Coated Nut Undercutting or Tapped Oversize
Hot dip spun galvanizing is a coating of corrosion-inhibiting, highly abrasion resistant molten zinc on bare steel, the original bolt becomes slightly thicker. When galvanising fastener threads the increased thickness is important and needs to be removed.
In consideration of the added thickness, nuts and threaded holes need to be overtapped (enlarged) in order to accommodate the increased size of the fastener to be inserted. While chasing or retapping the nuts after galvanizing results in an uncoated female thread, the zinc coating on the engaged male thread will protect both components from corrosion. For economy, nuts are usually galvanized as blanks and the threads tapped oversize after galvanizing.
Tapping of all holes after galvanizing is recommended to eliminate double-tapping costs and the possibility of cross-threading. If, after galvanizing, the hole is still not large enough, it can be chased or retapped. Chasing or retapping will not necessitate re-galvanizing. Because of the zinc coating’s cathodic properties, the coating on the mating fastener will protect chased or retapped nuts.
Nuts and other tapped fasteners that are Hot Dip Spun Galvanised, or have other heavy coatings deposited on the nut will require re-tapping to clear coating material from the threads in the nuts.
Nuts can also be tapped oversize before coating to allow for heavy coating deposits in the threads.
Solutions to avoid Under Cutting Threads
Galvanised zinc bolt threads increased thickness is accommodated by the normal practice of over tapping the nut thread. For more critical applications the minor reduction of strength of the nut and the overall strength of the nut and bolt assembly may be a factor. Improving the strength of the construction can be achieved by using thicker nut this will increase the number of threads in the nut and the bolt assembly the other solution is to increase to a higher class grade of nut.