þ100 and �300. The quality of edge

sharpening is checked visually with 10� magnifying glasses.

The edge must be sharp all along, and it must be free from

cracks, dents, deformations, stain, etc.

Knives are made of high-grade tool steel, or of structural

steel with high-grade tool steel edges. Manufacturers gener-

ally use high-grade cold working alloy steel of different types,

and typically 1.2631, A8 and D2 DIN grades. Knives are nor-

mally hardened to between 55 and 60 HRC (Rockwell scale).

During service, knives are generally sharpened with a wet

sharpener, after removing them from the chipper. Knives can

also be honed manually with a portable electric grinder,

without removing them from the chipper (dry sharpening).

The result is not as good as with a wet sharpener, because

manual grinding is not accurate enough and it cannot restore

a proper front angle. In fact, dry sharpening is normally used

to improve knife performance on the fly, between two wet

sharpening sessions. Dry sharpening was never meant to

replace wet sharpening, but just to integrate it. Regardless of

sharpening technique, knives become shorter after each

sharpening session, because grinding removes part of the

knife material. For this reason, knife offset must be adjusted

after sharpening. Knives are then moved forward, in order to

maintain the correct clearance between knife and anvil. That

is crucial for efficient shearing action. Clearance is generally

comprised between 0.5 and 0.8 mm, and is checked with a

spacer gauge. The anvil itself is made of high-grade steel

similar to that used for knive manufacturing. Furthermore,

the anvil edges can be layeredwithwear resistant alloy. Anvils

are also subject to wear, and they are turned when their edge

has a curvature radius of about 5 mm. When all four sides are

worn, then the anvil must be discarded. In turn, knives are

discarded when they become too short for safe operation.

The goals of this study were: 1) to gauge knife and anvil

service life; 2) to determine the effect of knife wear on chipper

productivity and chip quality; 3) to highlight the eventual

benefits of dry-sharpening between wet sharpening sessions.

Fig. 1 e Knife sharpness angles.

2. Materials

The chipper used for the experiment was a Biber 70 model,

manufactured by the Austrian company Eschlbock [15]. This

machine is a trailer-mounted, tractor-driven drum chipper,

designed for industrial use. The chipper features a massive

steel drum, with a diameter of 860 mm. The drum length is

subdivided into 6 sections, each carrying 2 knives, in opposite

positions. That means that each knife in a section performs

one cut for every full revolution of the drum. Every alternate

section is rotated 90� on the drum axis with respect to the

adjacent sections, so that the work face of the drum is

constituted of 4 rows of 3 knives each. The staggered knife

design is considered especially suitable for processing large

logs. It is also less vulnerable to knife damage, since the

eventual damage is contained to one or few specific knives.

The test chipper was driven by a 150 kW farm tractor.

The tests were conducted between April and November

2012 at the wood yard of the Mondi Group cellulose plant in

Stamboliyski, near Plovdiv in Bulgaria (coordinates:

42�09003.6600N, 24�31001.7000E). The machine was run in two

shifts: from 06:00 to 14:00, and from 14:00 to 22:00. Three kits

of knives were employed in turns. Each kit was used for 3

shifts. At the end of the third shift, the knife kit was removed

for wet sharpening, and another sharp kit was installed.

The chipper was used to process wood residues, trucked to

the plant from a number of sawmills locatedwithin a radius of

about 100 km from Stamboliyski. Sawmill residues consisted

of slabs, with a length between 1 and 4 m; main species were

pine, spruce, fir, poplar and aspen.Watermass fraction varied

from 18 to 40% on wet base. The chipper also processed small

quantities of pulpwood logs, rejected for different reasons.

The annual chip production amounted to about 100 dam3

loose volume, or about 30,000 tonnes.

