The answer to this question can be found in this blog and in many available references. References are listed below and an abbreviated table is listed here. You and your customer must decide upon the acceptable specification and then produce the product accordingly. The data provided here is a guide that does work but can vary from machine to machine. The exact settings must be determined by trial and error.
Slight modifications of these values will work for bare non coated steel.
A full list of materials and thicknesses and associated weld schedules can be obtained in AWS Standard C1.1. The RWMA Manual offers a full background in Resistance Welding including Machinery, Electrical and all components. The Tuffaloy Products Catalog offers weld schedules.
Related Article:
What are the typical resistance weld schedule settings to weld 1mm to 2mm low carbon steel?
References: AWS Standard C1.1 - Recommended Practices for Resistance Welding
RWMA – Resistance Welding Manual 4th Edition
Tuffaloy Products Catalog