AP05_3.vp

1 Introduction
Parallel kinematics machines (PKM) represent a new con-

cept in the design of machine tools [6, 7]. This concept has
been investigated since the early 1990s. Parallel kinematics
machines (PKM) enable the mechanical properties of manu-
facturing machines, especially the dynamics, to be improved.
This has been proven by several new machine tool concepts.
However, few PKMs have been successful on the market,
due to new design problems resulting from the parallel kine-
matics concept. This leads to reduced workspace and stiffness.
However, these problems can be removed and the potential
of PKM can be be increased by applying the principle of re-
dundant actuation [1]. This paper deals with the extension
of the concepts of redundantly actuated parallel kinematics
structures for five-sided five-axis machine tools and for a
free-forming sheet metal forming machine. These concepts
can be divided into full, hybrid and modular PKMs.

2 Full parallel kinematics machines
In full parallel kinematics machines, all DOFs are realized

by the motion of the platform, and the platform is suspended
by parallel links with drives on the frame. An example of spa-
tial redundant full parallel kinematics is Octapod [2] (Fig. 1)
corresponding to the traditional Hexapod. Eight links were
selected for Octapod, instead of six links of Hexapod. In the
initial concept, both the frame and the platform were cubes
(Fig. 2a). The links connecting the vertices of the cubes are
translational actuators, which are connected to the frame and
the platform by spherical joints. The links have variable
lengths. The first design problem was that the initial concept
with a cubic platform was singular in the whole workspace. It
was necessary to generate a modified platform concept. The
modified platform consists of skew mutually rotated rectan-
gles (Fig. 2b), and parameter optimisation of its dimensions
was provided. This PKM is non-singular, and has good dex-
terity in the whole workspace.

The workspace is in principle equal to the whole cube of
the frame without the boundary layer of the platform thick-
ness. The orientation capability of the platform is good (more
than +- 90 degrees have been achieved). If the platform is just

a fraction of the frame cube (e.g. 1:5), then the ratio between
the workspace and the machine space is much better than for
other parallel kinematics concepts.

However, these properties pose quite large demands on
the angular extent of the spherical joints, and these require-
ments have led to a special design of the new spherical joints
[3]. This involves the possibility of five-sided machining.
Five-sided machining means that a machine part (e.g. a cube)
is fixed within the workspace and all five free sides of this
cube are machined without any other fixing. Severe problems
are posed by collisions. In our case this is influenced by the
relative size of the platform, the position of the fixing table
within the workspace and the resulting cube being five-side
machinable. These parameters were thoroughly optimised,
with large computational demands. The result is that a cube
with the size of 25% of the Octapod frame can be really
five-side machined.

56 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 45 No. 3/2005 Czech Technical University in Prague

Study of Concepts of Parallel Kinematics
Machines for Advanced Manufacturing
M. Valášek, V. Bauma, Z. Šika

This paper deals with possible new concepts for machine tools based on parallel kinematics for advanced manufacturing. Parallel kinematics
machines (PKM) enable the mechanical properties of manufacturing machines to be improved. This has been proven by several new
machine tool concepts. However, this potential can be and must be increased by applying the principle of redundant actuation. This paper
deals with the extension of the concepts of redundantly actuated parallel kinematics structures for five-sided five-axis machine tools and for a
free-forming sheet metal forming machine. The design principles of previous successful PKMs are summarized and new concepts are
proposed. The most important requirement criteria are summarized. The proposed concepts are qualitatively and initially quantitatively
evaluated according to these criteria.

Keywords: parallel kinematics, machine tools, metal forming machines, conceptual design.

Fig. 1: Laboratory model of Octapod

Another example of a spatial full parallel kinematics ma-
chine is Octaslide [4]. It is a redundant version of Hexaslide.

It is a parallel kinematics machine the links of which have slid-
ing actuators. The basic concept of such a structure is shown
in Fig. 4. The platform is suspended on 8 links with actuators,
whereas Hexaslide or Pentaslide are suspended on only 6 or 5
links.

