determinig erros in complex surfaces machining with the

KOMISJA BUDOWY MASZYN PAN – ODDZIAŁ W POZNANIU
Vol. 26 nr 2
Archiwum Technologii Maszyn i Automatyzacji
2006
ANDRZEJ WERNER * , MAŁGORZATA PONIATOWSKA*
DETERMINING ERRORS IN COMPLEX SURFACES
MACHINING WITH THE USE OF CNC MACHINE TOOLS
The present paper concerns the question of determining errors in the process of producing
parts with the use of CNC machine tools in order to introduce intermediary (off-line) correction of
the processing programme. The elements were processed with a CNC FYS 16N milling machine
with a TNC 360 Heidenhain numerical control system. The methodology of determining the errors
was presented on the example of an object made from an aluminium alloy described with a free
surface panel. Both the geometrical model and the processing programme were created in the
Mastercam system. The production accuracy inspection were conducted on a MISTRAL Brown &
Sharpe coordinate measuring machine. The obtained information relating to the values and
distribution of processing errors was used to correct the CNC machine tool control programmes.
Key words: processing errors, complex surface, coordinate measuring
1. INTRODUCTION
Processing with the use of numerically controlled machine tools is inevitably
connected with errors in the machining method, fixing the processed object,
thermal expansion of the material as well as the processing system stiffness.
There are two methods of error correction in machining. One of them is the online method, applied during the processing. The other is the off-line method
involving intermediary correction of a processing programme after analysing the
machining accuracy [3], for which it is essential to collect information on the
geometry of the produced object. In the case of free shape surfaces, converting a
three-dimensional object into a discreet description of its geometry requires
applying a surface scanning method called digitalisation. This process aims at
machining a geometrical model of an object in the computer memory with the
use of a measuring points cloud. Both coordinate measuring machines and CNC
machine tools equipped with suitable measuring probes can be used for
*
Ph.D. Eng. – Faculty of Mechanical Engineering, Bialystok Technical University.
212
A. Werner, M. Poniatowska
measuring [1]. There are a few methods of surface digitalisation on coordinate
measuring machines (CMMs). The choice of the method depends mostly on the
type of a CMM, of the measuring probe head as well as of software. The process
of 3D surface digitalisation is most often carried out with a contact probe, taking
into account the correction of the measuring ball radius in the direction normal
to the surface at a particular measuring point. CMMs may be also equipped with
non-contact (optical) measuring probe systems. The distance between two
measuring points may be constant or may change depending on the shape of the
measured surface. The points obtained as a result of the measurement may be
distributed according to an regular or irregular network, consecutive sequential
lines, or even selectively. The coordinate values of measuring points in a digital
form may be further processed in order to determine the values of processing
errors.
The article presents a method of determining summary errors in processing
surfaces of complex spatial shapes with numerically controlled machine tools
with the use of a coordinate measuring machine and the CAD/CAM systems.
2. RESEARCH STAND
The experiment was conducted on a research stand including the following
elements (Fig. 1):
– a PC computer 3 integrated with a numerically controlled machine tool and
with a coordinate measuring machine; with the CAD/CAM (Mastercam) and the
PC-DMIS measuring software installed,
– a FYS 16N machine tool 1 with Heidenhain TNC 360 numerical control,
– a DEA (Brown & Sharpe) Mistral 070705 coordinate measuring machine 2.
Mastercam is an integrated CAD/CAM system which creates geometry,
prepares details and finished drawings, visualizes the graphic tool path and
generates the NC programme. Thanks to its possibilities it is an ideal tool for
preparing technological processes for any numerically controlled machine tools.
Mastercam allows the user to change the processing parameters or tool at any
time during the process, to modify the tool path, and later to visualise the
machining again. Before applying the programme directlyon a machine tool, it is
possible for a programmer to check the effect of his or her work on the screen. In
addition to creating geometry, the programme user can analyse it, control the
tool path, and see on the screen the machining effect, i.e. the real appearance of
the worked-on detail including the tool tracks formed in the machining process.
