THE SUITABILITY OF REPEATED MACHINING IN NESTING MILLING ON CNC MACHINING CENTERS

Abstract

The article deals with the suitability of repeated machining strategy in working by nesting milling on CNC machining center. Appropriateness of the strategy is assessed for MDF boards milling with thickness of 18 mm by router bits with one reversible razor blade and router bits with two reversible knives. The article presents maximum feed rate based on both, subjective assessment of the operator, as well as on the basis of objective criteria, the quality of the surface represented by the arithmetic mean deviation of the roughness profile. The article simultaneously deals with the distribution of the arithmetical mean deviation of the roughness profile over the height of cut and how it is affected by multiple transition of woodworking machine. At the end of the article, there is analysis why repeated machining strategy is inefficient for this type of machining.

Full text

CHIP AND CHIPLESS WOODWORKING PROCESSES, 10(1): 101–114, 2016
Zvolen, Technical University in Zvolen, ISSN 1339-8350 (online), ISSN 2453-904X (print)
Technical University in Zvolen, T. G. Masaryka 24, 960 53 Zvolen
e-mail: richard.kminiak@tuzvo.sk, sustek@tuzvo.sk
101
THE SUITABILITY OF REPEATED MACHINING IN NESTING
MILLING ON CNC MACHINING CENTERS
Richard Kminiak – Ján Šustek
Abstract
The article deals with the suitability of repeated machining strategy in working by nesting
milling on CNC machining center. Appropriateness of the strategy is assessed for MDF
boards milling with thickness of 18 mm by router bits with one reversible razor blade and
router bits with two reversible knives.
The article presents maximum feed rate based on both, subjective assessment of the
operator, as well as on the basis of objective criteria, the quality of the surface represented
by the arithmetic mean deviation of the roughness profile. The article simultaneously deals
with the distribution of the arithmetical mean deviation of the roughness profile over the
height of cut and how it is affected by multiple transition of woodworking machine. At the
end of the article, there is analysis why repeated machining strategy is inefficient for this
type of machining.
Key words: nesting milling, machining strategy, the quality of the surface, feed rate,
machining efficiency.
1. INTRODUCTION
The application of CNC technology offers a variety of machining possibilities of the given
material. In machining of agglomerated materials, so called nesting milling is becoming
extremely popular. Nesting milling is process when workpiece is extracted from the input
material by router bits milling while distribution of the individual workpiece within given
material determinates optimization software and created surface is considered as final and
therefore no further working with exception of grinding is necessary. Nesting milling can
be done by different machining strategies, for example, repeated machining belongs in one
of those basic ones whilst each strategy requires specific technological parameters.
As optimization criterion of choice of technological parameters has become a quality of the
product, because determinants for success of the given product on the market, in other
words in competitive environment, is quality and price.
For an objective indicator of the quality of the product, it is considered to be precision and
the surface quality of the workpiece. Under the precision of workpiece we understand the
degree of proximity of workpiece geometry to the values indicated on the drawings. We can
take in consideration the shape and dimensional accuracy. The question of sufficient
accuracy with CNC machining centers is solved by design of the machine and therefore
more significant question is related to the quality of finished surface. Surface quality can be

THE SUITABILITY OF REPEATED MACHINING IN NESTING MILLING ON CNC ...
102
precisely defined by parameters of surface irregularities. Surface roughness has kinematic,
technical and technological reasons.
Kinematic causes of inequality (causes of waviness) lie in the cycloid shape of relative
movement of the cutting edge of the knife in the wood, which makes absolutely flat surface
even theoretically unreachable by rotary tool.
Technological causes of inequality (causes of roughness) consist for example in vessels
cuts, fibers, annual rings, moisture, milling along and against fiber and type of wood and
etc.
Technical causes of inequality (causes of roughness and waviness) lie in the precision of
setting of knives in the cutter head (or in precision of grinding disc cutters to equal diameter
of all cutting edges) in the state of wear of the cutting edge of the blade and in vibration and
shaking of milling tool. They are expressed both in uprooting fibers (cutting edge wear) and
irregularities in the distance ripples on a milled surface.
This article sets as its objective to find the maximum feed rate for various machining
strategies and then assess the suitability of the given machining strategy with regards to the
efficiency of the process.
2. METHODOLOGY
Material
The characteristics of used material:
Raw medium hardboard (MDF) supplied by Bučina Ltd. Zvolen, Slovakia were used in
the experiment. MDF boards had thickness h = 18 mm format and width w = 2800, length
l = 2070 mm. Basic technical parameters provided by manufacturer are presented in Tab. 1.
Table 1 Technical parameters of raw medium-density fiberboard.
Property Test method Request
Thickness tolerance
STN EN 324-1
± 0,3 mm
Dimensions tolerance
STN EN 324-1
± 5,0 mm
Squareness tolerance
STN EN 324-2
± 2 mm.m-1
Humidity
STN EN 322
4 ÷ 11 %
Formaldehyde release
STN EN 120
< 8 mg / 100 g a.s. samples
Thickness range
> 6 >9 >12 >19 >30
< 9 <12 <19 <30 <45 (mm)
Bending strength
STN EN 310
23 22 20 18 17 (MPa)
Tensile strength
STN EN 319
0,65 0,60 0,55 0,55 0,50 (MPa)
Swelling after 24 hours
STN EN 317
17 15 12 10 8 (%)
Modulus of elasticity
STN EN 310
2800 2500 2200 2150 1900 (MPa)
Characteristics of the machine:
The experiment was conducted at 5 axes CNC machining center SCM Tech Z5 (Figure
1) supplied by BOTO Ltd., Nové Zámky, Slovakia. Basic technical and technological
parameters provided by the manufacturer are presented in Table 2.

RICHARD KMINIAK – JÁN ŠUSTEK
103
Figure 1 CNC machining center SCM Tech Z5
Table 2 Technical and technological parameters of CNC machining center SCM Tech Z5
Technical parameters of CNC machining center SCM Tech Z5
Useful desktop
x = 3050mm , y = 1300mm, z =300mm
Speed X axis
0 ÷ 70 m.min-1
Speed Y axis
0 ÷ 40 m.min-1
Speed Z axis
0 ÷ 15 m.min-1
Vector rate
0 ÷ 83 m.min-1
Technical parameters of the main spindle - electric spindle with HSK F63
connection
Rotation axis C
640°
Rotation axis B
320°
Revolutions
600 ÷ 24 000 ot.min-1
Power
11 kW 24 000 ot.min-1
7,5 kW 10 000 ot.min-1
Maximum tool diameter
D = 160 mm
L = 180 mm
Characteristics of tools:
For experiment, router bit with one reversible razor blade type designation KARNED 4451
and router bit with two reversible knives type designation KARNED 4551 were used, both
by manufacturer Karned Tools Ltd., Prague, Czech Republic. Basic technical and
technological parameters provided by the manufacturer are in Table 3. Router bits were
equipped with reversible blades HW 49.5 / 9 / 1.5 and HW 50/12 / 1.5 from sintered
carbide T03SMG (standard material used for the treatment of HDF, and MDF DTD), from
BOTO Ltd., Nové Zámky, Slovakia. Basic technical parameters provided by the
manufacturer of sintered carbide provide Table 4.