The concept of redundant actuation completely removes
the singularities from the workspace. The stiffness in the
whole workspace was increased in maximum values by
65–74 %, and in average values by 43–54 % compared to
Hexaslide. Then the Octaslide concept was optimised as
a machine tool for five-axis machining. The Hexaslide and
Octaslide concepts were simultaneously intensively opti-
mized. The result is that Octaslide is superior in all mecha-
nical properties. The resulting concept of Octaslide is an
asymmetric structure (Fig. 5), both in connections of the links
with the frame and in the conical form of the platform. The
increased orientation angle is � 33°.

3 Hybrid parallel kinematics
machines
In hybrid parallel kinematics machines, the DOFs of the

machine are split into two parts, at least one of which is real-
ized by the parallel kinematics concept. There are two groups

Czech Technical University in Prague Acta Polytechnica Vol. 45 No. 3/2005

1
2

5
6

7
8

8´ 7´

5´
6´

3´

2´1´

4´

(a) (b)

Fig. 2: The initial (a) and final (b) structure of OCTAPOD spatial redundant parallel kinematics

Fig. 3: Octapod variant for five-sided machining

Fig. 4: Kinematical concept of Octaslide

of concepts for five-axis / five-sided machining. They are
based on dividing the required 5 DOFs into two parts, with
3+2 DOFs or 4+1 DOFs.

One group of hybrid PKMs is based on a planar parallel
kinematics mechanism that realizes 3 DOFs in a large range,
and this mechanism is combined with a translational table
that can also rotate (two further DOFs). An example is shown
in Fig. 6. An important feature is redundant actuation, which
enables such a large range of motions to be realized.

Another other group of hybrid PKMs is based on a paral-
lel kinematics mechanism that realizes spatial translational
Cartesian motions and one rotation of the spindle (4 DOFs),

and this mechanism is combined with a rotary table (1 DOF).
An example is shown in Fig. 7. It uses modifications of the ad-
vantageous module of Trijoint [5] (Fig. 8) as the portal hori-
zontal mechanisms for the motion of the quill carrier, for quill
travelling and even for spindle rotation.

58 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 45 No. 3/2005 Czech Technical University in Prague

Fig. 5: Resulting variant of Octaslide with a conical platform

Fig. 6: An example of a PKM based on a redundant planar paral-
lel mechanism with a large range of motions

Top view

Back view

Fig. 7: An example of a PKM based on a portal mechanism with
redundant modules

4 Modular parallel kinematics
machines
Modular parallel kinematics machines form a special

group of hybrid PKMs that are based on PKM for spatial
translational Cartesian motions combined with a swivel head
for additional orientation of the spindle. They use the in-
creased stiffness and dynamics of both parts – basic PKM for
translational motions and the swivel head for orientation.
If the swivel head is realized by a PKM, it solves the great
problem of traditional swivel heads on composed rotational
axes – that the head cannot always move directly to the re-
quired orientation. The basic PKM can implement all three
Cartesian motions (3 DOFs) and the swivel head has only 2

DOFs (2 orientation angles) ,or the basic PKM can implement
only two Cartesian motions (2 DOFs) and the swivel head has
3 DOFs (2 orientation angles and translation, as in the case of
current parallel swivel heads).

This modular solution has many possible variants. The
basic PKM can be based on Sliding Delta [2] (Redundant
Uran) with three Cartesian motions (Fig. 9), or on Trijoint
(Fig. 8) or Sliding Star [2], with two Cartesian motions
(Fig. 10). The swivel head is mounted on the platform of the
basic PKM. The swivel head can be based on a traditional
Cardan mechanism or on parallel mechanisms. A redun-
dantly actuated parallel swivel head with 3 DOFs (2 rotations,
1 translations) with increased stiffness and dynamics is shown
in Fig. 11.

