Spot face milling for precision assembly

When we design and manufacture components with tight tolerances, we use precision processes (like grinding) on the mating surfaces in order to achieve the desired results. But even if we have perfect surfaces when we bolt together strongly the components, we observe that our surfaces don’t mate so perfect.

The problem lies under the “skin” of the mating surfaces. The thread-bolt and the produced forces that hold the components together are responsible for deformations on the threaded surface. The bolt through its action causes tensile stresses around the threaded surface; these stresses deform the surface around the thread, creating a “hill” a few micrometers high. No more a perfect mating.
The solution is a small spot face cut around the thread a few tenths of a millimeter deep. The diameter of the spot face must be 1.5-2 times the nominal diameter of the thread. Spot face is a simple detail that saves us from form inaccuracies.