THE SUITABILITY OF REPEATED MACHINING IN NESTING MILLING ON CNC ...
104
a) b)
Figure 2 router bits used in the experiment a) with one replaceable knife b) with two interchangeable
cutting knifes (D - diameter operation, I - working length, d - clamping diameter)
Table 3 Technical and technological parameters of router bits
Miller
Working
diameter
Working
length
Diameter of the
chucking shank
Dimensions of used
razor blades
Blades
material
D [mm]
l [mm]
d [mm]
L x š x h [mm]
KARNED 4451
16
49,5
12
49,5 x 9 x 1,5
T03SMG
KARNED 4551
16
50
12
50 x 12 x 1,5
T03SMG
Table 4 Technical data of cemented carbide
Classes of
TIGRA
ISO
CODE
US
CODE
Binder %
Hardness
Bending strength
HV10
HRA±0.2
N/mm2
psi
T03SMG
K1
C4++
3.5
2100
94.6
2400
348.000
Method
The experiment was conducted in the following steps:
1. Router bit was fitted with the hydraulic clamp SOBO. 302680291 GM 300 HSK 63F
from Gühring KG Albstadt company, Germany and then inserted into a CNC machine
magazine.
2. The input format of MDF board (2750*1840 mm) was divided in half (2*2750*868 mm)
3. Half- formatted MDF board was placed in a CNC machining center so that the longer
side was in the X-axis and the shorter side was in the Y-axis, attaching the MDF was
provided by 12 evenly placed suction cups measuring 120 x 120 x 35 mm (vacuum set was
0.9 bar) (suction cups distance from the edge of the MDF board was not more than 50 mm)
(see Figure 3). Since the MDF board format dwindled during the experiment, suction cups
location was corrected till the distance from the edge of the MDF board was less than 20
mm.
4. The experiment was carried out in a way that a router bit was gripped by CNC machining
center (KARNED 4451 or KARNED 4551) and by using the given strategy (per pass "I"
e=h, the two transitions "II" e = 1/2h or three transitions "III", e = 1/3h see Figure 4) 5 mm
thick strip of MDF board was cut off (ie. sample "L" as the left). Then CNC machine tool
gripped circular saw with 250 mm diameter and separated another 5 mm thick strip of MDF
boards (ie. Sample "P" as the right) from the format. After separation of required samples,
the MDF board was released and pushed to the end stop and the process was repeated with
a different combination of technological parameters. The process was carried out at
constant operation speed of router bit n = 20,000 min-1 and changing feeding speed from

RICHARD KMINIAK – JÁN ŠUSTEK
105
vf = 1 m.min-1 to speed where vf = vf / max / permissible , based on a professional judgment of
machine operator, machine did not produced surface of good quality or because of an
excess tool load, milling process was accompanied by an enormous increase in the noise
produced by CNC machine.
Figure 3 Scheme of samples preparation (1 - CNC machining center transverse beam , 2 - vacuum
suction cup, 3 - MDF board, 4 - sample "P", 5 - sample "L", 6 - router bit, vc - direction cutting
speed, vf - direction of sliding velocity)
a) b) c)
Figure 4 Machining strategy a) on the first transition, b) on two transitions, c) on three transitions

THE SUITABILITY OF REPEATED MACHINING IN NESTING MILLING ON CNC ...
106
5. L sample was left for further evaluation, and there were extracted samples from P sample
in order to determine surface roughness. Samples were extracted according to the
methodology by Siklienka and Adamcova (2012) see the diagram on figure 5.
Figure 5 Exctracting method of test samples for the determination of surface roughness (Siklienka
and Adamcova 2012)
3. MEASUREMENTS AND EVALUATION
Determination of surface roughness:
The inequality of the surface of the test piece was measured with a laser profilometer LPM-
4 (Figure 6) from the Kvant Ltd. Slovak Republic. Profilometer uses triangulation principle
of laser profilometry. The image of the laser line is scanned at an angle by digital camera.
Then an object profile in cross-section is evaluated from scanned image. Obtained data are
mathematically filtered and individual indicators of primary profile are set, profile of
waviness and roughness profile (Kminiak, Gaff, 2015)
For measuring surface roughness, methodology by Siklenka and Adamcova (2012) was
used reflecting the standard EN ISO 4287. On each test sample, measurements were
performed on three tracks located in the middle of samples, evenly spaced across the width
of the sample (4.5 / 7.5 / 10.5 / 13.5 from the edge of the sample), line length was 60 mm
and the track being oriented in the direction of displacement of the spindle in a milling
process (Figure 7). Surface roughness was evaluated using parameter of arithmetic mean
deviation of roughness profile Ra.
Figure 6 Laser Surface Profile LPM - 4 (1 - supporting structure allowing manual preset of working
distance and mounting of profilometric head and trolley system, 2 - profilometric head, 3 - feed
system of the XZ axis, 4 - control system of working desk shifts)

