THE
EFFECT OF MACHINING SPEED INCREASE ON THE ROUGHNESS OF MACHINED SURFACE FOR A
SELECTED CNC MACHINE TOOL
Krzysztof Walczak Wójciak, Zbigniew
Adamski
Department of Material Technology and Material
Engineering
Faculty of Technical Sciences
University of Warmia and Mazury in Olsztyn
Abstract
A simple method which a
technologist and operator may apply to achieve
a surface of low roughness is to increase the machining speed. This paper
presents a practical method of evaluation of the effect of machining speed on
the parameters which describe the surface roughness of an object being
machined. For a selected numerically controlled machine tool – a HOMA –
ECOCENTER V350-FMC miller – the machining speed has been determined which
allows for achieving a surface of minimum roughness.
INTRODUCTION
The degree of object roughness
determines its resistance to wear, corrosion, the size of transferred
pressures, fatigue strength and the general surface appearance. Surface
roughness achieved in machining is directly affected by the conditions of the
machining process, especially the vibrations generated in the
machine-grip-object-tool system, susceptibility to elastic and plastic strains,
the tool work time and built-up edge. The effect of factors of unidentified
origin on surface roughness should also be taken into account [3].
Some assistance in selecting the
appropriate machining conditions may be provided by tool catalogues, published
by manufacturers. However, the recommendations and suggestions contained in
them are frequently of
a general nature. The values of speed and feed are usually provided in wide
ranges and are based on the adopted cutting tool 15-minute life. Among the
exceptions are technological research into selection of machining parameters
which take into account the material hardness and the cutting tool life, with
the assumed surface roughness.
The
microgeometry of the surface following mechanical processing is estimated with
models used in mathematical statistics. If an unevenness
is represented arc-wise, its height is
equal to [4]:
|
|
(1) |
where:
|
|
– feed per turn; |
|
|
– plate corner radius. |
The theoretical values of
roughness parameters, calculated on the basis of the adopted model, may differ
by 1.5÷15 times from those achieved in practice [1].
It has been experimentally
confirmed that surface roughness is significantly affected by increasing
machining speed 
. The machining speeds available in numerically controlled machining
tools used today, e.g. in millers (the object of the authors’ study), are much
higher than those achieved in traditional tools. At the same time, owing to the
progress in material engineering, new types of tools and methods of their
fixing have been implemented, along with completely new machine types,
including parallel machine tools. In such cases, the experiments conducted so
far and the standards developed for them, describing the microgeometrical
condition of the surface are not applicable, and the method of increasing the
machining speed, applied intuitively by a technologist or a machine tool
operator, does not bring the desired results.
Hence the question arises: what
value of the machining speed 
(or its range) is appropriate for
a specific machine tool, material and the cutting tool, to achieve the minimum
surface roughness?
STUDY AIM AND METHODS
The aim of the study was to
determine the maximum range of machining speed for a specific CNC machine tool,
for which the roughness of the achieved surface is minimal.
The experiments were conducted on
elements made of 20P steel, with carbon content C=0.42÷0.50%. It is a
commonly used unalloyed steel. Under normal conditions its hardness is HB=150.
The elements used were cubic in shape, which made it possible to determine
specific surfaces and ensure fixing the elements in a machine vice without
damaging the machined surfaces. Each of the four machined surface was a
separate sample (Fig.1).
Fig. 1. The
shape of samples made of 20P steel and the method of their determination
The elements were machined on a
numerically controlled HOMA – ECOCENTER V350-FMC miller with a
FANUC OM programmer, with the power of 3.7 KW (Fig. 2).
All samples were machined at
various machining speeds with the same tool by face milling. TPKN 1603 PPR
N250/P25 multi-cutter plates, manufactured by Baildonit, were used in the face
mill.
Fig. 2. Machining
the sample surface on a HOMA – ECOCENTER V350-FMC machine tool
The range of speeds applied in the
study of machining and feed was selected taking into consideration the
recommended conditions of steel finishing machining with P plates – Table 1.
Table 1. Recommended conditions of
machining carbon steel
|
|
source |
||||||
|
|
PAFANA (catalogue) |
SANDWIK-Coromant |
Engineer’s Guide |
||||
|
Material |
HB |
machining speed |
feed |
machining speed |
feed |
machining speed |
feed |
|
vc [m·min-1] |
fz |
vc [m·min-1] |
fz |
vc [m·min-1] |
fz |
||
|
carbon steel |
150 |
150÷250 |
0,1÷0,2 |
395 |
0,2 |
180 |
0,04 |
In order to eliminate the effect
of cutter wear on the machined surface roughness, the sequence of machining
speed changes was determined with
a table of random numbers.
Conditions of control surface
machining:
– number of samples: 24,
– machining speed: 
= 100÷560 mŸmin-1,
– machining diameter: 
= 63 mm,
– feed speed per cutter: 
= 0,06 mmŸobr-1,
– machining depth: 
= 0,3 mm;
RESULTS
Roughness was measured by a
portable DIAVITE DH-5 device, shown in Fig. 3.
Fig. 3. Measurement of roughness with a DIAVITE DH-5 device
According to PN-ISO 4288,
which lays down the principles and procedures of microgeometric surface
evaluation by the profile method, the following were adopted for the arithmetic
average of the ordinates of profile 0.1 ≤ 
≤ 2.0 [2]:
|
– elementary section of roughness: |
|
|
– measurement section of roughness: |
|
The values of Rmax, provided by the DIAVITE DH-5 device were read as
parameter RZ, in
accordance with the guidelines laid down in standard PN-EN ISO 4287. The
roughness of each surface was measured five times. After rejecting the extreme
values, the remaining three were used to calculate the arithmetic average,
which was adopted as the final result. The machining parameters and the final
results are shown in Table 2.
