CALCULATING OVERSIZE THREADING ALLOWANCES

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. This is not the case. This assumption is fine in the case of plain bar, where the physical increase is measurable in vertical and horizontal planes. With threaded components the additional coating is applied to the threads at an angle, hence the resulting increase upon the "effective diameter" is greater than the increase upon the plain shank in terms of the engagement with the nuts.

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

Screw thread measurement is based upon a theoretical dimension known as the "Effective Diameter". Although there are still 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 (See figure). The same designation is applied to both internal and external threads.

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

In most cases, the Male fastener is assumed to be manufatured to nominal dimensions before coating applied. However, some manufacturers choose to reduce the effective diameter of the bolts by "underscrewing" the threads. With hot dip galvanising however, a re-tapping operation is still necessary because the threads of the nuts still tend to fill up when coated. Oversize tapping is the more widely adopted solution as in this case the original integrity of the male fastener is not compromised with a view to Tensile Strength