RICHARD KMINIAK – JÁN ŠUSTEK
107
Figure 7 Placement of surface roughness measuring tracks across the width of the sample (Siklienka
and Adamcova 2012)
4. RESULTS AND DISCUSSION
To verify the suitability of the repeated machining strategy of material (for one transition -
material removal e = h, for two transitions - material removal e = 1/2 h and three transitions
- material removal e = 1/3 h), by nesting milling of MDF board with thickness h = 18 mm,
it was necessary, in the first step, to determine feeding speed (vf / mi n ÷ vf / max ) for each
combination of parameters
Choice of sliding speed vf/real / real (vf / real < vf / max) is left to the CNC machine operator.
Helpful utility for the operator in choosing speed are charts, where, based on the width of
the cut B, it possible to determine the recommended level vf (for our chosen instrument, a
recommended sliding speed by the diagram is vf/recommended = 5m.min-1). In real life
however, the operator selects a sliding rate based on his experience - their subjective
assessment of the situation (usually vf/real ≤ vf/recommended, therefore recommended sliding
speed by diagram represents certain limit values of sliding speed). Operator manages the
process by subjective perception of noise of the milling and the subjective assessment of
the quality of the surface (for our chosen instrument operator usually chooses sliding speed
vf/real ≈5 m.min-1.)
Since we wanted to determine the maximum allowable feeding speed only based on an
assessment of the situation by the operator of CNC machining center without heaving
others factors influence our experiment, we chose the following procedure.
We left experienced operator (it is the operator of CNC machine working in small series
production with frequently changing production and the need for frequent adjustment of
cutting conditions, with 8 years of experience in machining of agglomerated materials
including MDF) to set the value of the permissible feeding speed on the basis of his
experience ( noise of milling process and the quality of the surface) without any real
knowledge of the current feeding velocity (avoidance of diagram influence in setting of
feeding speed limit values ) and even with the risk of damaging the instrument.

THE SUITABILITY OF REPEATED MACHINING IN NESTING MILLING ON CNC ...
108
During the experiment, there was no tool damage regardless of combination of the
parameters. The average values of maximum sliding speeds vf / max obtained by this
experiment presents graph on Figure 8. As can be seen from values, results obtained this
way are much higher than the results expected and as stated by the CNC machining
operator himself, with real feedback of the actual speed of the slide, he would turn down
sliding speeds.
Figure 8. Maximum feeding speed set by the operator VF / max / permissible
Then we proceeded further and chose arithmetic mean deviation of the roughness profile
Ra as optimization criteria. The first step was the analysis of values distribution of the
arithmetical mean deviation of the roughness profile along the height of cut - material
thickness, because in multiple transition of working tool, the tool is in contact with surface
multiple times and that may affect the surface quality in the area of machining.
The statistical evaluation (Table 5) indicated that the arithmetic mean deviation of the
roughness profile is dependent on the track in which it was measured, but it was not
demonstrated on the machining strategy though - the number of transitions of working tool
through the working zone.
Table 5 Results of multifactor analysis of diffusion for determination of the dependence of the
arithmetical mean deviation of roughness profile on track measuring and machining strategy
SS
Degr. of
Freedom
MS F p
Intercept
0,071458
1
0,071458
5268,284
0,000000
Machining strategy
0,000054
2
0,000027
1,995
0,137865
Track of measuring
0,000618
3
0,000206
15,195
0,000000
Machining strategy/
Track of measuring
0,000031 6 0,000005 0,377 0,893549
Error
0,004056
299
0,000014
vf/ recommended
1 miller knife on one transition 1 miller knife on two transitions
1 miller knife on two transitions 2 miller knife on one transition
2 miller knife on three transitions 2 miller knife on three transitions

