With a Swivelcheck, they can – for the first time – measure without dismantling
To accurately measure modern milling heads, many hydraulic screw fittings and lines must be removed to reach the rotary axis. This easily took a day, and could risk violating the milling head’s warranty. The API Swivelcheck can be mounted directly on the machine, and its software can even compensate for the softness of the spindle.
If any single construction has established itself worldwide for milling machines, it is the portal technology. In particular for large components such as wide-body aircraft or models for car bodies, portal milling machines are always the first choice when machining large parts from light or highly stable materials quickly and with high precision. As the global leader for milling solutions using the portal technique, Zimmermann has not only perfected this technology with 5-side, 5-axis machining, but has also opened up a new chapter in milling head technology.
The process with which the company sets up and calibrates machines and milling heads is also innovative: a new measurement system that no longer has to be mounted on the center of motion for the measurement guarantees the optimal setting and performance of the milling machines from Zimmermann.
Among other things, portal milling machines have the advantage that the moving masses are extremely low, so that large workpieces, above all, can be machined with it. But even before it comes to this, the manufacturer must prove that the machine can really carry out the required tasks – in the form of a customer acceptance that will not only take place at the manufacturer’s premises, but also later after the installation at the customer’s site. The proof that the machines meet the required specifications is the precondition for a successful project management – and if the manufacturer should ultimately fail in the documentation of this proof, costly rework may be necessary, and even the failure of the complete project can no longer be excluded.
The final acceptance of a portal milling machine thereby represents a very special challenge, and can easily last several weeks; the measurement of the linear and rotary axes according to VDI/ISO is as much a contractual part of the acceptance as the measurement of the geometry and the subsequent test milling. In order to carry out the check, a plug gauge is clamped in the machine and, using a dial gauge, the deviations are documented over multiple test runs along the dial gauge. The errors that arise are then entered in a compensation table.
The fatal flaw in this measurement method is that neither the positional accuracy nor the repetition accuracy is determined accord- ing to the corresponding VDI/ISO standard; even a large number of runs does not solve the fundamental problem (the standard prescribes a minimum of 3 runs, in which every position must be run to a total of at least 6 times).
In the meantime, laser interferometers have proved very successful in the measurement of linear axes. Furthermore, measurement systems for the measurement of swivel axes and tilting tables have also come onto the market that promise assistance in this area; mounted at the center of rotation / center of the swivel movement of the rotary axis, the required measurements can now be carried out. The devil is in the details. However, to take the measurements, these measurement systems must be mounted on the rotary axis. And that is the fundamental problem – especially with the arrival of modern, multi-axis milling heads of the new generation. The rotary axis is only accessible by dismantling the milling head – and hydraulic screw fittings also must be loosened and media feeds removed. In the case of bought-in milling heads, this would thereby not only make any warranty invalid, but would also cause the time required to change tools to drastically increase. What could be measured at all with a semi-dismantled milling head remains questionable. The time was ripe for a completely new approach.
Measurement directly on the spindle
Nothing seemed more natural for the measurement technicians from Zimmermann than to move the measurements directly to the spindle, but there was no corresponding measurement system for this. The visit of a supplier turned out to be even more interesting when, in the shape of the portable Swivelcheck from Automated Precision (API), he brought with him a measurement system for the factory acceptance of the milling machines he had ordered from Zimmermann that could indeed be mounted directly on the spindle of the milling head. The measurements with this system were central components of the acceptance documentation for the ordered systems. For the first time, the milling head did not have to be dismantled for the measurements, because the measurement system could be adapted directly to the spindle.
“The new system solved a number of problems at one stroke,” according to Rainer Quast, who works in the Measurement Technology department at Zimmermann. “With modern milling heads, a lot of parts have to be removed on order to reach the rotary axis – not only hydraulic screwed fittings and lines; tool times of a day can easily arise and, in the case of bought-in milling heads, there is also the problem of the warranty.” In addition, measurement directly at the spindle also removes the necessity for additional devices that were otherwise required previously when taking measurements on the rotary axis. “These additional devices were prone to softness or wobble, and produced far more measurement errors than were actually present in the system. We therefore mounted the measurement system directly on the spindle, without having to dismantle anything,” continued Rainer Quast.
After the first measurements, the measurement technology specialists at Zimmermann were so impressed by this device that the visitor had to fly home without his measurement system, leaving it behind for measurements on additional machines in Germany.
In the following period, the system, which was generally suitable for the assessment of the performance of rotary axes and tables, was subjected to an endurance test; it was not only used for the pre- acceptance in the factory, but also in the milling head assembly, as well as for final acceptances at the customers’ premises.
Good is good, but better carries it
But there’s nothing that can’t be improved further. It was quickly realized at Zimmermann that the holder provided by the manufacturer of the measurement system for recording directly at the spindle (so that the system could be fitted directly before the tool holder) was a very elegant and well-thought-out solution, but led to the fact that, in some cases, the softness of the spindle itself could also be included in the measurement result. Zimmerman wanted to exclude this, because the accuracy demanded by the customer required a precise consideration of the measurement results. “All that we needed in addition was a torsion-resistant holder for the Swivel- check measurement systems by means of a fixed lock”, explained Rainer Quast. “We therefore realised a very simple but effective adapter in the form of spindle-specific holder in order to exclude the influence of the spindle itself.”
The software of the Swivelcheck delivers a compensation table with the errors determined at the corresponding positions. The measurements themselves are carried out automatically by the measurement system and also provide information as to whether the internal measurement system of the machine is working cor- rectly. Whereas, previously, errors could not be clearly limited to the A-axis of the milling head, and no information could be pro- vided regarding the repeat accuracy when carrying out the mea- surements using a mandrel, all the mechanical errors can now be detected and localised. The Swivelcheck proved its value so well in such a short time that Zimmermann decided to acquire two systems via the European branch of API in Heidelberg. The portable measurement systems, no larger than a small shoe box – are not only in continual use in the factory, but also travel to customer lo- cations in the luggage of the acceptance team.
Founded in 1933, Zimmermann has been involved in the removal and modification of the external contours of different materials from the very start. Today, Zimmermann is a globally leading, high- tech provider of portal milling machines, who repeatedly achieves impressive results with its “Made in Germany” solutions. The automobile and aviation industries in particular have benefited from this, with portal milling machines from Zimmermann being widely used for 5-axis machining, even of parts with very large dimensions. The new 2- and 3-axis milling heads of the globally operating company with head office in Denkendorf near Stuttgart are highly innovative.
Contact: Rainer Quast, firstname.lastname@example.org