Cell phone towers are a nearly ubiquitous feature of every city, town and roadway in America. These towers send and receive signals to and from your cell phone. The towers are equipped with filter boxes that filter the high-frequency-range (i.e. for UMTS-nets) to generate a narrow-band signal for the data stream. Each filter box contains filter chambers that process the frequency spectrum. The clarity of the filters is adjusted by one or more setscrews, or threaded pins, which are mechanically positioned in each chamber to an axial depth. A counter-nut does the exact positioning and mechanically locks the setscrew.

However, when the nut counters the setscrew, it is possible for the adjustment to shift on its own because of a thread-backlash. To avoid this effect and to reduce repeated adjustments, manufacturers of filter boxes have to use custom-made setscrews, which are very expensive and still don’t solve this problem satisfactorily.

On average, each filter box can require between 50 to over 100 screw adjustments. Naturally, this is a major opportunity for automation. Tests of the assembly and adjustment sequence have clearly shown that most of the assembly time is spent on the preadjustment of the setscrew and the subsequent counteraction given to the nut. The best possible positioning accuracy of the setscrew is approximately ±0.02 millimeter. If such an accurate adjustment is achieved, then subsequent readjustments can be avoided and the total processing time can be shortened by as much as 50 percent.
Since the filter chambers have to filter different frequencies, the setscrews have to be adjusted to different heights. A multiaxis, programmable assembly machine is the best solution for the combined requirement of varying positions, multitude of filter chambers, and many different filter boxes.

For this requirement, an elegant solution was designed by DEPRAG. The screwdriving head was built to accept both the setscrew and the nut (see picture 1). Once both fasteners are supplied to the screwdriving head, the setscrew is driven through the nut into the cover of the filter box to the required depth. Thereafter, the nut is countered, without letting go of the setscrew, and its position is positively locked.

This extraordinary screwdriving head achieves the following results: