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Study of z-level finishing milling strategy

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The article presents the z-level milling strategy, which is one of the most important strategy in a CAM system. In case of z-level milling, we can use end mill with corner radus for finishing milling of steep walls. The aim of this article is present the effect of the parameters of part (gradient of the surface, direction of the cutting), tool (corner radii) and cutting process (depth of cut, feed per teeth) to the surface quality.
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Mikó Balázs: Study of z-level finishing milling strategy; Chapter 2.1 in Development in machining technology Scientific
research report Vol.2., Ed.by W. Zebala, I. Mankova, Cracow University of Technology Cracow 2012. ISBN 978-83-7242-
655-0 p.83-90.
STUDY OF Z-LEVEL FINISHING MILLING STRATEGY
Mikó B.
Óbuda University
Abstract: The article presents the z-level milling strategy, which is one of the
most important strategy in a CAM system. In case of z-level milling, we can use
end mill with corner radii for finishing milling of steep walls. The aim of this
article is present the effect of the parameters of part (gradient of the surface,
direction of the cutting), tool (corner radii) and cutting process (depth of cut,
feed per teeth) to the surface quality.
Keywords: CAM systems, z-level milling, surface quality
Introduction
A complex product, like an engine for a car, contains lot of different parts,
which have different shape and require different manufacturing process.
However the milling technology is not a primary manufacturing technology in
case of automotive parts, because of the use of sheet metal, plastic and casted
and forged parts, the production of manufacturing equipments, like die and
moulds etc., are required the milling technology.
The conventional milling machines ensure the production of simple geometry,
like planes and holes, but the complex part geometry is able to be
manufactured by CNC milling machine. The 2.5D milling means that the
milling cutters move in a curve in x-y plane and the z move means the depth of
cut. The 3D milling means, that the milling cutter moves in 3 axes parallel,
which ensures the manufacturing of complex surfaces.
In case of complex surfaces the CNC program is generated by CAM system,
because the tool path generation needs complicated calculation. The CAM
systems contain several milling strategies, which identify the type of the
manufacturing step. During the definition of a step the engineer (user) selects
the strategy, sets the date of the milling cutter, the parameters of the cutting
and the path, select the area of the manufacturing.
The most important strategies are the volume milling, the z-level milling and
the surface milling (Fig.1). In general, the volume milling is applied for
roughing, the z-level milling and the surface milling are applied for finishing.
Figure 1 Volume milling, z-level finishing and 3D surface milling
In this article, the z-level milling was studied. In case of z-level milling we can
use end mill with corner radius for milling of steep walls. The aim of this
article is present the effect of the parameters of part, tool and cutting process
to the surface quality.
Test environment and equipments
The test part was made of non-alloyed structural steel S355 (Fe 510). The part
contains two different test surfaces with different gradients. Three different
gradients were defined: A
1
= 65° / 75° / 85°. Every test surface contains two
surfaces: the first one ensures parallel milling with the x axes (A
2
= 0°), and the
second one is angled with x axes (A
2
= 45°) (Fig.2).
The CAD model and the NC programs was generated by Pro/Engineer WildFire
4 integrated CAD/CAM software, and the machining was performed by Mazak
Nexus 410-A II machining centre. The surface roughness was measured by
Mitutoyo SJ-301. The surface roughness is determined by average of 3
measured values.
Two milling cutter was used for the tests: Fraisa U5250.445 and U5250.450,
the cutting diameter is 10 mm, in both cases, and the corner radii are 0,5 and 1
mm. The number of teeth is 6, the cutting speed (v
c
) 200 m/min, the
revolution (n) 6.400 1/min. The feed per teeth (f
z
) and the depth of cut (a
p
)
were varied based on tool catalogue [1], feed per teeth were: 0.08/0.12/0.16
mm (feed speed: v
f
= 3000/4500/6000 mm/min) and the depth of cut:
0.15/0.20/0.25 mm. The profile milling strategy was selected in the CAM
system, and conventional milling was used.