Czech Technical University in Prague Acta Polytechnica Vol. 45 No. 3/2005

Fig. 9: Kinematical concept and laboratory model of Sliding Delta (redundant Uran)

Fig. 10: Kinematical concept and laboratory model of Sliding Star

Fig. 8: Kinematical concept and design of Trijoint

5 Parallel kinematics metal forming
machines
There is only one example of the application of parallel

kinematics for metal forming machines. This is Hexabend,
produced by IWU FhG Chemnitz – PKM for free forming of
tubes and similar rod profiles. Hexabend consists of a tradi-
tional feed mechanism for travelling the tube, together with
Hexapod for bending the tube in any direction. This concept
can be extended for metal forming of sheets (Fig. 12). One
parallel kinematics mechanism acts as the holder and manip-
ulator, and the other acts as the universal forming tool. It can
have planar and spatial variants.

6 Conclusion
This paper describes different ways of extending PKMs

for five-axis/ five-sided machining and for metal forming ma-
chines. The initial concept of parallel kinematics was oriented
towards such applications, especially five-sided – five-axis ma-
chining, but design problems leading to limited workspace
prevented the development of PKM with these applications.
Current progress in designing PKMs without these design
problems enables PKM to be proposed again for such de-

manding applications. The paper summarizes several ways
for designing such machines.

Acknowledgment
The authors appreciate the kind support by MSMT grant

J04/98:212200008 “Development of methods and tools of
integrated mechanical engineering”.

References
[1] Valasek, M., Sika, Z.: “Redundantly Actuated Parallel Ki-

nematics-New Concept for Machine Tools.” In: Proc. of
1st IFAC-Conference on Mechatronic Systems, Darm-
stadt 2000, p. 241–246.

[2] Valasek, M., Bauma, V., Sika, Z., Vampola, T.: “Redun-
dantly Actuated Parallel Structures – Principle,
Examples, Advantages.” In: Neugebauer, R., (ed.):
Development Methods and Application Experience of
Parallel Kinematics, PKS 2002, IWU FhG, Chemnitz
2002, p. 993–1009.

[3] Valasek, M., Sulamanidze, D., Bauma, V.: “Spherical
Joint with Increased Mobility for Octapod.” In: Neu-
gebauer, R., (ed.): Development Methods and Applica-
tion Experience of Parallel Kinematics, PKS 2002, IWU
FhG, Chemnitz 2002, p. 285–294.

[4] Bauma, V., Valasek, M., Sika, Z.: “Design and Properties
of Octaslide Redundant Parallel Kinematics.” In: Proc.
of International Conference on Advanced Engineering
Design AED 03, CTU, Prague 2003, p. C3.6/1–8

[5] Petru, F., Valasek, M.: “Concept, Design and Evaluated
Properties of TRIJOINT 900H.” In: Neugebauer, R.
(ed.): Proc. of PKS 2004 Parallel Kinematics Seminar,
Chemnitz 2004.

[6] Neugebauer, R., (ed.): “Development Methods and Ap-
plication Experience of Parallel Kinematics.” PKS 2002,
IWU FhG, Chemnitz 2002.

[7] Neugebauer, R., (ed.): “Parallel Kinematic Seminar PKS
2004.” IWU FhG, Chemnitz 2004.

Prof. Michael Valášek, DrSc.
valasek@fsik.cvut.cz

Ing. Václav Bauma, CSc.
steinb@fsik.cvut.cz

Ing. Zbyněk Šika, PhD.
sika@fsik.cvut.cz

Department of Mechanics

Czech Technical University in Prague
Faculty of Mechanical Engineering
Karlovo nám. 13
121 35 Praha 2, Czech Republic

Acta Polytechnica Vol. 45 No. 3/2005 Czech Technical University in Prague

Fig. 11: Redundantly actuated swivel head

Universal
forming
tool

Sheet metal

Holder

Fig. 12: Sheet metal forming by PKM