1-Trak provides a flexible high performance solution to a
demanding tool changing problem.
Task
Tool changing devices for CNC machines enable rapid transfer of tools in and out of the
machine and are available in standard format with either a circular carousel or oval track the
most common configurations.
However, when Walter Maschinenbau wanted to develop a new CNC controlled Helitronic
tool grinder they needed a product that could link 12 or 24 x 254mm diameter grinding discs
and traverse them around a circuit without them clashing. In principle this did not sound too
demanding until the space available to fit the tool changer into was revealed.
With no suitable solution evident Walter turned to linear motion experts HepcoMotion to see
if a standard PRT2 track system could fit into the limited available space.
Initial investigations were promising but the aspect of preventing the discs clashing as they
went around the curved sections of the track was proving challenging. The obvious option of
just lengthening the track was not possible with the space available. There was also the
requirement for the control system to be able to demand any wheel in the circuit and bring it
to a predetermined position within a positional accuracy of 0.050mm.
Solution
After discussing the key requirements with engineers from HepcoMotion the 1-Trak system
began to look like a viable solution as the product would allow any two dimensional track
shape to be produced. Whereas PRT track systems have standard end segments of the
same diameters linked by straight slides, 1-Trak allows a bespoke shape to be produced.
Fitting into the available space and preventing the discs from clashing meant a number of
hours using CAD software, developed by HepcoMotion, to plot the required circuit for the
discs.
The final shape for the 12 and 24 carriage systems, to prevent the discs from clashing, was
somewhat different as the same length space restrictions existed in the machine for both
options. In the 24 carriage case there was vertical room available and this was utilised by the
final C shape of the system.
254mm diameter discs
locate on tool holders
fitted to carriages
3 bearing carriage
used on both systems
Three bearing carriage solution
The novel three bearing carriage allows a specific geometry to be produced and provided
the flexibility required to achieve the necessary track width with a tighter bend radii where
needed. The reduction in capacity with three bearings over the normal four would have
caused a potential life problem. However by producing a slightly thicker track vee section a
larger bearing was fitted, with a higher load capacity to meet the life requirements.
Driving and mounting the system
The 1-Trak system was produced from a single plate of specialised material so that there
was no assembly of track parts required. The plate included the vee track machined in, plus
the cam profile for the carriage drive pins to follow. A drive wheel with slots to allow the pin
movement was driven by a helical bevel gearbox attached from the rear of the plate.
Achieving the required accuracy.
To achieve the motion around the track with
either 12 or 24 carriages there would need to
be a small amount of clearance at each link
attachment; this potentially could have been
problematical in achieving the positional
accuracy tolerance of 0.050mm. This tight
tolerance was necessary to ensure the shaft
holding the disc was not damaged when it was
returned to the holder on the tool changer.
A pneumatically operated locking device
physically locked the carriage in the stopped
position ensuring a positional accuracy of
0.030mm was achieved.
Ensuring a long and trouble free life.
Very little maintenance is required for any of the parts, the Hepco standard bleed lubrication
system for the track was installed to ensure a measured amount of lubricant was distributed
to the vee guide. The life of the system is many thousands of kilometers service and if
eventually, during the life of the machine, bearing wear is evident it is a simple task to
change the vee bearings within the carriage.
Ease of installation.
The complete assembled plate, over 1.5m long in respect of the 12 carriage system, had a
number of fixing holes that enabled fixing to the inside frame of the machine. Apart from the
attachment of the servo motor to the gearbox and fixing the locking system, there was very
little additional assembly work required.
Result.
The joint cooperation between HepcoMotion design team and Walter development
engineers produced a unique solution to a challenging problem. The 1-Trak system provides
a smooth, quiet robust solution that, following rigorous testing, has proved to be highly
reliable. The ease of installation coupled with the ability to achieve the positional accuracy in
such a confined space has opened up opportunities for similar solutions on a smaller or
larger scale.
Link to Video: http://youtu.be/FSbKCZ_VrIU