RICHARD KMINIAK – JÁN ŠUSTEK
109
The course of dependence of mean arithmetic deviation of the roughness profile from track
measurement f (Figure 9) can be explained by density distribution profile of MDF boards
(the edges of the MDF board are thicker compared to the center of the board).
Figure 9 Dependences of the arithmetic mean deviation of roughness profile Ra on track of
measuring for different machining strategies
Since it was not proved the effect of machining strategies on the distribution of the
arithmetical mean deviation of the roughness profile along the height of the cut, in the
further evaluation, we will not select its value by the number of passes through working
zone, but we will evaluate it as a whole.
The overall course of average values of the arithmetical mean deviation of the roughness
profile regarding to the sliding speed throughout the range of sliding speeds is shown on
Figure 10.

THE SUITABILITY OF REPEATED MACHINING IN NESTING MILLING ON CNC ...
110
Figure 10 The dependence of the average value of the arithmetical mean deviation of the roughness profile R
a
on the feeding
speed vf

RICHARD KMINIAK – JÁN ŠUSTEK
111
Table 6 presents the values of the arithmetical mean deviation of the roughness profile at
sliding speeds chose by the operator of CNC machining center based on his subjective
assessment considered by operator as limit (further increases would not be appropriate).
Table 6 Maximum feeding speeds set by the operator VF / max / permissible and reached value of the
arithmetical mean deviation of roughness profile Ra
Type
of
tool
Number of
transitions
Feeding
speed

vf/max
[m.min
-1
]
Arithmetical mean deviation of the roughness profile
Ra [µm]
Mean
Ra [µm]
Std.Err.
Ra [µm]
-95.00%
Ra [µm]
+95.00%
1 Knife
I. transition
3
17,71375
0,999107
15,58420
19,84330
1 Knife
II. transitions
5
22,29300
1,114218
20,01746
24,56854
1 Knife
III. transitions
7
23,04200
1,517589
19,98542
26,09858
2 Knife
I. transition
8
18,36675
1,042864
16,25172
20,48178
2 Knife
II. transitions
18
18,15725
1,088102
15,97836
20,33614
2 Knife
III. transitions
17
15,88425
1,209409
13,45626
18,31224
As already mentioned, the method is highly subjective as it depends on the experience of
the operator. If we want to set the maximum allowable feeding speed based on specific
measurable indicator, for example quality value of the surface required by experience Ra =
16 microns (the value is not defined by norm, we set it on the basis of a survey among
three MDF boards processors, who evaluated samples made by us and chose for them
eligible ones based on the quality of the surface), our conclusion is that the satisfactory
quality of the surface is achievable at feeding speeds lower than those established by the
operator of CNC machining center. Image 11.
Figure 11 Maximum feeding speed vf/max formed on the basis of the required quality of
surface Ra = 16 microns
We also subjected the dependence of the arithmetical mean deviation of the roughness
profile on the type of mill, feeding speed and the machining strategy to statistical analysis
1 miller knife on one transition 1 miller knife on two transitions
1 miller knife on three transitions 2 miller knife on one transition
2 miller knife on two transitions 2 miller knife on three transitions

THE SUITABILITY OF REPEATED MACHINING IN NESTING MILLING ON CNC ...
112
(Table 7) (analysis required limiting the range of the sliding speed to one constant vf = 1 ÷
5 m.min-1).
Statistical analysis indicated that all three assessed factors have statistically significant
effect on the value of the arithmetic mean deviation of the roughness profile and the order
of significance is the type of cutter, sliding speed and finally, machining strategy. The
interaction of machining strategies and feeding velocity has not been demonstrated.
Graphical representation of statistical analysis displays Figure 12.
Table 7 Results of multi-factorial analysis of variance to determine the dependence of the arithmetical
mean deviation of roughness profile on the type of cutter, the size of the sliding speed and machining
strategy.
SS
Degr. of
Freedom
MS
F
p
Intercept
0,023969
1
0,023969
6446,335
0,000000
Machining strategy
0,000029
2
0,000015
3,921
0,023298
Feeding speed
0,000166
4
0,000041
11,148
0,000000
Miller type
0,000361
1
0,000361
96,990
0,000000
Machining strategy/ Feeding Speed
0,000006
8
0,000001
0,193
0,991340
Machining strategy/ Miller type
0,000009
2
0,000004
1,198
0,306439
Feeding Speed/ Miller type
0,000011
4
0,000003
0,712
0,585751
Machining strategy/ Feeding Speed/
Miller type
0,000048 8 0,000006 1,617 0,131056
Error
0,000335
90
0,000004
Figure 12 Dependencies of the arithmetical mean deviation of the roughness profile Ra
from the type of cutters, feeding speed and machining strategy

