Projection weld nuts are used in many industrial applications. Many are piloted weld nuts as shown below.
THREE PROJECTION WELD NUT WITH PILOT RING
To make a good projection weld several issues must be considered:
They are:
1. The purpose for the pilot on a weld nut: The short answer is to help protect the internal portion of the weld nut from expulsion and other foreign material while being welded. The tooling and part associated with the projection welding (PW) of forged or coined weld nuts (think solid projections) must be properly designed, to include ways to minimize expulsion getting into the threads. A proper pilot on the nut can help in this regard.
2. The tooling used is also really important: The pin package and other portions of the tooling must be robust, able to hold the part in a repeatable location, and place the weld nut accurately and centered on the hole. Items that need to be considered as part of this design are the gauge of the material being welded, the thickness of the nut, the height of the projections, and where electrical insulation is needed.
3. The proper size of the hole in the part: The stated clearance value of 0.002” is too small for the size of the application. One PW design standard I am familiar with specifies for an M10 piloted weld nut (close to the 3/8 nut in this question) a 15.0-15.15 mm hole Ø for a pilot on the nut, with a pilot of between 12.4 – 12.7 mm Ø. The aforementioned standards would result in clearances of roughly double reported above.
4. The welding equipment must be capable, and the schedule proper for the application: This one is very application dependent. There is an article in the Welding Journal (Sep-2010 - RWMA Q & A) that might be helpful to address machine and weld schedule.
References: RWMA – Resistance Welding Manual 4th Edition
AWS – AWS Standard C1.1 Recommended Practices for Resistance Welding
Technical Input by - DONALD F. MAATZ Jr.
Milco Manufacturing Company