Figure 2 Test part
Table 1. Test sets
Test
part
No
R
tool
v
f
a
p
A
1
mm mm/min mm °
1
1 0.5 3000 0.15 65
2 0.5 6000 0.15 85
2
3 0.5 4500 0.20 75
4 0.5 6000 0.25 65
3
5 0.5 3000 0.25 85
6 1 6000 0.15 65
4
7 1 3000 0.15 85
8 1 4500 0.20 75
5
9 1 3000 0.25 65
10 1 6000 0.25 85
The Table 1 shows the 10 test sets, which was determined by design of
experiment (DOE) method. The DOE method ensures less number of tests with
same effectiveness.
Results
The tests sets make possible several analyses.
The Fig.3 shows the measured Ra surface roughness values. The first curve
(continuous blue curve) shows the surface roughness in case of x axes parallel
milling, and the second curve (interrupted red curve) shows the 45° milling.
Based on the test in case of parallel milling the surface roughness is larger in
nine cases, the maximum difference is up to 35% in case of 6
th
test surface.
The cause of it is the less vibration, because the parallel motion of the x and y
axes don’t permit to develop the harmful vibration.
Figure 3. Surface roughness in function of milling direction
The Fig.4 shows the effect of the other parameters to the surface roughness,
the parallel and 45° are separated.
The larger a
p
cause larger surface roughness, it is evident. However, in case of
45° milling and smaller a
p
it has smaller effect.
The wall gradient has advantageous influence to the Ra, because the high of
the rest material is smaller, as it will be presented later on. This is the reason
why the z-level milling is used for steep wall finishing. The milling direction
does not have a significant effect.
Figure 4. Effects of different parameters to the surface roughness
The tool corner radius shows inverse proportionality, the larger radius causes
better Ra, and significant difference between the parallel and 45° milling is
experienced.
The last parameter is the feed speed (v
f
) in the Fig.4. The larger feed speed
cause worse surface roughness, the influence of the A
2
is not significant.
The cause of the surface roughness is the rest material between the slices of
the milling. This rest material is characterized by the cusp height parameter.
As the Fig.5 shows, the cusp height (Ch) is determined by tool corner radius
(R), the depth of cut (a
p
) and the angle of the wall (α, in the actual research it
was marked A
1
).
Figure 5. Cusp height in case of z-level milling
Figure 6. Ra, Rz and cusp height in case of z-level milling
The connection between the cusp height and the surface roughness is evident,
but based on Fig.6, it is not a simple linear function, and other parameters
have significant effects too.
The next two estimation formulae were created by the MiniTab v14 analysis
software. As result of numerous iterations, beside the cusp height, the feed
speed and the angle of the wall, which was defined in radian, in this case, were
important effect to the surface roughness. The R-Sq(adj) is 83.8 %, which is
not toog good result, but based on cusp height values (see Fig.6) better result
is not realistic goal. The second suggested formulea work in the logarothmic
space, the structure and the R-Sq(adj) value are similar (R-Sq(adj) = 85.0 %).
v1 : Ra = 0.93 + 192 Ch + 0.000309 v
f
- 1.17 A
1_rad
(1)
v2 : ln Ra = - 0,52 + 0,660 ln Ch + 0,563 ln v
f
- 0,603 ln A
1_rad
(2)
Fig. 7 shows the estimated and the measured values of Ra. The estimated
values follow the measured curve generally, but in some cases, there are
larger error.
Figure 7. Estimation of the surface roughness
Conclusions
Z-level milling is essential milling strategy for finishing of free form surfaces.
Based on the tests, beside the tool parameters, the cutting parameters and the
surface gradient, the milling direction has also influence to the surface quality.
The regression analysis showed the mathematical connection between the
cusp height, which can be calculated, and the surface roughness parameter. In
order to eliminate the error of the estimation of the surface roughness
additional tests are required.
The project was realised through the assistance of the European Union, with
the co-financing of the European Social Fund, namely: MOP-4.2.1.B-
11/2/KMR-2011-0001 Researches on Critical Infrastructure Protection.
References
[1] Fraisa High-performance end mill tools 2011/12; Fraisa SA Bellach 2011.
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