Regal Cutting Tools Full Line Catalog
June 29, 2025
- 800-435-2948
- 800-435-2948
Applying protective or decorative plating to threaded fasteners inevitably alters critical dimensions, including thread gages and pitch diameters. To maintain the precise class of fit required for a specific application, manufacturers must calculate and compensate for these dimensional shifts before the tapping process even begins.
Failing to account for the thickness of a plating deposit on internal threads is a primary cause of failed quality inspections, customer rejections, and costly manufacturing delays. Selecting the correct oversize tap is the single most crucial step in ensuring your plated internal threads meet exact engineering tolerances.
When a coating is applied to an internal thread, it does not simply reduce the hole size by the raw thickness of the plating. Because threads consist of angled flanks, the plating material builds up on both sides of the thread profile, compounding the reduction of the pitch diameter (P.D.).
To determine the correct tap size, you must first calculate exactly how much the plating will reduce the pitch diameter. This is done using a specific mathematical ratio based on the thread angle. As the thread angle becomes steeper (narrower), the effect of the plating thickness on the pitch diameter becomes exponentially more severe.
| Thread Type | Thread Angle | Plating Thickness to Pitch Diameter Ratio |
|---|---|---|
| Standard Unified/Metric | 60-degree | 4:1 (P.D. decreases by 4x plating thickness) |
| Acme Threads | 29-degree | 8:1 (P.D. decreases by 8x plating thickness) |
| Square Threads | 10-degree | 23:1 (P.D. decreases by 23x plating thickness) |
Note: This assumes the plating is applied properly and uniformly across the entire thread profile.
To compensate for the material added during the plating process, the internal thread must be cut slightly larger than the final specification. When the plating is eventually applied, it will build up the surfaces, reducing the pitch diameter perfectly down to the required class of fit.
Manufacturers achieve this by utilizing oversize taps or taps with higher H-limits.
To find your target pre-plating dimensions, calculate the maximum and minimum plating adjustments. Add the maximum plating adjustment to the minimum allowable pitch diameter, and add the minimum plating adjustment to the maximum acceptable pitch diameter.
To illustrate the process, here is how you would determine the target pitch diameter and tap size for a standard 60-degree threaded hole that requires a plating thickness specification of .0002 to .0004 inches.
By precisely calculating these “before plate” tap limits, the final tapped threads will consistently fall within the required hole limits after the coating is applied. This success ultimately relies on the plating facility maintaining the deposit within the prescribed minimum and maximum thickness tolerances, while ensuring no excessive pooling occurs at the major diameter of the tapped thread. Utilizing high-quality, precision-ground oversize taps ensures the foundational machining is flawlessly prepped for downstream finishing.
