Spinning around

It might look like simple enough, but the flashing blue police light must run smoothly and accurately for a long time; presenting a number of challenges for designers.

Premier Hazard, Leeds based manufacturer of these systems, enlisted the help of Fishercast for a redesign, replacing a two piece rotator base in favour of a single die cast component in zinc alloy, increasing tolerance and ensuring the rotator gear runs smoothly.

To ensure the reflector rotates smoothly and accurately, the rotator gear must mesh precisely with the motor drive’s worm gear. This requires a sturdy rotator base built to exact dimensional, positioning and alignment tolerances.

Die casting the assembly as a single component holds the improved tolerances, thereby solving the gear meshing issue,and has simplified manufacture, bypassing the need to press, machine, tap, bend, deburr, and finally join the post to the flange by swaging. To produce the rotator base as a single die cast component, Fishercast used its six action tooling process.

At first glance, the rotator base appears to be an easy-to-cast component, requiring only four slide movements to form the post’s internal bore, the screw holes in the motor mount bracket, and the two radial post grooves. The reality was more complex, recalls Jurg Geiling, from Fishercast: ‘Like every other die caster, we faced the undesirable possibility of machining the post grooves later, which would have left sharp edges or burrs. By designing two submerged slides into the tool’s moveable die steel, we were able to eliminate this secondary operation.’

The submerged slides function along the X-Y axis in the moveable Z-axis portion of the tool. Located one on either side of the post, the slides activate and meet at the post’s centerline each time the tool closes. They retract into the die steel milliseconds before the tool opens, a design that employs a delayed two-stage ejection system.