3. Methods

Wear was determined for a kit of new knives put into opera-

tion on April 11th, 2012. Total knife length wasmeasured with

an electronic calliper after each wet sharpening sessions,

occurring every third shift. Length was recorded individually

for each knife, and the knife was then installed exactly on the

same mount as before. This was done specifically for the

experiment. In common practice, knives are always rotated

from shift to shift (central knives to the sides and vice versa) in

order to obtain even wear. The experiment lasted 7 months,

until November 1st, 2012, when the last of the 12 knives was

worn out and had to be scrapped. At this date, we also

measured wear on the anvil, which was brand new at the

beginning of the experiment. During the experiment, the anvil

was turned 3 times. Wear was determined as the length dif-

ference between one measurement and the next. Anvil wear

was measured as the reduction in the length of its diagonal

section. This was determinedwith a calliper at 10 cm intervals

along the anvil (Fig. 2). The price of a twelve-knife kit was 900

V, while the price of the anvil was 440 V.

The impact of blade wear on productivity was determined

through time studies. For three consecutive shifts, we

Fig. 2 e Measurement of anvil wear.

b i om a s s a n d b i o e n e r g y 6 2 ( 2 0 1 4 ) 1 1 7e1 2 2 119

determined total mass output and total time input. Chip

output was measured by taking to a weighbridge all truck

loads processed in a shift. Time input was reported on the

chipper electronic hour metre. This procedure was repeated

six times (or 18 shifts), each three-shift unit counting as one

repetition. During the experiment there were no major delay

events, capable of biasing the results. During the test, the

machine was fed with the same sawmill residue from conifer

tree species.

The effect of dry sharpening was determined in a similar

way, by repeating the same procedure on 18 more shifts, with

the only difference that the knives were dry sharpened at the

end of each shift. Repetitions for the two treatments (with and

without dry sharpening) were alternated in a random fashion.

Chipper cost was calculated with the method recently

developed within COST Action FP0902 [16], on an estimated

service life of 4 years, or 10,000 h. The calculated operational

costs of the chipper were increased by 20% in order to include

overheads and administration costs. Further detail is shown

in Table 1. Chipper cost was used to evaluate the financial

benefit possibly obtained by increasing the frequency of knife

sharpening.

Table 1 e Estimated machine cost.

Investment V 200,000

Resale value V 40,000

Service life Years 4

Utilization SMH year1 2500

Interest rate % 4

Depreciation V year1 40,000

Interests V year1 5600

Insurance V year1 2500

Diesel V year1 62,500

Lube V year1 6250

R&M V year1 20,000

Subtotal V SMH1 54.7

Labour V SMH1 6.0

Overheads V SMH1 12.1

Total rate V SMH1 72.9

Note: SMH ¼ scheduled machine hour, inclusive of all delays; R&M

excludes knife cost, which is estimated separately in Table 4.

Finally, the effect of knife wear on chip quality was gauged

by determining chip size distribution after each of the three

shifts spanned by the same knife kit, before wet sharpening.

This test was repeated three times, covering nine shifts. No

dry sharpening was performed at the end of the first and

second shifts. Chip size distribution was determined on an 8 L

chip sample per shift. Each sample was placed in an oscil-

lating screen, using four sieves to separate the following five

chip length classes: >63 mm (oversize particles), 63e46 mm

(large chips), 45e17mm (medium chips), 16e3.15 (small chips)

and <3.15 mm (fines). Bark was separated from the rest and

considered as an additional class. Each fraction was then

weighed with a precision scale.

4. Results

Knife service life depends on the position on the drum. Knives

in the two side sections lasted 1440 scheduled machine hours

(SMH) or 180 shifts. In contrast, knives in the two central

sections lasted only 1056 SMH or 132 shifts. The number of

times they could be wet sharpened amounted to 60 and 44,

respectively. The average amount of knife length removed per

sharpening sessionwas 0.6mm for side knives and 0.8mm for

centre knives. In common practice, knives are rotated among

positions in order to obtain evenwear of the original knife-set.

In that case, one can assume the average duration and wear

values of 1176 SMH, 147 shifts, 49 sharpening sessions and

0.73 mm per sharpening session.