The PC-DMIS programme is the software of a Mistral Standard CNC
coordinate measuring machine. PC-DMIS for Windows is a full geometrical
measurements package. It translates high level commands required in measuring
Determining errors in complex surfaces machining …
213
Fig. 1. Research stand scheme:
1 – numerically controlled milling
machine, 2 – coordinate measuring
machine, 3 – PC computer
Rys. 1. Schemat stanowiska badawczego: 1 – frezarka sterowana numerycznie, 2 – współrzędnościowa
maszyna pomiarowa, 3 – komputer
klasy PC
parts into detailed steps necessary to control coordinate measuring machines
(CMMs). PC-DMIS for Windows uses the Microsoft Windows interface both for
creating and executing programmes. It is easy for a user to start the measuring
process with the use of a drop-down menu, dialog boxes and icons. The PCDMIS interface versatility assures an easy way of adapting the software to the
user's individual needs.
3. RESEARCH INTO THE FREE SURFACE MACHINING ACCURACY
The method of determining errors in the process of machining a 3D object is
shown on the example of an object made from aluminium and described with a
free surface panel. The geometrical description of the object was created in the
Mastercam programme (Fig. 2).
Fig. 2. 3D free surface – a view
of the surface generated in the
Mastercam programme
Rys. 2. Powierzchnia swobodna
3D – widok powierzchni wygenerowanej w programie Mastercam
214
A. Werner, M. Poniatowska
3.1. Programming and machining
The programme controlling the object machining was created with the use of
the Mastercam system 3D milling module. In order to generate the tool path
(Fig. 3), the procedure of parallel passages was used (two-side machining –
without return lost motion). The applied tool was an end mill φ10 with a ball
end, made of high-speed steel (DIN 1889 BA 10 K-H-HSS-E).
Fig. 3. Tool path
Rys. 3. Ścieżka przejścia narzędzia
Conditions of machining: feed – 200 mm/min, rotational speed of spindle –
1600 rpm, parrarel passages distance – 0.3 mm, machining tolerances – 0.01 mm.
The machine tool controlling programme was generated with the use of a
postprocessor adapted to control a Heidenhain TNC 360.
After appropriate equipping and setting of the machine tool, a cubicoid
aluminium alloy workpiece was fixed in the device. Machining of the object was
realized in the DNC mode (the volume of the controlling programme exceeded
the memory capacity of the numerical control system).
3.2. Evaluation of machining surface accuracy
Control measurements were taken with a Mistral 070705 coordinate
measuring machine. A TP200 measuring probe equipped with a stylus with a
ball tip of 2 mm in diameter was used. After having defined the co-ordinate
system of the measured object in the manual control mode, the researchers
Determining errors in complex surfaces machining …
215
transferred the geometrical model of the object, prepared in the Mastercam
system to the PC- DMIS system controlling the measuring machine performance
[4]. In order to transfer the geometrical data, the researchers applied the IGES
format. With the surface model of the object, the surface scanning procedure
was automatically started (Fig. 4).
Fig. 4. The imported surface with
the system of measuring points, the
number of which is established by
the user
Rys. 4. Zaimportowana powierzchnia wraz z układem punktów pomiarowych, których liczbę użytkownik
The area of scanning was defined through setting the boundary values of the
u and v parameters (the parameterization directions used in a surface objects
group in CAD systems). The number of points for different scanning directions
was also established (10 and 15 respectively). As a result, 150 measuring points
were generated on the surface panel (Fig. 5).
Fig. 5. Surface panel with the 150 measuring
points marked
Rys. 5. Płat powierzchni z naniesionymi
150 punktami pomiarowymi
After accomplishing the measurement phase, the co-ordinate values of the
measuring points were obtained. The next step was to determine the Δ total
spatial error of the surface machining [2]. In order to determine the values of this
error for every measuring point, the obtained co-ordinate values were substituted
to the following formula:
216
A. Werner, M. Poniatowska
Δ = ( X T − X A) 2 + (Y T − Y A) 2 + ( Z T − Z A) 2
processing error [mm]
where: Δ
– total spatial error of machining,
X T , YT , Z T – co-ordinates of nominal points,
X A , Y A , Z A – co-ordinates of measuring points.
