Since allowance for tolerance pos3tion g is 0.038, the maximum thickness of coating which may be applied is equal to 0.038 divided by 6 or 0.0063. This is not sufficient for the required coating of 0.008 so additional adjustments to the before-coated pitch and major diameters must be made.
To determine the before coating maximum material sizes, decrease the maximum pitch diameter of 12.701 by 0.048 (6 x 0.008) to 12.653 and maximum major diameter of 14.000 by 0.024 (3 x 0.008) to 13.976. For the before coating minimum sizes, decrease the minimum pitch diameter of 12.601 by 0.032 (4 x 0.008) to 12.569 and the minimum major diameter of
13.720 by 0.016 (2 x 0.008) to 13.704. The before coating sizes should be included in the thread designation. (See para. 7.8.2.)
8.5.3 Adjusted Size limits. It should be noted in the above examples that the before coating material limit tolerances are less than the tolerances after coating. This is because the coating tolerance consumes some of the product tolerance. In some instances there may be insufficient pitch diameter tolerance available in the before coating condition so that additional adjustments and controls will be necessary.
8.5.4 Strength. On small thread sizes (5 mm and smaller) there is a possibility that coating thickness adjustments will cause base material minimum material conditions which may significantly affect strength of externally threaded parts. Limitations on coating thickness or part redesign may be necessary.
8.6 Internal Threads
Standard internal threads provide no allowance for coating thickness. To determine before coating limits for a uniformly coated thread, increase
(a) minimum pitch diameter by 4 times maximum coating thickness, if specified, or by 6 times minimum S.
or nominal coating thickness;
(c) minimum minor diameter by 2 times maximum coating thickness, if specified, or by 3 times minimum or nominal coating thickness;
(d) maximum minor diameter by 2 times minimum or nominal coating thickness.
8.7 Electrodeposited Coatings
Electroplated coatings do not cover threads uniformly. Deposits build up more on thread crests than on thread flanks and coating thickness is greater on the lead end of a thread than on threads in the center of a part. The guidelines for calculating pre-coating thread sizes for uniformly coated threads can be used as a starting place in determining the pre-coating thread sizes on a given part, but the exact sizes for any given part may have to be determined by trial and error experimentation. One of the effects of electro-deposited coatings not applying uniformly is that the functional diameter of an external thread will generally increase by a greater magnitude than the increase of the pitch diameter on a given part. Depending on an externally threaded part’s configuration, an electro-deposited coating of 0.0025 mm on a thread flank will probably result in a growth of the pitch diameter of approximately 0.0100 mm or more, but the functional diameter may change by as much as 0.0150 mm or more. In the case of internally threaded parts, electro-deposited coatings cause the pitch diameter and functional diameter to decrease in size, with the pitch diameter decreasing less than the functional diameter on a given part.
Another effect of the non-uniform build-up of electrodeposited coatings is that the coating thickness on the lead end of a thread is commonly greater than the thickness on the threads in the center of the part. This results in the pitch diameter and the functional diameter of a thread to be considerably larger on the end of the part than their comparable measurements away from the end. The longer the part, in the case of external threads, or the thicker the part, in the case of internal threads, the greater these differences tend to be. The part producer and the plater cannot control this phenomenon. It is the nature of electro-deposited coatings on threaded parts. It is conceivable that a part’s dimensions can measure within limits on the lead threads and be beyond their limits on the threads away from the lead threads.