Tap Selection for Plated Internal Threads

Applying plating to fasteners necessarily alters gages, pitch diameters, and other characteristics. Manufacturers must consider and make allowances for these changes in order to maintain the class of fit the application calls for and the customer desires. Tap selection becomes the crucial element in creating internal threads that meet the required tolerances. Neglecting to account for the alterations plating will bring about on internal threads can result in failed inspections, customer rejection, and lost profits.

Calculating the Effects

The first step towards compensating for the effects of plating is to determine exactly how and by how much the plating will change the threads’ dimensions. For instance, for the commonly specified 60-degree thread, the plating on the threads will decrease the pitch diameter of internal threads by four times its thickness (double the plate thickness on both sides of the threads) when it is applied properly and uniformly. This 4:1 ratio becomes even more pronounced as the thread angle decreases; 8:1 on 29-degree Acme threads; 23:1 on 10-degree square-threaded nuts.

The Solution

tap in use

The easiest and most common solution to a coating’s effect on internal threads’ pitch diameter is to start with a slightly larger P.D., so that applying the coating will reduce the diameter by the correct amount to make the finished product fall within class-of-fit specifications. Using oversize or higher H-limit taps will create the wider diameter pitch called for in the calculations. As a rule of thumb, increase the standard tap one “H” number for every .125 mil (3.2 micron) of plating thickness. To compensate to the correct “pre-plating” internal thread dimensions to be tapped, add the maximum “plating adjustment” (4:1 for 60-degree threads) to the minimum allowable pitch diameter. Then add the minimum plating adjustment to the maximum acceptable pitch diameter.

The Math

Here’s an example of how to determine the proper tap for a 60-degree threaded hole with a plate thickness specification of .0004 to .0002:

  • Multiply the max thickness by the “degree ratio” (4:1 in this case). 4 x .0004 - .0016. This is the maximum plating adjustment.
  • Multiply the min thickness by the degree ration. 4 x .0002 = .0008. This is the minimum plating adjustment.
  • Select a tap that will create internal threads within these tolerances. That is, tap the hole with a pitch dimension that is .0012 (midway between .0008 and .0016) larger than the finished product needs to be. Or use the 40-percent rule and make the P.D. .00096 larger. 

Using correct “before plate” tap thread limits will ensure the tapped threads will fall within the hole limits after plating, as long as the plate deposit remains within the prescribed minimum and maximum thickness with no excessive plating at the major diameter of the tapped thread.

Regal manufactures a complete line of metal-cutting tools. Our oversize taps deliver the precise tolerances required to ensure plated internal threads meet exact specifications. To discuss your drilling and tapping needs, contact us via our form, or call 800-435-2948 to speak to one of our experts.