RICHARD KMINIAK – JÁN ŠUSTEK
113
As it is clear from obtained results, applying repeated machining strategy allows us to use
higher feeding speeds.
In order to make given machining strategy successful, it is necessary to at least multiply
feeding speed to the number of passes, but only in case of closed objects without the need
of inter-transfer.
In the case of single-bladed cutter, this means that in double transition, feeding speed
should be 6 m.min-1 and in triple transition 9 m.min-1. And thus from the standpoint of
single-bladed cutter use, this strategy is inappropriate.
In a case of double-bladed cutter, this means that in double transition feeding speed should
be 16 m.min-1 and in triple transition 24 m.min-1. And therefore, even from the standpoint
of double-bladed cutter use, this strategy is inappropriate.
5. CONCLUSION
Based on the presented results we can make the following conclusions:
• Real feeding speed, when determined based on experience of the operator is higher than
the feeding speed based on optimization criteria of the arithmetical mean deviation of the
roughness profile.
• End users determine the value of the optimization criteria of the arithmetical mean
deviation of the roughness profile Ra = 16 microns.
• The repeated machining strategy allows the use of higher feeding speeds. But increase of
feeding velocity does not compensate for the extension of working time due to necessity of
multiple transitions on the same path and therefore in terms of efficiency, repeated
machining strategy is inappropriate.
AKNOWLEDGMETS
This work was supported by grants from VEGA no. 1/0725/16 „Prediction of the quality of
the generated surface during milling solid wood by razor endmills using CNC milling
machines.“
LITERATURE
1. Kminiak, R., and Gaff, M. (2015). “Roughness of surface created by transversal sawing
of spruce, beech, and oak wood,“ BioResources. 10(2): 2873-2887.
2. SIKLIENKA, M., JANDA, P.,(2013): Effect of method of clamping milling head to the
quality of the surface during milling on the four-sided milling machine. In Annals of
Warsaw University of Life Sciences. Forestry and wood technology. - ISSN 1898-5912.
- No. 81 (2013), p. 240-247.
3. Siklienka, M., and Adamcová, E. (2012).“ Výskum vplyvu materiálu reverzibilných
nožov stopkových nástrojov na kvalitu opracovaného povrchu MDF “ TRIESKOVÉ A
BEZTRIESKOVÉ OBRÁBANIE DREVA, 8(1): 315-323. (in Slovak with English
abstract)
4. Šustek, J., Siklienka, M., (2012). “Effect of saw blade overlap setting on the
cutting,“Acta Facultatis Xylologiae Zvolen, 54(1): 73−79 (in Slovak with English
abstract)

THE SUITABILITY OF REPEATED MACHINING IN NESTING MILLING ON CNC ...
114
5. Krilek, J., Kováč, J., Kučera, M. (2013). “Wood crosscutting process analysis for
circular saws,“ BioResources 9(1):1417-1429
6. Siklienka, M., Mišura, Ľ, (2008). “ Influence of saw blade clearance over the workpiece
on tool-wear,“ Drvna Industrija. 59(4):151-155
7. Dzurenda, L., et al. (2008) “ Vplyv techniky na kvalitu deleného a obrábaného dreva,“
TU vo Zvolene: 140p
8. StatSoft, Inc. 2004: STATISTICA data analysis software system, version 10.
www.statsoft.com.