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Calculating Oversize Allowance

Many people believe that to calculate the increase required for surface coatings, one must add the actual thickness of the coating to the component dimensions. Not so. This is fine in the case of plain bar, where the physical increase is measurable in vertical and horizontal planes.

It is necessary for Commercial Fasteners to be interchangeable throughout the World, so manufacturers have adopted universal standards to ensure dimensional compatability.

Screw thread measurement is based upon a theoretical dimension known as the "Effective Diameter". Although there are many different thread forms in use today, this system is used in virtually all instances. The Effective Diameter (ED) is a measurement taken from an axis drawn horizontally, at approximately half of the total thread height, to a mirror image position, diametrically opposite. The same method applies for both internal and external threads.

Manufacturing standards have been created around this method based on the "Nominal" or ideal dimension in each case, and based upon these standards, the allowance for oversize tapping is derived.

The Male fastener is assumed to be manufatured to standard dimensions before coating applied.


Formula for Oversize Tapping Allowance

The increase in ED is represented by the hypotenuse (AC) of the right angle triangle (shown in yellow) multiplied by 2. The actual coating thickness is represented by the "Opposite" side of the triangle (BA).

Allowance for a given coating is calculated by the following formula:-

2x (Dim A-C) = 2 x t (coating thickness) / Sine of Angle (x°) / 2

(x°) = the Thread Inclusive Angle - (UN & ISO Thread Form = 60°)
(Whitworth Thread Form, ie: BSW & BSF = 55°)