Table 2. The parameters of machining and the roughness measurement
results
|
Sample results |
Machining sequence |
Machining speed |
Spindle rotation |
Table feed speed |
Ra [µm] |
RZ [µm] |
|
1 |
12 |
100 |
505 |
91 |
1.507 |
11.300 |
|
2 |
13 |
120 |
610 |
109 |
1.950 |
14.233 |
|
3 |
21 |
140 |
708 |
128 |
2,343 |
15,833 |
|
4 |
3 |
160 |
809 |
145 |
1,597 |
12,100 |
|
5 |
17 |
180 |
910 |
164 |
2,357 |
15,400 |
|
6 |
8 |
200 |
1011 |
182 |
1,367 |
8,833 |
|
7 |
2 |
220 |
1112 |
200 |
1,070 |
8,867 |
|
8 |
15 |
240 |
1213 |
218 |
1,367 |
10,533 |
|
9 |
19 |
260 |
1314 |
236 |
0,953 |
7,400 |
|
10 |
4 |
280 |
1415 |
255 |
0,583 |
4,100 |
|
11 |
11 |
300 |
1516 |
273 |
0,830 |
5,900 |
|
12 |
24 |
320 |
1618 |
291 |
0,870 |
4,933 |
|
13 |
1 |
340 |
1719 |
309 |
0,607 |
5,233 |
|
14 |
14 |
360 |
1820 |
327 |
0,713 |
5,333 |
|
15 |
16 |
380 |
1921 |
346 |
0,887 |
5,233 |
|
16 |
20 |
400 |
2022 |
364 |
0,757 |
5,200 |
|
17 |
5 |
420 |
2123 |
382 |
0,543 |
3,700 |
|
18 |
6 |
440 |
2224 |
400 |
0,550 |
3,967 |
|
19 |
10 |
460 |
2325 |
418 |
0,540 |
4,333 |
|
20 |
9 |
480 |
2426 |
436 |
0,507 |
3,567 |
|
21 |
18 |
500 |
2527 |
455 |
0,840 |
6,000 |
|
22 |
23 |
520 |
2628 |
473 |
0,850 |
5,700 |
|
23 |
22 |
540 |
2730 |
491 |
0,867 |
6,833 |
|
24 |
7 |
560 |
2831 |
509 |
0,870 |
6,167 |
The line of regression was
determined on the basis of the final results. It is described by the following
polynomial equation:
= 0.0000123x2 – 0.0106x
+ 2.985
An analysis of the correlation
between the machining speed 
and parameter 
is shown in Fig. 4. The
regression line shows that an increase in the machining speed to about 
= 440 m·min-1 is accompanied by a decrease in
the roughness index 
, which approaches the value of 0.7 µm. Machining speeds exceeding 
= 480 m·min-1 do not significantly reduce
parameter 
, hence they can be applied if justified by other requirements, such as
the need to increase the machining efficiency.
Fig. 4. The effect of
machining speed 
on roughness 
for the feed speed
= 0.06 mmŸturn-1 and
machining depth 
= 0.3 mm
The characteristics
determined for parameter 
are similar (Fig. 5).
Its values do not exceed 6 µm if machining speed lies within the range
400 < 
< 480 m·min-1.
Fig. 5. The effect of
machining speed 
on the maximum height
of 
profile for the speed feed 
= 0.06 mmŸturn-1 and
machining depth 
= 0.3 mm
SUMMARY
Determination of the
effect of machining speed on one of the parameters (
,
) of surface microgeometry is justified. Increasing the machining speed 
in order to reduce
the roughness is effective for a specific machine tool only within a limited
range of machining speeds.
An analysis of the
effect of machining speed 
on roughness
(Fig. 4 and 5) shows that an increase in 
and 
is observed at machining speeds
exceeding 
= 440 m·min-1. It is therefore
unjustified to increase it further in order to achieve the roughness of a
machined surface of 
μm and 
μm. However, the achieved values of machining speeds
is much higher than the values recommended by the manufacturer of the TPKN 1603
PPR N250/P25 plate. According to the Baildonit catalogue, the machining speed
for carbon steels with carbon content higher than 0.4% should lie within the
range 
= 150÷240 m·min-1.
Increasing the value 2÷3 times will result in
a considerable reduction of the cutter’s life.
In order to achieve the desired
surface quality and the improve the effectiveness of machine use, it is
necessary to compare the properties of the machining tools which are owned by a
plant and to determine which of them ensures the desired parameters of
microgeometry of machined elements in terms of the condition of their surface.
The procedures are not too costly or labour-consuming, but they make it
possible to determine the appropriate range of speeds which produce the minimum
roughness of a machined object. In some cases, such information helps avoid additional
procedures or operations, especially in unit manufacture. The characteristics
should be primarily determined for the machines on which after-machining
procedures are performed. During machine operation, such data may be useful to
evaluate their wear.
Reference
1. Grzesik W. 1998. Podstawy skrawania materiałów
metalowych, Wydawnictwo Naukowo-Techniczne, Warszawa.
2. PN-ISO 4288. Zasady i procedury oceny struktury geometrycznej
powierz-chni metodą profilową. 1997. Polski Komitet Normalizacyjny.
3. Lubimow W., Oczoś K.: Wybrane
zagadnienia kształtowania nierówności powierzchni w procesach
obróbkowych. Mechanik nr 3/1997.
4. Katalog
narzędzi PAFANA 2002.