Coil Binders — Punching for Plastic Coil
By David Spiel
Punching for spiral binding of any kind is a tricky matter—and even trickier for plastic coil. Punching for double-loop wire has always been a snap: Merely open your die, place your sheet against the pins and pull the pins nearest the edges of the sheet. This way there is no chance of punching a partial hole. You have the luxury of centering your sheet and producing a generous, attractive margin (the distance between the first or last hole and the head or foot of the book).
Punching for spiral is not always so easy. If you have a plastic coil machine that can manipulate the spiral prior to insertion, consider yourself lucky; most cannot. Therefore you are stuck with whatever pitch the spiral may be. Let's use the industry standard of the 4:1 pitch.
In days gone by metal spiral binding used a 4:1 (or in Europe a 6mm center-to-center) pitch. But back in those halcyon days, the holes were small, 9⁄64˝ (3.5mm). They only needed to accommodate a very thin gauge of wire. Plastic coil gauges are much thicker, some as thick as 1⁄8˝ (3mm) and 9⁄64˝ holes just won't do. Standard plastic coil gauges range from .063˝ to .085˝ (1.5-2mm).
Some had turned to oval holes. Oval holes were, for many years, the accepted way to battle thicker books with spiral wire and, even for a time, plastic coil. The old standard was a double "D" die. Why is it called double "D"? Imagine two Ds mirrored against each other and turned counter-clockwise 90 degrees. It is actually a round hole with its sides sliced off. A double "D" die used to be 9⁄64˝ (3.5mm) wide and 11⁄64˝ (4.365mm) tall.
This, obviously, helped with insertion of spiral wire and hand-held plastic coil books. However, oval dies are much more expensive than round hole dies since they have to be EDM'd (laser burned). They can cost up to $2,500 more than a round hole die.