The Variable Arc is made of a super-tough and lightweight polycarbonate, and because of its unique design, will form a perfectly circular arc for all radius measurements when mounted on the IRG 3600 & ORG 3600 radius gauges.
The Variable Arc great for measuring the curvature of patterns (especially multi-radiused patterns) drawn on cardboard. Use it to measure arcs with shorter arc lengths. Or use it for drawing shapes, by simply tracing along its edge. Also ideal for drafting in that the Variable Arc is readily usable and easily set to any size drawing scale. Determine the lengths of the arcs by using the Imperial and Metric increments located along the edges of the Variable Arc.
This attachment is unique in that, when mounted on the Arc Master Radius Gauge(IRG 3600 or ORG 3600) (as shown below), the Variable Arc will form a perfectly circular arc with the same radius as indicated on the Measurement Scale.
Try drawing tighter arcs using the Variable Arc without the Arc Master Radius Gauge. 
Mounting the Variable Arc onto the IRG 3600 Radius Gauge
To use the Variable Arc with the IRG 3600, mount the Variable Arc into the slots near the contact points. To do this, slide the Pointer toward the 12″(300mm) mark on the scale, attach the Variable Arc’s center bracket, its 2 pins fitting into the 2 slots on the gauge.
Then, grab one of the Variable Arc’s outer pins and bend the Variable Arc until you are able to insert the pin into the slot provided near the outer contact points. Repeat this at the other bracket.
The Variable Arc is now attached and locked in place. See below for an example.
To attach the Variable Arc to the ORG 3600, set the pointer to around the 24″(600mm) mark, insert the pins into both of the outer slots then insert the 2 center pins. See below for an example.
