(Engineering Bulletin #/web963)
Special considerations are necessary when using plating processes that add material thickness to Acme screw threads. These processes include electroplating of zinc, chrome and cadmium as well as electroless nickel and many others. Processes such as black oxiding, which transform the surface but do not add thickness to the surface require no special attention.
Plating will build up the thickness of an Acme thread pitch diameter at 8 times the actual plating thickness. The major diameter will build up at 2 times the actual plating thickness (See table). Another factor that complicates plating of Acme threads is that Acme threaded parts are often very long and thin. Electroplating processes, because of higher electric currents at the ends of these parts, will deposit thicker coatings at the ends. A thickness of .0002 may be measured in the middle of a screw shaft while the ends measure .0004 on parts only 10 diameters in length. On longer parts, the difference can be even greater. If a screw and mating nut are both plated, fit-up problems can be expected.
One way to work around this build up problem is to use special undersized, pre-plated thread sizes. This may be impractical, because, it can require special tooling and gaging.
Another consideration when evaluating plating is wear. The first level of wear on a plated part will be the plating itself. Once the plating has worn away, bare metal is exposed to the environment. Since Acme screws often “wear-in” much like plain bearings, bare metal can be exposed rather quickly depending upon the loading, duty cycle and lubrication present.
For very long parts, and parts in critical applications, use of stainless steel screws is often the best solution. No special sizing is required and after “wear-in” the exposed metal still provides corrosion protection for the application.
Plating size changes for Acme screws
CIncrease in Major
DIncrease in Pitch