Fig. 3 shows the relationship between knife wear and use,

as represented by the cumulated length reduction and hours

in use, respectively. This relationship was modelled through

regression analysis. The resulting function was highly

KW = a SMH + b SMH * DM + c SMH *DC Count = 600 R2 = 0.980

Parameter Coefficient Std. Error t-Value P-Value

a 0.026 4.679 E-5 549.058 <0.0001

b 0.005 7.693 E-5 65.974 <0.0001

c 0.008 8.569 E-5 94.738 <0.0001Where: KW = knife wear in mm; SMH = scheduled machine hours; DM = Dummy Mid,

equal to 0 if knife is in a side or centre position, 1 if knife is in a mid position (i.e.

mounted on one of the two intermediate drum sections); DC = Dummy Centre, equal to 0

if knife is in side or mid position, 1 if knife is in centre position (i.e. mounted on one of the

two central drum sections).

Fig. 3 e Relationship between knife wear and time in use.

Fig. 4 e Graph of anvil wear as represented by the

reduction in the length of its diagonal section.

Table 3 e ANOVA table for productivity vs. shift and drysharpening.

Effect DF SS h2 F-Value p-Value

Shift 2 1365.48 0.61 84.252 <0.0001

Dry sharpening 1 350.31 0.16 43.230 <0.0001

Shift * Dry sharpening 2 275.32 0.12 16.988 <0.0001

Residuals 30 243.11 0.11

Note: DF ¼ Degrees of freedom; SS ¼ Sum of squares; h2 ¼ ratio

between the SS for a specific effect and the total SS for all effects,

interactions and residuals.

Table 4 e Unit chipping cost as a function of wetsharpening frequency.

Shifts per sharpening n 3 2

b i om a s s a n d b i o e n e r g y 6 2 ( 2 0 1 4 ) 1 1 7e1 2 2120

significant. It was forced through the origin, on the assump-

tion that before use (hours ¼ 0) wear must be 0.

Anvil wear was somewhat slower. Although installed new

with the test knife set, the anvil wasworn on three sideswhen

all knives were fully worn. The last knives were discarded

after 1440 h, and therefore the anvil can be expected to last

about 4/3 * 1440 ¼ 1920 SMH. Since the average duration of a

knife set was estimated to 1176 SMH, then the anvil may last

about 60% longer than a full knife set. Anvil wear followed the

same trend as knife wear, with central sections wearing down

much faster than side sections (Fig. 4). The two extreme sec-

tions (100 and 1100 mm) were those housed in the anvil

mount. Therefore they experienced no wear and could be

taken as a reference for the diagonal length of an anvil in

pristine conditions.

Table 2 shows the productivity per shift for the three

consecutive shifts before knives were sharpened. Productivity

dropped by over 10% from one shift to the next one, as a result

of blade wear. Productivity in the third shift was 23% lower

than in the first shift. Dry sharpening with a grinder allowed

mitigating the effect of blade wear, and reduced the drop in

productivity from one shift to the next. Dry sharpening

allowed increasing productivity by 6% in the second shift, and

by 19% in the third shift. However, productivity in the third

shift was still 9% lower than recorded for the first shift, despite

of dry sharpening.

The analysis of variance showed that many of these dif-

ferenceswere significant, but that the effect of wearwas twice

as strong as the effect of dry sharpening (Table 3).

Table 2 e Productivity in fresh tonnes per shift.

Dry sharpening D (%)

No Yes

1st shift 92.7A 92.7A 0.0

2nd shift 82.2B 87.6AB 6.6

3rd shift 70.9C 84.3B 18.9

Note: D ¼ % productivity increment due to dry sharpening, for the

specific shift; different letters in superscript indicate that the

means are statistically significant at the 1% level.

Using the knife duration and chipper productivity data, we

estimated the potential savings obtained by removing and

sharpening the knives at the end of the second shift, rather

than at the end of the third shift (Table 4). In that case, knives

would have a shorter service life, but the average productivity

per shift would be higher. This preliminary calculation

showed that increasing the frequency of sharpening may

accrue savings in the order of 5%, or 0.40 V per tonne. On an

average annual production of 30,000 tonnes, this would

amount to an annual saving in the order of 12,000 V.