The calculated results are illustrated in Fig. 6. It results from the graph that
the deviation range amounts to 0–0.4 mm. The main reason of occurred
machining errors was size (length, diameter, nose radius) and form errors of the
mill. Control measurements of it revealed thad the nose radius was 0.2 mm less
then nominal.
0,4
0,35
0,3
0,25
0,2
0,15
0,1
0,05
Fig. 6. Graph illustrating a error of
surface machining before correction
Rys. 6. Wykres przedstawiający błąd
obróbki powierzchni przed korekcją
145
133
121
97
109
85
73
61
49
37
25
1
13
0
m easuring point number
Information relating to the values and distribution of the machining errors
was used in correcting the CNC machine tool control programme. The influence
of the correction is presented in Fig. 7.
0,3
0,25
0,2
0,15
0,1
0,05
measuring point number
before correction
after correction
Fig. 7. Graph representing the influence of the correction
Rys. 7. Wykres przedstawiający wpływ korekcji
145
136
127
118
109
91
100
82
73
64
55
46
37
28
19
10
0
1
processing error [mm]
0,4
0,35
Determining errors in complex surfaces machining …
217
4. CONCLUSIONS
The coordinate measuring method is a quick and effective tool for
determining errors in machining objects with numerically controlled machine
tools. It is particularly significant in the case of machining objects of complex
spatial shapes. The obtained information on the values and distribution of
machining errors may be used to correct machine tools controlling programmes.
In consequence, significant increase in the production accuracy is obtained. This
fact is very important in the aspect of applying modern production systems
aiming at achieving high product quality.
REFERENCES
[1] Ainsworth M., Ristic M., Brujic D., Cad-Based Measurement Path Planning for Free-Form
Shapes Using Contact Probes, Advanced Manufacturing Technology, 2000, 16, p. 23–31.
[2] Ikua B. W.,Tanaka H., Obata F., Sakamoto S., Prediction of cutting forces and machining
error in ball end milling of curved surfaces – I theoretical analysis, Precision Engineering,
Journal of the International Societies for Precision Engineering and Nanotechnology, 2001,
vol. 25, p. 266–273.
[3] Lechniak Z., Werner A., Skalski K., Kędzior K., Methodology of the Off-line Software
Compensation for Errors in the Machining Process on the CNC Machine Tool, Journal of
Material Processing Technology, 1998, 73, p. 42–48.
[4] Min-Yang Y., Jin-ho P., Analysis of setting errors in precision ballscrew machining and the
automatic adjustable center, International Journal of Machine Tools & Manufacture, 1998,
38, p. 965–979.
Praca wpłynęła do redakcji 24.03.2006
Recenzent: dr hab. inż. Roman Staniek
WYZNACZANIE BŁĘDÓW WYKONANIA PRZEDMIOTÓW
NA OBRABIARKACH CNC
Streszczenie
Artykuł dotyczy zagadnienia wyznaczania błędów wykonania części na obrabiarkach CNC,
co pozwoli na wprowadzenie pośredniej korekcji (off-line) programu obróbkowego. Elementy
obrobiono na frezarce CNC FYS 16N z numerycznym sterowaniem TNC 360 Heidenhain.
Dokładność wykonania kontrolowano na współrzędnościowej maszynie pomiarowej MISTRAL
Brown&Sharpe. Wyniki pomiarów posłużyły do wyznaczenia błędów obróbki. Informacje
dotyczące wartości i rozkładu błędów obróbkowych wykorzystane zostały do skorygowania
programów obróbkowych.
Słowa kluczowe: błędy obróbki, powierzchnia złożona, pomiary współrzędnościowe