Updated October 30, 2022
CP: Stands for Circular Pitch. It’s the distance from a point on a gear tooth along the pitch circle of the gear to the corresponding point on the next gear tooth. Therefore, it's the difference in repeat between a plate cylinder and the next one in size.
DP: Stands for Diameter Pitch. It’s a ratio equal to the number of gear teeth per inch of printing diameter.
Terms
Printing Circumference: The measurement of the circle described by a point on a plate’s printing surface while printing. It is equal to the plate cylinder repeat.
Pi: The ratio between the diameter of a circle and its circumference.
Pi = 3.1416 (to four decimal places)
Pi x diameter = Circumference
Circumference / Pi = Diameter
1/8 C.P. defines the distance between gear teeth at the plate circumference.
Formula
# Teeth x 0.125 = Maximum Repeat
Example to find the repeat length;
82 tooth gear X 0.125” (the decimal equivalent of 1/8”) = 10.25" maximum repeat
To Calculate the minimum gear size needed to yield a given repeat length;
Formula
Repeat Length / 0.125 = Maximum Repeat
Calculate minimum gear size to print a 7.5" repeat:
7.500” / 0.125” = 60 teeth
Calculations involving DP gears differ from those involving CP gears in that the DP number is related to the diameter of the plate cylinder instead of its circumference.
Formula
(Pi x Diameter / 32) x # Teeth = Maximum Repeat
For example with a 2 inch diameter plate cylinder.
((3.1416 x 2)/32) x 51 = 10.013 Maximum Repeat
Conversely to find the gear size teeth needed to print a given repeat length
Formula
Repeat Size / (Pi / 32) = Gear Size Teeth
For Example; 7.5" Repeat
7.5 / (3.1416 / 32) = 76.39 Teeth