The Effects of Plating.. What you really need to know!
The KATO Tech Group receives many technical inquiries on the usage of various platings and coatings applied to helical coil inserts. In general, platings and coatings are used to protect components from corrosion and other environmental factors. They can also provide increased wear resistance, dimensional stability, increased hardness, and lubricity. The lubricity characteristics can minimize friction and galling with the mating screw or bolt, as well as improve installation properties of the inserts.
Figure 1: Plating build-up on various surfaces.
The specifications covering free running and locking Tangless Inserts, NAS1130, as well as locking Tanged inserts, NASM21209 both give the user the option of specifying either Cadmium Plating in accordance with QQ-P-416, or Dry Film Lubricant (Molybdenum Disulfide) in accordance with MIL-L-46010.
Stainless steel helical coil inserts without platings or coatings can be used in environments with temperatures up to 800° F. However, cadmium plating and dry film lubricant have limited high temperature capabilities. Please contact the KATO Tech Group for plated or coated inserts used in temperatures above 450° F. The Cadmium Plating and Dry Film Lubricant Coatings should not be used in vacuum type applications due to their out-gassing properties.
When designing threaded assemblies, compensation must be made in the tolerances for plating buildup, depending on the plating thickness. For discussion purposes, we will show the effect of plating buildup on various surfaces in the following examples, assuming a plating thickness of 0.0003 inches is used.
Figure 1 shows the buildup of plating and how it increases the overall dimensions of each example shown. In the case of a simple surface, the plating increases the thickness by .0003 in. Plating on a diameter increases that diameter by 2 x .0003 = .0006 in. Plating on an internal or external thread is a bit more complicated. Because of the 60° thread form, the buildup of plating on the pitch diameter is 4 x .0003 = .0012 in. The thread geometry is shown in Figure 2.
Figure 2: Plating build-up on a 60° thread form.
Here’s the part that may be new to you. Figure 2 details the plating build-up on a thread form. Again because of the 60° thread geometry combined with the fact that there are four sides being plated, the buildup of plating on the pitch diameter of a helical coil insert is 8 x .0003 = .0024.
In many cases it is necessary to compensate for plating on the tapped hole or insert by using STI taps with a larger pitch diameter. The tap manufacturer does this by specifying a larger “H limit” on the tap, depending on the amount of compensation needed for plating thickness. Each H limit represents 0.0005 inches over basic pitch diameter.
For more information on H limits and Class of fit, see Article 0702, The Science behind Pitch Diameter and Class of Fit.
Plating an insert is also a solution for correcting an oversize tapping condition. If an STI tapped hole was tapped oversize by .001 in., the final assembly could be brought back into specification by plating the insert with .000125 in. plating thickness, i.e. (8 x .000125 = .001).