Knife wear also resulted in a visible decay of chip quality,

resulting from the increase of oversize (>63 mm) and fine

(<3.15 mm) particles, as represented in Fig. 5.

The incidence of oversize particles was 5 times higher at

the end of the second shift than at the end of the first shift. At

the end of the third shift, the incidence of oversize particles

was 15 times higher than at the end of the first shift. The

proportion of fine particleswas 2 times higher at the end of the

second and the third shifts, than it was at the end of the first

shift. Non-parametric tests were used to test the statistical

significance of differences between mean values of the same

size class for different shifts. Significance was highest for the

extreme size classes and shifts (Table 5).

5. Discussion

To our knowledge, no other studies offer a detailed follow-up

of chipper knife wear all along its service life, nor they present

shift-level information about chipper performance decay, as

caused by knife wear.

Knife investment cost V 900 900

Total sharpening sessions n 49 49

Knife depreciation per session V 18 18

Sharpening cost per session V 3 3

Production per session t 245 174

Chipper cost per SMH V 72.9 72.9

Chipper use per session SMH 24 16

Chipper cost per session V 1750 1166

Chipping cost per tonne V 7.14 6.70

Knife depreciation per tonne V 0.07 0.11

Sharpening cost per tonne V 0.01 0.02

Total cost per tonne V 7.23 6.83

Note: SMH ¼ scheduled machine hour, inclusive of all delays.

0%

20%

40%

60%

80%

100%

First

shift

Secon

d shift

Third

shif t

Bark<3.15 mm16-3.15 mm45-17 mm63-46 mm>63 mm

Fig. 5 e Particle size distribution in percent over total mass

for the five particle length classes and the 3 work shifts.

b i om a s s a n d b i o e n e r g y 6 2 ( 2 0 1 4 ) 1 1 7e1 2 2 121

The amount of wood processed between two wet sharp-

ening sessions is comparable with the figures reported in

other studies. These indicate that a new set of knives can

process up to 300 tonnes [17]. In contrast, the figures for total

service life found in this study are twice as long as those re-

ported in a previous study by Spinelli and Magagnotti [18],

where a knife set could be sharpened up to 20 times, and not

44 times. That might be related to the different materials

being processed. In this study, the chipper processed clean

sawmill residue, which may have dulled the knives, without

denting them. In contrast, the chippers in Spinelli and Mag-

agnotti [18] processed forest residue, which is often contam-

inated with soil and stones. Hard contaminants dent the

knives, requiring that a larger proportion of knife material is

removed with each wet sharpening session. Raw material

type has the strongest effect on chipper knife wear.

Table 5 e Chip size distribution (in % of total weight) at the en

>63 mm 63e46 mm 45e17

End of 1st shift

Mean 0.5A 5.6A 49.1A

SD 0.8 0.6 5.4

Min 0.0 5.1 45.9

Max 1.4 6.2 55.3

Mean rank 2.0 2.0 5.3

End of 2nd shift

Mean 2.7B 8.2AB 46.5A

SD 0.6 1.5 6.1

Min 2.1 7.1 42.9

Max 3.2 9.9 53.6

Mean rank 5.0 5.3 3.7

End of 3rd shift

Mean 7.4C 10.2B 50.5A

SD 0.6 1.2 2.2

Min 6.9 8.8 48.5

Max 8.1 11.2 52.8

Mean rank 8.0 7.7 6.0

p-Value 0.0265 0.0390 0.561

Notes: SD ¼ Standard Deviations; Mean rank¼mean ranking order attribu

that the difference between ranks (treatments) is casual; Different supers

mean values of the same size class for different treatments.

The higherwear in the central sections of the drum and the

anvil is a well-known fact, and depends on feeding technique.

Loads are systematically fed to the centre of the infeed

opening, so that the central portion of the drum tends to

process more wood than its lateral portions.

