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.
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.
