Thread plating allowance calculator

Because they are different surfaces. The crest of a thread is a cylinder, so a coating of thickness t adds t to each side and the outside diameter grows by 2t. The flanks sit at 30 degrees to the thread axis, so the same coating displaces the flank by t divided by the sine of 30 degrees on each side. That works out to a pitch diameter growth of exactly 4t. A plated screw can measure correctly across the crests on a micrometer and still be rejected by a ring gauge, because the gauge is reading the pitch diameter.

The Class 2A pitch diameter allowance for 1/4-20 UNC computes to 0.0011 inch. Since coating consumes that allowance at four times its thickness, the gauge ceiling is 0.0011 divided by 4, which is 0.000275 inch, just under 7 micrometres. Read that as a ceiling on the maximum local thickness anywhere on the thread. ASTM B633 SC1 asks for a 5 micrometre minimum, which leaves under 2 micrometres of headroom, so it is geometrically possible but tight for production control. SC2 at 8 micrometres is already above the ceiling and will not gauge.

Not on a standard UNC thread. SC4 calls for 25 micrometres. The largest Class 2A allowance in the UNC series, at 4 inch diameter, computes to 0.0034 inch, giving a ceiling of 0.00085 inch or 21.59 micrometres. Every standard UNC size from #1 up to 4 inch has a ceiling below 25 micrometres, so SC4 does not fit any of them. The thread has to be cut undersize before plating, the class relaxed, or the coating specified thinner.

A Class 3A external thread is cut to basic size with no allowance at all, so there is no room for any coating whatsoever. Any plating at all pushes the pitch diameter over the maximum. A 3A thread that has to be plated must be cut undersize before plating.

Standard internal threads carry no allowance, so there is nothing to consume. The remedy is to tap oversize before plating. One tap H number is 0.0005 inch of pitch diameter, and since coating moves pitch diameter at four times its thickness, one H number corresponds to 0.000125 inch, about 3.2 micrometres, of plating.

Not because mechanical deposition changes the geometry. The four times relationship comes from the thread angle and applies to any coating however it is deposited, and ASTM B695 itself warns about buildup in thread roots. What makes heavy coatings assemble is the nut. ASTM A563 covers nuts tapped oversize for use with hot-dip and mechanically zinc-coated externally threaded fasteners, so the clearance for the coating comes from the nut rather than from the bolt thread allowance. If you need a heavy zinc coating on a threaded part, the overtapped nut is the mechanism to specify.

Yes, on anything longer than about five diameters. ISO 4042 Annex D.3 notes that local coating thickness at mid-length runs roughly one third to one half of the thickness at the extremities, because current density is higher at the ends. So plating enough to reach the specified minimum in the middle drives the ends to roughly two to three times that figure, and it is the ends that have to stay under the gauge ceiling. Enter a length in the calculator and it will flag this.