The productivity figures reported in this study are

compatible with those reported in other studies for this type

of machine and material [8]. Work at wood yard must have

allowed a drastic reduction of operational delay time, which

explains high productivity all along the work shift. That may

also explain the relatively even figures obtained from the

different repetitions, given that the occurrence of erratic delay

events was minimized [19].

Other studies report of the significant productivity losses

consequent to bladewear. These are estimated to 20% [11], 30%

[20] or 50% [21]. Differences are explained by the different type

of productivity considered in the threementioned studies: the

first two studies considered net productivity excluding delays,

whereas the third study considered pure chipping productiv-

ity, excluding all other work time, workplace time and delays

[22]. Given the limited incidence of delay time, our results are

closer to those reported in Nati et al. [11], but they are also

compatible with the results of the other studies.

However, none of those studies explored the effect of dry

sharpening, which is applied by several operators, but is not in

general use. This study shows that dry sharpening can miti-

gate the effect of knife wear, which remains dominant. In fact,

dry sharpening cannot restore the correct blade angle. Oper-

ators resorting to dry sharpening state that it slows down chip

quality decay, containing the incidence of oversize particles.

However, this study could not test this assumption, which

therefore remains anecdotal evidence. Further studies should

explore the effect of dry sharpening on chip quality, as well as

on knife service life.

d of the three work shifts.

mm 16e3.15 mm <3.15 mm Bark

37.9A 4.6A 3.1A

5.6 1.5 0.7

31.5 2.9 2.4

41.9 5.9 3.7

7.3 2.0 7.7

32.8AB 8.0B 1.8A

4.9 0.7 0.8

27.4 7.2 1.3

37.0 8.6 2.7

5.7 5.0 4.0

20.9B 9.4B 1.6A

2.8 0.7 0.4

17.9 8.9 1.1

23.3 10.2 1.9

2.0 8.0 3.3

1 0.0509 0.0273 0.1133

ted by the non-parametric KruskaleWallis test; p-Value¼ probability

cript letters represent statistically significant differences between the

b i om a s s a n d b i o e n e r g y 6 2 ( 2 0 1 4 ) 1 1 7e1 2 2122

Our calculations show that increasing the frequency of wet

sharpening may offer some financial benefit. In fact, such

benefit might be underestimated. Our calculations assume

that wear and fuel consumption remain the same for both

options (i.e. replacing knives every 2 shifts or every 3 shifts).

Fuel consumption is lowerwhenworkingwith sharper knives,

and it should soar in the third shift. Similarly, the amount of

knife material removed with each wet sharpening session is

likely smaller after working 2 shifts, than after working 3

shifts. Hence the possible underestimate of knife service life

for the 2 shifts replacement option. This said, we believe that

making unsubstantiated assumptions about fuel consump-

tion and service life is worse than underestimating the benefit

of a more frequent sharpening. At this stage, we can say that

increased sharpening frequency is cost-effective, even under

prudential assumptions.

Knifewear is known to affect chip quality, and especially to

increase the incidence of oversize particles and fines [11,21].

With increasing knife wear, the comminution process shifts

from proper shearing to tearing and breaking. That results in

higher chip size variation and in the irregular form of the

chips themselves.

6. Conclusions

Knife wear has a dominant effect on chipper productivity and

product quality. Dry sharpening with a portable grinder can

mitigate this effect, but it cannot replace proper wet sharp-

ening. Increasing the frequency of the wet sharpening ses-

sions may be an effective measure for reducing overall

chipping cost. Ideally, knives should be replaced when the

savings in sharpening and knife cost match the losses

incurred through lower productivity and higher fuel con-

sumption. Such break-even point is likely reached before

knives are fully worn, and future studiesmay try to determine

its position along a chipper knife service cycle.

Acknowledgements

This study was conducted within the scope of the EU INFRES

project, which has been funded by the European Union Sev-

enth Framework Programme (FP7/2012-2015) under grant

agreement n� 311881. Support for this studywas also provided

by COST Action FP0902 within the scope of its 1st STSM

programme.

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