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.
is fine in the case of plain bar, where the physical increase is
measurable in vertical and horizontal planes.
is necessary for Commercial Fasteners to be interchangeable throughout
the World, so manufacturers have adopted universal standards to
ensure dimensional compatability.
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
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.
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.
Male fastener is assumed to be manufatured to standard dimensions
before coating applied.